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An experimental study on charge optimization of a trans-critical CO2 cycle
Aprea, C.; Greco, A. & Maiorino, A.
Abstract
The phasing out of hydrochlorofluorocarbon
and chlorofluorocarbon fluids and further environmental
problems arising from new, synthetic working fluids
stimulate a continuously rising interest on natural candidates.
The nontoxic and nonflammable CO2 impacts neither
on ozone depletion nor on global warming if leaked to
the atmosphere. The critical temperature of CO2 (31.1 °C)
is almost ambient and, therefore, it undergoes a transcritical
refrigeration cycle. In cooling mode operation, a
trans-critical CO2 system, as compared to conventional airconditioners,
has a lower performance which, contrary to
conventional systems, strongly depends on the refrigerant
charge. In this paper, the performance of the trans-critical
system was evaluated experimentally under different
refrigerant charge amounts. The influence of charge on
coefficient of performance (COP), cooling capacity, compression
ratio, suction line superheat was analyzed in
detail. The experimental results indicate that the COP of
the trans-critical cycle attains a maximum at optimal
refrigerant charge. By varying the charge the cooling
capacity attains a maximum, as well, that corresponds to
the optimal charge. In order to understand the effect of
refrigerant charge on the performance of each device of the
plant, an exergetic analysis based on the experimental data
was carried out. The analysis shows that the exergy flow
destroyed in the compressor is one of the major causes of
overall exergy destruction. At the optimal refrigerant
charge, the compression losses attain a minimum.
Keywords
CO2; Trans-critical cycle; Refrigerant charge; Coefficient of performance; Optimization
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