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

Ground-source heat pumps (GSHPs) are a sustainable energy technology for space heating and cooling, obtained by coupling of heat pumps with the ground by means of vertical or horizontal ground heat exchangers (GHXs). The horizontal GHXs are commonly placed in shallow diggings few meters deep in soil, as opposed to the vertical deep solution where the GHXs are installed in boreholes drilled down up to hundred meters. Owing to their different depths of installation, the vertical solution exploits really a constant geothermal source, while the horizontal one profits from temporary underground solar energy storage. At the moment, the vertical solution is commonly more efficient than the horizontal one, but novel prototypes are improving the performance of shallow installations. For both cases, the weakest link of a GSHP system is the GHXs, because the heat transfer from the ground is mainly conductive and its thermal diffusivity is low as well. It means that the ground thermal response is slower than the heat pump requirement, which transfers thermal energy to and from the ground through the GHXs. Employing phase change materials (PCMs) may be considered as an effective measure to smooth out the thermal cycles of a GSHP system and improve the underground thermal energy storage. The approach is well known for vertical systems, where the PCMs are introduced directly in a tank within the closed loop. However, use of a tank containing PCMs could be an expensive solution for horizontal shallow systems, and also heat transfer may not be as effective for the bulky PCM tank. So, we have proposed to mix encapsulated PCMs directly with backfill material adjacent to the GHXs or installing them within a surrounding shell. There is little research reported in literature about this approach, and the performance is not yet well known. Use of the PCMs incorporated with a GHXs may meet some instantaneous heating demand by a GSHP and reduce the abrupt heating or cooling surge upon the ground loop coil. Therefore, the peak temperature should be lower with an equal GHX length, or the GHX length should be shorter with an equal peak temperature. Moreover, unlike the vertical GHXs, the energy exploitation given by shallow GHXs is periodically reset by the solar heat transfer over the soil surface and no underground thermal energy storage is possible. Nevertheless, this energy storage can be recovered using PCMs, owing to their natural properties of melting and solidification.