Experimental and numerical investigation on a new type of heat exchanger in ground source heat pump system

Based on deficiencies of existing underground heat exchangers, a new design is proposed which consists of one outlet and three inlet pipes. Experimental measurements to evaluate the performance of single-U, double-U, and the new design referred to as 3I, for three inlet pipes, were carried out which showed superior performance of the 3I design. Experimental results showed that when under a constant heating load, the average circulating water temperature (ACWT) of the 3I-type was 3.7 and 1 °C lower than that of single-U and double-U, respectively. The thermal resistance of the 3I-type was 15.8 and 31.1 % lower than that of double-U and single-U type. In addition, numerical simulations were performed to further investigate the heat transfer performance over a very long operating time. Numerical results showed that for long-term operation under constant heating load, the ACWT of the 3I-type was 6.6 and 5.8 % lower than that of double-U type. Under a constant inlet water temperature that mimics a condenser output, the heat exchange per unit borehole depth of 3I-type was 17.1 and 11.6 % higher than that of double-U type in different operation conditions.