Investigation on the thermal response of steel pipe energy piles with different backfill materials

The use of geothermal energy piles (GEPs) associated with ground source heat pump systems is a sustainable and cost effective technology to heat and cool buildings, based on the efficient application of available resources found at the building site. Currently, a new building with GEPs is under construction at the University of São Paulo campus in São Paulo City, Brazil. Part of the building loads will be supported by steel pipe piles equipped with single U-type absorber pipes for heat exchange. To find the optimum solution of pile backfill material in terms of cost, constructability, sustainability and thermal performance, field thermal response tests were conducted on 4 instrumented piles filled with different materials: water, saturated sand, grout, and steel fiber grout. Both analytical and numerical models were used to evaluate the tested alternatives. The results showed that the thermal performance of the 4 piles is similar; however, the costs and sustainability aspects (low CO2 emissions) of the solutions using water or saturated sand imply that they are more advantageous than those using grout. Additionally, the experiments showed that for the pile backfilled with water the convection effects have improved the heat transfer to the soil.