Page 27 - Low Temperature Energy Systems with Applications of Renewable Energy

16 Low-Temperature Energy Systems with Applications of Renewable Energy

Fig. 1.13 Heat pump processes for Example 1.

Q _ 3;4 ¼ _ m ðh 3 h 4 Þ

or
.
_ m ¼ Q _ 3;4 ðh 3 h 4 Þ¼ 350=ð597.78 403.59Þ¼ 1:8024 kg=s

The power required for the compression is found from the energy equation for the
compressor:

_
W 2;3 ¼ _ m ðh 3 h 2 Þ¼ 1:8024 ð597.78 520.09Þ¼ 140:0kW
Lastly, the COP HP is found from Eq. (1.4):

Q 3;4 350
COP HP ¼ ¼ ¼ 2:50
W 2;3 140:0
Using Eq. (1.3) or Fig. 1.8, the ideal heat pump operating over the same temperature
limits as the one in this example would have a COP HP of 3.77. This heat pump there-
fore achieves 66% of the ideal limit. Note that we have not accounted for the efficiency
of the electric motor that drives the compressor. Since motors are very efficient, the
actual electrical power might be 2e3% higher than calculated in this example.