Page 232 - Low Temperature Energy Systems with Applications of Renewable Energy

Heating with geothermal systems 219

Loss of exergy in the economizer EC is:

E _ EC ¼ T env ½ _ m EV ðs 9 s 6 Þþð _ m C _ m EV Þðs 8 s 7 ފ (5.30)
Dk
where ð _ m C _ m EV Þ is the R123 mass flow rate through the economizer.
Exergy loss in a throttle valve at the low-pressure side is:

E _ TV1 ¼ _ m EV T env ðs 10 s 9 Þ (5.31)
Dk
For the throttle valve at the high-pressure side, the loss of exergy is determined by:

E _ TV2 ¼ð _ m C _ m EV Þ T env ðs 7 s 6 Þ (5.32)
Dk
The overall exergy efficiency for the system is:

s
Q _ HPU cond
ex (5.33)
h ¼ P _ _
W comp þ Q s 0
0
5.7 Summary

Geothermal energy, either alone or in conjunction with heat pumps, is providing heat-
ing for residents and businesses in cities all over the world. Historical accounts of very
old systems in Europe and in the United States show that these systems have a long
life, give reliable service, and are environmentally friendly as well as economic.
Greenhouses warmed by geothermal fluids allow year-round crop growing even in se-
vere climates. A wide variety of fish and other water creatures such as alligators are
raised in geothermally-heated ponds. The outflow from geothermal electric power
plants can be used as the heating medium. Such facilities are good businesses and
can often be promoted as tourist attractions. The use of heat pumps with geothermal
energy offers a multiplier effect to achieve high performance even under cold ambient
temperatures.

Nomenclature

E _ exergy loss
C specific heat capacity
k isentropic index
_ m mass flow
p pressure
q specific heat transfer
q* specific cooling capacity
Q _ heat