Research On Heat Balance And Operating Strategy Of Hybrid Ground-source Heat Pump System

With the increasingly serious problems of energy and environment, the application of renewable energy in buildings is highly valued, ground-source heat pump(GSHP) which as a method of using renewable energy is widely used. In practice, the cumulative heat extraction and rejection of ground heat exchanger may probably not be balanced due to the imbalance between building cooling load and heating load, which causes heat balance problem and has adverse effects on the system in the long-term running. Therefore, hybrid ground-source heat pump system(HyGSHPs) which equipped with supplemental heat rejection or extraction device has developed rapidly in recent years. Taking HyGSHPs which equipped with cooling tower as the research subject and aiming at Nanjing area, this thesis studied the impacting factor on heat balance, heat balance condition and system performance of different operating strategies, and optimized control setpoint of each operating strategy and proposed recommended operating strategy. It can provide reference for designing and operation management.Based on the climatic characteristics of Nanjing area, this thesis determined cooling and heating seasons. Then based on an actual building, it analyzed the annual dynamic load which was generated by DeST. As a result, this building was cooling dominated, the ratio of cooling load to heating load was1.53:1, the ratio of annual cumulative cooling load to heating load was2.0:1. Also, it determined the form of HyGSHPs and designed important devices such as ground heat exchanger, heat pump unit and cooling tower.A conventional GSHPs model was created using the TRNSYS software and a transient simulation was carried out for a period of5-year, which mainly studied the impact on heat balance problem of borehole spacing, borehole depth and backfill material. The result showed that increasing the borehole spacing can mitigate the problem significantly, increasing the borehole depth can mitigate the problem to a certain extent, increasing the thermal conductivity of backfill material can only mitigate the problem to a very limited extent. However, none of these factors can solve heat balance problem essentially.The HyGSHPs model was created by adding cooling tower module to the GSHPs model and a transient simulation which studied heat balance condition and system performance was carried out for a period of20-year. This thesis proposed3different operating strategies and setpoints of each strategy for optimizing. As a result, there was never a situation that can get the best heat balance condition and the minimum system energy consumption at the same time when optimizing the setpoints or comparing different operating strategies. Thus, the system energy consumption should be the evaluation index. The recommended sequence of operating strategies according to priority is as follows:strategy2, strategy3, strategy1b and strategy1a.