Customise chiller performance curves
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The EnergyPlus chiller performance curves use performance information at reference conditions along with three curve fits for cooling capacity and efficiency to determine chiller operation at off-reference conditions. Chiller performance curves can be generated by fitting the manufacturer’s catalogue data or measured data. For more details, we recommend reviewing the EnergyPlus documentation.
Generating the three performance curves to model a real-world chiller accurately is a non-trivial task. Customising chiller performance curves would typically be completed in NABERS and Green Star energy modelling workflows, where the additional accuracy that this can bring to model chiller performance is typically expected. Generating the chiller performance curves involves the following key steps;
Gather data from the chiller manufacturer; complete full load operating specification and two sets of 10-point part load data are required: 1) 10-point part load data for constant condenser water temperatures and 2) 10-point part load data for AHRI relief condenser water temperatures.
Make two pragmatic simplifying assumptions: 1) the chiller's capacity does not vary as a function of condenser or evaporator fluid temperatures; 2) the chiller coefficient of performance (COP) does not vary as a function of chilled water supply temperatures.
Calculate the curve fit for the Energy Efficiency Ratio (EIR) vs Part load ratio (PLR) curve.
Calculate the curve fit for the EIR vs fluid temperature curves.
A spreadsheet implementation of this example can be found at this link.
Step 1 - Gather chiller details and enter them into a spreadsheet for analysis. In this example, we have entered the full load details shown in the image below for a hypothetical 300 kW chiller.
Qref (Chiller full load capacity)
300.0
COPref (Full load COP)
6.000
Pref (Chiller full load electrical power input)
50.0
EIR ref (Energy efficiency ratio, inverse of COP)
0.2
EEWT (Evaporator entering water temperature)
15.0
ELWT (Evaporator leaving water temperature)
7.0
CEWT (Condenser entering water temperature)
38.0
CLWT (Condenser leaving water temperature)
34.0
Evap flow (L/s, Chilled water flow rate at full load)
11.4
Evap min flow (Chiller water minimum flow rate)
10.0
Cond flow (L/s, Condenser water flow rate at full load)
17.5
Cond. Min flow (L/s, Condenser water minimum flow rate)
17.5
Step 2 - Enter the 10-point part load data for the AHRI relief temperatures.
100%
300.0
6.00
15.00
28.0
90%
270.0
6.40
14.00
26.1
80%
240.0
8.00
13.00
24.1
70%
210.0
10.00
12.00
22.2
60%
180.0
11.80
11.00
20.2
50%
150.0
12.20
10.00
18.3
40%
120.0
12.10
9.00
18.3
30%
90.0
10.50
8.00
18.3
20%
60.0
8.00
7.00
18.3
10%
30.0
2.00
6.00
18.3
Step 3 - Enter the 10-point part for the constant condenser water temperature.
100%
300.0
6.000
15.00
38.0
90%
270.0
6.200
14.00
38.0
80%
240.0
6.400
13.00
38.0
70%
210.0
6.800
12.00
38.0
60%
180.0
7.200
11.00
38.0
50%
150.0
7.400
10.00
38.0
40%
120.0
7.200
9.00
38.0
30%
90.0
6.500
8.00
38.0
20%
60.0
4.000
7.00
38.0
10%
30.0
2.000
6.00
38.0
Step 4 - Use the constant condenser water temperature to calculate the Part Load Ratio modifiers of the EIR vs PLR curve. The formula in the PLR Modifier column is Chiller power / (Full load COP * Full load electrical input * Full load Energy Input Ratio)
1.0
50.00
6.000
0.167
1.00
0.9
43.55
6.200
0.161
0.87
0.8
37.50
6.400
0.156
0.75
0.7
30.88
6.800
0.147
0.62
0.6
25.00
7.200
0.139
0.50
0.5
20.27
7.400
0.135
0.41
0.4
16.67
7.200
0.139
0.33
0.3
13.85
6.500
0.154
0.28
0.2
15.00
4.000
0.250
0.30
Step 5 - Use the LINEST function in Excel to calculate the ‘curve fit’ I.e. the coefficients of a quadratic equation that fits data. The formula is LINEST(‘Y axis data (PLR Modifier column), (‘X axis data’ - the PLR column)^{1,2}, True,True). This generates a grid of output; the first line is the coefficients of x^2,x,a in that order.
0.907
-0.150
0.264741652
0.130
0.159
0.042868565
0.994
0.023
#N/A
527.479
6.000
#N/A
0.553
0.003
#N/A
Step 6 - Use the AHRI data (and the PLR modifiers) to calculate the EIR vs Fluid Temperature curve. The formula in the EIR vs FT modifiers column is PChiller / (Full load Capacity*Full EIR * PLR Modifier).
1.0
50.0
6.00
28.00
1.00
0.9
42.2
6.40
26.07
0.97
0.8
30.0
8.00
24.13
0.80
0.7
21.0
10.00
22.20
0.68
0.6
15.3
11.80
20.20
0.61
0.5
12.3
12.20
18.30
0.61
0.4
9.9
12.10
18.30
0.60
0.3
8.6
10.50
18.30
0.62
0.2
7.5
8.00
18.30
0.50
Step 7 - Use the LINEST function in Excel to calculate the ‘curve fit’, I.e. the coefficients of a quadratic equation that fits data. The formula is LINEST(‘Y axis data (CWT column), (‘X axis data’ - the PLR column)^{1,2}, True, True). This generates a grid of output; the first line is the coefficients of x^2,x,a in that order.
0.003
-0.072
1.0410585
0.002
0.097
1.104608761
0.955
0.048
#N/A
31.788
3.000
#N/A
0.146
0.007
#N/A
