Theoretical Analysis And Experimental Research Of Air Source Heat Pump Performance Under Frosting Condition

Air source heat pump (ASHP) which has high efficiency and environment protection effect has been widely used in the south of our country and also in some northern parts. In winter, ASHP system always encounters the matter of frost formation and decline of heating load under unfavorable conditions. Contrary to dry environment of northern winter, the Hot Summer and Cold Winter Zone represented by Changsha has characteristic of high humidity, and faces worse situation of frost formation. Effective defrosting control is badly needed to deal with the problem. In this paper, frost of AHSP was theoretically and experimentally studied under condition of low temperature and high humidity, and defrosting control method was also discussed.In this paper, development of finite time thermodynamics was reviewed, and research of heat pump based on finite time thermodynamics was introduced. Based on the theory of endo-reversible cycle, an endo-reversible cycle of ASHP was modeled with employing of CYCLEPAD software, and implement of heat pump performance under variable status was also discussed.Frost mechanism and frost structure were analyzed. According to heat and mass transfer principle and molecule diffusion theory, a simple frost model and a steady-state lumped parametric model of evaporator were built. Temperature of frost surface was obtained through iterative computation, and then the ratio of latent heat to sensible heat was calculated which can reflect high humidity characteristic of environment. Based on the analytic and computational results mentioned before, an endo-reversible cycle of heat pump was built, and the change of ASHP performance was simulated.Dimensional theory was introduced. According to dimensional analysis, a correlation for frost accumulation which consists of five non-dimensional rules was developed, and an evolved formula was derived from the correlation. Variables of non-dimensional rules were analyzed about their effect on frost process, and exponents of correlation and evolved formula were calculated through data. A field test of frost situation of ASHP was performed under condition of low temperature and high humidity, and data of parameters such as frost accumulation were obtained. The evolved formula was modified according to the test data, and defrosting control method was also discussed.