CO 2Subcooling

CO2 refrigeration cycles require modifications or enhancements to reach the highest possible efficiencies, especially at high heat rejection temperatures. This chapter focuses on one of the most recent methods to increase the efficiency of CO2 refrigeration cycles, subcooling the refrigerant at the exit of the gas-cooler or condenser. Subcooling, achieved with different cycle architectures is probably the method that allows enhancing most the performance, is achieved with simple mechanisms and is easily to be built. Starting from a thermodynamic analysis of cycle modifications due to subcooling, the enhancement and the cost of subcooling, this chapter reviews the subcooling methods that have been devised and implemented around the World. It starts with the well know internal heat exchanger device; then analyses the dedicated mechanical subcooling cycle, which relies on the use of an external vapour compression cycles; and ends with the integrated mechanical subcooling system, developed to operate only with CO2 as refrigerant. For each method, thermodynamic aspects, improvements, experimental results and optimum working conditions are analysed and discussed.