Abstract
The high irreversibility caused by the expansion valve in the conventional transcritical CO2 heat pump cycle has been reported as the major drawback on the overall system performance. To overcome this problem and recover some of the energy lost, different isentropic expansion devices such as turbine expander and two phase ejector have been proposed. This study aims to numerically compare the performance of the transcrtical CO2 heat pump in terms of first and second law of thermodynamics. In addition, the energy recovered by the two phase ejector and the turbine expander cycles have been evaluated. The pressure recovery and entrainment ratio in the ejector device were investigated comprehensively. Two numerical models using MATLAB and ASPEN PLUS software have been developed, and REFPROP database was used to estimate the refrigerant thermophysical properties. The results showed that the heating coefficient of performance (COPh) of the ejector cycle is higher than that of the turbine and valve cycles by 10.15 % and 20.84 % respectively. In addition, the ejector cycle has the highest second law efficiency (0.1) and the recovered energy is (0.63 kW) compared to (0.107 kW) gained by the turbine cycle. The ejector device has the least exergy destruction (0.2 kW) and can recover 0.7 Mpa of the pressure losses.
Highlights
The environmental friendly behaviour and the high efficiency performance of the working fluid are essential demands by the heating and cooling industries for any modern heating systems
The energy recovered by the ejector cycle is significantly higher than that for the turbine cycle
The mathematical modelling is conducted using MATLAB software linked to REFPROP database in order to obtain the thermophysical properties of the refrigerant in different thermodynamic states across the cycles
Summary
The environmental friendly behaviour and the high efficiency performance of the working fluid are essential demands by the heating and cooling industries for any modern heating systems. The system performance of the conventional cycle which utilise an expansion valve is compromised by the high irreversibility (throttling losses) in the expansion process During this process, the CO2 velocity rises as a result of the gained kinetic energy. In the transcritical CO2 heat pump cycle, the heat rejection process take place in the supercritical region which requires higher pressure ratio. This will lead to higher throttling losses compared with the subcritical cycle [5, 17].
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