EFFECTS OF SEASONAL VARIATIONS ON THE THERMAL RESPONSE OF ENERGY PILES IN AN UNSATURATED BRAZILIAN TROPICAL SOIL

Abstract A large amount of Brazilian electricity consumption is building related, and mainly consumed by air-conditioning systems due to the warm tropical weather; therefore, geothermal energy piles integrated with heat pump seem to be an interesting alternative for the building cooling demand in this country. Tropical residual soils in unsaturated condition cover a significant part of the Brazilian territory, and due to the alternation of dry and rainy periods the groundwater level and the gravimetric water content of the soils vary seasonally. Considering that the efficiency of energy piles as heat exchangers is highly dependent on soil thermal properties, which in turn rely on the soil saturation, to evaluate the seasonal effects on the thermal response of energy piles in a typical site of Brazilian unsaturated tropical soil, an extensive program of field tests and monitoring supplemented by laboratory work was carried out during a period of four years, and is reported in this paper. The results demonstrate that the thermal response of the energy pile investigated is variable throughout the year. The thermal conductivity of the soil surrounding the pile determined at the end of the rainy season can be reduced approximately 32% at the end of the dry season. This finding indicates that the seasonal variation of soil thermal properties should be considered in the design of GSHP systems with energy piles in similar soil and climate conditions.