Numerical study of a solar district heating system with photovoltaic-thermal collectors and pit thermal energy storage

Solar district heating (SDH) systems with large pit thermal energy storage (PTES) are key for future heat demands. Photovoltaic-thermal (PVT) collectors, efficient in converting solar radiation into both electrical and thermal energy, are promising for such systems. This study proposes an SDH system combining a PVT collector field and a PTES, utilizing a water-to-water heat pump as an auxiliary heat source. The system is modelled in the TRNSYS simulation platform, wherein the PTES component is a self-developed modified Type 343 and has been validated against measured data from the Dronninglund PTES. Simulation results show that, for the base case, the annual thermal, electrical, and combined energy utilization ratios of the PVT collector field are 15.39 %, 24.13 % and 39.52 %, respectively. The PTES achieves an impressive 88.38 % energy efficiency without side or bottom insulation. The annual solar thermal and electrical fractions measure 77.4 % and 236.8 %, respectively, signifying a positive energy gain for the system. The avoided global warming potential is 3692.3 tCO2/year. The study also explores the impact of collector area and storage volume on overall system performance. These insights provide valuable references for designers and practitioners involved in the planning of SDH systems that employ PVT collectors and PTES.