THERMODYNAMICS OF A VAPOR-COMPRESSION REFRIGERATION CYCLE WITH MECHANICAL SUBCOOLING

Refrigeration and air-conditioning systems, when operating under large temperature differences between the condenser and evaporator, consume significant amounts of energy. A vapor-compression refrigeration cycle with a mechanical subcooling loop to increase system performance and reduce energy consumption is investigated by using both the first and second laws of thermodynamics. Although the first-law (energy-balance) approach to system analysis shows improvement in the system coefficient of performance (COP) with an increase in the temperature difference between the condenser and evaporator, it fails to locate sources of losses. Identifying and quantifying these sources can be a useful design tool, especially in developing or investigating new, more complex refrigeration cycles. A second-law analysis (in terms of irreversibility) has been carried out for both the simple and the vapor-compression refrigeration cycle with a mechanical subcooling loop. It is found that the performance of the system can be significantly improved by reducing the irreversibilities due to the expansion process. The low-temperature refrigeration system, when operating at the optimum subcooler saturation temperature, may have the following features: (i) 85% reduction in power input; (ii) 65% percent lower irreversibility rate; (iii) 20% reduction in the total refrigerant flow-rate.