| Air source heat pump used in providing heat in winter has advantages of a high coefficient of performance(COP)and no pollution.However,for the single-stage or cascade air source heat pump,the optimal coefficient of performance cannot be achieved under all circumstances when operating separately.Therefore,a novel air source heat pump system with switchable operation modes between single-stage and cascade heat pumps has been developed,by which a single-stage cycle or a cascade cycle is adopted to satisfy different heating demands in cold winter and maintain a high coefficient of performance.In this work,the performances for these two modes are theoretically studied,and the change trend of the power input and coefficient of performance with the change of the water temperature and ambient temperature of two heating modes is analyzed.The results show that the cascade cycle has a lower power input and a higher coefficient of performance than single one under the lower ambient temperature and high supply water temperature.The single cycle is better on the opposite situation.Besides,the switching ambient temperature of two heating modes is obtained for energy saving when the intermediate temperature of cascade heating mode is applied at the optimal value and some certain deviations from that.The results show that the switching ambient temperature between two heating modes increases with increasing the supply water temperature and reducing water temperature difference.Besides,the switching ambient temperature drops as the degree of deviation from the optimal intermediate temperature increases,and rises with the growing outlet water temperature.Frost formation may happen on the finned tubes when the outdoor heat exchangers of air source heat pumps operates at a low environment temperature,which can decrease the effects of heat transfer,influence the performance of heat pumps,even lead to failure accident.Therefore,it is of great significance to study the frost formation process.Based on several common assumptions in the past studies about frost,frost formation on the plat of a certain temperature is simulated at several conditions in a certain time according to the numerical simulation model and UDF programs by Ansys Fluent,the commercial computational fluid mechanics(CFD)software.The influence of different parameters about the wet air like air temperature,air humidity and external conditions like plat temperature on frost formation is studied.Moreover,the feasibility and accuracy of the model is verified through the comparison with the experiment results. |
