Coefficient of Performance (COP)
In the refrigerator or heat pump, a refrigerant, typically ammonia or
freon, is pumped around a loop. The refrigerant gas condenses to a
liquid in the condenser, giving out heat.
The liquid passes through an expansion valve and drops in pressure.
The liquid boils to a gas in the evaporator, absorbing heat. The
compressor compresses the gas.
Energy is conserved so:
Q2 = Q1 + W
The coefficient of performance as a refrigerator is defined as:
COP = Q1/W
For example, at a COP of 3, one watt of motor power in will produce 3
watts of cooling at the evaporator and 4 watts of heating at the
condensor.
The COP depends primarily on the temperatures of the evaporator and the
condenser, the closer the two temperatures are, the higher the COP. COP
also depends on the refrigerant gas and the model of compressor
used.
In the table below, typical COP is given for a screw compressor
operating on an ammonia refrigerant.
|
Condensing temperature
oC
|
Evaporating temperature
oC
|
COP
|
|
20
|
-40
|
1.3
|
|
20
|
-30
|
2.4
|
|
20
|
-20
|
3.9
|
|
20
|
-10
|
5.9
|
|
20
|
0
|
9.0
|
|
30
|
-40
|
0.9
|
|
30
|
-30
|
1.6
|
|
30
|
-20
|
2.7
|
|
30
|
-10
|
4.2
|
|
30
|
0
|
6.1
|
|
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