Page 39 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 27

_
_
DE CN ¼ _ me 3 E Q CN _ me 4 ¼ 1:8024 ð125:12 84:62Þ 61:88
¼ 11:12 kW

Notice that the middle term in this equation is numerically negative because Eq.
(1.20) assumed that the condenser-heat exergy flowed out.

At the same time, the heat that is received by the reservoir at 80 C brings an
_
amount of exergy E Q hs where
298:15
_ T 0 _
E Q hs ¼ 1 Q CN ¼ 1 350 ¼ 54:51 kW
T hs 353:15
Thus, the exergy destroyed at the condenser caused by heat transfer from the cycle
working fluid to the heated space across a large and variable temperature difference
amounts to 7.37 kW, i.e., about 12% of the exergy of the condensing heat released,
or about 2.1% of the total heat transfer. The efficiency of the exergy transfer is there-
fore about 88%.
Throttle: Since there is no work or heat transfer at the throttle, the exergy decrease
for the working fluid is the only factor. Thus,

_
DE TH ¼ _ me 4 _ me 1 ¼ 1:8024 ð84:62 59:96Þ¼ 44:45 kW
This seems quite large but it is the throttling process that creates the low tempera-
ture that makes the heat transfer from the surroundings possible.
Evaporator: Here heat is removed from an ambient-temperature space and delivered
to the R152a which is maintained at a lower temperature. The method used for the
condenser may be applied to the evaporator with the following results:

_ _
DE EV ¼ _ me 1 þ E Q EV _ me 2

_ T 0 _ 298:15
E Q EV ¼ 1 Q EV ¼ 1 1:8024 ð520:09 403:59Þ
T EV 293:15
¼ 3:581 kW

This negative result may seem surprising, given that heat is transferred from the sur-
roundings to the R152a. The term is negative because T 0 > T EV , meaning that the flow
of exergy is actually from the R152a to the surroundings. And since any exergy dis-
charged into the dead state is lost, this term constitutes part of the exergy destroyed
in the evaporator. Therefore, we find

_
_
DE EV ¼ _ me 1 þ E Q EV _ me 2 ¼ 1:8024 ð59:96 57:98Þþ 3:581
¼ 7:150 kW