Thermal response of energy foundations installed in unsaturated residual soils

This study focuses on the thermal response of energy foundations with different piping geometries installed in unsaturated soil. Energy foundations are an efficient alternative to traditional space heating and cooling approaches and can reduce energy demand for air conditioning in Brazil, where unsaturated residual soil deposits are abundant. A three-dimensional numerical model for heat transfer and subsurface flow is first validated against field data from a thermal response test at the University of São Paulo. The model is then used to compare the performance of triple and quadruple U-tube piping geometries and helical piping geometries of equivalent length. The helical geometries resulted in initial less uniformly heated foundations and lower heat flux at the foundation boundary compared with the U-tubes, but the differences between the U-tube geometries and their equivalent length helices were less than 1°C. All piping geometries exhibited increased heat output as the length of heat exchanger piping increased. The infinite line source solution was compared with the model results. The infinite line source solution underestimated the thermal response of the system during the first 25-30 days and overestimated it afterwards.

Changes in shaft resistance and pore water pressures during heating of an energy foundation

This study focuses on the evolution of shaft resistance during operation of a geothermal energy foundation installed in a saturated glacial till layer. Energy foundations are a sustainable alternative to traditional space heating and cooling approaches for buildings. Despite efficient operational performance, there are still valid concerns regarding the effects of heating on the structural performance of foundations. To investigate the effect of heating at the soil-pile interface, four drilled shafts are utilized as energy foundations on the Urbana-Champaign campus of the University of Illinois and instrumented. Although the energy foundations are not yet operational, a theoretical investigation is possible to understand the effects of heating on the evolution of thermally induced pore water pressures and the shaft resistance of an energy foundation. A thermo-poroelastic numerical model is validated against an analytical solution, then is used to analyze the thermo-mechanical response of the soil-structure system under different conditions. The results indicate that the evolution of pore water pressure is affected by the rate of heating and the hydraulic conductivity of the surrounding soil, as expected. Higher pore water pressures are generated in the case of low hydraulic conductivity and higher rates of heating. Prior to the dissipation of excess pore pressures, the changes in shaft resistance are variable and influenced by the thermally-induced deformation of the foundation and the surrounding soil.

NUMERICAL SIMULATION OF THE PROCESS OF GEOTHERMAL LOW-POTENTIAL GROUND ENERGY EXTRACTION

The aim of our research is to study the interaction of energy foundations with the ground mass and to develop methods for their construction on the example of the city of Perm. Field studies of ground were carried out in a specially chosen pilot site to determine temperature distribution in the ground mass, change of ground-water level and physical-mechanical and thermal-physical characteristics of the ground mass. The diagrams of depth temperature distribution in the ground and its seasonal variations were obtained on the results of monitoring, and also the average groundwater level. To carry out numerical simulation, software-complex “GeoStudio” was selected. Its basic differential equation is the fundamental heat conduction equation with an internal heat source. The purpose of the numerical simulation was quantitative evaluation of the thermal energy extracted from different energy foundations under soil conditions in the city of Perm. By results of the spent numerical experiments the equations of regress and nomographs dependences of size of received thermal energy on geometrical parameters of the projected power bases to hydro-geological and climatic conditions of the Perm region are constructed