ASHRAE Journal - April 2023 - 48

TECHNICAL FEATURE
FIGURE 3 The effect of changing the working fluid on the COP normalized
with the COP of isentropic cycles: (top) refrigeration and (bottom) heat pump
cycles.
1.2
R-134a
R-410A
β
βS
C02
Ammonia
Water
1.1
enthalpy at the end of the compression process, respec1.0
1.1
γ
γS
1.0
0.0
0.5
Q/Qhg
R-410A.
Natural refrigerants like CO2 and ammonia saw
greater increases in COP of 7% and 13%, respectively. The
greatest increase of COP was for water, which increased
by 20%. A similar trend was observed for the heat pump
cycle; however, increases were not as pronounced.
The increase in COP was found to be related to the
slope of the saturated vapor lines on the T-s diagram.
The comparison of the T-s diagram of R-134a, plotted
in Figure 4a, and the T-s diagram of ammonia, plotted
in Figure 4b, shows that the latter has a significantly
less steep saturated vapor line. This is mirrored by
the increase in COP of 13% for ammonia compared to
an increase in COP of only 1% for R-134a, which has a
steeper slope for its saturated vapor line. It was clear
from the results that natural refrigerants demonstrated
higher COP improvements than HFC type refrigerants
when using the proposed compression-expansion
fusion concept.
To quantify the potential increase in COP, a new
dimensionless parameter was developed by the present
authors, as
ξx=−∫
s
s
2
1
TT
hh
pg
pg
−
−
ds
(10)
where s1 and s2 refer to initial and final states for the
compression process from the evaporator pressure to
the condenser pressure, Tp and hp are temperature and
48
ASHRAE JOURNAL ashrae.o rg
A P R I L 2023
1.0
where C1 ≈ 52, C2 = 5/8 and it has a coefficient of determination
of R2 = 0.997.
FIGURE 4 A comparison of the T-s diagrams of (a) R-134a, a HFC refrigerant,
and (b) ammonia, a natural refrigerant, highlights the difference in the slope of
the saturated vapor curve as a function saturation temperature and entropy.
A
100
3000 kPa
2000 kPa
50
900 kPa
0.2
-50
0.00
B
100
50
-50
0.0
1.0
2.0
3.0
4.0
s (kJ/kg·K)
5.0
6.0
1000 kPa
2000 kPa
5000 kPa
9000 kPa
80 kPa
300 kPa
0.25
0.50
s (kJ/kg·K)
0.75
1.00
0.4
200 kPa
400 kPa
50 kPa
0.6
0.8
tively, and Tg and hg are saturated vapor temperature
and saturated vapor enthalpy corresponding to the
same pressure at the end of the compression. The new
number, ξ, represents the ratio of the heat needed to be
removed to the possible work reduction. The heat needed
to be removed can be visualized as the area between
the isentropic compression line marked by the blue
arrow in Figure 4, and the saturation line, marked by the
orange dotted line on the T-s diagram. This newly developed
dimensionless number was found to be successful
in correlating the maximum increase in COP for five
different refrigerants, over an order of magnitude variation
(Figure 5). The best fit correlation was also plotted in
Figure 5, and given by:
ΔCOP% = C1ξC2
(11)
T (°C)
T (°C)
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