Numerical modelling of a building integrated earth-to-air heat exchanger

Abstract

This paper investigates an innovative earth-to-air heat exchanger (EAHE), namely the geothermal foundation Fondatherm®. This system solves numerous technical problems inherent to a traditional EAHE. However, an accurate understanding of its thermal behavior is crucial to ensure the best energy gain and its economic viability. This current paper focuses on a numerical study of the ventilated foundation. A time-dependent three-dimensional finite volume numerical model is developed. This model considers coupled heat and moisture transfers as well as complex boundary conditions such as the weather, the building and the water table simultaneous influences. A matric-head based system of equations is used to represent the heat, vapor and liquid transfers within the soil. It is combined to a capillary pressure-based formulation of the heat and vapor balance equation for the concrete foundation. Furthermore, a numerical method built on a switching from a potential- to a flux-based system of equations is proposed. It is added to a variable time-step method of resolution and allow to handle the greatest hygric loads variations at the ground surface. The numerical code is successfully validated against analytical solutions for simple cases, and against experimental and in situ measurements.