Feasibility Study of a Novel Ground Heat Exchanger using Phase-Change Materials

The performance of a novel ground heat exchanger, the Underground Thermal Battery (UTB), was investigated in this paper. The UTB is an alternative to the conventional ground heat exchanger (GHE) which is designed to be installed in the shallow subsurface of the ground (less than 20 ft. deep). This can reduce the cost of drilling associated with the installation of boreholes in the conventional vertical bore ground heat exchanger (VBGHE). The UTB is a tank, 2-3 ft. in diameter, which is filled with water. The tank has a large thermal capacity which reduces the temperature response of the tank for a given thermal load. PCM is suspended in the tank water to further increase the thermal capacity. A simplified 1-D model has been developed to simulate the UTB over a year of operation. This model was validated against a 3-D CFD model. The performance of the UTB is compared with the conventional VBGHE under various operating conditions. The results indicate that a UTB with two tanks can offer similar performance as the VBGHE under most conditions. Simulation results indicate that, by integrating with UTB with the lawn irrigation system of a residence, the performance of the UTB is improved and thereby the size needed to meet the thermal demands can be reduced without any additional water consumption. A preliminary cost analysis suggests that the UTB may offer significant cost savings potential over the conventional VBGHE.