Model for an indirect steam heated absorption chiller based on performance curves. The model uses performance curves similar to the EnergyPlus model Chiller:Absorption:Indirect.

The model uses six functions to predict the chiller cooling capacity, power consumption for the chiller pump and the generator heat flow rate and the condenser heat flow. These functions use the performance data stored in the record per. The computations are as follows:

The capacity function of the evaporator is

capFuneva = A1 + A2 Teva,lvg + A3 T2eva,lvg + A4 T3eva,lvg.

The capacity function of the condenser is

capFuncon = B1 + B2 Tcon,ent + B3 T2con,ent + B4 T3con,ent.

These capacity functions are used to compute the available cooling capacity of the evaporator as

eva,ava = capFuneva   capFuncon   Q̇eva,0,

where eva,0 is obtained from the performance data per.QEva_flow_nominal. Let eva,set denote the heat required to meet the set point TSet. Then, the model computes the part load ratio as

PLR =min(Q̇eva,set/Q̇eva,ava, PLRmax).

Hence, the model ensures that the chiller capacity does not exceed the chiller capacity specified by the parameter per.PLRMax. The cycling ratio is computed as

CR = min(PLR/PLRmin, 1.0),

where PRLmin is obtained from the performance record per.PLRMin. This ratio expresses the fraction of time that a chiller would run if it were to cycle because its load is smaller than the minimal load at which it can operate. Note that this model continuously operates even if the part load ratio is below the minimum part load ratio. Its leaving evaporator and condenser temperature can therefore be considered as an average temperature between the modes when the compressor is off and on.

Using the part load ratio, the energy input ratio of the chiller pump is


The generator heat input ratio is

genHIR = D1 + D2PLR+D3PLR2+D4PLR3.

Two additional curves modifiy the heat input requirement based on the condenser inlet water temperature and the evaporator outlet water temperature. Specifically, the generator heat modifier based on the condenser inlet water temperature is

genTcon = E1 + E2 Tcon,ent + E3 T2con,ent + E4 T3con,ent,

and the generator heat modifier based on the evaporator inlet water temperature is

genTeva= F1 + F2 Teva,lvg + F3 T2eva,lvg + F4 T3eva,lvg.

The main outputs of the model that are to be used in energy analysis are the required generator heat QGen_flow and the electric power consumption of the chiller pump P. For example, if the chiller were to be regenerated with steam, then QGen_flow is the heat that must be provided by a steam loop. This model computes the required generator heat as

gen = -Q̇eva,ava genHIR genTcon genTeva CR.

The pump power consumption is


where P0 is the pump nominal power obtained from the performance data per.P_nominal. The heat balance of the chiller is

con = -Q̇eva + Q̇gen + P.

Performance data

The equipment performance data is obtained from the record per, which is an instance of Buildings.Fluid.Chillers.Data.AbsorptionIndirectSteam. Additional performance curves can be developed using two available techniques (Hydeman and Gillespie, 2002). The first technique is called the Least-squares Linear Regression method and is used when sufficient performance data exist to employ standard least-square 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 Hydeman and Gillespie (2002).



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