Study Of Thermodynamics And Heat Transfer On Frost Prevention And Retardation Of Air Source Heat Pump

Air source heat pumps(ASHPs) commonly used as the major heating and cooling source for buildings become increasingly popular in China. However, frost formation will occur on the outdoor evaporator’s surface when ambient temperature just reaches only below dew point in central-south China, which is characteristic of low temperature and high moisture content. As the frost accumulates with time, the influence of the frost accumulation will increase the pressure drop rapidly and decrease the heat transfer dramatically, which thus degrades the performance of heat pump. The most widely used traditional defrosting methods for ASHPs are the reverse cycle defrosting(RCD) method and hot gas bypass defrosting method. The long-term RCD operation would definitely lead to deterioration of the performance of the ASHPs and damage the components of the system. While whenever the HGBD is activated, as for heating originating from the evaporator, the heating capacity cannot be guaranteed with this solution. Not only does the defrosting time remain long, but the overall indoor thermal performance of the amenity is also degraded.(1) This paper presented a new dimensionless artificial neural network(DANN) correlation involving frost accumulation for air source heat pump(ASHP) on the air-side of fin-and-tube. The dimensionless parameters in this DANN were based on the ambient conditions and structures of heat exchangers. In particular, more kinds of refrigerants including R12, R134 a, R22, R410 A, R407 C and R600 a were taken into account. To enhance the reliability of the DANN, emphasis is also placed on this DANN model for its self-adaptive algorithm.(2) A new frost retardation theory based on frost accumulation is proposed. It can help analyze the complicated frosting process and help the manufactures design the new ASHPs more reasonable in different regions with different ambient conditions.(3) This study firstly presented a novel design of ASHP equipped with auxiliary electric heaters(AEH)(two sets of heaters installed on tube, between capillary and evaporator for the B-AEH and between evaporator and compressor for the A-AEH) for frost prevention and retardation. In our solution, the refrigerant flowing through the evaporator unit can be heated by the installed electric heater. The electric heating increases the refrigerant’s temperature at the inlet to the outdoor evaporator and heats the incoming air flow, thus preventing frosting on the surface wall. As a consequence, the evaporator’s temperature can be maintained high enough to preclude frost formation and enable proper efficient performance even in frosting range. The proposed design can also delay and resolve the frequent ON/OFF problem of the compressor that is characteristic of the RCD method.(4) Orthogonal experiment design(OED) is applied in this study to evaluate the performance of the air source heat pump(ASHP) with different magnitudes of AEH power under a range of ambient frosting conditions. The L25(56) orthogonal array is selected and analyzed by means of analysis of range(ANORA) and analysis of variance(ANOVA). The optimum parameter combination studied in this paper affecting the performance of ASHP was determined and the most significant parameters were also identified.(5) In this study, a computational model for an ASHP equipped with the B-AEH heater for retardation of frosting was proposed. The new ASHP system involves an AEH installed on the tube between the capillary and evaporator(B-AEH). The computational model considered variable ambient conditions including dry conditions, wet conditions and frosting conditions. The computational model allows both steady state and transient analyses.(6) The performance of the ASHP(in terms of the COP and heating capacity Qh) as a function of time was calculated using this computational model, and results were validated by comparing computed values with data from the experiments. Other performance parameters were also predicted using this computational model, including refrigerant heat transfer coefficient, refrigerant mass flow rate and tube wall temperature. The computational model offers a convenient tool for practicing engineers that allow the analysis and design of ASHP systems for a wide range of operating conditions.To summarize, the presented frost accumulation can help the manufactures design the new ASHPs to better accommodate typical severe weather conditions in regions for which frosting is common. In additon, the AEH design can be applied to existing commercial heat pumps. By adjusting the magnitude of AEH heating, the performance of ASHP is enhanced and frost formation is prevented or delayed. The manufacturing processes for such a design can be easily accommodated on the tubes of outdoor evaporator.