Impact of soil surface finishing on horizontal ground heat exchangers

Ground-source heat pumps (GSHPs) have been regarded as a sustainable energy technology for space heating and cooling of commercial, industrial and residential buildings. In closed loop plants, the heat pump is linked by means of piping with ground heat exchangers (GHXs), which may be installed vertically or horizontally. In horizontal technologies, the GHXs are placed inside shallow trenches, few meters deep in soil. Unlike in vertical solutions, the GHXs are installed in boreholes drilled down up to hundred meters. Taking into account the different depth, the vertical technology really exploits a steady geothermal source/sink, while the horizontal solution uses an unsteady source/sink, due to the seasonal shallow energy balance. So, the finishing of the soil surface could be an important factor to improve the energy performance in horizontal applications. To evaluate the significance, a numerical approach is pursued to assess the energy performance of a shallow GHX. Here, a novel type of GHX is analyzed, consisting of a flat panel installed horizontally and edgeways at shallow depth. A 2D finite elements domain is composed for solving the heat transfer problem with two different soil surface finishing: grassland and asphalt pavement. The domain is implemented with specific thermal properties for soil and finishing, as reported in literature. For simplicity, all aspects related to the water are neglected, such as heat latent and rain. Monthly boundary conditions are set to represent the energy balance at the soil surface, the GHX energy requirement for heating and cooling and the deep thermal energy exchange. The energy balance at soil surface is introduced taking into account the solar radiation and the natural convection, which are strictly linked to the emissivity and reflectivity of the finishing type, the convective heat transfer coefficient and the air temperature. The GHX energy requirement is defined by means of a thermal power operating in wintertime and summertime, and representing the heating and cooling monthly requirement. The deep thermal energy exchange is introduced by mean of a fixed temperature set at the bottom of the domain, which is taken ten meter deep from the soil surface. The two models have run for five years to check the presence of a yearly thermal drift, owing to the GHX exploitation. The results show that no difference are evident between the maximum and minimum temperatures achieved by the GHX; only a different distribution of the thermal field and a different surface temperature are present. The asphalt pavement seems to operate like an insulating layer, and its shallow underground temperatures are on average warmer. Unlike for the grassland, the temperatures are cooler due to its lower insulation in wintertime and its higher reflectivity in summertime. The underground thermal field shows a time shift and some minimal differences, that do not impact on the leaving temperature at the GHX