Performance Investigation And Optimization On Air Source Heat Pump System Combined With Latent Heat Thermal Energy Storage Unit

Air source heat pump systems have been gradually used for space heating in northern part of China in recent years due to their high efficiency and cleanness.However,the decrease of ambient temperature at night will cause poor heating performance.It is a promising method to combine the air source heat pump with latent heat thermal energy storage technology to solve the problem.Condensing heat storage unit combines condenser and heat storage unit together,in which heat is transferred directly between refrigerant and phase change material(PCM)in order to increase heat transfer rate.With a proper size of heat storage unit,the system can realize the operation mode that heat is stored during the day and released at night,avoiding various problems of the air source heat pump system running at low temperatures.However,nowadays there are still few related studies on operating characteristics of air source heat pump systems with direct heat exchange between refrigerant and PCM and the combination of multi-stage PCM.Therefore,main work of this article is listed as follows:(1)Experiments on the heat storage and release characteristics of the system are carried out.Results shows that the average COP of charging process is 2.6,and heat storage capacity can reach 28.9 kWh.The average heat release power during the heat release process is 4.5 kW,which can theoretically provide heat for a room of 18 m~2 for8 h.A numerical model of direct heat transfer between refrigerant and PCM is established,and validated by experimental data.Results show that simulation results are in good agreement with experimental data,and the average relative error is less than10%.It is calculated when this system is under continuous operating condition and results show that when inlet water temperature is 35℃,heat storage and release efficiency of this system after the sixth cycle reaches 99.1%,and it enters a stable operating state.(2)In order to improve heat storage and release performance of the condensing heat storage unit,parameter analysis including melting point of PCM and structural parameters is conducted.Results show that when melting point of PCM is reduced,heat storage time can be shortened,but heat storage capacity is slightly reduced,and the average heat release power is significantly reduced.When the average meltinf point of PCM is equal,compared with a fixed melting point,charging time will be shortened and the average heat release power will be larger if PCM has a melting point range.But heat storage capacity is reduced.Increasing fin thickness or reducing fin spacing will speed up heat storage rate and increase heat storage capacity slightly.However,the change of tube diameter shows little impact on heat storage time and heat storage capacity.Finally,uneven fin spacing and thermal conductivity of fins are set in order to explore whether they could improve the heat transfer rate.Results show that these two methods do not work well and more insight investigation is still required.(3)A multi-stage PCM dynamic heat transfer model is established to explore its effect on the system performance.Assuming that only melting points are different in multi-stage PCM configurations.Results show that compared with single-stage PCM,heat storage time is shortened by 9.6%,COP is increased by 4.0%and total exergy efficiency is improved by 4.5%when applying three-stage PCM.When two-stage PCM is used,if only the melting point of PCM in subcooled zone is reduced,system performance is similar to that of three-stage PCM.If only the melting point of PCM in superheated zone is increased,system performance is similar to that of single-stage PCM.In addition,the actual thermophysical parameters of paraffin are inserted into the mode and system performance of 5 different combinations of paraffin are compared.Results show that reducing the melting point of PCM in subcooled zone plays a key role in improving the system performance,while increasing the melting point of PCM in superheated zone has little effec.Reducing the melting point of PCM in two-phase zone has both positive and nagetive effect on the system performance.Considering the heat storage and release performance and structure of the system,Combination2(RT48/RT48/RT44)is recommended as the preferred form of PCM for the system.