Performance of ground-source heat exchangers using short residential foundation piles

Experiments on the performance of ground-source heat exchangers using short residential foundation piles were performed in Saga, Japan. U-tube, double-tube, and multitube heat exchangers were installed in the ground to depths of 20 m. Water was used as the working fluid. Temperatures were measured on pipe walls, pile walls, and in the ground. From the measured data, thermal resistances of pipes, grout, and soil were calculated. We also investigated the effect of the ground surface temperature on heat transfer rate. Total thermal resistances of the double-tube, multitube, and U-tube exchangers were 0.231 (m K)/W, 0.295 (m K)/W, and 0.356 (m K)/W, respectively. Of these total resistances, soil, grout, and pipe thermal resistances accounted for 89%, 0%, and 11%, respectively, for the double-tube heat exchanger; 50%, 27%, and 23%, respectively, for the multitube heat exchanger; and 62%, 21%, and 17%, respectively, for the U-tube heat exchanger. Heat transfer rates of the tested heat exchangers could be predicted within 8.6% by using a measured value for the surface temperature coefficient. For atmospheric temperatures around 30 °C, the effect of the surface ground temperature on heat transfer rates was approximately 10%-15%.