Study of Performance on Recharging the Borehole by Means of Exhaust-air Energy

In this paper, the performance analysis of recharging the borehole by means of exhaust-air energy is carried out. The results show that a vertical borehole used as heat source for a Ground Source Heat Pump (GSHP) can be recharged in high efficiency. With equal heat transfer capabilities of exhaust-air coil and borehole collector, the system provides a maximum overall efficiency. However, due to ground infinite capacity, the optimum brine flow rate is different from conventional twoexchanger system. The recharging system provides two peak overall efficiencies when the capacity ratio Cr = 5 for laminar flow and Cr = 15 for turbulent flow respectively.The overall efficiency is independent of exhaust-air temperature and undisturbed ground temperature,although the fluid properties depend on temperature. In practical system lower ethyl percentage brine should be chosen if the freezing point meets the system request,which can provide a higher overall efficiency.