Electric Chiller Model Based on Condenser Entering Temperature
This model (EnergyPlus name Chiller:Electric:EIR) simulates the performance of an electric liquid chiller. The model is based on the compression chiller model (COMREF) in the DOE-2.1 building energy simulation program. The EnergyPlus model contains all of the features of the DOE-2.1 chiller model, plus additional abilities for modelling evaporatively-cooled 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 modelling an air-cooled or evaporatively-cooled condenser. This model does not simulate the thermal performance or the power consumption of associated pumps or cooling towers.
The chiller model uses user-supplied performance information at reference conditions along with three performance curves (curve objects) for cooling capacity and efficiency to determine chiller operation at off-reference 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 entering condenser fluid temperature. The output of this curve is multiplied by the reference capacity to give the full-load 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.
ChillerCapFTemp = a + b(Tcw,l) + c(Tcw,l)2 ,+ d(Tcond,e) + e(Tcond,e)2 + f(Tcw,l )(Tcond,e)
where:
ChillerCapFTemp = cooling capacity factor, equal to 1 at reference conditions
Tcw,l = leaving chilled water temperature, ˚C
Tcond,e = entering condenser fluid temperature, ˚C. For a water-cooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air- or evap-cooled condensers this will be the entering outdoor air dry-bulb or wet-bulb temperature, respectively.
The energy input to cooling output ratio function of temperature curve is a biquadratic performance curve that parameterises 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 full-load 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.
ChillerEIRFTemp = a + b(Tcw,l) + c(Tcw,l)2 ,+ d(Tcond,e) + e(Tcond,e)2 + f(Tcw,l )(Tcond,e)
where:
ChillerEIRFTemp = energy input to cooling output factor, equal to 1 at reference conditions
Tcw,l = leaving chilled water temperature, ˚C
Tcond,e = entering condenser fluid temperature, ˚C. For a water-cooled condenser this will be the water temperature returning from the condenser loop (e.g., leaving the cooling tower). For air- or evap-cooled condensers this will be the entering outdoor air dry-bulb or wet-bulb temperature, respectively.
The energy input to cooling output ratio function of part-load ratio curve is a quadratic performance curve that parameterizes the variation of the energy input ratio (EIR) as a function of the part-load ratio. The EIR is the inverse of the COP, and the part-load 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 part-load ratio at which the chiller is operating. This curve should have a value of 1.0 when the part-load ratio equals 1.0. The quadratic curve should be valid for the range of part-load ratios anticipated for the simulation.
ChillerEIRFPLR = a + b PLR + c PLR 2
where:
ChillerEIRFPLR = energy input to cooling output factor, equal to 1 at reference conditions
PLR = part-load ratio = (cooling load) / (chiller’s available cooling capacity)
All three of the performance curves are accessed through EnergyPlus’ built-in 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).
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:
Qavail = Qref ChillerCapFTemp
where:
Qref = chiller capacity at reference conditions (reference temperatures and flow rates defined by the user), W
Q avail = 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 Setpoint manager and referenced in the Plant loop). 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.