Heat emissions from buildings are a crucial component
influencing climate change and urban microclimate, e.g., urban
heat island effect. The heat emissions from a building include
heat releases from three levels of building systems: building
envelope, zone/space, and HVAC systems. Quantifying heat
emissions from buildings and its spatiotemporal distribution
have significant implications on urban environment and climate
studies. Knowing how much heat is released into the atmosphere
can help quantify the magnitude of its impact on urban
microclimate, e.g., urban heat island effect and climate
change. Spatial and temporal distribution of heat emissions
can serve to locate hot spots in space and time, therefore
useful for prioritizing recourse to mitigate heat emissions
from buildings.
This routine calculates heat emissions from buildings and
the scope is restricted to report the buildings’ heat directly
discharged to the ambient air. As shown in the figure, the
emissions include heat releases from three levels of building
components: (1) building envelope - exterior surface
convection and radiative heat transfer to the ambient air, (2)
zone - exhaust air or exfiltration to ambient, (3) HVAC system
- relief/exhaust air from AHUs, and (4) HVAC system - heat
rejected by air-cooled condensers or central plants, including
cooling towers of water-cooled chillers, gas-fired boilers,
furnaces and water heaters. It should be noted that any
under-ground zone or envelope is excluded, since heat from
those spaces is rejected to ground instead of to ambient
air.
Building released heat
compositions [fig:building-released-heat-compositions]
The calculated heat emissions measure the heat transfer
from buildings to their ambient environment, and can have both
positive and negative values. The positive values indicate the
building injects heat to the environment, while the negative
values indicate the building extracts heat from the
environment.
For heat transfer via air flow, both the sensible and
latent heat emissions are calculated and reported.
Exterior surfaces emit heat to ambient air in the form of
convection and long-wave radiation, which could be absorbed by
particles and gas molecules in air. The internal sources
modify the outside surface temperature through envelope
conduction, along with incident solar radiation, which
triggers the heat exchange with the ambient air. This part
does not include the long-wave radiation from surfaces to the
sky or the ground. The surface outside face heat emission rate
is calculated as:
where \(Q_{conv, surf}\)
is the surface outside face convection heat gain rate, \(Hr\) is the surface outside face
thermal radiation to air heat transfer coefficient, \(A\) is the surface area, \(T_{surf}\) and \(T_{out}\) are the surface and
outdoor air temperature.
Discharge air carries heat to ambient at the zone level
from exfiltration and exhaust fan. Exfiltration is unintended
airflow to outside through envelope cracks, as opposite to
infiltration. Zones such as restroom, kitchen and laundry room
may be equipped with exhaust fans for active venting. These
items can be quantified by:
where \(m_{exf}\) is the
exfiltration air mass flow rate, \(m_{exh}\) is the zone exhaust air
mass flow rate from the zone exhaust node, \(h_{zone}\) and \(h_{out}\) are the zone and
outdoor air enthalpy. Among these, the sensible parts are:
where \(c\) is the
specific heat of air, \(T_{zone}\) and \(T_{out}\) are the zone and
outdoor air temperature. And the latent parts are determined
by taking the difference between the total and the sensible
rate. For each zone, its exfiltration rate is calculated by
solving a mass flow balance on the zone including
infiltration, ventilation, outdoor air mixing, zone-to-zone
mixing, and all of the zone inlet, exhaust, and return
nodes.
This part of zone exhaust only includes the exhaust heat at
the zone level defined by Fan:ZoneExhaust
or AirflowNetwork:MultiZone:Component:ZoneExhaustFan.
For each zone, we aggregated this part by searching its
exhaust fans. The exhaust heat from Zone
HVAC system nodes is included in the next item.
Heat Emission from HVAC
Systems through Relief Air[LINK]
HVAC systems relieve heat from system outdoor air relief
nodes, and the total and sensible emission rate are calculated
as:
HVAC system equipment also directly exchanges heat with
ambient air via refrigeration cycle and its energy usage. For
example, for gas-fired boilers/heaters, waste heat is
exhausted to outside together with combustion byproduct. For
space cooling, heat is removed from indoors and dumped to
outdoor at the condenser side or corresponding cooling
equipment, such as cooling tower. In addition, energy consumed
by outdoor equipment will directly dissipate to the ambient
air, such as compressors and fans in packaged DX systems. The
surface outside face heat emission rate is calculated as:
a. Air-cooled: cooling rate + electric power of condenser
fan and compressor
b. Water-cooled: total heat transfer rate with outdoor
air
Detailed calculation of HVAC rejected heat varies with
different HVAC object groups and component types, as
summarized in Table 1.
Heat Rejection Calculation for Different HVAC
Component Types
Object Group
Component
Calculation Methods
Coils
Chilled water cooling coil
Emit heat in condensing unit
defined in Condenser Equipment
Heating coil (fuel heated)
Fuel Consumed + Heating Coil
Defrost Electric Consumption + Heating Coil Crankcase Heater
Electric Consumption – Heating Coil Total Heating Energy
Hot water/steam heating
coil
Emit heat in plant source
defined in Plant Heating and Cooling Equipment
DX cooling coil
(air-cooled)
Cooling Coil Source Side Heat
Transfer Rate + Cooling Coil Electric Power + Cooling Coil
Crankcase Heater Electric Energy + Fuel Waste Heat
DX cooling coil
(Evaporatively-cooled)
Evaporative Cooler Condenser
Pump Electricity Consumption + Cooling Coil Basin Heater
Electric Energy + Evaporative Cooler Water Volume * rho *
evaporation heat of water (\(\delta
H_{we}\))
DX VRF cooling coil
Emit heat in condensing unit
defined in Variable Refrigerant Flow Equipment
DX heating coil
(air-cooled)
Heating Coil Electric Power /
Fuel Consumed + Heating Coil Defrost Electric Power + Heating
Coil Crankcase Heater Electric Power – Heating Coil Total
Heating Rate
Water to air heat pump
Emit heat in condensing unit
defined in Condenser Equipment
DX VRF cooling /heating
coil
Emit heat in condensing unit
defined in Variable Refrigerant Flow Equipment
Evaporative Coolers
Evaporative Coolers
Evaporative Cooler Condenser
Pump Electricity Consumption + Cooling Coil Basin Heater
Electric Energy + Evaporative Cooler Water Volume * rho *
evaporation heat of water (\(\delta
H_{we}\))
Variable Refrigerant Flow
Equipment
VRF air-to-air heat pump
condensing unit (air-cooled)
VRF Heat Pump Total Cooling Rate
+ VRF Heat Pump Total Electric Power (cooling
mode)<br>VRF Heat Pump Total Heating Electric Power /
Fuel Rate - VRF Heat Pump Total Heating Rate (heating
mode)
VRF air-to-air heat pump
condensing unit (evaporative-cooled)
Evaporative Cooler Condenser
Pump Electricity Consumption + Cooling Coil Basin Heater
Electric Energy + Evaporative Cooler Water Volume * rho *
evaporation heat of water (\(\delta
H_{we}\))
VRF air-to-air heat pump
condensing unit (water-cooled)
VRF Heat Pump Condenser Heat
Transfer Rate
Plant Heating and Cooling
Equipment
Hot water/Steam boiler
Boiler <Fuel Type> Rate +
Boiler Ancillary Electric Energy - Boiler Heating Rate
Chiller water-cooled
Emit heat in Condenser
Equipment
Chiller air-cooled /
evap-cooled
Chiller Condenser Heat Transfer
Rate
Condenser Equipment
Cooling Tower
Cooling Tower Heat Transfer Rate
+ Cooling Tower Fan Electric Power
Water Heaters and Thermal
Storage
Water heater
Water Heater <Fuel Type>
Rate - Water Heater Heating Rate
Zone HVAC Forced Air Units
Window air conditioner /
Packaged terminal air conditioner (PTAC) / Packaged terminal
heat pump (PTHP); Energy recovery ventilator (ERV); Unit
ventilator/heater; Zone outdoor air unit
<unit type> Total Cooling
Rate + <unit type> Electric Energy (cooling
mode)<br><unit type> Heating Electric Power / Fuel
Rate + <unit type> Electric Power - <unit type>
Total Heating Rate (heating mode)
Water-to-air heat pump
Emit heat in condensing unit
defined in Condenser Equipment
Zone evaporative cooler
Emit heat in condensing unit
defined in Evaporative Coolers
Hybrid Unitary HVAC
System dependent; Calculated
according to the equipment specification as above
There are several types of components for which the heat
rejection is not clearly defined. The following are not
included in the heat emissions report:
User defined systems have a variety of possible sources
of heat emission including outdoor air relief for exhaust or a
condenser outlet for heat rejection. These parts can be
counted by tracing the condensing unit linked to the air
connection inlet and outlet (for user defined zone HVAC and
plant component).
For user defined coil, this part depends on how the
coil is designed and equipped. We do not count this in
report.
ThermalStorage:Ice:* do not indicate where losses go,
so we neglect this.
By summing up these components, the heat emissions are
calculated and reported from buildings by systems and
components as well as in total.
Heat Emissions[LINK]
Heat emissions from buildings are a crucial component influencing climate change and urban microclimate, e.g., urban heat island effect. The heat emissions from a building include heat releases from three levels of building systems: building envelope, zone/space, and HVAC systems. Quantifying heat emissions from buildings and its spatiotemporal distribution have significant implications on urban environment and climate studies. Knowing how much heat is released into the atmosphere can help quantify the magnitude of its impact on urban microclimate, e.g., urban heat island effect and climate change. Spatial and temporal distribution of heat emissions can serve to locate hot spots in space and time, therefore useful for prioritizing recourse to mitigate heat emissions from buildings.
This routine calculates heat emissions from buildings and the scope is restricted to report the buildings’ heat directly discharged to the ambient air. As shown in the figure, the emissions include heat releases from three levels of building components: (1) building envelope - exterior surface convection and radiative heat transfer to the ambient air, (2) zone - exhaust air or exfiltration to ambient, (3) HVAC system - relief/exhaust air from AHUs, and (4) HVAC system - heat rejected by air-cooled condensers or central plants, including cooling towers of water-cooled chillers, gas-fired boilers, furnaces and water heaters. It should be noted that any under-ground zone or envelope is excluded, since heat from those spaces is rejected to ground instead of to ambient air.
The calculated heat emissions measure the heat transfer from buildings to their ambient environment, and can have both positive and negative values. The positive values indicate the building injects heat to the environment, while the negative values indicate the building extracts heat from the environment.
For heat transfer via air flow, both the sensible and latent heat emissions are calculated and reported.
Heat Emission from Building Envelope[LINK]
Exterior surfaces emit heat to ambient air in the form of convection and long-wave radiation, which could be absorbed by particles and gas molecules in air. The internal sources modify the outside surface temperature through envelope conduction, along with incident solar radiation, which triggers the heat exchange with the ambient air. This part does not include the long-wave radiation from surfaces to the sky or the ground. The surface outside face heat emission rate is calculated as:
\[\label{eq:he-1} Q_{emission, surf} = Q_{conv, surf} + Hr * A * (T_{surf} - T_{out})\]
where \(Q_{conv, surf}\) is the surface outside face convection heat gain rate, \(Hr\) is the surface outside face thermal radiation to air heat transfer coefficient, \(A\) is the surface area, \(T_{surf}\) and \(T_{out}\) are the surface and outdoor air temperature.
Heat Emission from Zones[LINK]
Discharge air carries heat to ambient at the zone level from exfiltration and exhaust fan. Exfiltration is unintended airflow to outside through envelope cracks, as opposite to infiltration. Zones such as restroom, kitchen and laundry room may be equipped with exhaust fans for active venting. These items can be quantified by:
\[\label{eq:he-2} Q_{exf, zone} = m_{exf}(h_{zone} - h_{out})\]
\[\label{eq:he-3} Q_{exh, zone} = m_{exh}(h_{zone} - h_{out})\]
where \(m_{exf}\) is the exfiltration air mass flow rate, \(m_{exh}\) is the zone exhaust air mass flow rate from the zone exhaust node, \(h_{zone}\) and \(h_{out}\) are the zone and outdoor air enthalpy. Among these, the sensible parts are:
\[\label{eq:he-4} Q_{exf, zone, sensible} = m_{exf} * c * (T_{zone} - T_{out})\]
\[\label{eq:he-5} Q_{exh, zone, sensible} = m_{exh} * c * (T_{zone} - T_{out})\]
where \(c\) is the specific heat of air, \(T_{zone}\) and \(T_{out}\) are the zone and outdoor air temperature. And the latent parts are determined by taking the difference between the total and the sensible rate. For each zone, its exfiltration rate is calculated by solving a mass flow balance on the zone including infiltration, ventilation, outdoor air mixing, zone-to-zone mixing, and all of the zone inlet, exhaust, and return nodes.
This part of zone exhaust only includes the exhaust heat at the zone level defined by Fan:ZoneExhaust or AirflowNetwork:MultiZone:Component:ZoneExhaustFan. For each zone, we aggregated this part by searching its exhaust fans. The exhaust heat from Zone HVAC system nodes is included in the next item.
Heat Emission from HVAC Systems through Relief Air[LINK]
HVAC systems relieve heat from system outdoor air relief nodes, and the total and sensible emission rate are calculated as:
\[\label{eq:he-6} Q_{exh, sys} = m_{exh}(h_{node} - h_{out})\]
\[\label{eq:he-7} Q_{exh, sys, sensible} = m_{exh} * c * (T_{node} - T_{out})\]
And the latent parts are determined by taking the difference between the total and sensible rate.
The air exchange with outdoor is represented with an OutdoorAir:Mixer object in the AirLoopHVAC:OutdoorAirSystem object. This part is aggregated and reported by each Controller:OutdoorAir object.
Heat Rejection from HVAC System Equipment[LINK]
HVAC system equipment also directly exchanges heat with ambient air via refrigeration cycle and its energy usage. For example, for gas-fired boilers/heaters, waste heat is exhausted to outside together with combustion byproduct. For space cooling, heat is removed from indoors and dumped to outdoor at the condenser side or corresponding cooling equipment, such as cooling tower. In addition, energy consumed by outdoor equipment will directly dissipate to the ambient air, such as compressors and fans in packaged DX systems. The surface outside face heat emission rate is calculated as:
(1) Gas-powered combustion unit: Fuel generated heat - fuel heat supply
(2) Condensing unit:
a. Air-cooled: cooling rate + electric power of condenser fan and compressor
b. Water-cooled: total heat transfer rate with outdoor air
Detailed calculation of HVAC rejected heat varies with different HVAC object groups and component types, as summarized in Table 1.
There are several types of components for which the heat rejection is not clearly defined. The following are not included in the heat emissions report:
User defined systems have a variety of possible sources of heat emission including outdoor air relief for exhaust or a condenser outlet for heat rejection. These parts can be counted by tracing the condensing unit linked to the air connection inlet and outlet (for user defined zone HVAC and plant component).
For user defined coil, this part depends on how the coil is designed and equipped. We do not count this in report.
ThermalStorage:Ice:* do not indicate where losses go, so we neglect this.
By summing up these components, the heat emissions are calculated and reported from buildings by systems and components as well as in total.
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This documentation is made available under the EnergyPlus Open Source License v1.0.