Abstract
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.
Highlights
Heating and cooling buildings consumes a large quantity of electricity that is often sourced from fossil fuels across the world
Taylor and Cavazza [2] performed laboratory scale experiments in which temperature gradients were imposed on unsaturated silt to observe flow of water vapor from warm to cool regions accompanied by a return flow of liquid water from cool to warm regions
The helical geometries resulted in less uniformly heated foundations and lower heat flux at the foundation boundary compared with the U-tube, but the differences between the U-tube geometries and their equivalent length helices were less than 1°C
Summary
Heating and cooling buildings consumes a large quantity of electricity that is often sourced from fossil fuels across the world. Energy foundations exchange heat between structural elements of foundations and surrounding soil for heating and cooling of buildings They function by circulating a fluid through polyethylene pipes installed with different configurations within foundations, including U-tube and helical configurations. Philip and de Vries [3] formulated the coupled heat transfer and liquid water and vapor flow in unsaturated porous materials based on those driving forces They found that water transfer is low in very dry or very wet media and maximized at some intermediate water content. [3] noted significant latent heat transfer by vapor condensation in soils at intermediate water contents Baser et al [6] and Baser and McCartney [7] investigated the role of coupled heat and moisture transfer in unsaturated soils in the context of shallow geothermal systems through numerical analysis, tank-scale experiments, and full scale field-scale experiments and concluded that consideration of coupled thermal and hydraulic material properties and heat transfer mechanisms was important
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