Operational Faults[LINK]
Introduction
to Operational Faults Modeling[LINK]
Most of the buildings, either new or old, have operational
faults in the sensors, controllers, meters, equipment and
systems. Being able to model and simulate these faults and
their impact on energy performance of buildings is crucial to
improve accuracy of building simulations and to support the
retrofit of buildings.
To date, the main practitioner use of EnergyPlus has been
for new construction design. With the new high priority
attached by USDOE to retrofit and improved operation of
existing buildings, there is a need to extend the capabilities
of EnergyPlus to model existing buildings, including faulty
operation:
- Retrofit analysis: starts with calibrated simulation; the
ability to estimate the severity of common faults is expected
to improve the accuracy and transparency of the calibrated
model and hence the increase accuracy of the analysis of
different retrofit measures.
- Commissioning providers can use the fault models to
demonstrate the saving to be expected from fixing faults found
in retro-commissioning
- Support for building operation by using the calibrated
model, including unfixed faults, as a real-time reference
model to detect, and verify the diagnosis of, newly occurring
faults.
The users in these cases are practitioners, not power
users, so it is needed to implement the fault models using
conventional EnergyPlus objects rather than the EMS, which, in
any case, could only be used to model limited types of
faults.
EnergyPlus contains a number of objects to model
operational faults of sensors, meters, equipment and systems.
The current implementation allows the modeling of the
following fault types: (1) sensor faults with air economizers,
(2) thermostat/humidistat offset faults, and (3) heating and
cooling coil fouling faults.
Sensor Faults with
Air Economizers[LINK]
The sensor readings deviate from the actual air conditions
due to sensor offset, which leads to inappropriate operations
of the air economizer and thus undesired resulting indoor
conditions.
Modeling Approach[LINK]
There are a number of sensors installed to support the air
economizer operations. The sensors may be of different types.
The objects used by EnergyPlus to model the sensor faults are
as follows:
- FaultModel:TemperatureSensorOffset:OutdoorAir
- FaultModel:HumiditySensorOffset:OutdoorAir
- FaultModel:EnthalpySensorOffset:OutdoorAir
- FaultModel:TemperatureSensorOffset:ReturnAir
- FaultModel:EnthalpySensorOffset:ReturnAir
Thermostat/Humidistat
Offset[LINK]
The zone air temperature/relative humidity readings deviate
from the actual zone air conditions due to
thermostat/humidistat offset, and thus leads to inappropriate
operations of the HVAC system and undesired resulting indoor
conditions.
Modeling Approach[LINK]
The thermostat offset fault is described in the object FaultModel:ThermostatOffset,
which refers to the object ZoneControl:Thermostat.
The humidistat offset fault is described in the object FaultModel:HumidistatOffset,
which refers to the object ZoneControl:Humidistat.
The effect of an offset in a thermostat/humidistat whose sole
use is for the calculation of difference between the
set-points and the design values can be modeled as an equal
and opposite offset in the thermostat/humidistat:
\[T_{s,f} = T_{s,ff} - \Delta
T\]
\[RH_{s,f} = RH_{s,ff} - \Delta
RH\]
Where,
\(T_{s,f}\) thermostat
value in the faulty case, C
\(T_{s,ff}\) thermostat
value in the fault-free case (design value), C
\(RH_{s,f}\) humidistat
value in the faulty case,
\(RH_{s,ff}\) humidistat
value in the fault-free case (design value),
\(\Delta T / \Delta RH\)
difference between the thermostat/humidistat readings and the
actual zone air conditions. Positive values mean that the
readings is higher than the actual air conditions.
For the humidistat that is independent of the thermostat,
\(\Delta RH\) can be
described by a pre-defined schedule. For the humidistat offset
that is caused by the thermostat offset, \(\Delta RH\) is related with both
the thermostat offset level as well as the indoor air
conditions which are dynamic, and therefore cannot be
described with a pre-defined schedule. In this case, the
humidistat offset level is calculated each time step.
\[\Delta RH = RH_{s,ff} -
f(T_{real}, W_{s,f})\]
Where,
\(T_{real}\) real-time
temperature of the indoor air (real value), C
\(W_{s,f}\) humidistat
ratio corresponding to \(T_{real} -
\Delta T and RH_{s,ff, kgWater/kgDryAir}\)
Note that the thermostat/humidistat settings are related
with two major processes within EnergyPlus: one is the design
load calculations and HVAC system sizing, and the other is the
HVAC system operations. Only the latter is affected by the
thermostat/humidistat offset fault, while the former is not.
Therefore, the size of the corresponding HVAC equipment in the
faulty cases is the same as that in the fault-free cases.
When EMS is used to overwrite the
ZoneControl:Thermostat/ZoneControl:Humidistat values, the
offsets are applied to the EMS values rather than the original
Thermostat/Humidistat values.
Heating and Cooling
Coil Fouling[LINK]
Reduced overall heat transfer coefficient (UA) causes
reduced coil capacity, resulting in unmet loads and/or
increased water flow rate and decreased water side temperature
difference (“low ΔT” syndrome).
Modeling Approach[LINK]
The coil fouling fault is described in the object FaultModel:Fouling:Coil.
The fault model currently applies only to the ‘simple’ water
coils: Coil:Heating:Water
and Coil:Cooling:Water.
The FaultModel:Fouling:Coil
object allows the user to describe the fouling information in
either of the two methods: FouledUARated or FoulingFactor.
Using FouledUARated method, user specifies the value of
UAfouled directly. Using FoulingFactor method user specifies
air/water side fouling factor, and the UAfouled value is
further calculated via the equations shown below.
\[UA_{fouled} = [UA_{air} -
1 + R_{foul} + UA_{water} - 1]-1\]
Where,
\(U_{Aair}\) heat transfer
coefficient of the coil on the air side, W/K
\(UA_{fouled}\) overall
heat transfer coefficient of the fouled coil, W/K
\(UA_{water}\) heat
transfer coefficient of the coil on the water side, W/K
\(R_{foul}\) fouling
factor, K/W
\(R_{foul}\) is determined
by:
\[R_{foul} = r_{air}/A_{air} +
r_{water} / A_{water}\]
Where,
\(r_{air}\) Air side
fouling factor, m2-K/W
\(r_{water}\) Water side
fouling factor, m2-K/W
\(A_{air}\) Air side coil
surface area, m2
\(A_{water}\) Water side
coil surface area, m2
The pressure drop associated with the fouling is ignored in
the current implementation.
Operational Faults[LINK]
Introduction to Operational Faults Modeling[LINK]
Most of the buildings, either new or old, have operational faults in the sensors, controllers, meters, equipment and systems. Being able to model and simulate these faults and their impact on energy performance of buildings is crucial to improve accuracy of building simulations and to support the retrofit of buildings.
To date, the main practitioner use of EnergyPlus has been for new construction design. With the new high priority attached by USDOE to retrofit and improved operation of existing buildings, there is a need to extend the capabilities of EnergyPlus to model existing buildings, including faulty operation:
The users in these cases are practitioners, not power users, so it is needed to implement the fault models using conventional EnergyPlus objects rather than the EMS, which, in any case, could only be used to model limited types of faults.
EnergyPlus contains a number of objects to model operational faults of sensors, meters, equipment and systems. The current implementation allows the modeling of the following fault types: (1) sensor faults with air economizers, (2) thermostat/humidistat offset faults, and (3) heating and cooling coil fouling faults.
Sensor Faults with Air Economizers[LINK]
Symptom[LINK]
The sensor readings deviate from the actual air conditions due to sensor offset, which leads to inappropriate operations of the air economizer and thus undesired resulting indoor conditions.
Modeling Approach[LINK]
There are a number of sensors installed to support the air economizer operations. The sensors may be of different types. The objects used by EnergyPlus to model the sensor faults are as follows:
Thermostat/Humidistat Offset[LINK]
Symptom[LINK]
The zone air temperature/relative humidity readings deviate from the actual zone air conditions due to thermostat/humidistat offset, and thus leads to inappropriate operations of the HVAC system and undesired resulting indoor conditions.
Modeling Approach[LINK]
The thermostat offset fault is described in the object FaultModel:ThermostatOffset, which refers to the object ZoneControl:Thermostat. The humidistat offset fault is described in the object FaultModel:HumidistatOffset, which refers to the object ZoneControl:Humidistat. The effect of an offset in a thermostat/humidistat whose sole use is for the calculation of difference between the set-points and the design values can be modeled as an equal and opposite offset in the thermostat/humidistat:
\[T_{s,f} = T_{s,ff} - \Delta T\]
\[RH_{s,f} = RH_{s,ff} - \Delta RH\]
Where,
\(T_{s,f}\) thermostat value in the faulty case, C
\(T_{s,ff}\) thermostat value in the fault-free case (design value), C
\(RH_{s,f}\) humidistat value in the faulty case,
\(RH_{s,ff}\) humidistat value in the fault-free case (design value),
\(\Delta T / \Delta RH\) difference between the thermostat/humidistat readings and the actual zone air conditions. Positive values mean that the readings is higher than the actual air conditions.
For the humidistat that is independent of the thermostat, \(\Delta RH\) can be described by a pre-defined schedule. For the humidistat offset that is caused by the thermostat offset, \(\Delta RH\) is related with both the thermostat offset level as well as the indoor air conditions which are dynamic, and therefore cannot be described with a pre-defined schedule. In this case, the humidistat offset level is calculated each time step.
\[\Delta RH = RH_{s,ff} - f(T_{real}, W_{s,f})\]
Where,
\(T_{real}\) real-time temperature of the indoor air (real value), C
\(W_{s,f}\) humidistat ratio corresponding to \(T_{real} - \Delta T and RH_{s,ff, kgWater/kgDryAir}\)
Note that the thermostat/humidistat settings are related with two major processes within EnergyPlus: one is the design load calculations and HVAC system sizing, and the other is the HVAC system operations. Only the latter is affected by the thermostat/humidistat offset fault, while the former is not. Therefore, the size of the corresponding HVAC equipment in the faulty cases is the same as that in the fault-free cases.
When EMS is used to overwrite the ZoneControl:Thermostat/ZoneControl:Humidistat values, the offsets are applied to the EMS values rather than the original Thermostat/Humidistat values.
Heating and Cooling Coil Fouling[LINK]
Symptom[LINK]
Reduced overall heat transfer coefficient (UA) causes reduced coil capacity, resulting in unmet loads and/or increased water flow rate and decreased water side temperature difference (“low ΔT” syndrome).
Modeling Approach[LINK]
The coil fouling fault is described in the object FaultModel:Fouling:Coil. The fault model currently applies only to the ‘simple’ water coils: Coil:Heating:Water and Coil:Cooling:Water.
The FaultModel:Fouling:Coil object allows the user to describe the fouling information in either of the two methods: FouledUARated or FoulingFactor. Using FouledUARated method, user specifies the value of UAfouled directly. Using FoulingFactor method user specifies air/water side fouling factor, and the UAfouled value is further calculated via the equations shown below.
\[UA_{fouled} = [UA_{air} - 1 + R_{foul} + UA_{water} - 1]-1\]
Where,
\(U_{Aair}\) heat transfer coefficient of the coil on the air side, W/K
\(UA_{fouled}\) overall heat transfer coefficient of the fouled coil, W/K
\(UA_{water}\) heat transfer coefficient of the coil on the water side, W/K
\(R_{foul}\) fouling factor, K/W
\(R_{foul}\) is determined by:
\[R_{foul} = r_{air}/A_{air} + r_{water} / A_{water}\]
Where,
\(r_{air}\) Air side fouling factor, m2-K/W
\(r_{water}\) Water side fouling factor, m2-K/W
\(A_{air}\) Air side coil surface area, m2
\(A_{water}\) Water side coil surface area, m2
The pressure drop associated with the fouling is ignored in the current implementation.
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This documentation is made available under the EnergyPlus Open Source License v1.0.