Improving the long-term sustainability of photovoltaic thermal - ground source heat pump system by soil thermal balance regulation

The hybrid system combining photovoltaic thermal collectors (PVT) with ground source heat pump (GSHP) system has become a promising technology for realizing net-zero carbon emission buildings and PVT-GSHP system is capable to provide sufficient energy for building in cold region. However, the long-term utilization of soil thermal energy has seriously affected environment and system sustainable operation. Therefore, for exploring system design solution that achieves long-term stable operation, this work proposed an optimization method based on thermal imbalance (TIR), energy efficiency ratio (EER), life cycle cost (LCC) and carbon dioxide emission reduction (CDER). The system performance influence of PVT area, heat pump capacity, flow rate of PVT circulating pump and buried pipe drilling depth was studied. And the optimization research of the system in Beijing under long-term operation conditions was carried out. Besides, the weights of objective functions in the optimization and the system performance under different optimization cases were discussed. The results showed that when the solution considering TIR, TIR and LCC were reduced by 49.8%, 18.91% and EER, CDER were increased by 2.8%, 1.34 times when compared with no-TIR solution. Lower TIR ensures better comprehensive performance in long-term operation. Weight analysis showed that when TIR was considered, the weight distribution changed and the weight of LCC increased, which meant economic performance dominated in the optimization under long-term operation. The significance of this work proves the significance of soil thermal balance in PVT-GSHP systems and provide a practical method for PVT-GSHP system design.