Operating Characteristics And Optimization Of The Solar-ground Coupled Heat Pump System With Seasonal Storage

During the practical application of ground-coupled heat pump in severe cold area, due to the large heat extraction of the heat pump, the soil temperature would descend year by year for the system operation all the year round. As a result, the heating performance of the heat pump would drop year by year, too. Finally, the heat pump output would be insufficient. Therefore, based on the idea of transferring the solar energy between the seasons, this paper presented the solar-ground coupled heat pump system with seasonal storage (SGCHPSS), which integrated the solar energy, the soil heat storage and the ground-coupled heat pump technology organically. In this way, the abundant solar energy in the non-heating seasons was transferred to winter and the problems mentioned above could be solved. The system provides a new method for space heating in severe cold area.To find out the practical effect and heating performance of the system in severe cold area, the experimental platform of SGCHPSS system was established, the tentative operation way and control method of the system were given and the annual experiment was performed. The operating characteristics of solar soil heat storage and system heating as well as the temperature variation characteristics of the soil were tested and analysed. It can be concluded that the soil temperature could be increased effectively via solar seasonal soil heat storage, the COP of the heat pump achieved 4.29, the system annual coefficient of performance reached 6.14 and 87% of the total heat supply in winter came from the solar energy.Considering the complexity of the system and time constraints, further study of the system can not be realized though the experimental method one by one. Therefore, the mathematical model of each device in the system was established based on the experiment. Combining the operation way and control method given in the experiment, the system dynamic simulation was realized. To reduce the computional time, the thermal interference between the U-tubes of the ground heat exchanger was neglected. For verifying the rationality of the simplification, the single tube mathematical model was verified by the multi-tube mathematical model. The results show that the maximum relative errors of the heat storage capacity and heat extraction capacity of single borehole, computed separately by the single tube mathematical model and the multi-tube mathematical model, were 5.9% and 7.9%, respectively, which were all in the allowable range of less than 10%. The system mathematical models were further verified through the comparison between the simulation results and the experimental results. The system mathematical models established were first used to conduct the simulative study on varing the parameters. Impacts on system operating characteristics caused by varing the system configuration parameters including solar collector area, buried depth of ground heat exchanger and heat output of the heat pump were analyzed in detail. The simulation results show that increasing the solar collector area and the capacity of the heat pump were all favourable for improving the heating performances of the heat pump and the system as well as reducing the operating costs. However, the buried depth of the ground heat exchanger was not the deeper the better. When the buried depth increased to a certain extent, the heating performances of the heat pump and the system were enhanced little, while the operating costs would increase.Secondly, the sustainability of the system operation under the conditions of storing heat seasonally or not was analyzed respectively. The following conclusions can be drawn: in severe cold area, if the ground-coupled heat pump was used to heat the building all the year round, the heat output of the heat pump would be insufficient and the power consumption would increase continually, while the indoor temperature could not meet the requirements; the SGCHPSS could raise the soil temperature effectively, the heating performance of the heat pump was improved year by year, the power consumption of the system was also reduced year by year and the annual heat balance could be maintained in the soil. Then, in view of the existing problems of operation way in the experiment, three new and feasible operation ways were put forword and a comparative analysis with that in the experiment was conducted. It can be drawn that the heating performance of the system could be promoted and the proportion of the energy output of each device could be changed by varying the operation way; the heating effect could be improved while saving the electric energy if the heating period was divided into phases to vary the operation way.Finally, the possible system configuration and operation way were given, and the dynamic programming method was used to optimize the operation way for different system configuration. By this way, the optimal operation way for the heating period was obtained for different system configuration. Then, the annual cost was taken as the objective function to optimize the system configuration. Consequently, the optimal system configuration and operation way were gained. This optimization method can provide guidance for engineering applications of SGCHPSS system.The studies in this paper have proved that the SGCHPSS has great energy-saving advantages and better heating performance, which can provide theoretical basis and technical support for the optimization design and operation of the system.