An experimental study on charge optimization of a trans-critical CO₂ cycle

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 trans-critical refrigeration cycle. In cooling mode operation, a trans-critical CO₂ system, as compared to conventional air-conditioners, 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.

List of symbols

Symbols

BPV:

Back pressure valve

COP:

Coefficient of performance

ex:

Specific exergy (J/kg)

${\dot{\text{E}}\text{x}}$ :

Exergy flow (W)

EEV:

Electronic expansion valve

h :

Specific enthalpy (kJ/kg)

IHX:

Internal heat exchanger

$\dot{m}_{\text{r}}$ :

Refrigerant mass flow rate (kg/s)

NC:

Normalized charge

$\dot{Q}$ :

Heat transfer rate (kW)

RC:

Refrigerant charge (kg)

SC:

Specific charge (kg/kW)

T :

Temperature (°C, K)

T _o :

Ambient temperature (°C, K)

v :

Specific volume (m³/kg)

$\dot{V}$ :

Volumetric flow rate (m³/s)

$\dot{W}$ :

Compression power (kW)

Greek symbols

β :

Compression ratio

η _v :

Volumetric efficiency

τ :

Dimensionless exergetic temperature

Subscripts

a:

Air

des:

Destroyed

cp:

Compressor

ev:

Evaporative

ev:

Evaporator

gc:

Gas cooler

in:

Inlet

IHX:

Internal heat exchanger

max:

Maximum

TMT:

Mean thermodynamic

out:

Outlet

r:

Refrigerant fluid

suc:

Suction

v:

Expansion valve