Page 23 - Low Temperature Energy Systems with Applications of Renewable Energy

12 Low-Temperature Energy Systems with Applications of Renewable Energy

70
o
60 20 C
50

COP R 40 30 C
30
o
20
10
40 C
o
0
-25 -20 -15 -10 -5 0 5 10 15 20
o
Refrigera on temperature, C
Fig. 1.9 Ideal refrigerator cycle COP versus refrigeration temperature for three values of
ambient (or sink) temperature.

Figure 1.9 gives the ideal refrigerator COP R as a function of the cold space (refrig-
erator) temperature for three different heat sink (or ambient) temperatures. Contrary to
the case of the heat pump, the lower the sink temperature, the higher the COP R . In fact
from Eq. (1.4) it is clear that the performance grows infinitely large as the temperature
difference across the heat pump decreases toward zero. Of course, if the temperature
difference across the refrigerator approaches zero, there is no need for a refrigerator at
all.
Figure 1.10 shows a comparison between semi-ideal refrigeration machines and
heat pumps, where the temperature differences needed to allow heat transfer to take
place are also shown. Heat transfer in a heat exchanger must always occur from the
hotter body to the cooler one. This also illustrates Thomson’s multiplier effect since
the output heat at the higher temperature is equal to the sum of the heat input at the

Fig. 1.10 Schematic of heat and work flow for refrigeration machines (RMs) and heat pumps
(HPs); see Nomenclature.