The search for sustainable renewable energy matrices has motivated the use of geothermal piles to take advantage of deep building foundation structures to, in tropical climate countries, dissipate heat from building to soil layers. Heating with geothermal piles is a technique widely used by European countries, where severe winters make spending on heating system mandatory. The geothermal pile system is coupled to the Ground Source Heat Pump (GSHP) assembly and is based on the circulation of a fluid through a closed circuit of pipes connected to the pile and to the heat pump. The present work aims to study the thermal behaviour of geothermal piles in a saturated sandy soil using numerical simulations to identify the variables of greatest interest to the project. The numerical model was developed with the ANSYS CFX version 19.2 program, a high performance CFD (Computational Fluid Dynamics) tool. Data from a Thermal Response Test (TRT) test carried out at the Polytechnic School of the University of São Paulo were used to calibrate the numerical model. In the steady state simulations, the variables studied were: diameter and length of piles; thermal conductivity of soil, concrete and pipe; degree of soil saturation; system inlet temperature; and circulating fluid velocity. Fluid inlet temperature, pile diameter and pile length are the variables that cause the greatest increases in the total heat transfer rates. However, the variables pile length and pile diameter do not really affect the thermal efficiency of the system, as normalized heat transfer rates showed unimportant variation. Regarding the heat transfer rates per unit of parameter, the most relevant variables are contact resistance, degree of saturation and thermal conductivity of the concrete.

The worldwide consumption of electric energy destined for air conditioners, expected to triple by 2050, can be lessened by geothermal piles, which transfer heat from the internal environment of buildings to the subsoil. This paper shows the influence of pile geometry and properties of soil, pile, and pipe materials on the heat transfer of a geothermal pile to the surrounding soil, to support design from the viewpoint of thermal performance optimization. A numerical model was developed with ANSYS CFX 19.2, a high-performance Computational Fluid Dynamics tool, and calibrated using data from a thermal response test performed in a saturated sandy soil in São Paulo, Brazil. A parametric analysis was carried out varying pile length, diameter, and slenderness; soil and pile material conductivities; degree of saturation; fluid inlet temperature; fluid flow rate; and pipe thermal resistance. Results show that the fluid inlet temperature is the most influential parameter on the thermal performance of the pile. Heat transfer grows when geometrical parameters (diameter and length) are increased mainly due to an increase in heat exchange surface area, whereas the normalized heat transfer rate per unit of surface area of the pile is practically unaltered. Higher soil, pipe and pile thermal conductivities improve thermal performance. The degree of saturation increases the thermal conductivity of the soil; however, the effect is not remarkable on the system’s thermal performance for saturation degrees higher than 20%. The fluid flow must be turbulent but increases above a certain flow rate do not improve the thermal performance.

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