Characteristics of heat fluxes in subsurface shallow depth soil layer as a renewable thermal source for ground coupled heat pumps

Abstract

The soil heat fluxes (G) in subsurface shallow depth soil layer (SSDSL) are characterized by dynamics of diurnal, monthly, seasonal and annual behaviour. The soil heat very often is underestimated as a part of heat balance of the pedosphere, because its values are relatively small in comparison with sensible turbulent heat and latent heat fluxes. These fluxes concern mainly with net solar radiation and natural thermal properties of soils and plant cover. The soil heat in SSDSL is jointed with commonly widely available and relatively easy acquiring energy resources. This paper aims to present the thermal regime of heat fluxes in SSDSL and its climatic conditions in central Europe, taking into consideration the agriculture periphery characteristics of Wroclaw (Poland). The measurement results gathered across ten years (August 2007–July 2017) and the conducted thermal analysis aim to evaluate the utilisation of SSDSL as a heat source for ground coupled heat pumps (GCHP). The measurements of G (positive soil heat fluxes G > 0 and negative G < 0) were done between the active surface (for both of bare soil and grassy area) and their lower layers on an 8 cm depth. The soil temperature data between 5 cm and 10 cm below the ground and the temperatures of the active soil surfaces are also represented and discussed. The ten-year annual average sums for G > 0 reached 331,8 MJ∙m−2 (92,2 kWh∙m−2) for the bare soil and 179,0 MJ∙m−2 (49,7 kWh∙m−2) for the grassy area. The similar ten-year values for G < 0 reached 330.3 MJ∙m−2 (91.8 kWh/m2) for the bare soil and 170.6 MJ/m2 (47.4 kWh/m2) for the grassy area. The heat fluxes of the 5 cm depth soil were higher than for the 8 cm and 10 cm soil. The obtained results fortify the importance and engineering role of the soil heat fluxes in SSDSL. The current results of the thermal analysis and detailed clarifications of the soil positive and negative heat fluxes confirms the energy potential of SSDSL as an environmental friendly, efficient heat source for operation of GCHP through the year.