Heat recovery of exhaust air from HVAC using plate-fin harmonica-tube oscillating heat pipes

To meet the low-carbon transition of energy consumption, the waste heat recovery using heat pipe heat exchangers has been considered an effective approach to enhance the energy-saving performance of heating, ventilation, and air conditioning (HVAC) systems, particularly in large public buildings. In this paper, we developed a novel exhaust air heat recovery system utilizing two identical plate-fin harmonica-tube oscillating heat pipes (OHPs), and refrigerant R134a was employed as the working fluid at an optimal filling ratio of 50%. At fresh-air temperatures of 1-2 °C, 2-3 °C, and 6-7 °C, the effective thermal conductivities of these two OHPs close to the exhaust air inlet and outlet can exceed 19 and 3 kW/(m·K), respectively, at an exhaust air inlet temperature of 24 °C. The competitive behavior in effective thermal conductivity was evidenced for these two OHPs and enhanced the overall heat recovery performance. The heat recovery performance shows sensitivity to variations in fresh-air temperature, and the reduction in fresh-air temperature increases the energy-saving ratio of the heat-recovery system. Additionally, a linear empirical correlation was developed relating to the energy-saving ratio to the fresh-air temperature. Further predictive analysis suggests that the energy-saving ratio can reach 47%-58% in cold regions and 71%-77% in extremely cold regions with outdoor air temperatures below −15°C. The harmonica-tube OHP provides a feasible low-carbon and high-efficiency technology for sustainable buildings, offering a technically viable pathway for air-to-air heat recovery in winter.