Numerical investigation on thermal performance of ground heat exchangers using phase change materials as grout for ground source heat pump system

In order to solve the increasing deterioration of the energy shortage problem, ground source heat pump (GSHP) systems have been widely used. The ground heat exchanger (GHE) is a significant component of GSHP systems and the backfill materials have a dominating effect on thermal performance of GHEs. Therefore, phase change materials (PCM) were proposed to use as backfill material instead of common materials to improve the performance of GSHP systems. This paper first established a finite element model to investigate thermal performance of GHE, which was verified by experimental data of other researchers. Evaluation methodologies, such as effectiveness and thermal effects radius, were then proposed. The thermal performance of four backfill materials, including soil, paraffin RT27, acid and enhanced acid, was discussed, respectively. Finally, the effects and the combined effects of initial ground temperatures and pipe spacing on thermal performance of GHEs were studied. The results show that PCM has certain advantages over ordinary soil, because of its small thermal effects radius and consistent temperature in the phase change process. Meanwhile, the lower initial ground temperature and the larger pipe spacing of U-tube were beneficial to heat transfer, while it increased thermal effects radius and decreased liquid fraction of PCM. Furthermore, the thermal effects radius growth rate with pipe spacing increased with an increase in initial ground temperature.