Experimental study on space heating performance of PVT-air dual heat source direct expansion heat pump system based on micro heat pipe arrays

To address the growing demand for efficient and renewable heating, a novel dual source direct expansion PVT-air heat pump system integrating a PVT-air evaporator based on micro heat pipe arrays is proposed. This design overcomes limitations of conventional flat-plate PVT evaporators, such as high-pressure loss, low solar energy utilization and unstable system energy supply. The system was experimentally evaluated under winter space heating conditions. Results showed a maximum thermal efficiency of 35%, power generation efficiency of 17%, COP of 3.52, and exergy efficiency of 21% under average solar irradiance of 654 W/m2, ambient temperature of 5.1 °C, and wind speed of 2.6 m/s. COP increased by up to 12.5% and exergy efficiency by 43.4% per 100 W/m2 rise in solar irradiance, while a 5 K rise in load water temperature reduced COP by 8.4% and exergy efficiency by 4.7%. The system demonstrated stable switching between solar and air heat sources. Compared to the conventional tube-and-sheet PVT direct expansion heat pump, the proposed system achieved 4.5%, 33.5%, and 20% improvements in COP, power generation efficiency, and COPPVT, respectively. The dimensionless pressure loss of the PVT-air evaporator based on micro heat pipe arrays was also 87% lower than that of the fin-tube heat exchanger. These results confirm the system's excellent performance, reliability, and potential for practical heating applications.