Modern Engineering Thermodynamics

14.15 Reversed Stirling Cycle Refrigeration 573
A theoretical analysis of the Stirling cycle was finally carried out by I. A. Wyshnegradski in 1871. Because of its inherent
safety, it was extensively used for refrigeration of food products (especially frozen meat) on ships from 1880 to
1900. The safe operation of a reversed Stirling cycle also made it ideal for use in deep mines until about 1930, when
synthetic refrigerants (especially R-11 and R-12) made vapor-compression machines safer and more efficient.
The reversed Stirling cycle refrigerator is most effective when hydrogen gas is the working fluid and a very
efficient regenerator is employed. Hydrogen has the largest specific heat of all common gases and does not condense
at temperatures above 35 K (helium is used for lower temperatures). Under these conditions, Stirling
refrigerators are often used to produce temperatures in the −80.0°C to−200.°C (−112°F to−328°F) range.
Recall that the Stirling cycle consists of two constant temperature processes and two constant volume processes
(see Figure 14.29). Reciprocating piston technology is still the most effective mechanism used to form this cycle,
with the constant volume processes being approximated by the relatively small piston motion near the top and
bottom dead center crankshaft positions. Since the Stirling and Carnot cycles have the same thermal efficiency,
the reversed Stirling and the reversed Carnot cycles have the same coefficient of performance,
COP reversed Stirling
ASC HP
= T H
T H − T L
(14.26)
and
COP reversed Stirling
ASC R/AC
=
T L
T H − T L
(14.27)
Q H
Q L
W (compressor piston)
4
1
4
T H = c
T
V = c V = c
p
3 T L = c
1
3 2
W (disp. piston)
s
(a) T–s diagram
V
(b) p–V diagram
2
Q regen
3 2
Constant
volume
regenerator
4 1
Q L
Q H
Displacement piston
(c) Equipment schematic
Compressor piston
FIGURE 14.29
The reversed Stirling cycle operating as a refrigerator, air conditioner, or heat pump.
CRITICAL THINKING
Stirling engines and refrigerators work best when hydrogen gas is used as the working fluid, because hydrogen has a very
large specific heat (c p-hydrogen = 14.32 kJ/kg·K, whereas c p-air = 1.004 kJ/kg·K). Since the hydrogen is not burned, but merely
passes through the cycle described in Figure 14.29, then why would having a high specific heat make any difference? Hint:
Think about the heat transfer process in the regenerator.