Absorption Chiller[LINK]
The input object Chiller:Absorption provides a model for absorption chillers that is an empirical model of a standard absorption refrigeration cycle. The condenser and evaporator are similar to that of a standard chiller, which are both watertowater heat exchangers. The assembly of a generator and absorber provides the compression operation. Lowpressure vapor from the evaporator is absorbed by the liquid solution in the absorber. A pump receives lowpressure liquid from the absorber, elevates the pressure of the liquid, and delivers the liquid to the generator. In the generator, heat from a high temperature source (hot water or steam) drives off the vapor that has been absorbed by the solution. The liquid solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to maintain the pressure difference between the generator and absorber. The heat supplied to the absorber can be waste heat from a diesel jacket, or the exhaust heat from diesel, gas, and steam turbines. For more information on absorption chillers, see the Input/Output Reference Document (Object: Chiller:Absorption).
The partload ratio of the absoprtion chiller’s evaporator is simply the actual cooling effect produced by the chiller divided by the maximum cooling effect available.
where
= partload ratio of chiller evaporator
= chiller evaporator load [W]
= rated chiller evaporator capacity [W]
This absorption chiller model is based on a polynomial fit of absorber performance data. The Generator Heat Input Part Load Ratio Curve is a quadratic equation that determines the ratio of the generator heat input to the demand on the chiller’s evaporator (Q_{evap}).
The Pump Electric Use Part Load Ratio Curve is a quadratic equation that determines the ratio of the actual absorber pumping power to the nominal pumping power.
Thus, the coefficient sets establish the ratio of heat power intocooling effect produced as a function of part load ratio. The ratio of heatpowerin to coolingeffectproduced is the inverse of the coefficient of performance.
If the operating partload ratio is greater than the minimum partload ratio, the chiller will run the entire time step and cycling will not occur (i.e. CyclingFrac = 1). If the operating partload ratio is less than the minimum partload ratio, the chiller will be on for a fraction of the time step equal to CyclingFrac. Steam (or hot water) and pump electrical energy use are also calculated using the chiller partload cycling fraction.
where
= chiller partload cycling fraction
= chiller minimum partload ratio
= generator input power [W]
= absorbtion chiller pumping power [W]
The evaporator water mass flow rate is calculated based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= chiller evaporator water mass flow rate (kg/s)
= chiller design evaporator water mass flow rate (kg/s)
= chiller evaporator water temperature difference (ºC)
= chiller evaporator inlet water temperature (ºC)
= chiller evaporator outlet water setpoint temperature (ºC)
= specific heat of water entering evaporator (J/kg•ºC)
The evaporator outlet water temperature is then calculated based on the cooling effect produced and the evaporator entering water temperature.
where
= chiller evaporator outlet water temperature [ºC]
= chiller evaporator inlet water temperature [ºC]
= specific heat of chiller evaporator inlet water [J/kg/ºC]
= chiller evaporator water mass flow rate [kg/s]
The condenser heat transfer and condenser leaving water temperature are also calculated.
where
= chiller condenser heat transfer rate [W]
= chiller condenser outlet water temperature [ºC]
= chiller condenser inlet water temperature [ºC]
= specific heat of chiller condenser inlet water [J/kg/ºC]
= chiller condenser water mass flow rate [kg/s]
The absorption chiller can model the impact of steam or hot water entering the generator, although the connection of the steam (hot water) nodes to a plant is not actually required. The calculations specific to the generator depend on the type of fluid used and are described here in further detail.
Steam Loop Calculations[LINK]
When a steam loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a steam loop), the generator outlet node steam mass flow rate and temperature are calculated based on the generator input power, latent heat of steam, the specific heat of water, and the amount of subcooling in the steam generator. The model assumes dry saturated steam enters the absorption chiller’s generator and exits the generator as a subcooled liquid. The temperature leaving the generator is calculated based on the user entered amount of liquid subcooling in the generator. The effect of subcooling of the liquid (condensate) in the pipe returning to the boiler is not modeled.
where
= chiller steam mass flow rate [kg/s]
= latent heat of steam [J/kg]
= specific heat of saturated water in the generator [J/Kg ºK]
= amount of subcooling in steam generator [ºC]
= generator steam outlet node temperature [ºC]
= generator steam inlet node temperature [ºC]
Hot Water Loop Calculations[LINK]
When a hot water loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a hot water loop), the generator outlet node temperature is calculated based on the generator input power, mass flow rate of water, and the specific heat of water entering the hot water generator. The calculations are based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= generator hot water mass flow rate [kg/s]
= generator design hot water mass flow rate (kg/s)
= generator design hot water temperature difference (ºC)
Indirect Absorption Chiller[LINK]
The Chiller:Absorption:Indirect object is an enhanced version of the absorption chiller model found in the Building Loads and System Thermodynamics (BLAST) program. This enhanced model is nearly identical to the existing absorption chiller model (Ref. Chiller:Absorption) with the exceptions that: 1) the enhanced indirect absorption chiller model provides more flexible performance curves and 2) chiller performance now includes the impact of varying evaporator, condenser, and generator temperatures. Since these absorption chiller models are nearly identical (i.e., the performance curves of the enhanced model can be manipulated to produce similar results to the previous model), it is quite probable that the Chiller:Absorption model will be deprecated in a future release of EnergyPlus.
The indirect absorption chiller’s condenser and evaporator are similar to that of a standard chiller, which are both watertowater heat exchangers. The assembly of a generator and absorber provides the compression operation. A schematic of a singlestage absorption chiller is shown in the figure below. Lowpressure vapor from the evaporator is absorbed by the liquid solution in the absorber. A pump receives lowpressure liquid from the absorber, elevates the pressure of the liquid, and delivers the liquid to the generator. In the generator, heat from a high temperature source (hot water or steam) drives off the vapor that has been absorbed by the solution. The liquid solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to maintain the pressure difference between the generator and absorber. The heat supplied to the generator can be either hot water or steam, however, connection to an actual plant loop is not required. For more information on indirect absorption chillers, see the Input/Output Reference Document (Object: Chiller:Absorption:Indirect).
The chiller cooling effect (capacity) will change with a change in condenser water temperature. Similarly, the chiller cooling effect will change as the temperature of the evaporator water changes. The chiller cooling effect will also change with a change in or generator inlet water temperature and only applies when Hot Water is used as the generator heat source. A quadratic or cubic equation is used to modify the rated chiller capacity as a function of both the condenser and generator inlet water temperatures and the evaporator outlet water temperature. If any or all of the capacity correction factor curves are not used, the correction factors are assumed to be 1.
(Hot Water only)
where
= Capacity correction (function of evaporator temperature) factor
= Capacity correction (function of condenser temperature) factor
= Capacity correction (function of generator temperature) factor
= evaporator outet water temperature [C]
= condenser inlet water temperature [C]
= generator inlet water temperature [C]
= maximum chiller capacity [W]
= rated chiller capacity [W]
The partload ratio of the indirect absoprtion chiller’s evaporator is simply the actual cooling effect required (load) divided by the maximum cooling effect available.
where
= partload ratio of chiller evaporator
= chiller evaporator operating capacity [W]
The generator’s heat input is also a function of several parameters. The primary input for determining the heat input requirements is the Generator Heat Input function of PartLoad Ratio Curve. The curve is a quadratic or cubic equation that determines the ratio of the generator heat input to the chiller’s maximum capacity (Q~evap, max~) and is soley a function of partload ratio. Typical generator heat input ratios at full load (i.e., PLR=1) are between 1 and 2. Two additional curves are available to modifiy the heat input requirement based on the generator inlet water temperature and the evaporator outlet water temperature.
where
GeneratorHIR = ratio of generator heat input to chiller operating capacity
GenfCondT = heat input modifier based on generator inlet water temperature
GenfEvapT = heat input modifier based on evaporator outlet water temperature
The Pump Electric Use function of PartLoad Ratio Curve is a quadratic or cubic equation that determines the ratio of the actual absorber pumping power to the nominal pumping power.
If the chiller operating partload ratio is greater than the minimum partload ratio, the chiller will run the entire time step and cycling will not occur (i.e. CyclingFrac = 1). If the operating partload ratio is less than the minimum partload ratio, the chiller will be on for a fraction of the time step equal to CyclingFrac. Generator heat input and pump electrical energy use are also calculated using the chiller partload cycling fraction.
where
= chiller partload cycling fraction
= chiller minimum partload ratio
= generator heat input [W]
= chiller pumping power [W]
The evaporator water mass flow rate is calculated based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= chiller evaporator water mass flow rate (kg/s)
= chiller design evaporator water mass flow rate (kg/s)
= chiller evaporator water temperature difference (ºC)
= chiller evaporator inlet water temperature (ºC)
= chiller evaporator outlet water setpoint temperature (ºC)
= specific heat of water entering evaporator (J/kg ºC)
The evaporator outlet water temperature is then calculated based on the cooling effect produced and the evaporator entering water temperature.
where
= chiller evaporator outlet water temperature [ºC]
= chiller evaporator inlet water temperature [ºC]
= specific heat of chiller evaporator inlet water [J/kg/ºC]
= chiller evaporator water mass flow rate [kg/s]
The condenser heat transfer and condenser leaving water temperature are also calculated.
where
= chiller condenser heat transfer rate [W]
= chiller condenser outlet water temperature [ºC]
= chiller condenser inlet water temperature [ºC]
= specific heat of chiller condenser inlet water [J/kg/ºC]
= chiller condenser water mass flow rate [kg/s]
The absorption chiller can model the impact of steam or hot water entering the generator, although the connection of the steam (hot water) nodes to a plant is not actually required. The calculations specific to the generator depend on the type of fluid used and are described here in further detail.
Steam Loop Calculations[LINK]
When a steam loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a steam loop), the generator outlet node steam mass flow rate and temperature are calculated based on the generator heat input, latent heat of steam, the specific heat of water, and the amount of subcooling in the steam generator. The model assumes dry saturated steam enters the generator and exits the generator as a subcooled liquid. The temperature leaving the generator is calculated based on the user entered amount of liquid subcooling in the generator. The effect of subcooling of the liquid (condensate) in the pipe returning to the boiler is also modeled using the user entered abount of steam condensate loop subcooling.
where
= chiller steam mass flow rate [kg/s]
= latent heat of steam [J/kg]
= specific heat of water [J/Kg ºC]
= amount of subcooling in steam generator [ºC]
= amount of condensate subcooling in steam loop [ºC]
= generator steam outlet node temperature [ºC]
= generator steam inlet node temperature [ºC]
Hot Water Loop Calculations[LINK]
When a hot water loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a hot water loop), the generator outlet node temperature is calculated based on the generator heat input, mass flow rate of water, and the specific heat of water entering the hot water generator. The calculations are based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= generator hot water mass flow rate [kg/s]
= generator design hot water mass flow rate (kg/s)
= generator design hot water temperature difference (ºC)
Combustion Turbine Chiller[LINK]
The input object Chiller:CombustionTurbine provides a chiller model that is the empirical model from the Building Loads and System Thermodynamics (BLAST) program. Fitting catalog data to a third order polynomial equations generates the chiller performance curves. Three sets of coefficients are required to model the open centrifugal chiller as discussed in the section, titled, ‘Electric Chiller Based on BLAST Centrifugal Chiller Model’.
The gas turbinedriven chiller is an open centrifugal chiller driven directly by a gas turbine. The BLAST model of an open centrifugal chiller is modeled as standard vapor compression refrigeration cycle with a centrifugal compressor driven by a shaft power from an engine. The centrifugal compressor has the incoming fluid entering at the eye of a spinning impeller that throws the fluid by centrifugal force to the periphery of the impeller. After leaving the compressor, the refrigerant is condensed to liquid in a refrigerant to water condenser. The heat from the condenser is rejected to a cooling tower, evaporative condenser, or well water condenser depending on which one is selected by the user based on the physical parameters of the plant. The refrigerant pressure is then dropped through a throttling valve so that fluid can evaporate at a low pressure that provides cooling to the evaporator. The evaporator can chill water that is pumped to chilled water coils in the building. For more information, see the Input/Output Reference Document.
This chiller is modeled like the electric chiller with the same numerical curve fits and then some additional curve fits to model the turbine drive. Shown below are the definitions of the curves that describe this model.
The chiller’s temperature rise coefficient which is defined as the ratio of the required change in condenser water temperature to a given change in chilled water temperature, which maintains the capacity at the nominal value. This is calculated as the following ratio:
Where:
TCEnt_{required} = Required entering condenser air or water temperature to maintain rated capacity.
TCEnt_{rated} = Rated entering condenser air or water temperature at rated capacity.
TELv_{required} = Required leaving evaporator water outlet temperature to maintain rated capacity.
TELv_{rated} = Rated leaving evaporator water outlet temperature at rated capacity.
The Capacity Ratio Curve is a quadratic equation that determines the Ratio of Available Capacity to Nominal Capacity. The defining equation is:
Where the Delta Temperature is defined as:
TempCondIn = Temperature entering the condenser (water or air temperature depending on condenser type).
TempCondInDesign = Temp Design Condenser Inlet from User input above.
TempEvapOut = Temperature leaving the evaporator.
TempEvapOutDesign = Temp Design Evaporator Outlet from User input above.
TempRiseCoefficient = User Input from above.
The following three fields contain the coefficients for the quadratic equation.
The Power Ratio Curve is a quadratic equation that determines the Ratio of Full Load to Power. The defining equation is:
The Full Load Ratio Curve is a quadratic equation that determines the fraction of full load power. The defining equation is:
The Fuel Input Curve is a polynomial equation that determines the Ratio of Fuel Input to Energy Output. The equation combines both the Fuel Input Curve Coefficients and the Temperature Based Fuel Input Curve Coefficients. The defining equation is:
Where FIC represents the Fuel Input Curve Coefficients, TBFIC represents the Temperature Based Fuel Input Curve Coefficients, Rload is the Ratio of Load to Combustion Turbine Engine Capacity, and AT_{air}is the difference between the current ambient and design ambient temperatures.
The Exhaust Flow Curve is a quadratic equation that determines the Ratio of Exhaust Gas Flow Rate to Engine Capacity. The defining equation is:
**
Where GTCapacity is the Combustion Turbine Engine Capacity, and AT_{air} is the difference between the current ambient and design ambient temperatures.
The Exhaust Gas Temperature Curve is a polynomial equation that determines the Exhaust Gas Temperature. The equation combines both the Exhaust Gas Temperature Curve Coefficients (Based on the Part Load Ratio) and the (Ambient) Temperature Based Exhaust Gas Temperature Curve Coefficients. The defining equation is:
Where C represents the Exhaust Gas Temperature Curve Coefficients, TBC are the Temperature Based Exhaust Gas Temperature Curve Coefficients, RLoad is the Ratio of Load to Combustion Turbine Engine Capacity, and AT_{air} is the difference between the actual ambient and design ambient temperatures.
The Recovery Lubricant Heat Curve is a quadratic equation that determines the recovery lube energy. The defining equation is:
Where Pload is the engine load and RL is the Ratio of Load to Combustion Turbine Engine Capacity
The UA is an equation that determines the overall heat transfer coefficient for the exhaust gasses with the stack. The heat transfer coefficient ultimately helps determine the exhaust stack temperature. The defining equation is:
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
ChillerHeater:Absorption:DirectFired[LINK]
This model (object name ChillerHeater:Absorption:DirectFired) simulates the performance of a direct fired twostage absorption chiller with optional heating capability. The model is based on the direct fired absorption chiller model (ABSORGCHLR) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus some additional capabilities.
This model simulates the thermal performance of the chiller and the fuel consumption of the burner(s). This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. Cooling Tower:Single Speed).
Model Description[LINK]
The chiller model uses usersupplied performance information at design conditions along with five performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offdesign conditions. Two additional performance curves for heating capacity and efficiency are used when the chiller is operating in a heating only mode or simultaneous cooling and heating mode.
The following nomenclature is used in the cooling equations:
AvailCoolCap=available fullload cooling capacity at current conditions [W]
CEIR=user input “Electric Input to Cooling Output Ratio”
CEIRfPLR=electric input to cooling output factor, equal to 1 at full load, user input “Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CEIRfT=electric input to cooling output factor, equal to 1 at design conditions, user input “Electric Input to Cooling Output Ratio Function of Temperature Curve Name”
CFIR=user input “Fuel Input to Cooling Output Ratio”
CFIRfPLR=fuel input to cooling output factor, equal to 1 at full load, user input “Fuel Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CFIRfT=fuel input to cooling output factor, equal to 1 at design conditions, user input “Fuel Input to Cooling Output Ratio Function of Temperature Curve Name”
CondenserLoad=condenser heat rejection load [W]
CoolCapfT=cooling capacity factor, equal to 1 at design conditions, user input “Cooling Capacity Function of Temperature Curve Name”
CoolElectricPower=cooling electricity input [W]
CoolFuelInput=cooling fuel input [W]
CoolingLoad=current cooling load on the chiller [W]
CPLR=cooling partload ratio = CoolingLoad / AvailCoolCap
HeatingLoad=current heating load on the chiller heater [W]
HFIR=user input “Fuel Input to Heating Output Ratio”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
T_{cond}=entering or leaving condenser fluid temperature [C]. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower) if the entering condenser fluid temperature option is used. For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively, if the entering condenser fluid temperature option is used.
T_{cw,l}=leaving chilled water temperature [C]
Five performance curves are used in the calculation of cooling capacity and efficiency:
 Cooling Capacity Function of Temperature Curve
 Fuel Input to Cooling Output Ratio Function of Temperature Curve
 Fuel Input to Cooling Output Ratio Function of Part Load Ratio Curve
 Electric Input to Cooling Output Ratio Function of Temperature Curve
 Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve
The cooling capacity function of temperature (CoolCapfT) curve represents the fraction of the cooling capacity of the chiller as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature. The output of this curve is multiplied by the nominal cooling capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The available cooling capacity of the chiller is then computed as follows:
The fuel input to cooling output ratio function of temperature (CFIRfT) curve represents the fraction of the fuel input to the chiller at full load as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature. The output of this curve is multiplied by the nominal fuel input to cooling output ratio (CFIR) to give the fullload fuel input to cooling capacity ratio at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The fuel input to cooling output ratio function of part load ratio (CFIRfPLR) curve represents the fraction of the fuel input to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The fraction of the time step during which the chiller heater is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The cooling fuel input to the chiller is then computed as follows:
The electric input to cooling output ratio as function of temperature (CEIRfT) curve represents the fraction of electricity to the chiller at full load as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature.
The electric input to cooling output ratio function of part load ratio (CEIRfPLR) curve represents the fraction of electricity to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The cooling electric input to the chiller is computed as follows:
All five of these cooling performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (objects Curve:Linear, Curve:Quadratic and Curve:Biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s CFIRfPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero).
The condenser load is computed as follows:
The following nomenclature is used in the heating equations:
AvailHeatCap=available fullload heating capacity at current conditions [W]
CPLRh=cooling partload ratio for heating curve = CoolingLoad / NomCoolCap
HeatCapfCPLR=heating capacity factor as a function of cooling part load ratio, equal to 1 at zero cooling load, user input “Heating Capacity Function of Cooling Capacity Curve Name”
HeatCoolCapRatio=user input “Heating to Cooling Capacity Ratio”
HeatElectricPower=heating electricity input [W]
HeatFuelInput=heating fuel input [W]
HeatingLoad=current heating load on the chiller [W]
HEIR=user input “Electric Input to Heating Output Ratio”
HFIR=user input “Fuel Input to Heating Output Ratio”
HFIRfHPLR=fuel input to heating output factor, equal to 1 at full load, user input “Fuel Input to Heat Output Ratio During Heating Only Operation Curve Name”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
TotalElectricPower=total electricity input [W]
TotalFuelInput=total fuel input [W]
Cooling is the primary purpose of the Direct Fired Absorption Chiller so that function is satisfied first and if energy is available for providing heating that is provided next.
The two performance curves for heating capacity and efficiency are:
 Heating Capacity Function of Cooling Capacity Curve
 FuelInputto Heat Output Ratio Function
The heating capacity function of cooling capacity curve (HeatCapfCool) determines how the heating capacity of the chiller varies with cooling capacity when the chiller is simultaneously heating and cooling. The curve is normalized so an input of 1.0 represents the nominal cooling capacity and an output of 1.0 represents the full heating capacity. An output of 1.0 should occur when the input is 0.0.
The available heating capacity is then computed as follows:
The fuel input to heat output ratio curve (HFIRfHPLR) function is used to represent the fraction of fuel used as the heating load varies as a function of heating part load ratio. It is normalized so that a value of 1.0 is the full available heating capacity. The curve is usually linear or quadratic and will probably be similar to a boiler curve for most chillers.
 The fuel use rate when heating is computed as follows:
The fraction of the time step during which the chiller is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The heating electric input to the chiller is computed as follows:
If the chiller is delivering heating and cooling simultaneously, the parasitic electric load will be doublecounted, so the following logic is applied:
The total fuel and electric power input to the chiller is computed as shown below:
ChillerHeater:Absorption:DoubleEffect[LINK]
This model (object name ChillerHeater:Absorption:DoubleEffect) simulates the performance of an exhaust fired twostage (double effect) absorption chiller with optional heating capability. The model is based on the direct fired absorption chiller model (ABSORGCHLR) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus some additional capabilities. The model uses the exhaust gas output from Microturbine.
This model simulates the thermal performance of the chiller and the thermal energy input to the chiller. This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. Cooling Tower:Single Speed).
Model Description[LINK]
The chiller model uses usersupplied performance information at design conditions along with five performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offdesign conditions. Two additional performance curves for heating capacity and efficiency are used when the chiller is operating in a heating only mode or simultaneous cooling and heating mode.
The following nomenclature is used in the cooling equations:
AvailCoolCap=available fullload cooling capacity at current conditions [W]
CEIR=user input “Electric Input to Cooling Output Ratio”
CEIRfPLR=electric input to cooling output factor, equal to 1 at full load, user input “Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CEIRfT=electric input to cooling output factor, equal to 1 at design conditions, user input “Electric Input to Cooling Output Ratio Function of Temperature Curve Name”
TeFIR=user input “Thermal Energy Input to Cooling Output Ratio”
TeFIRfPLR=thermal energy input to cooling output factor, equal to 1 at full load, user input “Thermal Energy Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
TeFIRfT=thermal energy input to cooling output factor, equal to 1 at design conditions, user input “Thermal Energy Input to Cooling Output Ratio Function of Temperature Curve Name”
CondenserLoad=condenser heat rejection load [W]
CoolCapfT=cooling capacity factor, equal to 1 at design conditions, user input “Cooling Capacity Function of Temperature Curve Name”
CoolElectricPower=cooling electricity input [W]
CoolThermalEnergyInput=cooling thermal energy input [W]
CoolingLoad=current cooling load on the chiller [W]
CPLR=cooling partload ratio = CoolingLoad / AvailCoolCap
HeatingLoad=current heating load on the chiller heater [W]
HFIR=user input “Thermal Energy Input to Heating Output Ratio”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
**= exhaust air mass flow rate from microturbine (kg/s)
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
= exhaust air outlet temperature from microturbine entering the chiller
(^{o}C)
= Temperature of exhaust leaving the chiller (the generator component of the absorption chiller)
T_{cond}=entering condenser fluid temperature [°C]. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively.
T_{cw,l}=leaving chilled water temperature [°C]
The selection of entering or leaving condense fluid temperature can be made through the optional fieldTemperature Curve Input Variable.
Five performance curves are used in the calculation of cooling capacity and efficiency:
 Cooling Capacity Function of Temperature Curve
 Thermal Energy Input to Cooling Output Ratio Function of Temperature Curve
 Thermal Energy Input to Cooling Output Ratio Function of Part Load Ratio Curve
 Electric Input to Cooling Output Ratio Function of Temperature Curve
 Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve
The cooling capacity function of temperature (CoolCapfT) curve represents the fraction of the cooling capacity of the chiller as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and the entering condenser fluid temperature. The output of this curve is multiplied by the nominal cooling capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The available cooling capacity of the chiller is then computed as follows:
The thermal energy input to cooling output ratio function of temperature (TeFIRfT) curve represents the fraction of the thermal energy input to the chiller at full load as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and the entering condenser fluid temperature. The output of this curve is multiplied by the nominal thermal energy input to cooling output ratio (TeFIR) to give the fullload thermal energy input to cooling capacity ratio at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The thermal energy input to cooling output ratio function of part load ratio (TeFIRfPLR) curve represents the fraction of the thermal energy input to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The fraction of the time step during which the chiller heater is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The cooling thermal energy input to the chiller is then computed as follows:
To make sure that the exhaust mass flow rate and temperature from microturbine are sufficient to drive the chiller, the heat recovery potential is compared with the cooling thermal energy input to the chiller (CoolThermalEergyInput). The heat recovery potential should be greater than the CoolThermalEnergyInput. Heat recovery potential is calculated as:
T_{abs,gen,o}is the minimum temperature required for the proper operation of the doubleeffect chiller. It will be defaulted to 176°C.
The electric input to cooling output ratio as function of temperature (CEIRfT) curve represents the fraction of electricity to the chiller at full load as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature.
The electric input to cooling output ratio function of part load ratio (CEIRfPLR) curve represents the fraction of electricity to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The cooling electric input to the chiller is computed as follows:
All five of these cooling performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (objects Curve:Linear, Curve:Quadratic and Curve:Biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s TeFIRfPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero). A set of curves derived from manufacturer’s data are also provided in the dataset (ExhaustFiredChiller.idf) is provided with E+ installation.
The condenser load is computed as follows:
The following nomenclature is used in the heating equations:
AvailHeatCap=available fullload heating capacity at current conditions [W]
CPLRh=cooling partload ratio for heating curve = CoolingLoad / NomCoolCap
HeatCapfCPLR=heating capacity factor as a function of cooling part load ratio, equal to 1 at zero cooling load, user input “Heating Capacity Function of Cooling Capacity Curve Name”
HeatCoolCapRatio=user input “Heating to Cooling Capacity Ratio”
HeatElectricPower=heating electricity input [W]
HeatThermalEnergyInput= heating thermal energy input [W]
HeatingLoad=current heating load on the chiller [W]
HEIR=user input “Electric Input to Heating Output Ratio”
HFIR=user input “Thermal Energy Input to Heating Output Ratio”
HFIRfHPLR=thermal energy input to heating output factor, equal to 1 at full load, user input “Thermal Energy Input to Heat Output Ratio During Heating Only Operation Curve Name”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
TotalElectricPower=total electricity input [W]
TotalThermalEnergyInput=total thermal energy input [W]
Cooling is the primary purpose of the Exhaust Fired Absorption Chiller so that function is satisfied first and if energy is available for providing heating that is provided next.
The two performance curves for heating capacity and efficiency are:
 Heating Capacity Function of Cooling Capacity Curve
 ThermalEnergyInputto Heat Output Ratio Function
The heating capacity function of cooling capacity curve (HeatCapfCPLR) determines how the heating capacity of the chiller varies with cooling capacity when the chiller is simultaneously heating and cooling. The curve is normalized so an input of 1.0 represents the nominal cooling capacity and an output of 1.0 represents the full heating capacity. An output of 1.0 should occur when the input is 0.0.
The available heating capacity is then computed as follows:
The thermal energy input to heat output ratio curve (HFIRfHPLR) function is used to represent the fraction of thermal energy used as the heating load varies as a function of heating part load ratio. It is normalized so that a value of 1.0 is the full available heating capacity. The curve is usually linear or quadratic and will probably be similar to a boiler curve for most chillers.
 The thermal energy use rate when heating is computed as follows:
The fraction of the time step during which the chiller is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The heating electric input to the chiller is computed as follows:
If the chiller is delivering heating and cooling simultaneously, the parasitic electric load would be doublecounted, so the following logic is applied:
The total thermal energy and electric power input to the chiller is computed as shown below:
Personal communications with various absorption chiller manufacturers, March 2011.
Absorption Chillers and Heat Pumps, Keith Herold, Reinhard Radermacher and Sanford A. Klein (Mar 18, 1996).
Absorption systems for combined heat and power: The problem of partload operation, ASHRAE Transactions, 2003, Vol 109, Part1.
Constant COP Chiller[LINK]
The input object Chiller:ConstantCOP provides a chiller model that is based on a simple, constant COP simulation of the chiller. In this case, performance does not vary with chilled water temperature or condenser conditions. The nominal capacity of the chiller and the COP are user specified along with the connections to the plant and condenser loop and mass flow rates. Such a model is useful when the user does not have access to detailed performance data.
The chiller power is calculated from the load divided by the COP. This chiller will meet the load as long as it does not exceed the nominal capacity specified by the user.
QEvaporator = Load
Power = Load / ConstCOPChiller(ChillNum)%COP
Then the evaporator temperatures are calculated from the load
EvapDeltaTemp = QEvaporator/EvapMassFlowRate/CPwater
EvapOutletTemp = Node(EvapInletNode)%Temp  EvapDeltaTemp
The condenser load and temperatures are calculated from the evaporator load and the power to the chiller.
QCondenser = Power + QEvaporator
IF (ConstCOPChiller(ChillNum)%CondenserType == WaterCooled) THEN
IF (CondMassFlowRate > WaterMassFlowTol) THEN
CondOutletTemp = QCondenser/CondMassFlowRate/CPCW(CondInletTemp) + CondInletTemp
ELSE
CALL ShowSevereError('CalcConstCOPChillerModel: Condenser flow = 0, for CONST COP Chiller='// &
TRIM(ConstCOPChiller(ChillNum)%Name))
CALL ShowContinueErrorTimeStamp(' ')
CALL ShowFatalError('Program Terminates due to previous error condition.')
END IF
ELSE ! Air Cooled or Evap Cooled
! Set condenser outlet temp to condenser inlet temp for Air Cooled or Evap Cooled
! since there is no CondMassFlowRate and would divide by zero
CondOutletTemp = CondInletTemp
END IF
See the InputOutput Reference for additional information.
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
Hot Water Heat Recovery from Chillers[LINK]
The electric chillers (e.g., Chiller:Electric, Chiller:EngineDriven, Chiller:CombustionTurbine, Chiller:Electric:EIR, and Chiller:Electric:ReformulatedEIR) all have the option of connecting a third plant loop for heating hot water at the same time the chiller cools the chilled water. The engine and combustion turbine chillers models include curves for heat recovery from oil and or jacket coolers. The other three chillers can model heat recovery where part of its condenser section is connected to a heat recovery loop for what is commonly known as a double bundled chiller, or single condenser with split bundles. The heat recovery chiller is simulated as a standard vapor compression refrigeration cycle with a double bundled condenser. A double bundle condenser involves two separate flow paths through a split condenser. One of these paths is condenser water typically connected to a standard cooling tower; the other path is hot water connected to a heat recovery loop. After leaving the compressor, the refrigerant is condensed to liquid in a refrigerant to water condenser. In a split bundle, the chiller’s internal controls will direct a part of the refrigerant to heat recovery condenser bundle and/or to the tower water condenser bundle depending on the chilled water load, the condenser inlet temperatures and internal chiller controls (and possibly a leaving hot water temperature setpoint). The refrigerant pressure is then dropped through a throttling valve so that fluid can evaporate at a low pressure that provides cooling to the evaporator.
The algorithm for the heat recovery portion of the chiller needs to be determined from relatively simple inputs to estimate the amount of the heat that is recovered and then send the rest of the heat to the cooling tower. For the chiller models associated with the object Chiller:Electric, air or evaporativelycooled condensers are allowed to be used with heat recovery and, when used, the condenser specific heat, mass flow rate, and temperatures shown below refer to outdoor air. A condenser air volume flow rate must be specified when using heat recovery with air or evaporativelycooled chillers.
The basic energy balance for the condenser section of a heat recovery chiller is
In practice, if the entering temperature of the heat recovery hot fluid is too high, the chiller’s internal controls will redirect refrigerant away from the heat recovery bundle. A user input is available for declaring the inlet high temperature limit, and if it is exceeded, the chiller will shut down heat recovery and request no flow and will not reject any condenser heat to that fluid.
The heat recovery condenser bundle is often physically smaller than the tower water condenser bundle and therefore may have limited heat transfer capacity. User input for the relative capacity of the heat recovery bundle, , is used to define a maximum rate of heat recovery heat transfer using
This capacity factor is also used to autosize the heat recovery design fluid flow rate when it is set to autosize. The design heat recover flow rate is calculated by multiplying by the condenser tower water design flow rate. If no capacity factor is input, it is assumed to be 1.0.
A heat recovery chiller may control the temperature of heat recovery fluid leaving the device by modulating the flow of refrigerant to the heat recovery condenser bundle. There are two different algorithms used depending on if the input has declared a leaving setpoint node.
If no control setpoint node was named, then the model developed by Liesen and Chillar (2004) is used to approximate the relative distribution of refrigerant flow and condenser heat transfer between the bundles. This model approximates the heat transfer situation by using average temperatures in and out of the condenser section.
Then the inlet temperature is flowweighted to determine lumped inlet and outlet conditions.
The lumped outlet temperature is then used for an approximate method of determining the heat recovery rate
This rate is then limited by the physical size of the heat recovery bundle.
If user input for the leaving temperature setpoint is available, then a second model is used to distribute refrigerant flow and condenser heat transfer between the bundles that attempts to meet the heat recovery load implied by the leaving setpoint. When setpoint control is used, the desired rate of heat recovery heat transfer is:
Then the heat recovery rate is simply modeled as the lower of the three different heat flow rates: the desired capacity, the maximum capacity, and the current total heat rejection rate.
For both models, the condenser heat transfer rate is then
The outlet temperatures are then calculated using
A heat recovery chiller may need to work harder because the refrigeration system faces a higher lift owing to an elevated effective temperature for heat rejection. With heat recovery, the condenser temperature used with the chiller’s performance curves is determined using one of the following heatflowweighted methods. The first is used for the chiller model for the objects Chiller:Electric, and Chiller:Electric:EIR which use the condensing entering temperature for performance.
The second is used for the chiller model for the object Chiller:Electric:ReformulatedEIR which uses the leaving condenser fluid temperature.
Both of these are available as an output variable called Chiller Effective Heat Rejection Tempeature, in C.
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
Reference
Leisen and Chillar. 2004. Variable Heat Recovery In Double Bundle Electric Chillers. SimBuild 2004, IBPSAUSA National Conference Boulder, CO, August 46, 2004.
Electric Chiller Model Based on Fluid Temperature Differences[LINK]
The centrifugal chiller model (object name Chiller:Electric) was originally developed for the BLAST program. The model is based on a ‘capacity ratio’ curve, which is a quadratic equation that determines the Ratio of Available Capacity to Nominal Capacity. The defining equation is:
Where the Delta Temperature is defined as:
where the temperature rise coefficient is defined as the ratio of the required change in condenser water temperature to a given change in chilled water temperature, which maintains the capacity at the nominal value. If the chiller is a heat recovery chiller,then the condenser inlet temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers. This is calculated as the following ratio:
Where:
T_{cond,in,required} = Required entering condenser air or water temperature to maintain rated capacity (C)
T_{cond,in,rated} = Rated entering condenser air or water temperature at rated capacity (C)
T_{evap,out,required} = Required leaving evaporator water outlet temperature to maintain rated capacity (C)
T_{evap,out,rated} = Rated leaving evaporator water outlet temperature at rated capacity (C)
The Power Ratio Curve is a quadratic equation that determines the Ratio of Full Load to Power. The defining equation is:
where the part load ratio, PLR is defined as:
The Load Ratio Curve is a quadratic equation that determines the Ratio of Actual Cooling Load to Full Cooling Load. The defining equation is:
The evaporator heat transfer rate and the power required by the chiller are then calculated as:
Electric Chiller Model Based on Condenser Entering Temperature[LINK]
This model (object name Chiller:Electric:EIR) simulates the performance of an electric liquid chiller. The model is based on the compression chiller model (COMREF) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus additional abilities for modeling evaporativelycooled condensers and heat recovery for water heating.
This model simulates the thermal performance of the chiller and the power consumption of the compressor(s). It also models the power consumption of condenser fans if modeling an aircooled or evaporativelycooled condenser. This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. CoolingTower:SingleSpeed).
Model Description[LINK]
The chiller model uses usersupplied performance information at reference conditions along with three performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offreference conditions. The three performance curves are:
 Cooling Capacity Function of Temperature Curve
 Energy Input to Cooling Output Ratio Function of Temperature Curve
 Energy Input to Cooling Output Ratio Function of Part Load Ratio Curve
 The cooling capacity function of temperature curve is a biquadratic performance curve with two independent variables: the leaving chilled water temperature and the entering condenser fluid temperature. The output of this curve is multiplied by the reference capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the reference temperatures). The curve should have a value of 1.0 at the reference temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation. If the chiller is a heat recovery chiller,then the condenser inlet temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
where
ChillerCapFTemp = cooling capacity factor, equal to 1 at reference conditions
T_{cw,l}= leaving chilled water temperature, ˚C
T_{cond,e}= entering condenser fluid temperature, ˚C. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively.
 The energy input to cooling output ratio function of temperature curve is a biquadratic performance curve that parameterizes the variation of the energy input to cooling output ratio (EIR) as a function of the leaving chilled water temperature and the entering condenser fluid temperature. The EIR is the inverse of the COP. The output of this curve is multiplied by the reference EIR (inverse of the reference COP) to give the fullload EIR at specific temperature operating conditions (i.e., at temperatures different from the reference temperatures). The curve should have a value of 1.0 at the reference temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
where
ChillerEIRFTemp = energy input to cooling output factor, equal to 1 at reference conditions
T_{cw,l} = leaving chilled water temperature, ˚C
T_{cond,e}= entering condenser fluid temperature, ˚C. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively. If the chiller is a heat recovery chiller,then the condenser inlet temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
 The energy input to cooling output ratio function of partload ratio curve is a quadratic performance curve that parameterizes the variation of the chiller input power ratio as a function of the partload ratio. The partload ratio is the actual cooling load divided by the chiller’s available cooling capacity. The output of this curve is multiplied by the reference EIR (inverse of the reference COP) and the Energy Input to Cooling Output Ratio Function of Temperature Curve to give the EIR at the specific temperatures and partload ratio at which the chiller is operating. This curve should have a value of 1.0 when the partload ratio equals 1.0. The quadratic curve should be valid for the range of partload ratios anticipated for the simulation.
where
ChillerEIRFPLR = energy input to cooling output factor, equal to 1 at reference conditions
PLR = partload ratio = (cooling load) / (chiller’s available cooling capacity)
P_{chiller} = chiller power at specific PLR
P_{ref} = /COP_{ref}
All three of the performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (curve:quadratic and curve:biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s ChillerEIRFPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero). Performance curves for more than 160 chillers, including the default DOE2.1E reciprocating and centrifugal chillers, are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf).
Note: Chiller:Electric:EIR objects and their associated performance curve objects are developed using performance information for a specific chiller and should normally be used together for an EnergyPlus simulation. Changing the object input values, or swapping performance curves between chillers, should be done with caution.
For any simulation time step, the chiller’s available cooling capacity is calculated as follows:
where
= chiller capacity at reference conditions (reference temperatures and flow rates defined by the user), W
= available chiller capacity adjusted for current fluid temperatures, W
The model then calculates the evaporator heat transfer rate required to bring the entering chilled water temperature down to the leaving chilled water setpoint temperature (established using a SetpointManager object and referenced in the PlantLoop object). If this calculated heat transfer rate is greater than the heat transfer rate being requested by the plant equipment operation scheme, then the evaporator heat transfer rate is reset to the requested cooling rate.
The evaporator heat transfer rate is then compared to the available capacity. If the available chiller capacity is sufficient to meet the evaporator heat transfer rate, the leaving chilled water temperature is set equal to the chilled water setpoint temperature. If the requested evaporator heat transfer rate is larger than the available capacity the chilled water leaving the evaporator is allowed to float upward. For this case, the exiting chilled water temperature is calculated based on the water temperature entering the evaporator, the available cooling capacity, and the evaporator mass flow rate as follows:
where
T_{cw,l}= water temperature leaving the evaporator, ˚C
T_{cw,e}= water temperature entering the evaporator, ˚C
= evaporator mass flow rate, kg/s
C_{p,evap}= specific heat of water entering evaporator at T_{cw,e}, J/kg˚C
The partload ratio is then calculated as the ratio of the evaporator heat transfer rate to the available chiller capacity. The partload ratio is not allowed to be greater than the maximum partload ratio specified by the user or less than zero as follows:
where
PLR= partload ratio
= load to be met by the chiller, W
PLR_{max}= maximum partload ratio (specified by the user in the input data file)
The model assumes that the cooling load is met through chiller unloading down to the minimum unloading ratio. False loading (e.g. hotgas bypass) is assumed to occur between the minimum unloading ratio and the minimum part load ratio yielding constant electrical power consumption under these conditions. Below the minimum part load ratio, the chiller cycles on and off to meet very small loads and the power consumption during the on cycle is the same as when the chiller is operating at the minimum part load ratio. When the chiller part load ratio is less than the minimum part load ratio, the onoff cycling ratio of the chiller is calculated as follows and is available as an output variable.
To properly account for chiller electric power consumption when PLR is less than the minimum unloading ratio, the PLR is reset to the greater of the PLR calculated above and the PLR at the minimum unloading ratio. The result is available as the output variable Chiller Part Load Ratio.
This revised PLR accounts for the “false loading” (e.g., hot gas bypass) that is assumed to occur whenever the PLR (based on cooling load divided by available capacity) is less than the minimum unloading ratio specified. The amount of false loading on the chiller is calculated using this revised PLR and is reported as an output variable as follows:
The electrical power consumption for the chiller compressor(s) for any simulation time step is then calculated using the following equation:
where
P_{chiller}= chiller compressor power, W
COP_{ref}= reference coefficient of performance, W/W
Heat rejected by the chiller condenser includes the heat transferred in the evaporator plus a portion or all of the compressor electrical energy consumption. For electric chillers with hermetic compressors, all compressor energy consumption is rejected by the condenser (compressor motor efficiency = eff_{motor} = 1.0). For chillers with semihermetic or open compressors, only a portion of the compressor energy use is rejected by the condenser. The heat transfer rate for the chiller condenser is calculated as follows:
where
= condenser heat transfer rate, W
= compressor motor efficiency = fraction of compressor electrical energy consumption rejected as condenser heat
For watercooled chillers, the water temperature leaving the condenser is then calculated as shown below.
where:
T_{cond,l}= water temperature leaving the condenser, ˚C
T_{cond,e}= water temperature entering the condenser, ˚C
= mass flow rate through the condenser, kg/s
= specific heat of water entering the condenser at T_{cond,e}, J/kg˚C
For air and evaporativelycooled condensers, the exiting air temperature is not calculated and is set equal to the entering air or wetbulb temperature, respectively.
The model then calculates the condenser fan energy for air and evaporativelycooled condensers. The amount of condenser fan energy is assumed to be proportional to the chiller cycling ratio and is calculated as follows:
where
P_{cond}= chiller condenser fan electric power, W
P_{condfanratio}= condenser fan power ratio, W/W
The final calculations determine the total heat transfer energy for the condenser and evaporator, as well as the total electric energy consumed by the chiller compressor motor(s) and condenser fan(s). The results are available as output variables.
where
Q_{cond}= chiller condenser heat transfer energy, J
Q_{evap}= chiller evaporator cooling energy, J
E_{chiller}= chiller (compressor) electric energy, J
E_{cond}= chiller condenser fan electric energy, J
TimeStepSys = HVAC system simulation time step, hr
= conversion factor, sec/hr
Electric EIR Chiller with Heat Recovery Option[LINK]
Heat from the electric EIR chiller condenser may be recovered when a watercooled condenser is selected for simulation. The heat recovery water flow rate is specified by the user along with the input and output nodes connected to the heat recovery loop. The algorithms are identical to those used for Chiller:Electric. Refer to the section entitled Chillers with Plant Heat Recovery for details.
Standard Rating (Integrated Part Load Value)[LINK]
For the Chiller:Electric:EIR and Chiller:Electric:ReformulatedEIR objects in EnergyPlus, the industry standard rating of Integrated Part Load Value (IPLV) is calculated according to ANSI/AHRI Standard 550/590 (2011). This standard rating is not direct input to the model and is calculated using userentered information for these objects. These standard rating values are provided in the eplusout.eio output file (Ref. Output Details document) and also in the predefined tabular output reports (Output:Table:SummaryReports object, Equipment Summary).
Note: The standard ratings described in this section require that the EIR/Reformulated EIR chiller models be evaluated at specific operating conditions (e.g., specific evaporator outlet temperature (6.67 C) and drybulb temperatures for air entering the aircooled [outdoor] condenser). If the chiller performance curves can not be evaluated at the required test conditions, then the standard rating value will be determined at user specified curve limit and warning error message is provided. For example, if the curve object (Curve:Biquadratic) for Cooling Capacity Function of Temperature Curve has a minimum value of 21C for drybulb temperature entering the aircooled condenser coil, the IPLV calculation requires that EER_{D} be calculated at 13 C – so, this would result in IPLV value calculated at 21C and reported in the output and a warning message in the eplusout.err file.
The IPLV is a single number partload performance figure of merit for WaterChilling Packages. The IPLV equations and procedures described below are taken from Appendix D of ANSI/AHRI Std. 550/590, 2011 and provide a consistent method for calculating IPLV. These equations provide representative average partload efficiency for a single chiller. For equipment covered by this standard, the IPLV is calculated using the following formula:
where,
= EER or COP at 100% capacity at AHRI standard rating conditions
= EER or COP at 75% capacity and reduced ambient (see Table 49
= EER or COP at 50% capacity and reduced ambient (see Table 49)
= EER or COP at 25% capacity and reduced ambient (see Table 49)
The Coefficient of Performance (COP) at the various load capacity points (100%, 75%, 50%, and 25% partload ratios) are calculated using the procedure outlined below and applicable test conditions specified in Table 49.
EER at desired reduced capacity (75%, 50%, and 25%) is calculated as follows
![](media/image3046.png)
= Reference chiller capacity specified by the user, (W).
= Reference coefficient of performance specified by the user, (W/W).
= Userspecified biquadratic curve for modifying EIR as a function of leaving chilled water temperature (6.7°C) and entering condenser temperature obtained from Table 49 for reduced capacities, (dimensionless).
= Userspecified quadratic curve for modifying EIR as a function of part load ratio.
= Capacity (W) of the chiller determined per the ANSI/AHRI Standard 550/590 reduced ambient test conditions as shown in Table 49.
If the equipment cannot be unloaded to the desired reduced capacity (75%, 50%, and 25%) i.e. if the minimum unloading ratio of the chiller is greater than desired reduced capacity, then the model is run at the minimum unloading PLR of the equipment at the condenser entering conditions defined in Table 49 and the efficiency is adjusted for cyclic performance.
where,
= EIR of chiller at minimum unloading ratio
= degradation coefficient to account for cycling of the compressor for capacity less than the minimum capacity.
The degradation coefficient C_{D} for the desired reduced load points (75%, 50%, or 25%) is determined using the following equation:
The load factor (LF) for the desired reduced load points (75%, 50%, or 25%) calculated from the following equation:
Where,
**= standard rating part load ratio (PLR) points, 75%, 50%, 25%.
=Full load heating capacity (W) of the airsource heat pump equipment determined from ANSI/AHRI Standard 550/590 and test conditions shown in Table 49 for 100% load.
=Part load heating capacity (W) of the airsource heat pump units determined from ANSI/AHRI Standard 550/590 at the standard desired reduced ambient test conditions as shown in Table 49 and the minimum part load capacity of the unit.
where,
= Reference capacity specified by the user, (W).
= Userspecified biquadratic curve evaluated at full load (100%) test conditions shown in Table 49, (dimensionless).
= Part load capacity (W) of the chiller determined from ANSI/AHRI Standard 550/590 at the standard desired reduced ambient test conditions as shown in Table 49.
= Minimum PLR up to which chiller can be unloaded
Standard Rating (Integrated Part Load Value)


Evaporator (All types)

^{1}Condenser (Water Cooled)

^{1}Condenser (Air Cooled)

^{1}Condenser (Evaporatively Cooled)

AirCooled Without

Water and Evaporatively

^{1} If the unit Manufacturer’s recommended minimum temperatures are greater than those specified in Table 3, then those may be used in lieu of the specified temperatures.

: Standard Rating (Integrated Part Load Value)
Electric Chiller Model Based on Condenser Leaving Temperature[LINK]
This model (object name Chiller:Electric:ReformulatedEIR) simulates the thermal performance of an electric liquid chiller and the power consumption of its compressor(s). The model, developed by Hydeman et al. (2002) as part of the CoolTools™ project sponsored by Pacific Gas and Electric Company (PG&E), is an empirical model similar to EnergyPlus’ Chiller:Electric:EIR model. The model uses performance information at reference conditions along with three curve fits for cooling capacity and efficiency to determine chiller operation at offreference conditions. The model has the same capabilities as the Chiller:Electric:EIR model, but can potentially provide significant accuracy improvement over the Chiller:Electric:EIR model for chillers with variablespeed compressor motor drives and/or variable condenser water flow applications.
Chiller performance curves can be generated by fitting manufacturer’s catalog data or measured data. Performance curves developed primarily from manufacturer’s performance data are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf). This chiller model can be used to predict the performance of various chiller types (e.g., reciprocating, screw, scroll, and centrifugal) with watercooled condensers. The model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. CoolingTower:SingleSpeed).
The main difference between this model and the Chiller:Electric:EIR model is the condenser fluid temperature used in the associated performance curves: the Chiller:Electric:ReformulatedEIR model uses the LEAVING condenser water temperature while the Chiller:Electric:EIR model uses the ENTERING condenser water temperature. In addition, the Energy Input to Cooling Output Function of Part Load Ratio curve for this reformulated EIR chiller model includes the condenser leaving water temperature as an independent variable in addition to partload ratio. Since the leaving condenser water temperature is a function of load, chiller performance, and condenser entering water temperature, EnergyPlus must iterate to converge on a solution for each simulation time step.
Model Description[LINK]
The chiller model uses usersupplied performance information at reference conditions along with three performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offreference conditions. The three performance curves are:
 Cooling Capacity Function of Temperature Curve
 Energy Input to Cooling Output Ratio Function of Temperature Curve
 Energy Input to Cooling Output Ratio Function of Part Load Ratio Curve
The cooling capacity function of temperature curve is a biquadratic performance curve with two independent variables: the leaving chilled water temperature and the leaving condenser water temperature. The output of this curve is multiplied by the reference capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the reference temperatures). The curve should have a value of 1.0 at the reference temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation (otherwise the program issues warning messages).
where
ChillerCapFTemp = Cooling capacity factor, equal to 1 at reference conditions
T_{cw,l}= leaving chilled water temperature, ˚C
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
 The energy input to cooling output ratio function of temperature curve is a biquadratic performance curve that parameterizes the variation of the energy input to cooling output ratio (EIR) as a function of the leaving chilled water temperature and the leaving condenser water temperature. The EIR is the inverse of the COP. The output of this curve is multiplied by the reference EIR (inverse of the reference COP) to give the fullload EIR at specific temperature operating conditions (i.e., at temperatures different from the reference temperatures). The curve should have a value of 1.0 at the reference temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation (otherwise the program issues warning messages).
where
ChillerEIRFTemp = Energy input to cooling output factor, equal to 1 at reference conditions
T_{cw,l} = leaving chilled water temperature, ˚C
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
 The energy input to cooling output ratio function of partload ratio curve is a bicubic performance curve that parameterizes the variation of the chiller input power ratio as a function of the leaving condenser water temperature and the partload ratio. The partload ratio is the actual cooling load divided by the chiller’s available cooling capacity. The output of this curve is multiplied by the reference EIR (inverse of the reference COP) and the Energy Input to Cooling Output Ratio Function of Temperature Curve to give the EIR at the specific temperatures and partload ratio at which the chiller is operating. This curve should have a value of 1.0 at the reference leaving condenser water temperature with partload ratio equal to 1.0. It is recommended that this performance curve be developed using both full and partload performance data. The bicubic curve should be valid for the range of condenser water temperatures and partload ratios anticipated for the simulation (otherwise the program issues warning messages).
where
ChillerEIRFPLR = Energy input to cooling output factor, equal to 1 at the reference leaving condenser water temperature and PLR = 1.0
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
PLR = Part load ratio = (cooling load) / (chiller’s available cooling capacity)
P_{chiller} = chiller power at specific PLR
P_{ref} = /COP_{ref}
Note: Although a bicubic curve requires 10 coefficients (ref. Curve:Bicubic), coefficients 7, 9 and 10 are typically not used in the performance curve described here and should be entered as 0 unless sufficient performance data and regression accuracy exist to justify the use of these terms of the bicubic curve.
All three of the performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (curve:biquadratic and curve:bicubic). Note that the above three performance curves use the leaving condenser water temperature as an independent variable, instead of the entering condenser water temperature used in the performance curves for the Chiller:Electric:EIR model. Since the leaving condenser water temperature is calculated based on the condenser heat transfer rate, which is a function of the load to be met by the chiller, chiller compressor power, and the false loading (detailed calculations are given below), iterative calculations are required to determine the actual (converged) leaving condenser water temperature. The program uses the leaving condenser water temperature from the previous iteration to calculate values for each of the three performance curves described above. After obtaining the condenser heat transfer rate, the leaving condenser water temperature is recalculated. When the difference between the leaving condenser water temperature calculated on successive iterations is less than 0.0001^{°}C, the solution is assumed to have converged. Warning messages are issued if the calculated solution for leaving condenser water temperature and/or partload ratio falls outside the valid range specified for the chiller’s performance curves. If these warnings are issued, the user may chose to extend the range for the performance curves (only if a small extension is required since model extrapolation may produce significant errors) or a different chiller and associated performance curves with extended performance range can be located and used for the simulation.
Note: Chiller:Electric:ReformulatedEIR objects and their associated performance curve objects are developed using performance information for a specific chiller and should almost always be used together for an EnergyPlus simulation. Changing the object input values, or swapping performance curves between chillers, should be done with extreme caution. For example, if the user wishes to model a chiller size that is different from the reference capacity, it is highly recommended that the reference flow rates be scaled proportionately to the change in reference capacity. Although this model can provide more accurate prediction than the Chiller:Electric:EIR model, it requires more performance data to develop the associated performance curves (at least 12 points from fullload performance and 7 points from partload performance).
Although performance curve data sets for 160 chillers are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf), they may not meet the requirements for specific applications. One can develop performance curves from performance data using two available techniques (Hydeman and Gillespie 2002). The first technique is called the Leastsquares Linear Regression method and is used when sufficient performance data exist to employ standard leastsquare linear regression techniques. The second technique is called Reference Curve Method and is used when insufficient performance data exist to apply linear regression techniques. A detailed description of both techniques can be found in the reference mentioned above.
For any simulation time step, the chiller’s available cooling capacity is calculated as follows:
where
= chiller capacity at reference conditions (reference temperatures and flow rates defined by the user), W
= available chiller capacity adjusted for current water temperatures, W
The model then calculates the evaporator heat transfer rate required to bring the entering chilled water temperature down to the leaving chilled water setpoint temperature (established using a SetpointManager object and referenced in the PlantLoop object). If this calculated heat transfer rate is greater than the heat transfer rate being requested by the plant equipment operation scheme, then the evaporator heat transfer rate is reset to the requested cooling rate.
The evaporator heat transfer rate is then compared to the available capacity. If the available chiller capacity is sufficient to meet the evaporator heat transfer rate, the leaving chilled water temperature is set equal to the chilled water setpoint temperature. If the requested evaporator heat transfer rate is larger than the available capacity the chilled water leaving the evaporator is allowed to float upward. For this case, the exiting chilled water temperature is calculated based on the water temperature entering the evaporator, the available cooling capacity, and the evaporator mass flow rate as follows:
where
T_{cw,l}= water temperature leaving the evaporator, ˚C
T_{cw,e}= water temperature entering the evaporator, ˚C
= evaporator mass flow rate, kg/s
C_{p,evap}= specific heat of water entering evaporator at T_{cw,e}, J/kg˚C
The partload ratio is then calculated as the ratio of the evaporator heat transfer rate to the available chiller capacity. The partload ratio is not allowed to be greater than the maximum partload ratio specified by the user or less than zero as follows:
where
PLR= partload ratio
= load to be met by the chiller, W
PLR_{max}= maximum partload ratio (specified by the user in the input data file)
Note that the maximum partload ratio (PLR_{max}, specified in the Chiller:Electric:ReformulatedEIR object) used in the equation should be less than or equal to the maximum partload ratio specified in the “Energy Input to Cooling Output Ratio Function of PartLoad Ratio” performance curve object.
The model assumes that the cooling load is met through chiller unloading down to the minimum unloading ratio. False loading (e.g. hotgas bypass) is assumed to occur between the minimum unloading ratio and the minimum partload ratio yielding constant electrical power consumption under these conditions. Below the minimum partload ratio, the chiller cycles on/off to meet very small loads and the power consumption during the on cycle is the same as when the chiller is operating at the minimum part load ratio. When the chiller partload ratio is less than the minimum partload ratio, the onoff cycling ratio of the chiller is calculated as follows and is available as an output variable.
To properly account for chiller electric power consumption when PLR is less than the minimum unloading ratio, the PLR is reset to the greater of the PLR calculated above and the PLR at the minimum unloading ratio. The result is available as the output variable Chiller Part Load Ratio.
This revised PLR accounts for the “false loading” (e.g., hotgas bypass) that is assumed to occur whenever the PLR (based on cooling load divided by available capacity) is less than the minimum unloading ratio specified. The amount of false loading on the chiller is calculated using this revised PLR and is reported as an output variable as follows:
The electrical power consumption for the chiller compressor(s) for any simulation time step is then calculated using the following equation:
where
P_{chiller}= Chiller compressor power, W
COP_{ref}= Reference coefficient of performance, W/W
Heat rejected by the chiller condenser includes the heat transferred in the evaporator plus a portion or all of the compressor electrical energy consumption. For electric chillers with hermetic compressors, all compressor energy consumption is rejected by the condenser (compressor motor efficiency = eff_{motor} = 1.0). For chillers with semihermetic or open compressors, only a portion of the compressor energy use is rejected by the condenser. The heat transfer rate for the chiller condenser is calculated as follows:
where
= condenser heat transfer rate, W
= compressor motor efficiency = fraction of compressor electrical energy consumption rejected as condenser heat
The above curve values are calculated based on the leaving condenser water temperature found through iteration. After obtaining the condenser heat transfer rate, the final leaving condenser water temperature is then calculated as:
where:
T_{cond,l}= water temperature leaving the condenser, ˚C
T_{cond,e}= water temperature entering the condenser, ˚C
= mass flow rate through the condenser, kg/s
= specific heat of water entering the condenser at T_{cond,e}, J/kg˚C
The final calculations determine the total heat transfer energy for the condenser and evaporator, as well as the total electric energy consumed by the chiller compressor motor(s) and condenser fan(s). The results are available as output variables.
where
Q_{cond}= chiller condenser heat transfer energy, J
Q_{evap}= chiller evaporator cooling energy, J
E_{chiller}= chiller (compressor) electric energy, J
E_{cond}= chiller condenser fan electric energy, J
TimeStepSys = HVAC system simulation time step, hr
= conversion factor, sec/hr
Heat from the electric reformulated EIR chiller condenser may be recovered. The heat recovery water flow rate is specified by the user along with the input and output nodes connected to the heat recovery loop. The algorithms are identical to those used for Chiller:Electric and Chiller:Electric:EIR. Refer to the section entitled Chillers with Plant Heat Recovery for details.
Standard Rating (Integrated Part Load Value)[LINK]
Integrated Part Laod Value (IPLV) calculations for Reformulated EIR chiller are similar to what are described above for EIR chillers. The only difference with Reformulated EIR chiller is that it calls an iterative subroutine (SolveRegulaFalsi) to obtain a condenser water outlet temperature which corresponds to condenser inlet temperature at reduced capacity conditions as outlined in Table 49 above. SolveRegulaFalsi is a general utility routine for finding the zero of a function. In this case it finds the condenser inlet temperature that will zero the residual function – the difference between calculated condenser inlet temperature and desired condenser inlet temperature per ANSI/AHRE 550/590, 2011 (table 42 above) divided by desired condenser inlet temperature.
Hydeman, M., N. Webb, P. Sreedharan, and S. Blanc. 2002. Development and Testing of a Reformulated RegressionBased Electric Chiller Model. ASHRAE Transactions HI02182.
Hydeman, M. and K.L. Gillespie. 2002. Tools and Techniques to Calibrate Electric Chiller Component Models. ASHRAE Transactions AC0291.
Hydeman, M., K. Gillespie, and R. Kammerud. 1997. PG&E’s CoolTools project: A toolkit to improve evaluation and operation of chilled water plants. Presented at the Cool$ense National Forum on Integrated Chilled Water Retrofits, Sep. 1997. Berkeley California: Lawrence Berkeley National Laboratory.
Engine Driven Chiller[LINK]
The engine driven chiller (Object name: Chiller:EngineDriven) is the empirical model from the Building Loads and System Thermodynamics (BLAST) program. Fitting catalog data to a third order polynomial equations generates the chiller performance curves. Three sets of coefficients are required to model the open centrifugal chiller as discussed in the section, titled, ‘Electric Chiller Based on BLAST Centrifugal Chiller Model’. Additional curve fits are required to model the engine. Because the model inherently involves the lower heating value of the fuel, a reference temperature is also involved in the calculations, which manufacturers present at 25°C.
The engine model was also developed for the BLAST program. It was adapted for use in EnergyPlus. This model is used for both the engine driven generator and the engine driven chiller. It uses the following set of equations all of which are quadratic fits to the PLR (Part Load Ratio) of the generator. The coefficients must be derived from manufacturers data.
The exhaust gas temp and flow rate are used if a stack heat exchanger is used to recover waste heat from the exhaust. This temperature is the inlet temperature to the heat exchanger which is modeled in a UAeffectiveness form:
The exhaust flow rate is then calculated as:
where T_{reference} is the reference temperature for the fuel lower heating value, and is given as 25°C in manufacturer’s data, and
Finally, heat recovered from the lube oil and the water jacket are accounted for as follows:
Chiller Basin Heater[LINK]
Calculations are also made to estimate the electric power input to the basin heater for chillers with evaporativelycooled condensers. The chillers which calculate basin heater power are Chiller:Electric:EIR, Chiller:Electric, Chiller:ConstantCOP, Chiller:EngineDriven and Chiller:CombustionTurbine. A schedule may be used to disable the basin heater during regular maintenance periods or other time periods (e.g., during summer). If a schedule is not provided, the basin heater is assumed to be available the entire simulation time period. The basin heater operates when it is scheduled on, the outdoor air drybulb temperature is below the basin heater setpoint temperature, and the chiller is not active (i.e., chiller is not operating for the simulation time step — for example, when there is no cooling load to be met by the chiller, or if there is no water flow through the chiller due to a chiller or pump availability schedule, etc.). The user is required to enter a basin heater capacity (watts per degree Kelvin) and a heater setpoint temperature (^{o}C) if they want to model basin heater electric power.
where:
= Chiller basin heater electric power (W)
= Chiller basin heater electric consumption (J)
T_{setpoint,basin} = Basin heater setpoint temperature, user input (^{o}C)
T_{db,outdoor} = Outdoor air drybulb temperature (^{o}C)
CAP_{heater,basin} = Basin heater capacity, user input (W/K)
Schedule_{heater,basin} = Basin heater schedule, user input (schedule value > 0 means ON)
ChillerIsOFF = Logical variable denoting that the chiller is not operating for the current simulation time step (e.g., there is no cooling load to be met by the chiller, or if there is no water flow through the chiller due to a chiller or pump availability schedule, etc.)
Chillers[LINK]
Absorption Chiller[LINK]
The input object Chiller:Absorption provides a model for absorption chillers that is an empirical model of a standard absorption refrigeration cycle. The condenser and evaporator are similar to that of a standard chiller, which are both watertowater heat exchangers. The assembly of a generator and absorber provides the compression operation. Lowpressure vapor from the evaporator is absorbed by the liquid solution in the absorber. A pump receives lowpressure liquid from the absorber, elevates the pressure of the liquid, and delivers the liquid to the generator. In the generator, heat from a high temperature source (hot water or steam) drives off the vapor that has been absorbed by the solution. The liquid solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to maintain the pressure difference between the generator and absorber. The heat supplied to the absorber can be waste heat from a diesel jacket, or the exhaust heat from diesel, gas, and steam turbines. For more information on absorption chillers, see the Input/Output Reference Document (Object: Chiller:Absorption).
The partload ratio of the absoprtion chiller’s evaporator is simply the actual cooling effect produced by the chiller divided by the maximum cooling effect available.
where
= partload ratio of chiller evaporator
= chiller evaporator load [W]
= rated chiller evaporator capacity [W]
This absorption chiller model is based on a polynomial fit of absorber performance data. The Generator Heat Input Part Load Ratio Curve is a quadratic equation that determines the ratio of the generator heat input to the demand on the chiller’s evaporator (Q_{evap}).
The Pump Electric Use Part Load Ratio Curve is a quadratic equation that determines the ratio of the actual absorber pumping power to the nominal pumping power.
Thus, the coefficient sets establish the ratio of heat power intocooling effect produced as a function of part load ratio. The ratio of heatpowerin to coolingeffectproduced is the inverse of the coefficient of performance.
If the operating partload ratio is greater than the minimum partload ratio, the chiller will run the entire time step and cycling will not occur (i.e. CyclingFrac = 1). If the operating partload ratio is less than the minimum partload ratio, the chiller will be on for a fraction of the time step equal to CyclingFrac. Steam (or hot water) and pump electrical energy use are also calculated using the chiller partload cycling fraction.
where
= chiller partload cycling fraction
= chiller minimum partload ratio
= generator input power [W]
= absorbtion chiller pumping power [W]
The evaporator water mass flow rate is calculated based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= chiller evaporator water mass flow rate (kg/s)
= chiller design evaporator water mass flow rate (kg/s)
= chiller evaporator water temperature difference (ºC)
= chiller evaporator inlet water temperature (ºC)
= chiller evaporator outlet water setpoint temperature (ºC)
= specific heat of water entering evaporator (J/kg•ºC)
The evaporator outlet water temperature is then calculated based on the cooling effect produced and the evaporator entering water temperature.
where
= chiller evaporator outlet water temperature [ºC]
= chiller evaporator inlet water temperature [ºC]
= specific heat of chiller evaporator inlet water [J/kg/ºC]
= chiller evaporator water mass flow rate [kg/s]
The condenser heat transfer and condenser leaving water temperature are also calculated.
where
= chiller condenser heat transfer rate [W]
= chiller condenser outlet water temperature [ºC]
= chiller condenser inlet water temperature [ºC]
= specific heat of chiller condenser inlet water [J/kg/ºC]
= chiller condenser water mass flow rate [kg/s]
The absorption chiller can model the impact of steam or hot water entering the generator, although the connection of the steam (hot water) nodes to a plant is not actually required. The calculations specific to the generator depend on the type of fluid used and are described here in further detail.
Steam Loop Calculations[LINK]
When a steam loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a steam loop), the generator outlet node steam mass flow rate and temperature are calculated based on the generator input power, latent heat of steam, the specific heat of water, and the amount of subcooling in the steam generator. The model assumes dry saturated steam enters the absorption chiller’s generator and exits the generator as a subcooled liquid. The temperature leaving the generator is calculated based on the user entered amount of liquid subcooling in the generator. The effect of subcooling of the liquid (condensate) in the pipe returning to the boiler is not modeled.
where
= chiller steam mass flow rate [kg/s]
= latent heat of steam [J/kg]
= specific heat of saturated water in the generator [J/Kg ºK]
= amount of subcooling in steam generator [ºC]
= generator steam outlet node temperature [ºC]
= generator steam inlet node temperature [ºC]
Hot Water Loop Calculations[LINK]
When a hot water loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a hot water loop), the generator outlet node temperature is calculated based on the generator input power, mass flow rate of water, and the specific heat of water entering the hot water generator. The calculations are based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= generator hot water mass flow rate [kg/s]
= generator design hot water mass flow rate (kg/s)
= generator design hot water temperature difference (ºC)
Indirect Absorption Chiller[LINK]
The Chiller:Absorption:Indirect object is an enhanced version of the absorption chiller model found in the Building Loads and System Thermodynamics (BLAST) program. This enhanced model is nearly identical to the existing absorption chiller model (Ref. Chiller:Absorption) with the exceptions that: 1) the enhanced indirect absorption chiller model provides more flexible performance curves and 2) chiller performance now includes the impact of varying evaporator, condenser, and generator temperatures. Since these absorption chiller models are nearly identical (i.e., the performance curves of the enhanced model can be manipulated to produce similar results to the previous model), it is quite probable that the Chiller:Absorption model will be deprecated in a future release of EnergyPlus.
The indirect absorption chiller’s condenser and evaporator are similar to that of a standard chiller, which are both watertowater heat exchangers. The assembly of a generator and absorber provides the compression operation. A schematic of a singlestage absorption chiller is shown in the figure below. Lowpressure vapor from the evaporator is absorbed by the liquid solution in the absorber. A pump receives lowpressure liquid from the absorber, elevates the pressure of the liquid, and delivers the liquid to the generator. In the generator, heat from a high temperature source (hot water or steam) drives off the vapor that has been absorbed by the solution. The liquid solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to maintain the pressure difference between the generator and absorber. The heat supplied to the generator can be either hot water or steam, however, connection to an actual plant loop is not required. For more information on indirect absorption chillers, see the Input/Output Reference Document (Object: Chiller:Absorption:Indirect).
Schematic Diagram of a SingleStage Absorption Chiller
The chiller cooling effect (capacity) will change with a change in condenser water temperature. Similarly, the chiller cooling effect will change as the temperature of the evaporator water changes. The chiller cooling effect will also change with a change in or generator inlet water temperature and only applies when Hot Water is used as the generator heat source. A quadratic or cubic equation is used to modify the rated chiller capacity as a function of both the condenser and generator inlet water temperatures and the evaporator outlet water temperature. If any or all of the capacity correction factor curves are not used, the correction factors are assumed to be 1.
(Hot Water only)
where
= Capacity correction (function of evaporator temperature) factor
= Capacity correction (function of condenser temperature) factor
= Capacity correction (function of generator temperature) factor
= evaporator outet water temperature [C]
= condenser inlet water temperature [C]
= generator inlet water temperature [C]
= maximum chiller capacity [W]
= rated chiller capacity [W]
The partload ratio of the indirect absoprtion chiller’s evaporator is simply the actual cooling effect required (load) divided by the maximum cooling effect available.
where
= partload ratio of chiller evaporator
= chiller evaporator operating capacity [W]
The generator’s heat input is also a function of several parameters. The primary input for determining the heat input requirements is the Generator Heat Input function of PartLoad Ratio Curve. The curve is a quadratic or cubic equation that determines the ratio of the generator heat input to the chiller’s maximum capacity (Q~evap, max~) and is soley a function of partload ratio. Typical generator heat input ratios at full load (i.e., PLR=1) are between 1 and 2. Two additional curves are available to modifiy the heat input requirement based on the generator inlet water temperature and the evaporator outlet water temperature.
where
GeneratorHIR = ratio of generator heat input to chiller operating capacity
GenfCondT = heat input modifier based on generator inlet water temperature
GenfEvapT = heat input modifier based on evaporator outlet water temperature
The Pump Electric Use function of PartLoad Ratio Curve is a quadratic or cubic equation that determines the ratio of the actual absorber pumping power to the nominal pumping power.
If the chiller operating partload ratio is greater than the minimum partload ratio, the chiller will run the entire time step and cycling will not occur (i.e. CyclingFrac = 1). If the operating partload ratio is less than the minimum partload ratio, the chiller will be on for a fraction of the time step equal to CyclingFrac. Generator heat input and pump electrical energy use are also calculated using the chiller partload cycling fraction.
where
= chiller partload cycling fraction
= chiller minimum partload ratio
= generator heat input [W]
= chiller pumping power [W]
The evaporator water mass flow rate is calculated based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= chiller evaporator water mass flow rate (kg/s)
= chiller design evaporator water mass flow rate (kg/s)
= chiller evaporator water temperature difference (ºC)
= chiller evaporator inlet water temperature (ºC)
= chiller evaporator outlet water setpoint temperature (ºC)
= specific heat of water entering evaporator (J/kg ºC)
The evaporator outlet water temperature is then calculated based on the cooling effect produced and the evaporator entering water temperature.
where
= chiller evaporator outlet water temperature [ºC]
= chiller evaporator inlet water temperature [ºC]
= specific heat of chiller evaporator inlet water [J/kg/ºC]
= chiller evaporator water mass flow rate [kg/s]
The condenser heat transfer and condenser leaving water temperature are also calculated.
where
= chiller condenser heat transfer rate [W]
= chiller condenser outlet water temperature [ºC]
= chiller condenser inlet water temperature [ºC]
= specific heat of chiller condenser inlet water [J/kg/ºC]
= chiller condenser water mass flow rate [kg/s]
The absorption chiller can model the impact of steam or hot water entering the generator, although the connection of the steam (hot water) nodes to a plant is not actually required. The calculations specific to the generator depend on the type of fluid used and are described here in further detail.
Steam Loop Calculations[LINK]
When a steam loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a steam loop), the generator outlet node steam mass flow rate and temperature are calculated based on the generator heat input, latent heat of steam, the specific heat of water, and the amount of subcooling in the steam generator. The model assumes dry saturated steam enters the generator and exits the generator as a subcooled liquid. The temperature leaving the generator is calculated based on the user entered amount of liquid subcooling in the generator. The effect of subcooling of the liquid (condensate) in the pipe returning to the boiler is also modeled using the user entered abount of steam condensate loop subcooling.
where
= chiller steam mass flow rate [kg/s]
= latent heat of steam [J/kg]
= specific heat of water [J/Kg ºC]
= amount of subcooling in steam generator [ºC]
= amount of condensate subcooling in steam loop [ºC]
= generator steam outlet node temperature [ºC]
= generator steam inlet node temperature [ºC]
Hot Water Loop Calculations[LINK]
When a hot water loop is used and the inlet and outlet node names are specified (i.e. the nodes are connected to a hot water loop), the generator outlet node temperature is calculated based on the generator heat input, mass flow rate of water, and the specific heat of water entering the hot water generator. The calculations are based on the Chiller Flow Mode as follows.
Constant Flow Chillers:
Variable Flow Chillers:
where
= generator hot water mass flow rate [kg/s]
= generator design hot water mass flow rate (kg/s)
= generator design hot water temperature difference (ºC)
Combustion Turbine Chiller[LINK]
The input object Chiller:CombustionTurbine provides a chiller model that is the empirical model from the Building Loads and System Thermodynamics (BLAST) program. Fitting catalog data to a third order polynomial equations generates the chiller performance curves. Three sets of coefficients are required to model the open centrifugal chiller as discussed in the section, titled, ‘Electric Chiller Based on BLAST Centrifugal Chiller Model’.
The gas turbinedriven chiller is an open centrifugal chiller driven directly by a gas turbine. The BLAST model of an open centrifugal chiller is modeled as standard vapor compression refrigeration cycle with a centrifugal compressor driven by a shaft power from an engine. The centrifugal compressor has the incoming fluid entering at the eye of a spinning impeller that throws the fluid by centrifugal force to the periphery of the impeller. After leaving the compressor, the refrigerant is condensed to liquid in a refrigerant to water condenser. The heat from the condenser is rejected to a cooling tower, evaporative condenser, or well water condenser depending on which one is selected by the user based on the physical parameters of the plant. The refrigerant pressure is then dropped through a throttling valve so that fluid can evaporate at a low pressure that provides cooling to the evaporator. The evaporator can chill water that is pumped to chilled water coils in the building. For more information, see the Input/Output Reference Document.
This chiller is modeled like the electric chiller with the same numerical curve fits and then some additional curve fits to model the turbine drive. Shown below are the definitions of the curves that describe this model.
The chiller’s temperature rise coefficient which is defined as the ratio of the required change in condenser water temperature to a given change in chilled water temperature, which maintains the capacity at the nominal value. This is calculated as the following ratio:
Where:
TCEnt_{required} = Required entering condenser air or water temperature to maintain rated capacity.
TCEnt_{rated} = Rated entering condenser air or water temperature at rated capacity.
TELv_{required} = Required leaving evaporator water outlet temperature to maintain rated capacity.
TELv_{rated} = Rated leaving evaporator water outlet temperature at rated capacity.
The Capacity Ratio Curve is a quadratic equation that determines the Ratio of Available Capacity to Nominal Capacity. The defining equation is:
Where the Delta Temperature is defined as:
TempCondIn = Temperature entering the condenser (water or air temperature depending on condenser type).
TempCondInDesign = Temp Design Condenser Inlet from User input above.
TempEvapOut = Temperature leaving the evaporator.
TempEvapOutDesign = Temp Design Evaporator Outlet from User input above.
TempRiseCoefficient = User Input from above.
The following three fields contain the coefficients for the quadratic equation.
The Power Ratio Curve is a quadratic equation that determines the Ratio of Full Load to Power. The defining equation is:
The Full Load Ratio Curve is a quadratic equation that determines the fraction of full load power. The defining equation is:
The Fuel Input Curve is a polynomial equation that determines the Ratio of Fuel Input to Energy Output. The equation combines both the Fuel Input Curve Coefficients and the Temperature Based Fuel Input Curve Coefficients. The defining equation is:
Where FIC represents the Fuel Input Curve Coefficients, TBFIC represents the Temperature Based Fuel Input Curve Coefficients, Rload is the Ratio of Load to Combustion Turbine Engine Capacity, and AT_{air}is the difference between the current ambient and design ambient temperatures.
The Exhaust Flow Curve is a quadratic equation that determines the Ratio of Exhaust Gas Flow Rate to Engine Capacity. The defining equation is:
**
Where GTCapacity is the Combustion Turbine Engine Capacity, and AT_{air} is the difference between the current ambient and design ambient temperatures.
The Exhaust Gas Temperature Curve is a polynomial equation that determines the Exhaust Gas Temperature. The equation combines both the Exhaust Gas Temperature Curve Coefficients (Based on the Part Load Ratio) and the (Ambient) Temperature Based Exhaust Gas Temperature Curve Coefficients. The defining equation is:
Where C represents the Exhaust Gas Temperature Curve Coefficients, TBC are the Temperature Based Exhaust Gas Temperature Curve Coefficients, RLoad is the Ratio of Load to Combustion Turbine Engine Capacity, and AT_{air} is the difference between the actual ambient and design ambient temperatures.
The Recovery Lubricant Heat Curve is a quadratic equation that determines the recovery lube energy. The defining equation is:
Where Pload is the engine load and RL is the Ratio of Load to Combustion Turbine Engine Capacity
The UA is an equation that determines the overall heat transfer coefficient for the exhaust gasses with the stack. The heat transfer coefficient ultimately helps determine the exhaust stack temperature. The defining equation is:
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
ChillerHeater:Absorption:DirectFired[LINK]
Overview[LINK]
This model (object name ChillerHeater:Absorption:DirectFired) simulates the performance of a direct fired twostage absorption chiller with optional heating capability. The model is based on the direct fired absorption chiller model (ABSORGCHLR) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus some additional capabilities.
This model simulates the thermal performance of the chiller and the fuel consumption of the burner(s). This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. Cooling Tower:Single Speed).
Model Description[LINK]
The chiller model uses usersupplied performance information at design conditions along with five performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offdesign conditions. Two additional performance curves for heating capacity and efficiency are used when the chiller is operating in a heating only mode or simultaneous cooling and heating mode.
Cooling[LINK]
The following nomenclature is used in the cooling equations:
AvailCoolCap=available fullload cooling capacity at current conditions [W]
CEIR=user input “Electric Input to Cooling Output Ratio”
CEIRfPLR=electric input to cooling output factor, equal to 1 at full load, user input “Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CEIRfT=electric input to cooling output factor, equal to 1 at design conditions, user input “Electric Input to Cooling Output Ratio Function of Temperature Curve Name”
CFIR=user input “Fuel Input to Cooling Output Ratio”
CFIRfPLR=fuel input to cooling output factor, equal to 1 at full load, user input “Fuel Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CFIRfT=fuel input to cooling output factor, equal to 1 at design conditions, user input “Fuel Input to Cooling Output Ratio Function of Temperature Curve Name”
CondenserLoad=condenser heat rejection load [W]
CoolCapfT=cooling capacity factor, equal to 1 at design conditions, user input “Cooling Capacity Function of Temperature Curve Name”
CoolElectricPower=cooling electricity input [W]
CoolFuelInput=cooling fuel input [W]
CoolingLoad=current cooling load on the chiller [W]
CPLR=cooling partload ratio = CoolingLoad / AvailCoolCap
HeatingLoad=current heating load on the chiller heater [W]
HFIR=user input “Fuel Input to Heating Output Ratio”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
T_{cond}=entering or leaving condenser fluid temperature [C]. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower) if the entering condenser fluid temperature option is used. For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively, if the entering condenser fluid temperature option is used.
T_{cw,l}=leaving chilled water temperature [C]
Five performance curves are used in the calculation of cooling capacity and efficiency:
The cooling capacity function of temperature (CoolCapfT) curve represents the fraction of the cooling capacity of the chiller as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature. The output of this curve is multiplied by the nominal cooling capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The available cooling capacity of the chiller is then computed as follows:
The fuel input to cooling output ratio function of temperature (CFIRfT) curve represents the fraction of the fuel input to the chiller at full load as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature. The output of this curve is multiplied by the nominal fuel input to cooling output ratio (CFIR) to give the fullload fuel input to cooling capacity ratio at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The fuel input to cooling output ratio function of part load ratio (CFIRfPLR) curve represents the fraction of the fuel input to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The fraction of the time step during which the chiller heater is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The cooling fuel input to the chiller is then computed as follows:
The electric input to cooling output ratio as function of temperature (CEIRfT) curve represents the fraction of electricity to the chiller at full load as it varies by temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature.
The electric input to cooling output ratio function of part load ratio (CEIRfPLR) curve represents the fraction of electricity to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The cooling electric input to the chiller is computed as follows:
All five of these cooling performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (objects Curve:Linear, Curve:Quadratic and Curve:Biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s CFIRfPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero).
The condenser load is computed as follows:
Heating[LINK]
The following nomenclature is used in the heating equations:
AvailHeatCap=available fullload heating capacity at current conditions [W]
CPLRh=cooling partload ratio for heating curve = CoolingLoad / NomCoolCap
HeatCapfCPLR=heating capacity factor as a function of cooling part load ratio, equal to 1 at zero cooling load, user input “Heating Capacity Function of Cooling Capacity Curve Name”
HeatCoolCapRatio=user input “Heating to Cooling Capacity Ratio”
HeatElectricPower=heating electricity input [W]
HeatFuelInput=heating fuel input [W]
HeatingLoad=current heating load on the chiller [W]
HEIR=user input “Electric Input to Heating Output Ratio”
HFIR=user input “Fuel Input to Heating Output Ratio”
HFIRfHPLR=fuel input to heating output factor, equal to 1 at full load, user input “Fuel Input to Heat Output Ratio During Heating Only Operation Curve Name”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
TotalElectricPower=total electricity input [W]
TotalFuelInput=total fuel input [W]
Cooling is the primary purpose of the Direct Fired Absorption Chiller so that function is satisfied first and if energy is available for providing heating that is provided next.
The two performance curves for heating capacity and efficiency are:
The heating capacity function of cooling capacity curve (HeatCapfCool) determines how the heating capacity of the chiller varies with cooling capacity when the chiller is simultaneously heating and cooling. The curve is normalized so an input of 1.0 represents the nominal cooling capacity and an output of 1.0 represents the full heating capacity. An output of 1.0 should occur when the input is 0.0.
The available heating capacity is then computed as follows:
The fuel input to heat output ratio curve (HFIRfHPLR) function is used to represent the fraction of fuel used as the heating load varies as a function of heating part load ratio. It is normalized so that a value of 1.0 is the full available heating capacity. The curve is usually linear or quadratic and will probably be similar to a boiler curve for most chillers.
The fraction of the time step during which the chiller is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The heating electric input to the chiller is computed as follows:
If the chiller is delivering heating and cooling simultaneously, the parasitic electric load will be doublecounted, so the following logic is applied:
The total fuel and electric power input to the chiller is computed as shown below:
ChillerHeater:Absorption:DoubleEffect[LINK]
Overview[LINK]
This model (object name ChillerHeater:Absorption:DoubleEffect) simulates the performance of an exhaust fired twostage (double effect) absorption chiller with optional heating capability. The model is based on the direct fired absorption chiller model (ABSORGCHLR) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus some additional capabilities. The model uses the exhaust gas output from Microturbine.
This model simulates the thermal performance of the chiller and the thermal energy input to the chiller. This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. Cooling Tower:Single Speed).
Model Description[LINK]
The chiller model uses usersupplied performance information at design conditions along with five performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offdesign conditions. Two additional performance curves for heating capacity and efficiency are used when the chiller is operating in a heating only mode or simultaneous cooling and heating mode.
Cooling[LINK]
The following nomenclature is used in the cooling equations:
AvailCoolCap=available fullload cooling capacity at current conditions [W]
CEIR=user input “Electric Input to Cooling Output Ratio”
CEIRfPLR=electric input to cooling output factor, equal to 1 at full load, user input “Electric Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
CEIRfT=electric input to cooling output factor, equal to 1 at design conditions, user input “Electric Input to Cooling Output Ratio Function of Temperature Curve Name”
TeFIR=user input “Thermal Energy Input to Cooling Output Ratio”
TeFIRfPLR=thermal energy input to cooling output factor, equal to 1 at full load, user input “Thermal Energy Input to Cooling Output Ratio Function of Part Load Ratio Curve Name”
TeFIRfT=thermal energy input to cooling output factor, equal to 1 at design conditions, user input “Thermal Energy Input to Cooling Output Ratio Function of Temperature Curve Name”
CondenserLoad=condenser heat rejection load [W]
CoolCapfT=cooling capacity factor, equal to 1 at design conditions, user input “Cooling Capacity Function of Temperature Curve Name”
CoolElectricPower=cooling electricity input [W]
CoolThermalEnergyInput=cooling thermal energy input [W]
CoolingLoad=current cooling load on the chiller [W]
CPLR=cooling partload ratio = CoolingLoad / AvailCoolCap
HeatingLoad=current heating load on the chiller heater [W]
HFIR=user input “Thermal Energy Input to Heating Output Ratio”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
**= exhaust air mass flow rate from microturbine (kg/s)
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
= exhaust air outlet temperature from microturbine entering the chiller
(^{o}C)
= Temperature of exhaust leaving the chiller (the generator component of the absorption chiller)
T_{cond}=entering condenser fluid temperature [°C]. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively.
T_{cw,l}=leaving chilled water temperature [°C]
The selection of entering or leaving condense fluid temperature can be made through the optional fieldTemperature Curve Input Variable.
Five performance curves are used in the calculation of cooling capacity and efficiency:
The cooling capacity function of temperature (CoolCapfT) curve represents the fraction of the cooling capacity of the chiller as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and the entering condenser fluid temperature. The output of this curve is multiplied by the nominal cooling capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The available cooling capacity of the chiller is then computed as follows:
The thermal energy input to cooling output ratio function of temperature (TeFIRfT) curve represents the fraction of the thermal energy input to the chiller at full load as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and the entering condenser fluid temperature. The output of this curve is multiplied by the nominal thermal energy input to cooling output ratio (TeFIR) to give the fullload thermal energy input to cooling capacity ratio at specific temperature operating conditions (i.e., at temperatures different from the design temperatures). The curve should have a value of 1.0 at the design temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation.
The thermal energy input to cooling output ratio function of part load ratio (TeFIRfPLR) curve represents the fraction of the thermal energy input to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The fraction of the time step during which the chiller heater is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The cooling thermal energy input to the chiller is then computed as follows:
To make sure that the exhaust mass flow rate and temperature from microturbine are sufficient to drive the chiller, the heat recovery potential is compared with the cooling thermal energy input to the chiller (CoolThermalEergyInput). The heat recovery potential should be greater than the CoolThermalEnergyInput. Heat recovery potential is calculated as:
T_{abs,gen,o}is the minimum temperature required for the proper operation of the doubleeffect chiller. It will be defaulted to 176°C.
The electric input to cooling output ratio as function of temperature (CEIRfT) curve represents the fraction of electricity to the chiller at full load as it varies with temperature. This a biquadratic curve with the input variables being the leaving chilled water temperature and either the entering or leaving condenser fluid temperature.
The electric input to cooling output ratio function of part load ratio (CEIRfPLR) curve represents the fraction of electricity to the chiller as the load on the chiller varies at a given set of operating temperatures. The curve is normalized so that at full load the value of the curve should be 1.0. The curve is usually linear or quadratic.
The cooling electric input to the chiller is computed as follows:
All five of these cooling performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (objects Curve:Linear, Curve:Quadratic and Curve:Biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s TeFIRfPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero). A set of curves derived from manufacturer’s data are also provided in the dataset (ExhaustFiredChiller.idf) is provided with E+ installation.
The condenser load is computed as follows:
Heating[LINK]
The following nomenclature is used in the heating equations:
AvailHeatCap=available fullload heating capacity at current conditions [W]
CPLRh=cooling partload ratio for heating curve = CoolingLoad / NomCoolCap
HeatCapfCPLR=heating capacity factor as a function of cooling part load ratio, equal to 1 at zero cooling load, user input “Heating Capacity Function of Cooling Capacity Curve Name”
HeatCoolCapRatio=user input “Heating to Cooling Capacity Ratio”
HeatElectricPower=heating electricity input [W]
HeatThermalEnergyInput= heating thermal energy input [W]
HeatingLoad=current heating load on the chiller [W]
HEIR=user input “Electric Input to Heating Output Ratio”
HFIR=user input “Thermal Energy Input to Heating Output Ratio”
HFIRfHPLR=thermal energy input to heating output factor, equal to 1 at full load, user input “Thermal Energy Input to Heat Output Ratio During Heating Only Operation Curve Name”
HPLR=heating partload ratio = HeatingLoad / AvailHeatCap
MinPLR=user input “Minimum Part Load Ratio”
NomCoolCap=user input “Nominal Cooling Capacity” [W]
RunFrac=fraction of time step which the chiller is running
TotalElectricPower=total electricity input [W]
TotalThermalEnergyInput=total thermal energy input [W]
Cooling is the primary purpose of the Exhaust Fired Absorption Chiller so that function is satisfied first and if energy is available for providing heating that is provided next.
The two performance curves for heating capacity and efficiency are:
The heating capacity function of cooling capacity curve (HeatCapfCPLR) determines how the heating capacity of the chiller varies with cooling capacity when the chiller is simultaneously heating and cooling. The curve is normalized so an input of 1.0 represents the nominal cooling capacity and an output of 1.0 represents the full heating capacity. An output of 1.0 should occur when the input is 0.0.
The available heating capacity is then computed as follows:
The thermal energy input to heat output ratio curve (HFIRfHPLR) function is used to represent the fraction of thermal energy used as the heating load varies as a function of heating part load ratio. It is normalized so that a value of 1.0 is the full available heating capacity. The curve is usually linear or quadratic and will probably be similar to a boiler curve for most chillers.
The fraction of the time step during which the chiller is operating is computed as a function of the cooling and heating partload ratios and the userinput minimum partload ratio:
The heating electric input to the chiller is computed as follows:
If the chiller is delivering heating and cooling simultaneously, the parasitic electric load would be doublecounted, so the following logic is applied:
The total thermal energy and electric power input to the chiller is computed as shown below:
References[LINK]
Personal communications with various absorption chiller manufacturers, March 2011.
Absorption Chillers and Heat Pumps, Keith Herold, Reinhard Radermacher and Sanford A. Klein (Mar 18, 1996).
Absorption systems for combined heat and power: The problem of partload operation, ASHRAE Transactions, 2003, Vol 109, Part1.
Constant COP Chiller[LINK]
The input object Chiller:ConstantCOP provides a chiller model that is based on a simple, constant COP simulation of the chiller. In this case, performance does not vary with chilled water temperature or condenser conditions. The nominal capacity of the chiller and the COP are user specified along with the connections to the plant and condenser loop and mass flow rates. Such a model is useful when the user does not have access to detailed performance data.
The chiller power is calculated from the load divided by the COP. This chiller will meet the load as long as it does not exceed the nominal capacity specified by the user.
Then the evaporator temperatures are calculated from the load
The condenser load and temperatures are calculated from the evaporator load and the power to the chiller.
See the InputOutput Reference for additional information.
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
Hot Water Heat Recovery from Chillers[LINK]
The electric chillers (e.g., Chiller:Electric, Chiller:EngineDriven, Chiller:CombustionTurbine, Chiller:Electric:EIR, and Chiller:Electric:ReformulatedEIR) all have the option of connecting a third plant loop for heating hot water at the same time the chiller cools the chilled water. The engine and combustion turbine chillers models include curves for heat recovery from oil and or jacket coolers. The other three chillers can model heat recovery where part of its condenser section is connected to a heat recovery loop for what is commonly known as a double bundled chiller, or single condenser with split bundles. The heat recovery chiller is simulated as a standard vapor compression refrigeration cycle with a double bundled condenser. A double bundle condenser involves two separate flow paths through a split condenser. One of these paths is condenser water typically connected to a standard cooling tower; the other path is hot water connected to a heat recovery loop. After leaving the compressor, the refrigerant is condensed to liquid in a refrigerant to water condenser. In a split bundle, the chiller’s internal controls will direct a part of the refrigerant to heat recovery condenser bundle and/or to the tower water condenser bundle depending on the chilled water load, the condenser inlet temperatures and internal chiller controls (and possibly a leaving hot water temperature setpoint). The refrigerant pressure is then dropped through a throttling valve so that fluid can evaporate at a low pressure that provides cooling to the evaporator.
Diagram of Chiller:Electric with Heat Recovery
The algorithm for the heat recovery portion of the chiller needs to be determined from relatively simple inputs to estimate the amount of the heat that is recovered and then send the rest of the heat to the cooling tower. For the chiller models associated with the object Chiller:Electric, air or evaporativelycooled condensers are allowed to be used with heat recovery and, when used, the condenser specific heat, mass flow rate, and temperatures shown below refer to outdoor air. A condenser air volume flow rate must be specified when using heat recovery with air or evaporativelycooled chillers.
The basic energy balance for the condenser section of a heat recovery chiller is
In practice, if the entering temperature of the heat recovery hot fluid is too high, the chiller’s internal controls will redirect refrigerant away from the heat recovery bundle. A user input is available for declaring the inlet high temperature limit, and if it is exceeded, the chiller will shut down heat recovery and request no flow and will not reject any condenser heat to that fluid.
The heat recovery condenser bundle is often physically smaller than the tower water condenser bundle and therefore may have limited heat transfer capacity. User input for the relative capacity of the heat recovery bundle, , is used to define a maximum rate of heat recovery heat transfer using
This capacity factor is also used to autosize the heat recovery design fluid flow rate when it is set to autosize. The design heat recover flow rate is calculated by multiplying by the condenser tower water design flow rate. If no capacity factor is input, it is assumed to be 1.0.
A heat recovery chiller may control the temperature of heat recovery fluid leaving the device by modulating the flow of refrigerant to the heat recovery condenser bundle. There are two different algorithms used depending on if the input has declared a leaving setpoint node.
If no control setpoint node was named, then the model developed by Liesen and Chillar (2004) is used to approximate the relative distribution of refrigerant flow and condenser heat transfer between the bundles. This model approximates the heat transfer situation by using average temperatures in and out of the condenser section.
Then the inlet temperature is flowweighted to determine lumped inlet and outlet conditions.
The lumped outlet temperature is then used for an approximate method of determining the heat recovery rate
This rate is then limited by the physical size of the heat recovery bundle.
If user input for the leaving temperature setpoint is available, then a second model is used to distribute refrigerant flow and condenser heat transfer between the bundles that attempts to meet the heat recovery load implied by the leaving setpoint. When setpoint control is used, the desired rate of heat recovery heat transfer is:
Then the heat recovery rate is simply modeled as the lower of the three different heat flow rates: the desired capacity, the maximum capacity, and the current total heat rejection rate.
For both models, the condenser heat transfer rate is then
The outlet temperatures are then calculated using
A heat recovery chiller may need to work harder because the refrigeration system faces a higher lift owing to an elevated effective temperature for heat rejection. With heat recovery, the condenser temperature used with the chiller’s performance curves is determined using one of the following heatflowweighted methods. The first is used for the chiller model for the objects Chiller:Electric, and Chiller:Electric:EIR which use the condensing entering temperature for performance.
The second is used for the chiller model for the object Chiller:Electric:ReformulatedEIR which uses the leaving condenser fluid temperature.
Both of these are available as an output variable called Chiller Effective Heat Rejection Tempeature, in C.
Chiller Basin Heater[LINK]
This chiller’s basin heater (for evaporativelycooled condenser type) operates in the same manner as the Engine driven chiller’s basin heater. The calculations for the chiller basin heater are described in detail at the end of the engine driven chiller description (Ref. Engine Driven Chiller).
Reference
Leisen and Chillar. 2004. Variable Heat Recovery In Double Bundle Electric Chillers. SimBuild 2004, IBPSAUSA National Conference Boulder, CO, August 46, 2004.
Electric Chiller Model Based on Fluid Temperature Differences[LINK]
The centrifugal chiller model (object name Chiller:Electric) was originally developed for the BLAST program. The model is based on a ‘capacity ratio’ curve, which is a quadratic equation that determines the Ratio of Available Capacity to Nominal Capacity. The defining equation is:
Where the Delta Temperature is defined as:
where the temperature rise coefficient is defined as the ratio of the required change in condenser water temperature to a given change in chilled water temperature, which maintains the capacity at the nominal value. If the chiller is a heat recovery chiller,then the condenser inlet temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers. This is calculated as the following ratio:
Where:
T_{cond,in,required} = Required entering condenser air or water temperature to maintain rated capacity (C)
T_{cond,in,rated} = Rated entering condenser air or water temperature at rated capacity (C)
T_{evap,out,required} = Required leaving evaporator water outlet temperature to maintain rated capacity (C)
T_{evap,out,rated} = Rated leaving evaporator water outlet temperature at rated capacity (C)
The Power Ratio Curve is a quadratic equation that determines the Ratio of Full Load to Power. The defining equation is:
where the part load ratio, PLR is defined as:
The Load Ratio Curve is a quadratic equation that determines the Ratio of Actual Cooling Load to Full Cooling Load. The defining equation is:
The evaporator heat transfer rate and the power required by the chiller are then calculated as:
Electric Chiller Model Based on Condenser Entering Temperature[LINK]
Overview[LINK]
This model (object name Chiller:Electric:EIR) simulates the performance of an electric liquid chiller. The model is based on the compression chiller model (COMREF) in the DOE2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE2.1 chiller model, plus additional abilities for modeling evaporativelycooled condensers and heat recovery for water heating.
This model simulates the thermal performance of the chiller and the power consumption of the compressor(s). It also models the power consumption of condenser fans if modeling an aircooled or evaporativelycooled condenser. This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. CoolingTower:SingleSpeed).
Model Description[LINK]
The chiller model uses usersupplied performance information at reference conditions along with three performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offreference conditions. The three performance curves are:
where
ChillerCapFTemp = cooling capacity factor, equal to 1 at reference conditions
T_{cw,l}= leaving chilled water temperature, ˚C
T_{cond,e}= entering condenser fluid temperature, ˚C. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively.
where
ChillerEIRFTemp = energy input to cooling output factor, equal to 1 at reference conditions
T_{cw,l} = leaving chilled water temperature, ˚C
T_{cond,e}= entering condenser fluid temperature, ˚C. For a watercooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air or evapcooled condensers this will be the entering outdoor air drybulb or wetbulb temperature, respectively. If the chiller is a heat recovery chiller,then the condenser inlet temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
where
ChillerEIRFPLR = energy input to cooling output factor, equal to 1 at reference conditions
PLR = partload ratio = (cooling load) / (chiller’s available cooling capacity)
P_{chiller} = chiller power at specific PLR
P_{ref} = /COP_{ref}
All three of the performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (curve:quadratic and curve:biquadratic). It is not imperative that the user utilize all coefficients in the performance curve equations if their performance equation has fewer terms (e.g., if the user’s ChillerEIRFPLR performance curve is linear instead of quadratic, simply enter the values for a and b, and set coefficient c equal to zero). Performance curves for more than 160 chillers, including the default DOE2.1E reciprocating and centrifugal chillers, are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf).
For any simulation time step, the chiller’s available cooling capacity is calculated as follows:
where
= chiller capacity at reference conditions (reference temperatures and flow rates defined by the user), W
= available chiller capacity adjusted for current fluid temperatures, W
The model then calculates the evaporator heat transfer rate required to bring the entering chilled water temperature down to the leaving chilled water setpoint temperature (established using a SetpointManager object and referenced in the PlantLoop object). If this calculated heat transfer rate is greater than the heat transfer rate being requested by the plant equipment operation scheme, then the evaporator heat transfer rate is reset to the requested cooling rate.
The evaporator heat transfer rate is then compared to the available capacity. If the available chiller capacity is sufficient to meet the evaporator heat transfer rate, the leaving chilled water temperature is set equal to the chilled water setpoint temperature. If the requested evaporator heat transfer rate is larger than the available capacity the chilled water leaving the evaporator is allowed to float upward. For this case, the exiting chilled water temperature is calculated based on the water temperature entering the evaporator, the available cooling capacity, and the evaporator mass flow rate as follows:
where
T_{cw,l}= water temperature leaving the evaporator, ˚C
T_{cw,e}= water temperature entering the evaporator, ˚C
= evaporator mass flow rate, kg/s
C_{p,evap}= specific heat of water entering evaporator at T_{cw,e}, J/kg˚C
The partload ratio is then calculated as the ratio of the evaporator heat transfer rate to the available chiller capacity. The partload ratio is not allowed to be greater than the maximum partload ratio specified by the user or less than zero as follows:
where
PLR= partload ratio
= load to be met by the chiller, W
PLR_{max}= maximum partload ratio (specified by the user in the input data file)
The model assumes that the cooling load is met through chiller unloading down to the minimum unloading ratio. False loading (e.g. hotgas bypass) is assumed to occur between the minimum unloading ratio and the minimum part load ratio yielding constant electrical power consumption under these conditions. Below the minimum part load ratio, the chiller cycles on and off to meet very small loads and the power consumption during the on cycle is the same as when the chiller is operating at the minimum part load ratio. When the chiller part load ratio is less than the minimum part load ratio, the onoff cycling ratio of the chiller is calculated as follows and is available as an output variable.
To properly account for chiller electric power consumption when PLR is less than the minimum unloading ratio, the PLR is reset to the greater of the PLR calculated above and the PLR at the minimum unloading ratio. The result is available as the output variable Chiller Part Load Ratio.
This revised PLR accounts for the “false loading” (e.g., hot gas bypass) that is assumed to occur whenever the PLR (based on cooling load divided by available capacity) is less than the minimum unloading ratio specified. The amount of false loading on the chiller is calculated using this revised PLR and is reported as an output variable as follows:
The electrical power consumption for the chiller compressor(s) for any simulation time step is then calculated using the following equation:
where
P_{chiller}= chiller compressor power, W
COP_{ref}= reference coefficient of performance, W/W
Heat rejected by the chiller condenser includes the heat transferred in the evaporator plus a portion or all of the compressor electrical energy consumption. For electric chillers with hermetic compressors, all compressor energy consumption is rejected by the condenser (compressor motor efficiency = eff_{motor} = 1.0). For chillers with semihermetic or open compressors, only a portion of the compressor energy use is rejected by the condenser. The heat transfer rate for the chiller condenser is calculated as follows:
where
= condenser heat transfer rate, W
= compressor motor efficiency = fraction of compressor electrical energy consumption rejected as condenser heat
For watercooled chillers, the water temperature leaving the condenser is then calculated as shown below.
where:
T_{cond,l}= water temperature leaving the condenser, ˚C
T_{cond,e}= water temperature entering the condenser, ˚C
= mass flow rate through the condenser, kg/s
= specific heat of water entering the condenser at T_{cond,e}, J/kg˚C
For air and evaporativelycooled condensers, the exiting air temperature is not calculated and is set equal to the entering air or wetbulb temperature, respectively.
The model then calculates the condenser fan energy for air and evaporativelycooled condensers. The amount of condenser fan energy is assumed to be proportional to the chiller cycling ratio and is calculated as follows:
where
P_{cond}= chiller condenser fan electric power, W
P_{condfanratio}= condenser fan power ratio, W/W
The final calculations determine the total heat transfer energy for the condenser and evaporator, as well as the total electric energy consumed by the chiller compressor motor(s) and condenser fan(s). The results are available as output variables.
where
Q_{cond}= chiller condenser heat transfer energy, J
Q_{evap}= chiller evaporator cooling energy, J
E_{chiller}= chiller (compressor) electric energy, J
E_{cond}= chiller condenser fan electric energy, J
TimeStepSys = HVAC system simulation time step, hr
= conversion factor, sec/hr
Electric EIR Chiller with Heat Recovery Option[LINK]
Heat from the electric EIR chiller condenser may be recovered when a watercooled condenser is selected for simulation. The heat recovery water flow rate is specified by the user along with the input and output nodes connected to the heat recovery loop. The algorithms are identical to those used for Chiller:Electric. Refer to the section entitled Chillers with Plant Heat Recovery for details.
Standard Rating (Integrated Part Load Value)[LINK]
For the Chiller:Electric:EIR and Chiller:Electric:ReformulatedEIR objects in EnergyPlus, the industry standard rating of Integrated Part Load Value (IPLV) is calculated according to ANSI/AHRI Standard 550/590 (2011). This standard rating is not direct input to the model and is calculated using userentered information for these objects. These standard rating values are provided in the eplusout.eio output file (Ref. Output Details document) and also in the predefined tabular output reports (Output:Table:SummaryReports object, Equipment Summary).
The IPLV is a single number partload performance figure of merit for WaterChilling Packages. The IPLV equations and procedures described below are taken from Appendix D of ANSI/AHRI Std. 550/590, 2011 and provide a consistent method for calculating IPLV. These equations provide representative average partload efficiency for a single chiller. For equipment covered by this standard, the IPLV is calculated using the following formula:
where,
= EER or COP at 100% capacity at AHRI standard rating conditions
= EER or COP at 75% capacity and reduced ambient (see Table 49
= EER or COP at 50% capacity and reduced ambient (see Table 49)
= EER or COP at 25% capacity and reduced ambient (see Table 49)
The Coefficient of Performance (COP) at the various load capacity points (100%, 75%, 50%, and 25% partload ratios) are calculated using the procedure outlined below and applicable test conditions specified in Table 49.
EER at desired reduced capacity (75%, 50%, and 25%) is calculated as follows
= Reference chiller capacity specified by the user, (W).
= Reference coefficient of performance specified by the user, (W/W).
= Userspecified biquadratic curve for modifying EIR as a function of leaving chilled water temperature (6.7°C) and entering condenser temperature obtained from Table 49 for reduced capacities, (dimensionless).
= Userspecified quadratic curve for modifying EIR as a function of part load ratio.
= Capacity (W) of the chiller determined per the ANSI/AHRI Standard 550/590 reduced ambient test conditions as shown in Table 49.
If the equipment cannot be unloaded to the desired reduced capacity (75%, 50%, and 25%) i.e. if the minimum unloading ratio of the chiller is greater than desired reduced capacity, then the model is run at the minimum unloading PLR of the equipment at the condenser entering conditions defined in Table 49 and the efficiency is adjusted for cyclic performance.
where,
= EIR of chiller at minimum unloading ratio
= degradation coefficient to account for cycling of the compressor for capacity less than the minimum capacity.
The degradation coefficient C_{D} for the desired reduced load points (75%, 50%, or 25%) is determined using the following equation:
The load factor (LF) for the desired reduced load points (75%, 50%, or 25%) calculated from the following equation:
Where,
**= standard rating part load ratio (PLR) points, 75%, 50%, 25%.
=Full load heating capacity (W) of the airsource heat pump equipment determined from ANSI/AHRI Standard 550/590 and test conditions shown in Table 49 for 100% load.
=Part load heating capacity (W) of the airsource heat pump units determined from ANSI/AHRI Standard 550/590 at the standard desired reduced ambient test conditions as shown in Table 49 and the minimum part load capacity of the unit.
where,
= Reference capacity specified by the user, (W).
= Userspecified biquadratic curve evaluated at full load (100%) test conditions shown in Table 49, (dimensionless).
= Part load capacity (W) of the chiller determined from ANSI/AHRI Standard 550/590 at the standard desired reduced ambient test conditions as shown in Table 49.
= Minimum PLR up to which chiller can be unloaded
: Standard Rating (Integrated Part Load Value)
Electric Chiller Model Based on Condenser Leaving Temperature[LINK]
Overview[LINK]
This model (object name Chiller:Electric:ReformulatedEIR) simulates the thermal performance of an electric liquid chiller and the power consumption of its compressor(s). The model, developed by Hydeman et al. (2002) as part of the CoolTools™ project sponsored by Pacific Gas and Electric Company (PG&E), is an empirical model similar to EnergyPlus’ Chiller:Electric:EIR model. The model uses performance information at reference conditions along with three curve fits for cooling capacity and efficiency to determine chiller operation at offreference conditions. The model has the same capabilities as the Chiller:Electric:EIR model, but can potentially provide significant accuracy improvement over the Chiller:Electric:EIR model for chillers with variablespeed compressor motor drives and/or variable condenser water flow applications.
Chiller performance curves can be generated by fitting manufacturer’s catalog data or measured data. Performance curves developed primarily from manufacturer’s performance data are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf). This chiller model can be used to predict the performance of various chiller types (e.g., reciprocating, screw, scroll, and centrifugal) with watercooled condensers. The model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers. This auxiliary equipment must be modeled using other EnergyPlus models (e.g. CoolingTower:SingleSpeed).
The main difference between this model and the Chiller:Electric:EIR model is the condenser fluid temperature used in the associated performance curves: the Chiller:Electric:ReformulatedEIR model uses the LEAVING condenser water temperature while the Chiller:Electric:EIR model uses the ENTERING condenser water temperature. In addition, the Energy Input to Cooling Output Function of Part Load Ratio curve for this reformulated EIR chiller model includes the condenser leaving water temperature as an independent variable in addition to partload ratio. Since the leaving condenser water temperature is a function of load, chiller performance, and condenser entering water temperature, EnergyPlus must iterate to converge on a solution for each simulation time step.
Model Description[LINK]
The chiller model uses usersupplied performance information at reference conditions along with three performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at offreference conditions. The three performance curves are:
The cooling capacity function of temperature curve is a biquadratic performance curve with two independent variables: the leaving chilled water temperature and the leaving condenser water temperature. The output of this curve is multiplied by the reference capacity to give the fullload cooling capacity at specific temperature operating conditions (i.e., at temperatures different from the reference temperatures). The curve should have a value of 1.0 at the reference temperatures and flow rates specified in the input data file by the user. The biquadratic curve should be valid for the range of water temperatures anticipated for the simulation (otherwise the program issues warning messages).
where
ChillerCapFTemp = Cooling capacity factor, equal to 1 at reference conditions
T_{cw,l}= leaving chilled water temperature, ˚C
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
where
ChillerEIRFTemp = Energy input to cooling output factor, equal to 1 at reference conditions
T_{cw,l} = leaving chilled water temperature, ˚C
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
where
ChillerEIRFPLR = Energy input to cooling output factor, equal to 1 at the reference leaving condenser water temperature and PLR = 1.0
T_{cond,l}= leaving condenser water temperature, ˚C. This will be the water temperature entering the condenser loop (e.g., entering the cooling tower). If the chiller is a heat recovery chiller,then the condenser leaving temperature is adjusted to account for both fluid streams as described in the section above on heat recovery chillers.
PLR = Part load ratio = (cooling load) / (chiller’s available cooling capacity)
P_{chiller} = chiller power at specific PLR
P_{ref} = /COP_{ref}
All three of the performance curves are accessed through EnergyPlus’ builtin performance curve equation manager (curve:biquadratic and curve:bicubic). Note that the above three performance curves use the leaving condenser water temperature as an independent variable, instead of the entering condenser water temperature used in the performance curves for the Chiller:Electric:EIR model. Since the leaving condenser water temperature is calculated based on the condenser heat transfer rate, which is a function of the load to be met by the chiller, chiller compressor power, and the false loading (detailed calculations are given below), iterative calculations are required to determine the actual (converged) leaving condenser water temperature. The program uses the leaving condenser water temperature from the previous iteration to calculate values for each of the three performance curves described above. After obtaining the condenser heat transfer rate, the leaving condenser water temperature is recalculated. When the difference between the leaving condenser water temperature calculated on successive iterations is less than 0.0001^{°}C, the solution is assumed to have converged. Warning messages are issued if the calculated solution for leaving condenser water temperature and/or partload ratio falls outside the valid range specified for the chiller’s performance curves. If these warnings are issued, the user may chose to extend the range for the performance curves (only if a small extension is required since model extrapolation may produce significant errors) or a different chiller and associated performance curves with extended performance range can be located and used for the simulation.
Although performance curve data sets for 160 chillers are provided in the EnergyPlus Reference DataSets (Chillers.idf and AllDataSets.idf), they may not meet the requirements for specific applications. One can develop performance curves from performance data using two available techniques (Hydeman and Gillespie 2002). The first technique is called the Leastsquares Linear Regression method and is used when sufficient performance data exist to employ standard leastsquare linear regression techniques. The second technique is called Reference Curve Method and is used when insufficient performance data exist to apply linear regression techniques. A detailed description of both techniques can be found in the reference mentioned above.
For any simulation time step, the chiller’s available cooling capacity is calculated as follows:
where
= chiller capacity at reference conditions (reference temperatures and flow rates defined by the user), W
= available chiller capacity adjusted for current water temperatures, W
The model then calculates the evaporator heat transfer rate required to bring the entering chilled water temperature down to the leaving chilled water setpoint temperature (established using a SetpointManager object and referenced in the PlantLoop object). If this calculated heat transfer rate is greater than the heat transfer rate being requested by the plant equipment operation scheme, then the evaporator heat transfer rate is reset to the requested cooling rate.
The evaporator heat transfer rate is then compared to the available capacity. If the available chiller capacity is sufficient to meet the evaporator heat transfer rate, the leaving chilled water temperature is set equal to the chilled water setpoint temperature. If the requested evaporator heat transfer rate is larger than the available capacity the chilled water leaving the evaporator is allowed to float upward. For this case, the exiting chilled water temperature is calculated based on the water temperature entering the evaporator, the available cooling capacity, and the evaporator mass flow rate as follows:
where
T_{cw,l}= water temperature leaving the evaporator, ˚C
T_{cw,e}= water temperature entering the evaporator, ˚C
= evaporator mass flow rate, kg/s
C_{p,evap}= specific heat of water entering evaporator at T_{cw,e}, J/kg˚C
The partload ratio is then calculated as the ratio of the evaporator heat transfer rate to the available chiller capacity. The partload ratio is not allowed to be greater than the maximum partload ratio specified by the user or less than zero as follows:
where
PLR= partload ratio
= load to be met by the chiller, W
PLR_{max}= maximum partload ratio (specified by the user in the input data file)
Note that the maximum partload ratio (PLR_{max}, specified in the Chiller:Electric:ReformulatedEIR object) used in the equation should be less than or equal to the maximum partload ratio specified in the “Energy Input to Cooling Output Ratio Function of PartLoad Ratio” performance curve object.
The model assumes that the cooling load is met through chiller unloading down to the minimum unloading ratio. False loading (e.g. hotgas bypass) is assumed to occur between the minimum unloading ratio and the minimum partload ratio yielding constant electrical power consumption under these conditions. Below the minimum partload ratio, the chiller cycles on/off to meet very small loads and the power consumption during the on cycle is the same as when the chiller is operating at the minimum part load ratio. When the chiller partload ratio is less than the minimum partload ratio, the onoff cycling ratio of the chiller is calculated as follows and is available as an output variable.
To properly account for chiller electric power consumption when PLR is less than the minimum unloading ratio, the PLR is reset to the greater of the PLR calculated above and the PLR at the minimum unloading ratio. The result is available as the output variable Chiller Part Load Ratio.
This revised PLR accounts for the “false loading” (e.g., hotgas bypass) that is assumed to occur whenever the PLR (based on cooling load divided by available capacity) is less than the minimum unloading ratio specified. The amount of false loading on the chiller is calculated using this revised PLR and is reported as an output variable as follows:
The electrical power consumption for the chiller compressor(s) for any simulation time step is then calculated using the following equation:
where
P_{chiller}= Chiller compressor power, W
COP_{ref}= Reference coefficient of performance, W/W
Heat rejected by the chiller condenser includes the heat transferred in the evaporator plus a portion or all of the compressor electrical energy consumption. For electric chillers with hermetic compressors, all compressor energy consumption is rejected by the condenser (compressor motor efficiency = eff_{motor} = 1.0). For chillers with semihermetic or open compressors, only a portion of the compressor energy use is rejected by the condenser. The heat transfer rate for the chiller condenser is calculated as follows:
where
= condenser heat transfer rate, W
= compressor motor efficiency = fraction of compressor electrical energy consumption rejected as condenser heat
The above curve values are calculated based on the leaving condenser water temperature found through iteration. After obtaining the condenser heat transfer rate, the final leaving condenser water temperature is then calculated as:
where:
T_{cond,l}= water temperature leaving the condenser, ˚C
T_{cond,e}= water temperature entering the condenser, ˚C
= mass flow rate through the condenser, kg/s
= specific heat of water entering the condenser at T_{cond,e}, J/kg˚C
The final calculations determine the total heat transfer energy for the condenser and evaporator, as well as the total electric energy consumed by the chiller compressor motor(s) and condenser fan(s). The results are available as output variables.
where
Q_{cond}= chiller condenser heat transfer energy, J
Q_{evap}= chiller evaporator cooling energy, J
E_{chiller}= chiller (compressor) electric energy, J
E_{cond}= chiller condenser fan electric energy, J
TimeStepSys = HVAC system simulation time step, hr
= conversion factor, sec/hr
Electric Reformulated EIR Chiller with Heat Recovery Option[LINK]
Heat from the electric reformulated EIR chiller condenser may be recovered. The heat recovery water flow rate is specified by the user along with the input and output nodes connected to the heat recovery loop. The algorithms are identical to those used for Chiller:Electric and Chiller:Electric:EIR. Refer to the section entitled Chillers with Plant Heat Recovery for details.
Standard Rating (Integrated Part Load Value)[LINK]
Integrated Part Laod Value (IPLV) calculations for Reformulated EIR chiller are similar to what are described above for EIR chillers. The only difference with Reformulated EIR chiller is that it calls an iterative subroutine (SolveRegulaFalsi) to obtain a condenser water outlet temperature which corresponds to condenser inlet temperature at reduced capacity conditions as outlined in Table 49 above. SolveRegulaFalsi is a general utility routine for finding the zero of a function. In this case it finds the condenser inlet temperature that will zero the residual function – the difference between calculated condenser inlet temperature and desired condenser inlet temperature per ANSI/AHRE 550/590, 2011 (table 42 above) divided by desired condenser inlet temperature.
References[LINK]
Hydeman, M., N. Webb, P. Sreedharan, and S. Blanc. 2002. Development and Testing of a Reformulated RegressionBased Electric Chiller Model. ASHRAE Transactions HI02182.
Hydeman, M. and K.L. Gillespie. 2002. Tools and Techniques to Calibrate Electric Chiller Component Models. ASHRAE Transactions AC0291.
Hydeman, M., K. Gillespie, and R. Kammerud. 1997. PG&E’s CoolTools project: A toolkit to improve evaluation and operation of chilled water plants. Presented at the Cool$ense National Forum on Integrated Chilled Water Retrofits, Sep. 1997. Berkeley California: Lawrence Berkeley National Laboratory.
Engine Driven Chiller[LINK]
The engine driven chiller (Object name: Chiller:EngineDriven) is the empirical model from the Building Loads and System Thermodynamics (BLAST) program. Fitting catalog data to a third order polynomial equations generates the chiller performance curves. Three sets of coefficients are required to model the open centrifugal chiller as discussed in the section, titled, ‘Electric Chiller Based on BLAST Centrifugal Chiller Model’. Additional curve fits are required to model the engine. Because the model inherently involves the lower heating value of the fuel, a reference temperature is also involved in the calculations, which manufacturers present at 25°C.
The engine model was also developed for the BLAST program. It was adapted for use in EnergyPlus. This model is used for both the engine driven generator and the engine driven chiller. It uses the following set of equations all of which are quadratic fits to the PLR (Part Load Ratio) of the generator. The coefficients must be derived from manufacturers data.
The exhaust gas temp and flow rate are used if a stack heat exchanger is used to recover waste heat from the exhaust. This temperature is the inlet temperature to the heat exchanger which is modeled in a UAeffectiveness form:
The exhaust flow rate is then calculated as:
where T_{reference} is the reference temperature for the fuel lower heating value, and is given as 25°C in manufacturer’s data, and
Finally, heat recovered from the lube oil and the water jacket are accounted for as follows:
Chiller Basin Heater[LINK]
Calculations are also made to estimate the electric power input to the basin heater for chillers with evaporativelycooled condensers. The chillers which calculate basin heater power are Chiller:Electric:EIR, Chiller:Electric, Chiller:ConstantCOP, Chiller:EngineDriven and Chiller:CombustionTurbine. A schedule may be used to disable the basin heater during regular maintenance periods or other time periods (e.g., during summer). If a schedule is not provided, the basin heater is assumed to be available the entire simulation time period. The basin heater operates when it is scheduled on, the outdoor air drybulb temperature is below the basin heater setpoint temperature, and the chiller is not active (i.e., chiller is not operating for the simulation time step — for example, when there is no cooling load to be met by the chiller, or if there is no water flow through the chiller due to a chiller or pump availability schedule, etc.). The user is required to enter a basin heater capacity (watts per degree Kelvin) and a heater setpoint temperature (^{o}C) if they want to model basin heater electric power.
where:
= Chiller basin heater electric power (W)
= Chiller basin heater electric consumption (J)
T_{setpoint,basin} = Basin heater setpoint temperature, user input (^{o}C)
T_{db,outdoor} = Outdoor air drybulb temperature (^{o}C)
CAP_{heater,basin} = Basin heater capacity, user input (W/K)
Schedule_{heater,basin} = Basin heater schedule, user input (schedule value > 0 means ON)
ChillerIsOFF = Logical variable denoting that the chiller is not operating for the current simulation time step (e.g., there is no cooling load to be met by the chiller, or if there is no water flow through the chiller due to a chiller or pump availability schedule, etc.)
Documentation content copyright © 19962014 The Board of Trustees of the University of Illinois and the Regents of the University of California through the Ernest Orlando Lawrence Berkeley National Laboratory. All rights reserved. EnergyPlus is a trademark of the US Department of Energy.
This documentation is made available under the EnergyPlus Open Source License v1.0.