Study on the thermal imbalance characteristics and optimal operation strategy of ground source heat pump system in hot summer and cold winter areas

This paper presents a comprehensive analysis on the thermal imbalance characteristics and optimal operational strategy of a hybrid ground source heat pump system in the regions with hot summer and cold winter. To have a solid knowledge of thermal performance of the system under various operational conditions, component models for the overall hybrid ground source heat pump system are established which are validated by in-situ field test. Subsequently, a composite simulated annealing genetic algorithm is formulated in search of the optimal operational settings. Multi-objective optimization metrics highlights the integration of system energy efficiency, thermal balance indicator as well as load imbalance rate between ground source and end user. The results reveal that there is significant heat and cold accumulation around the borehole heat exchangers under constant operating conditions during a single cooling or heating season. On the other hand, the heat transfer performance of the borehole heat exchangers tends to degrade with prolonged operation, leading to a notable mismatch between the source and building load. By way of optimal control, the energy efficiency of the hybrid ground source heat pump system is markedly improved with a maximum increase of 50% and 83.3% during the heating and cooling season, respectively. Additionally, the thermal imbalance effect of the borehole heat exchanger cluster underground is significantly mitigated, evidenced by the improved thermal balance indicator from 0.75 to 0.85 under cooling conditions and from 0.85 to 0.9 under heating conditions. Furthermore, the equipment group within the hybrid ground source heat pump system can be switched on and off adaptively. The load imbalance rate between the ground source and building demand is maintained below 25%, thus enabling on-demand cooling or heating supply.