| In order to improve the adaptability, heating capacity and heating performance of traditional air source heat pump (ASHP) unit in case of low air temperature, a hybrid single-stage and double cascade ASHP heating system that is suitable for cold region has been developed in this paper, combining two-stage compression coupling heat pump technology with cascade heat pump technology. In this system the actions of both four-way valve and solenoid valve can be controlled according to user's need so as to switch among three modes of operation, which are single-stage cooling, single-stage heating and double-stage cascade heating respectively.In order to experimentally study the system's improved characteristics for expanding ASHP's suitable range of ambient temperature zone, a hybrid single-stage and double cascade ASHP prototype was designed, and the intrinsic operating parameters and heating performance of the prototype in different operating conditions and modes were tested in an enthalpy potential method laboratory. The results showed that the coefficient of performance in heating (COPh) of the unit was 2.14 with a heating capacity of 7.61kW, which operated in condition of ambient temperature at -20℃and 45℃of hot water supplying, and the compression ratio was less than 4.5, the discharge temperature was below 80℃. Moreover, the ambient temperature with 0℃was taken as the switch temperature for the modes of single stage and double cascade heating operation based on the optimization of COPh.In addition, steady-state mathematical simulation model of hybrid single-stage and double cascade ASHP unit was also established in the paper, of which accuracy is validated by the experimental results and meets the requirement of simulation. On the basis of the calculated results of the simulation model, optimal matching of heat transfer area of evaporative condenser in the prototype unit has been conducted, and switch temperatures for modes of single stage and double cascade heating operation in case of different supply water temperatures were optimized and analyzed. |
