Heat Pump Objective: to determine the power input, heat output and coefficient of performance of an air to water heat pump.
Apparatus: a Hilton air and water heat pump test stand was used. The unit had two evaporators and a change over switch that allows one or the other to be selected. One evaporator takes heat from ambient air while the other takes heat from flowing water. In this test the air flow evaporator was used. The main components of this device are: An electrically powered refrigerant compressor which displays its power consumption on a meter, an expansion valve which causes a pressure drop vaporising some of the refrigerant, a condenser which allows heat transfer to the water to be heated, an evaporator which converts the fluid into vapour, three temperature sensors at the compressor water inlet and outlet, and the condenser water outlet displayed on a temperature indicator, and R134a refrigerant fluid.
Schematic:
2
Procedure: The water supply to the switch and the main switch were turned on. The air evaporator was selected by pressing the evaporator change over switch down. The condenser gauge pressure was set to a desired value between 700 and 1100 kN/m
2
by adjusting the
condenser cooling water flow rate. Time was allowed for all the system parameters to reach a stable condition. The test readings were taken. The process was then repeated with the condenser gauge pressure being changed each time until 5 reading had been achieved.
Where Q=rate of heat transfer W m = mass flow rate kg/s Cw=Specific heat of water J/kgK 0
T=Conventional temperature C Qcnd
20.9W
T7 ( 33.6 33.6 273 273)K
QH mw Cw T7
QH
T6
3
1.567 1.567 10 W
Win 480W
Finding Coefficient of Performance
COP
QH W in
COP 3.26 3.266 6
Spreadsheet results: 1 Compressor waste heat rate
Q cmp cmp/W
Total heat rate delivered
Q H/W
2
3
4
5
21.40
8.56
27.39
25.68
27.39
1567.50
1580.04
1605.12
1559.98
1404.48
3.266
3.160
3.087
2.786
2.341
COP
4
Plots:
Q H, WIn, COP vs T7 1800.00
3.500
1600.00
3.000
P O1400.00 C &1200.00 ) W1000.00 ( n i W 800.00 , ) 600.00 W ( h 400.00 Q
2.500 2.000 Qh 1.500 1.000
Win COP
0.500
200.00 0.00
0.000 30
35
40
45
50
55
60
65
T7 (0C)
Q H, WIn, COP, T7 vs ṁw 1800.00
70
) c 1600.00 0 ( 7 1400.00 T d n a 1200.00 P O1000.00 C , ) W 800.00 ( n i W 600.00 , ) 400.00 W ( h Q 200.00
60 50 40
Qh
30
Win
20
COP T7
10
0.00
0 5
10
15
20
25
30
ṁw(gs-1)
5
Conclusion: It can be seen that from the first graph, as the power used by compressor is increased, the energy flow rate decreases, this causes a decrease in the Coefficient of performance and as flow rate decreases and power input increases the COP also decreases. It can be seen from the second graph that the lower the mass flow rate of water, the higher the temperature of the water leaving the compressor as lower flow rate gives the water more time to be heated. It can also be seen that the energy flow rate out of the system increases with the mass flow rate of the water.
As T7 decreases, the mass flow rate increases to give a greater energy flow rate from the system. This combined with a lower compressor power usage gives a higher COP at higher flow rates.
Comment and Discussion: COP or coefficient of performance is a measure of the efficiency of a heat pump. The heat pump used in the experiment had a COP greater than 1 which is 3.26 and that means at this condition, 3.26 kJ of heat energy could be extracted from the system with the input of 1kJ of work. Efficiency can never be greater than 1% but here COP is above 1 because COP is not a percentage its just a coefficient so by definition it should be greater than 1.
