Hybrid photovoltaic/thermal and ground source heat pump: Review and perspective

Ground source heat pump (GSHP) is widely studied for building energy efficiency but suffers from soil thermal imbalance and performance deterioration in heating-dominant regions. Photovoltaic (PV) collector is commonly used for renewable energy, but the efficiency is constrained by the PV module temperature. The combination of GSHP and photovoltaic/thermal (PVT) is promising to improve the performance of both individual technologies. A comprehensive review has been conducted to present the state-of-the-art of the hybrid PVT-GSHP, in terms of the principles, configurations, and functions. The basic PVT-GSHP systems are classified into four categories: the hybrid PVT-GSHP with PVT for direct heating, the hybrid PVT-GSHP with PVT for temperature increasing, the hybrid PVT-GSHP with multiple energy sources, and the hybrid-GSHP with energy storage/borehole recharge. These hybrid systems can be further combined to achieve advanced hybrid PVT-GSHP systems with more functions and improved performance. For the hybrid PVT-GSHP with PVT for direct heating, preheating and full heating have been used; the preheating yields a higher PV efficiency while the full heating recovers more heat. The hybrid PVT-GSHP with PVT for temperature increasing is more widely adopted, with both PVT electrical efficiency and GSHP heating efficiency significantly improved. For the hybrid PVT-GSHP with multiple energy sources, the current systems were limited to the integration of an air source heat pump as an additional heat source or a cooling tower as an additional heat sink. The hybrid PVT-GSHP with energy storage/ground recharge received the most intensive investigations owing to the reduced thermal imbalance and thus enhanced long-term performance. While most studies used normal flat-plate PVT, advanced collectors including concentrating PVT, building-integrated PVT, and solar-road PVT have also been studied. To facilitate performance improvement and application promotion, some perspectives on future development are presented: (1) advanced ground heat exchangers, i.e., energy geo-structures including the energy pile, energy wall, and energy tunnel; (2) advanced PVT types, i.e., more involvements in the high-temperature PVT and building-integrated PVT; (3) advanced hybrid systems, including driving sorption chillers, regenerating desiccant dehumidifiers, charging thermal batteries; and (4) optimal design and operation, considering local soil properties (e.g. seepage) and climate conditions through transient modeling. This study can facilitate future research, development, and promotion of the hybrid PVT-GSHP technology towards low-carbon buildings.