Subsurface shallow depth soil layers thermal potential for ground heat pumps in Poland

Subsurface shallow depth soil layer (SSDSL) is characterized by dynamic temperature changes, which may be eligible to constitute as a heat source for ground heat pumps. This paper aims to present the regime of heat flow in SSDSL and its climatic conditions in central Europe, taking into consideration the agriculture periphery characteristics of Wroclaw (Poland). Emphasis was laid on the relation between the radiative properties and heat balance of two contrasting types of analyzed active surfaces (bare soil and grassy surface). The obtained experimental results gathered across a span nine years (August 2007-July 2016) and the conducted thermal analysis aim to evaluate the utilization of SSDSL as a heat source for ground heat pumps. The heat fluxes were measured between the active surface of the soils and their lower layers on an 8 cm depth. The soil temperature data between 5 cm to 10 cm under the ground and the temperatures of the active soil surfaces are represented. The results showed that the biggest monthly sums of ground positive fluxes (G > 0) reached about 80 MJ/m2 (22.2 kWh/m2) for the bare soil from May to July and the adequate average monthly sums reached maximum 55 MJ/m2 (15.3 kWh/m2) in July. The sums for the grassy surface were nearly twice lower, with the monthly maximum equals to 42.5 MJ/m2 (11.7 kWh/m2) in June and average value of 32 MJ/m2 (8.9 kWh/m2) in July. The nine-year average annual sums for G > 0 reached 337.6 MJ/m2 (93.8 kWh/m2) for the bare soil and 170.8 MJ/m2 (59.9 kWh/m2) for the grassy surface. The heat fluxes of the 5 cm depth soil were higher than for the 8 cm and 10 cm soil. Thus, the conclusion of the study suggests that the thermal performance of SSDSL is efficient enough to be used as an environment friendly heat source for interseasonal heat pump operation.