TYPE 77:  	DIRECT-FIRED DOUBLE-EFFECT WATER-LiBr ABSORPTION 		CHILLER

General Description
	This component models the thermal performance and power (gas and electric) requirements of a direct-fired (natural gas) double-effect water-lithium bromide (LiBr) absorption chiller in parallel flow configuration.  A diagram of the chiller cycle is given in Figure 1.  The model was written in a general format with respect to the inputs and parameters, and calculates the cooling load, heat input, COP, outlet chilled and cooling water temperatures, and the electrical energy consumed by the chiller, for a range of operating loads, chilled water setpoints, chilled and cooling water flow rates, and inlet cooling water temperatures.  Model parameters are available from a manufacturer's catalog.  The chiller was modeled after the parallel flow configuration, patented by York/Hitachi (see manufacturer catalogs - York Applied Systems [1992,1993]).  Other manufacturers use some type of series flow configuration, and so the model may or may not be applicable to these configurations.  As part-load performance data are not given by these other manufacturers (only the York catalogs), this model may be used to predict the chiller performance at any operating load and conditions.  These chillers are used in commercial cooling applications, with typical rated (nominal) capacities of 100 tons or greater.
	The model has been developed using the performance results from a steady state, mechanistic computer simulation model of the chiller developed from first principles by Koeppel, Klein and Mitchell [1995] (also Koeppel [1994]) using an equation solver program, EES-Engineering Equation Solver (Klein and Alvarado [1993]).  The performance results are independent of the unit size (nominal capacity) and rated COP.  A brief description of the model is given.  Mass and energy equations were written for each of the components, along with the appropriate equilibrium equations, and all heat transfer rates were expressed as a function of the log-mean temperature difference.  The model calculates cycle state point temperatures, pressures, and concentrations and the required input for a given load and operating conditions (chilled and cooling water flow rates and temperatures).  The model nominal full-load performance was calibrated against manufacturer nominal full-load operating data and COP (York Applied Systems [1992,1993] and Aizawa, Nagao and Kazuo [1981]): unknown model parameters such as the component nominal UA values and nominal parallel flow split parameter were determined by matching the model's calculated state points and COP against the data and COP.  After the UA values were determined, they were adjusted for part-load operation using appropriate mass flow rates and accepted heat transfer correlations.  The model calculated performance due to lowering the inlet cooling water temperature to the chiller at full-load conditions agrees with the catalog's performance (York Applied Systems [1992]).  Model performance for increasing the outlet chilled water temperature at full-load conditions slightly underpredicts the catalog performance (York Applied Systems [1992]) with a maximum 2 percent difference.  For chiller part-load performance at nominal temperatures, the model slightly underpredicts the catalog data (York Applied Systems [1992]) with a maximum 6 percent difference.  
	Two York chillers are available depending on the rated capacity.  Units with a rated capacity of 422.4 kW (120 tons) through 704 kW (200 tons) have a nominal COP of 1.0, and units with a rated capacity of 880 kW (250 tons) through 5279.8 kW (1500 tons) have a nominal COP of 0.92.  The difference in the rated COPs is due to slightly different equipment configuration and sizes.  The mechanistic model was developed for the larger units, but the performance ratings given by York for both units are identical and it is assumed that the results obtained from the mechanistic model are applicable to the smaller units with a higher nominal COP.

Figure 1  Double-effect absorption cycle diagram.  Solid lines represent the
 solution flow and broken lines the refrigerant flow.

Nomenclature
	The subscript e refers to the chilled water loop, the subscript c refers to the cooling water loop.
	COP		-	coefficient of performance (HHV)
	COPnom	-	coefficient of performance at the nominal full-load conditions
	Cpw		-	specific heat of the chilled water
	M0-4		- 	curve fit coefficients
			-	cooling water mass flow rate
		-	nominal cooling water mass flow rate 
			-	chilled water mass flow rate
		-	nominal chilled water mass flow rate 
			-	combustion efficiency (HHV)
	Pelectric	-	total chiller electric energy consumed for solution pumps, controls, 				etc..
	PLF		-	part-load factor
	Qin		-	required total heat input to the generator
	Qload 		-	total chilled water cooling load for a single chiller
	Qload,max	-	nominal (rated) total chilled water cooling load for a single chiller
	Tci		-	inlet cooling water temperature
	Tco		-	outlet cooling water temperature
	Tei		-	inlet chilled water temperature
	Teo		-	outlet chilled water temperature
	Tchw,s		-	chilled water setpoint (outlet chilled water temperature)

Mathematical Description
	From manufacturer catalogs, York Applied Systems [1992,1993], the gas input to the chiller is modulated to maintain a constant outlet chilled water temperature setpoint, Tchw,s.  The gas input is modulated continuously between 30 and 100 percent of full-load and the chiller cycles off below 30 percent.  Figure 1 shows the type inputs and outputs.  The nominal, full-load conditions refer to an outlet chilled water temperature of 6.7°C (44°F), an inlet chilled water temperature of 12.2°C (54°F), an inlet cooling water temperature of 29.4 °C (85°F), and an outlet cooling water temperature of 35°C (95°F).
	The inlet cooling water temperature range for the component model is 18.3°C (65°F) to 32.2°C (90°F), and the range of chilled water setpoint is 5.6°C (42°F) to 8.9°C (48°F).  The chilled and cooling mass flow rate range is 50 percent to 125 percent of the nominal value.  There are some operating constraints with respect to the load, and the cooling water temperature and flow rate due to the possibility of crystallization and these will be discussed.  
	The cooling load met by the chiller model (output 5) is calculated from the given inputs as,

							(1)

where the outlet chilled water temperature (output 1), Teo, is equal to the chilled water setpoint (input 1).  The part-load factor, PLF, is then calculated as,

							(2)

If the part-load factor is less than 0.30, the chiller model does not meet the load and the outlet conditions are set equal to the inlet conditions and the value of Qin is 0.0.  If the part-load factor is greater than 1.0, the load (output 5) is set equal to the nominal load and the outlet chilled water temperature is re-calculated.  The inlet and outlet mass flow rates are equal.
	Since it was determined that changes in the chilled water flow rate have negligible effect on the COP within the allowable range of operation (Koeppel [1994]), the COP is a function of three variables,

	COP = f( PLF ; Tchw,s ; Tci ; )				(3)

Curve fits of the COP ratio (COP/COPnom) were obtained from the mechanistic EES model and are used to calculate the COP.  The curve fits are 4th order polynomials, fit in terms of the part-load factor.  There are three sets of the COP ratio, corresponding to a chilled water setpoint of: 5.6°C (42°F), 6.7°C (44°F), and 8.9°C (48°F).  Each COP ratio is a double array, in which the first number corresponds to the cooling water mass flow rate ratio, , and the second number refers to the inlet cooling water temperature, Tci.  The array is the curve fit, and is expressed as,

		(4)

The coefficient of determination for all curve fits is at least equal to or greater than 0.99914.  The TRNSYS and EES model COPs were compared at various operating conditions with almost exact agreement.
	A COP ratio at two chilled water setpoints is calculated by linearly interpolating equation (4) between two different cooling water temperatures and flow rate ratios.  Then the actual COP ratio at the desired chilled water setpoint is determined by linear interpolation of the calculated COP ratios at the two chilled water setpoints above and below the desired setpoint.  The COP is calculated from the determined COP ratio and the nominal COP.  
	The energy input (output 6) is calculated from the load and COP, as,

								(5)
The outlet cooling water temperature (output 7) is calculated from an energy balance on chiller,

					(6)

The combustion efficiency, , is approximated from a curve fit of the combustion efficiency as a function of the load at nominal conditions, obtained from the EES model.  The chiller electrical power consumed (output 8) is estimated from field monitored data of the chiller electrical consumption for two separate chillers of 200 and 400 ton capacity (Koeppel [1994]).  A simple linear function for the chiller electrical power in kW as a function of the load in tons was developed based on this information, 

				(4.7)

	A corresponding integer value describing the chiller operation is output number 9, and the values of this variable are given in Table 1.  Checks are done on the required load, cooling water and chilled water temperatures and flow rates, and operating constraints as a result of the possibility of crystallization.  If an operating constraint is encountered, such as a low inlet cooling water temperature (< 65°F), the integer corresponding to this constraint is output and a warning message is also printed.  For nominal operation the output is 0, and it is -1 when the chiller shuts off due to a part-load factor less than 0.30.  An output of 1 corresponds to a part-load factor greater than 1.0  Outputs 2 through 12 correspond to an operating constraint being exceeded.  Even though a constraint is encountered, a COP is calculated.  The performance is calculated at the constraint only for a violation of the minimum/maximum inlet cooling water temperature; otherwise the COP is calculated at the operating point.  Operation of the chiller at the nominal chilled and cooling water flow rates and inlet cooling water temperature will not result in exceeding an operating constraint.  The need for constraints arose because the input variables in a simulation might be out of a particular allowable operating range.
	In developing the curve fits, a high temperature generator temperature limit of 165.56°C (330°F) was used to develop operating constraints due to possible crystallization.  This temperature cutoff, as investigated by Koeppel [1994], limits the concentrated solution leaving the expansion valve (before entering the absorber) from becoming 65 % or greater.  As long as this concentration is less than 65 %, crystallization is not possible (see water-LiBr property chart, ASHRAE [1993]).  The operating constraints that might produce crystallization are given in Table 2.  In Table 2, values of the chilled water setpoint temperature, inlet cooling water temperature, cooling water flow rate ratio, and part-load factor that result in this constraint are listed.  The values in Table 2 are approximate; exact values resulting in the constraint were determined and then these exact values were extended into ranges as an approximation.  Also given in Table 2 is the required cooling water flow rate ratio () to operate at the given chilled water setpoint, inlet cooling water temperature and part-load factor.  The reason for the required cooling water flow rate ratio is that in case a constraint is encountered, the flow rate coming out of the cooling water pump could be increased to the required value because the chiller operation variable would correspond to the required cooling water flow rate ratio (see Table 1) and could be used as a control signal to the pump.  In general, as the load and the inlet cooling water temperature increase, and as the cooling water flow rate decreases, the chance of encountering a constraint increases.  The constraints are for the most part at extreme operating conditions, i.e. for a large cooling load and a low cooling water flow rate.

Table 1
Integer values of the chiller operation variable.

		Value		Operating Condition
		-1		PLF < 0.30
		0		O.K.
		1		PLF > 1.0
		2		Tci < 18.3 °C (65 °F)
		3		Tci > 32.2 °C (90 °F)
		4		Tchw,s < 5.6 °C (42 °F)
		5		Tchw,s > 8.9 °C (48 °F)
		6		 < 0.50
		7		> 1.25
		8		 < 0.50
		9		 > 1.25
		10		 must be increased to at least 0.75.
		11		 must be increased to at least 1.00.
		12		 must be increased to at least 1.25.

Table 2
Operating constraints due to possibility of crystallization as determined from a high temperature generator temperature limit of 165°C (330°F).

Tchw, s
(°F)	Tci
(°F)		PLF	Required 
	
42 =   = 43.5	88 >	0.49 =    < 0.70	0.65 <   =  0.80	0.75	
			0.80 <   =  0.90	1.0	
			0.90 <	1.25	
		0.70 =    < 0.90	0.80 <   =  0.90	1.0	
			0.90 <	1.25	

		0.90 =    = 1.0	0.90 <	1.25	
	85 <   = 88	0.49 =    < 0.70	0.70 <   =  0.85	0.75	
			0.85 <   =  0.95	1.0	
			0.95 <	1.25	
		0.70 =    < 0.90	0.85 <   =  0.95	1.0	
			0.95 < 	1.25	
		0.90 =    = 1.0	0.95 <	1.25	
	83 <   = 85	0.49 =    < 0.70	0.75 <   =  0.90	0.75	
			0.90 < 	1.0	
		0.70 =    < 0.90	0.90 <	1.0	
		0.90 =    < 0.97	0.95 =	1.0	
	80 <   = 83	0.49 =    < 0.70	0.80 <   =  0.95	0.75	
			0.95 < 	1.0	
		0.70 =    < 0.90	0.95 <	1.0	
		0.90 =    < 0.97	0.975 <	1.0	
	79 <   = 80	0.49 =    < 0.70	0.85 <	0.75	
43.5 <   = 45.5	88 >	0.49 =    < 0.70	0.65 <   =  0.85	0.75	
			0.85 <   =  0.95	1.0	
			0.95 <	1.25	
		0.70 =    < 0.90	0.85 <   =  0.95	1.0	
			0.95 <	1.25	
		0.90 =    = 1.0	0.95 <	1.25	
	85 <   = 88	0.49 =    < 0.70	0.70 <   =  0.90	0.75	
			0.90 < 	1.0	
		0.70 =    < 0.90	0.90 < 	1.0	
		0.90 =    < 0.98	0.95 <	1.0	
	83 <   = 85	0.49 =    < 0.70	0.75 <   =  0.95	0.75	
			0.95 < 	1.0	
		0.70 =    < 0.90	0.95 <	1.0	
	80 <   = 83	0.49 =    < 0.70	0.80 <	0.75	
	79 <   = 80	0.49 =    < 0.70	0.85 <	0.75	
45.5 < 	88 >	0.49 =    < 0.70	0.75 <   =  0.90	.75	
			0.90 <	1.0	
		0.70 =    < 0.90	0.90 <	1.0	
	85 <   = 88	0.49 =    < 0.70	0.80 <   =  0.95	.75	
			0.95 <	1.0	
		0.70 =    < 0.90	0.95 <	1.0	
	83 <   = 85	0.49 =    < 0.70	0.85 <	0.75	
	82 <   = 83	0.49 =    < 0.70	0.90 <	0.75	

TRNSYS Component Configuration
	The type parameters are available from a manufacturer's catalog with the units for capacity in tons, and for flow rates in gpm.

PARAMETER NO.		DESCRIPTION
	1					-	nominal (rated) cooling capacity (kJ/hr)
	2				COPnom	-	nominal COP - HHV 
	3					-	nominal chilled water mass flow rate (kg/hr)
	4  					-	nominal cooling water mass flow rate 								(kg/hr)

INPUT NUMBER		DESCRIPTION
	1				Tchw,s		-	chilled water setpoint, outlet chilled water
 								temperature (°C),  5.6°C (42°F) - 8.9°C 								(48°F)
	2				Tei		-	inlet chilled water temperature (°C)
	3						-	chilled water flow rate (kg/hr),  50% - 125%
								of the nominal
	4				Tci		-	inlet cooling water temperature (°C), 18.3°C 								(65°F) to 32.2°C (90°F)
	5						-	cooling water flow rate (kg/hr),  50% - 								125% of the nominal

OUTPUT NUMBER					DESCRIPTION
	1				Teo		-	outlet chilled water temperature (°C)
	2						-	chilled water flow rate (kg/hr)
	3				Tco		-	outlet cooling water temperature (°C)
	4						-	cooling water flow rate (kg/hr)
	5				Qload		-	total chilled water cooling load (kJ/hr)
	6				Qin		-	required total heat input to the generator 								(kJ/hr)
	7				COP		- 	coefficient of performance
	8				Pelectric	-	total chiller electric energy consumed (kW)
	9				Chiller 	-	integer value describing the operating 					Operation	    	conditions of the chiller (will signal when an 					Variable 		operating constraint is encountered), see 								Table 1

Parameters
1.			
2.	COPnom - HHV
3.	
4.	 
References
Aizawa, M., Nagao, M., and Kazuo, W. 1981. "Direct-fired absorption chiller-heater." 
Hitachi Review  30 (1): 19-24. 

ASHRAE. 1993. 1993 ASHRAE Handbook-Fundamentals, p. 17.82-17.83. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Klein, S.A., and Alvarado, F.L. 1993. EES - Engineering Equation Solver. Middleton, WI: F-Chart Software. 

Koeppel, E.A. 1994. “The Modeling, Performance and Optimal Control of Commercial Absorption Chillers.” M.S. Thesis, University of Wisconsin-Madison.

Koeppel, E.A., Klein, S.A., and Mitchell, J.W. 1995. “Commercial Absorption Chiller Models for Evaluation of Control Strategies.” To be presented at the ASHRAE Winter Meeting Symposium, Absorption Chiller and Heat Pump Technology Development I. January 31, 1995. Chicago, IL.

York Applied Systems. 1992. Form 155.17-EG1 (291). York, PA: York International Corporation 1992.

York Applied Systems. 1993. Form 155.17-EG3 (493). York, PA: York International Corporation 1993.

Sample Inputs, Outputs and Parameters
	
	The sample inputs, outputs and parameters are for a 400 ton unit operating at a cooling load of 280 tons (70 percent).
	Parameter No.			
		1	 	=	5064268.1 kJ/hr
		2	 COPnom	=	0.921 
		3	 	=	218243.1 kg/hr
		4  	=	416047.2 kg/hr

	Input No.				

		1	Tchw,s		=	6.67°C
		2	Tei		=	10.547°C
		3			=	218243.1 kg/hr
		4	Tci		=	29.4°C
		5			=	416047.2 kg/hr

	Output No.				
		1	Teo		=	6.67°C
		2			=	218243.1 kg/hr
		3	Tco		=	33.32
		4			=	416047.2 kg/hr
		5	Qload		=	3.545e6 kJ/hr
		6	Qin		=	3.889e6 kJ/hr
		7	COP		=	0.9117
		8	Pelectric	=	9.332 kW
		9	C.O.V. 	=	0

Sample TRNSYS Deck
	Given is a sample deck used that was used to simulate a chiller system consisting of a chiller, cooling tower, pumps and a cooling tower bypass valve, and investigate the optimal supervisory control.  A schematic of the chiller system is given in Figure 2.

ASSIGN RUN1.LST 6
ASSIGN RUN1.OUT 13
ASSIGN COST1.OUT 15
ASSIGN LW.DAT 18

***************************************************************************
*
*                  ABSORPTION CHILLER SYSTEM DECK 
*
*         400 TON CHILLER
*          30 HP CHILLED WATER PUMP BHP 
*          40 HP COOLING WATER PUMP BHP
*          50 HP COOLING TOWER FAN  BHP-VARIABLE SPEED DRIVE
*                  
***************************************************************************

EQUATIONS 4
START=0.0
STOP=191.75
STEP=0.25
RUNTIME = STOP - START

SIMULATION START STOP STEP
WIDTH 132
TOLERANCES 0.0001 0.0001
LIMITS 999999 999999

EQUATIONS 49
************* SET POINTS
Teset = 6.67   
************* WEATHER 
Tdb=[2,3]      
Twb=[2,2]   
************* LOAD 
Tei=[2,1]   
************* NOMINAL FLOW RATES AND LOAD
Men=218243.1     
Mcn=416047.2     
Qmax=5064268.1
*
************* PUMP CONTROL FUNCTIONS GAMMAS
Ge=1.0
Gc=[31,1]
************* COOLING TOWER FAN CONTROL FUNCTION 
Gf_1=[31,2]
Gf=max(0.4,Gf_1)
*
************* BYPASS CONTROL AND EQUATIONS
Gbyp_1=min(Gf_1,0.40)
Gbyp_2=2.5*Gbyp_1
Gbyp=min(1.0,Gbyp_2)
Mco=[1,4]
Mti=Gbyp*Mco
Mbyp=(1-Gbyp)*Mco
Tbyp=[1,3]
Mto=[15,2]
Tto=[15,1]
Mci=Mto + Mbyp
denom=max(.5, Mci )
Tci=(Mto*Tto + Mbyp*Tbyp)/(denom)
*
************* MASS FLOW RATES
Me=Ge*Men
Mc=Gc*Mcn
************* PUMP SIZES
* e = chilled water
* c = cooling water
Pemax=30*2684.564    
Pcmax=40*2684.564
* UNIT CONVERSION to kW
Pe=[3,3]/3600
Pc=[5,3]/3600
*
************* COOLING LOAD CONVERSION TO TONS
TONS=[1,5]*1000/1055.056/12000
*
************* Tower Brake Horshpower
EFFmech=-1.3323E-15 + 2.8047*Gf -2.756*Gf**2 +.90133*Gf**3 
BHPfan=[15,3]/EFFmech
*
************* ENERGY CONSUMPTION
QIN=[1,6]*STEP
TOW=[16,1]*STEP
CWPump=[6,1]*STEP
ChWPump=[4,1]*STEP
Chiller=[1,8]*STEP
************* COST ANALYSIS
GasTherm=QIN*1000/1055.056/100000
GaskWh=QIN/3600
GasP=.4227
GasCost=GasTherm*GasP
Ratio=4
ElecP=Ratio*(GasP/100000/1055.056*1000*3600)
ElecE=TOW + CWPump + ChWPump + Chiller
ElecCost=ElecE*ElecP
TotCost=GasCost + ElecCost
PerElecCost=ElecCost/(max(.5,TotCost))*100
Penalty=[1,10]
ObjFunc=GasCost + ElecCost + Penalty
*
*__________________________________________________________________________
UNIT 31 TYPE 76 OPTIMIZER
PARAMETERS 14
*  STime  Stop   cnt1   T    VMORelative  eps     rt     nt  
  111.75    114     2    8.0     .10        1.E-6   0.2   3.0
*lowerbnd1 def1   upperbnd1  lowerbnd2  def2   upperbnd2
   0.5     .67     1.0        0.01      .41      1.0
INPUTS 3
ObjFunc   31,1   31,2
*
*initial guesses
3.0         1.0    1.0
*__________________________________________________________________________
UNIT 2 TYPE 9 DATA READER (FREE)
PARAMETERS 14
*mode  N   delt  i  m  a   i  m  a   i  m  a  LU  freeFRMT
 2     3   STEP  -1  1  0  -2  1  0  -3  1  0  18 -1
*___________________________________________________________________________
UNIT 1 TYPE 77 ABSORPTION CHILLER
PARAMETERS 4
*CAP        COP    MEN        MCN
Qmax       .921    Men        Mcn 
INPUTS 5
*Teset   Tei       Mei      Tci       Mci  
Teset    Tei       3,2      5,1       5,2
Teset    9.9       Men      21.2      2.5E5
*__________________________________________________________________________
UNIT 3 TYPE 3 CHW PUMP
PARAMETERS 8 
*Mmax  CP     Pmax    FPAR     CO     C1    C2    C3
Men   4.19    Pemax   .10     0      0     0     1
INPUTS 3
*Tei     Me    Ge   
Tei      Me    Ge   
9.9      Men   1.0 
*__________________________________________________________________________
UNIT 4 TYPE 73 VARIABLE SPEED DRIVE/MOTOR
PARAMETERS 1
*NOM EFFICIENCY DRIVE/MOTOR
.873
INPUTS 2
*Ge  BHP
Ge     Pe
1.0    22.4
*__________________________________________________________________________
UNIT 5 TYPE 3 COOL W PUMP
PARAMETERS 8
*Mmax  CP     Pmax    FPAR     CO     C1    C2    C3
Mcn    4.19   Pcmax   .0      0      0     0     1.0
INPUTS 3
*Tci   Mc     Gc  
Tci    Mci    Gc   
21.2   2.5E5  .67   
*
*__________________________________________________________________________
UNIT 6 TYPE 73 VARIABLE SPEED DRIVE/MOTOR
PARAMETERS 1
*NOM EFFICIENCY DRIVE/MOTOR
.873
INPUTS 2
*Gc  BHP
Gc   Pc
.67  29.8
*__________________________________________________________________________
UNIT 15 TYPE 51 COOLING TOWER 
PARAMETERS 11
*    Cross Cell  Va       Pfan   Vaoff    Vs  Tsumpi  C      N    print
  1    2    1   291939.2  35.4   29193.9  0   29.44  3.75  -.75    2
INPUTS 6
*Twi  Mwi     Tdb    Twb   Tmain   Gamma
1,3   Mti     Tdb    Twb    0,0     Gf  
26    2.5E5    20     10.5   10      .41
*__________________________________________________________________________
UNIT 16 TYPE 73 VARIABLE SPEED/DRIVE MOTOR
PARAMETERS 1
*NOMINAL DRIVE/MOTOR EFF
.917
INPUTS 2
*GAMMA-FAN     BHP
Gf             BHPfan
.41            37.3
*__________________________________________________________________________
UNIT 27 TYPE 25 PRINTER 
PARAMETERS 5
STEP START STOP 13 2
INPUTS 10
TotCost Twb Tci 15,12 Gc Gbyp 15,1 TONS 1,7 1,9  
  $     Twb Tci  Gf   Gc Gb   Tto  Load COP Oper
*__________________________________________________________________________
UNIT 28 TYPE 25 PRINTER 
PARAMETERS 5
STEP START STOP 15 2
INPUTS 6
GasTherm ElecE GasCost ElecCost TotCost PerElecCost 
Therm    kWh   Gas-$    E-$       $       %Elec

END