Ground-source heat pumps : benefits of using phase change materials

Abstract

Ground-source heat pumps (GSHPs) have been regarded as a sustainable energy technology for space heating and cooling in commercial, industrial and residential buildings, as well as a profitable solution when correctly designed. Coupling a heat pump with the ground is obtained by means of ground heat exchangers (GHXs), which can be installed vertically or horizontally. In the horizontal installation, the heat exchangers are placed in shallow diggings a few meters deep in soil, as opposed to the vertical solution where the heat exchangers are installed in boreholes drilled down up to a hundred meters deep. Owing to their different depths of installation, the vertical solution exploits a real geothermal source, while for the horizontal one, the ground source may mainly serve as a solar energy buffer. However, the weakest link in a GSHP system is the GHX, because the heat transfer in the ground is mainly conductive and its thermal diffusivity is also low. This means that the ground thermal response is much slower than the heat pump behaviour, resulting in transfer of thermal waves to the ground through the GHXs by means of the closed loop. This may cause lower COP at the GSHPs. Employing Phase Change Materials (PCMs) is an effective measure to store thermal energy [1,2] and it may also be considered as an effective method to smooth the thermal wave generated from operation of a GSHP. The approach is known when the PCMs are introduced directly in a tank within a closed loop, especially for vertical closed loop. However, use of a tank containing PCMs could be an expensive solution for the horizontal closed loop GHXs system, due to their low energy performance. Moreover, the heat transfer may not be effective for the bulky PCM tank. So, we have proposed to mix the PCMs directly with backfill material, which is close to the GHXs or install them in a surrounding shell. There is little research reported in literature about this idea, and the performance is not yet well investigated [3,4]. Use of the PCMs incorporated with GHXs may meet some instantaneous heating demand by a GSHP, thus reducing the sudden heating or cooling wave upon the ground. Therefore, the peak temperature would be lower with an equal GHX length, or the GHX length could be shorter with an equal peak temperature. Moreover, the depletion of the latent heat due to the full solidification is regenerated during the summer season, which increases the underground thermal energy storage. We are currently analysing the performance of a novel GHX design with PCMs by means of an experimental setup and a numerical approach. The latter is presented here.