Dynamic Simulation And Experimental Study On Dual Heat Sources Heat Pump System Based On Heat Sources Priority

In the long-term operation of the ground source heat pump system(GSHPS),the attenuation of underground heat transfer not only affects the storage and extraction ability of soil,but also affects the performance and economic efficiency of GSHPS.The controllable intermittent technology can alleviate the deterioration situation of heat transfer in underground soil and control the attenuation of underground heat transfer.However,due to the influence of soil heat transfer characteristics,the soil temperature can not be fully recovered within a certain period of time,which results in a significant decrease in GSHPS's performance under the long-term operating condition.Based on the GSHPS experiment platform in cold region,this paper realizes the forced recovery of underground soil by introducing solar energy as the auxiliary heat source.At the same time,based on the theory that multiple energy systems can complement each other,the multi-condition hourly dynamic simulation model of the dual heat source heat pump system(DHSHPS)including the solar energy and the geothermal energy has been set up.Different heat source priority and operation dynamic characteristics of DHSHPS have been studied.The internal relevance between the heat source priority,the ratio of heat source and the heat source switch temperature with the performance of the DHSHPS are revealed,which can provide a theory basis for long-term operation of the DHSHPS.The specific work is as follows:Firstly,an experimental apparatus of the DHSHPS was built.The natural recovery and forced recovery characteristics of soil temperature,the influence of the operation mode on the performance of DHSHPS,and the short-term heat storage characteristics of solar recharging the boreholes were experimentally investigated.The results indicated that:?The use of the solar energy existing storage tank to recharge the boreholes can effectively improve the soil temperature recovery rate in the process of soil temperature recovery.?The mode in which the water tank was connected with the ground heat exchangers in series was recommended for the DHSHPS in the coldest month in Dalian.?Due to the discontinuity of solar energy and that soil thermal diffusion was greater than the heat accumulation in the late period of heat storage,the soil temperature did not rise but fall after a cycle of solar recharging boreholes.Therefore,the solar energy should be used in time to avoid energy waste.Secondly,the multi-condition hourly dynamic simulation model was established considering the complexity and diversity of the control modes of the DHSHPS.Among them,for the underground part:the equivalent heat transfer coefficient method was used to replace the traditional heat flux method to describe the influence of heat transfer in the borehole to the outside soil.Furthermore,the expressions of the equivalent heat transfer coefficient in the process of intermittent and operation were deduced.In turn,the dynamic simulation of the ground heat exchanger in the process of intermittent and operation were implemented.For the aboveground part:the multi-condition modeling method for the heat collecting/storage/using of the solar thermal energy was proposed.The dynamic simulation of the solar thermal energy in the process of heat storage and using,heat storage but not using,heat using but not storage and no heat transfer was realized.On the basis of this,the dynamic simulation platform of multi-condition dynamic simulation of DHSHPS was developed.In addition,the dynamic simulation model of DHSHPS was validated experimentally,which provided a reliable tool for the long-term dynamic performance analysis of the DHSHPS.Again,from the perspective of DHSHPS thermal performance and efficient for long-term operation,the basis of the dynamic balance of the ground temperature for many years was proposed,the curves of the borehole wall average temperature variated with the solar fraction and the heat source switch temperature were established under different heat source priority,the reasonable ratios of the heat source and the heat source switch temperature were determined on the basis of the dynamic balance of the ground temperature for many years,the action of the dynamic balance of the ground temperature in the long-term operation of the DHSHPS was revealed,and the problem of the low thermal performance of DHSHPS in long-term operation was improved.Finally,the influences of the external parameters(including the terminal load and the solar radiation)and the controllable parameters(including the fluid flow rate of load side,the fluid flow rate of source side,and the fluid flow rate of collector side)on the performance of the DHSHPS were analyzed.The variation of inlet temperature of heat pump and the COP of unit with those factors were fitted using the 1 stopt software.The relationship of the external parameters and controllable parameters,with the inlet temperature of heat pump and COP of unit for the DHSHPS have been developed.It was found that the variation of the COP and the inlet temperature of heat exchangers of the heat pump unit varied at an exponential function with the change of the external parameters and the controlled parameters under the series and parallel operation modes for the DHSHPS.These relations have been verified experimentally to provide a reliable basis for the quantitative prediction of the system performance when the fluid flow rate were changed.