Under low-temperature climate conditions

the compressor suction pressure and temperature are reduced owning to the decrease of the temperature of the inlet water and the outlet CO2of the gas cooler in the trans-critical cycle CO2 heat pump system.When the compressor suction pressure is less than the lower limit of the suction pressure

it leads to the unstable operation of the system.In order to overcome this drawback

a method of mixing water at the inlet of cold water in the gas cooler is presented.Proper quantities hot water in hot water tank is supplied to the cold water inlet of the gas cooler by a by-pass pipe.Adjusting the three-way valve to change the proportion of mixing water

the inlet temperature of gas cooler is increased to make the system run steadily under low-temperature climate conditions.Test results show that the method of mixing water could not only ensure the system of CO2 air source heat pump hot water system works steadily under low-temperature conditions

but also increase operation time of the system and reduce the frequency of frosting.However

the heating power and COP(coefficient of performance)of the system declines slightly.Considering the operation stability and thermal performance simultaneously

the suitable temperature of mixing water for this system is 12-18℃ when ambient temperature is -20℃ and heating temperature is 60℃.