They conducted experimental and analytical studies on the dual expansion CO2 system and the effect of gas-cooler pressure on the system performance was investigated. It was shown that there are maximum COP and maximum heat capacity with respect to change of gas-cooler pressure. Baek et al. (2013) studied the cooling performance of the CO2 heat pump system varying refrigerant charge amount, EEV opening, compressor frequency and outdoor fan speed at various outdoor temperatures.