Page 31 - Low Temperature Energy Systems with Applications of Renewable Energy

20 Low-Temperature Energy Systems with Applications of Renewable Energy

Q A Q H Q L
þ (1.11)
T A T H T L
Notice that the reservoirs that lose heat have negative entropy changes, et vice
versa.
Substituting Eq. (1.8) into Eq. (1.11) and rearranging the terms yields an important
result:

Q L T L T H T A
(1.12)
Q H T A T L T H
This ratio may be used as the absorption refrigerator coefficient of performance
COP AR since it gives the ratio of the desired energy objective to the input energy
needed to operate the system.

Q L T L T H T A
COP AR ¼ (1.13)
Q H T A T L T H
or

T L T H T A
COP AR;ideal ¼ (1.14)
T A T L T H
When the system is ideal, i.e., perfectly reversible, we find:

Q L Q L
¼ (1.15)
Q H ¼
T L T H T A COP CR h CPP
T A T L T H
where COP CR is the coefficient of performance for a Carnot refrigerator operating
between the surroundings and the cold space, and h CPP is the thermal efficiency for a
Carnot power plant operating between the high-temperature heat reservoir and the
ambient heat sink.
Figure 1.15 shows the ideal COP for an absorption refrigerator as a function of the
heat source temperature for several cold space temperatures; ambient temperature was
taken to be 25 C. The best results are obtained for modest cold space temperatures and

high heat source temperatures. Thus, we can find the minimum required heat Q H at a
high temperature that must be supplied continuously to the AR to remove any amount
of heat Q L in order to create and maintain the cold space at a given low temperature.
Any real refrigerator will require more heat than the ideal case.

1.5.5 Heat-driven, 3-fluid absorption refrigerator

This section presents a unique absorption refrigerator. It is possible to achieve refrig-
eration with an absorption refrigerator without using any form of energy input except