Energy storage and heat transfer characteristics of ground heat exchanger with phase change backfill materials

Abstract: Ground source heat pump (GSHP) has been recognized as being among the cleanest, most energy efficient and cost effective systems for residential and commercial space's heating and cooling applications. The main advantage of using the ground as the heat source or sink of the system is that the soil temperature at tens to hundreds of meters in depth is relatively constant and is generally lower in summer and higher in winter than that of ambient air temperature. This results in an overall improvement of the system performance and thus reduces operation costs. Therefore, GSHP systems have become increasingly popular in commercial and institutional buildings. Heat transfer around vertical ground heat exchanger (GHE) is a common problem for the design and operation of GSHP. The energy storage performance of GHE and its influences on the temperature thermal response characteristics of soil around it are important for a long-term high-efficient and steady operation of GSHP systems. Thus, to enhance energy storage performance of GHE and, at the same time, reduce the effects of thermal diffuse on soil temperature are key points for the application of GSHP. In this paper, a new type of GHE with phase change backfill materials was presented to change its thermal response characteristics and heat transfer performance. Theoretically, the thermal interference radius of soil can be reduced by the phase change of phase change materials (PCM), and the energy storage performance of GHE can be improved due to the release of phase change latent heat. In order to further investigate the influences of solid-liquid phase change of phase change backfill materials on energy storage and heat transfer performance of GHE, a quasi-three dimensional heat transfer model with phase change was developed for the vertical U-bend GHE, which couples the one-dimensional fluid heat transfer in vertical direction with the two-dimensional soil transient heat transfer in level. The model was discreted by the control volume method and solved by the apparent heat capacity method. Based on the numerical solution of the model, the influences of solid-liquid phase change of PCM on energy storage performance of GHE and thermal response characteristics of soil temperature around GHE were analyzed for winter and summer mode respectively. The effects of phase change temperature and phase change latent heat of PCM on the thermal diffusivity and energy storage characteristics of GHE were discussed. The results indicate that under same conditions, the soil temperature variations trend is slow down and the thermal interference region is reduced due to the heat extraction and release during the phase change of PCM. The heat exchange performance of GHE can be evidently improved by backfilling materials with low and high phase change temperature for summer and winter respectively. At the same time, the energy storage performance can be enhanced by grouting the materials with large latent heat. The study is significant for releasing the thermal interference region of soil and improving the energy storage and heat transfer performance of GHE.