Impact of soil moisture on the long-term energy performance of an earth-air heat exchanger system

The soil moisture contents affect significantly the soil thermal properties and consequently the thermal efficiency of shallow geothermal systems. This effect becomes more complex to be evaluated for an Earth-Air Heat Exchanger (EAHE) because of its non-stable energy performance due to a large fluctuation of the temperature of air. In this study, the impact of soil moisture content and soil thermal properties has been investigated on the long-term energy performance of an instrumented EAHE site. First, a full-scale experimental EAHE site in University of Strasbourg as well as its measured data are presented. The thermal properties of different soil layers present in the site were experimentally and theoretically characterized with different soil moisture contents. Based on these results, an analytical solution was proposed to simulate the soil temperature of the field and the output air temperature of the EAHE. A computer program based on this analytical solution was developed to assess the performance of the system for a period of three years. The numerical calculation was validated for an average saturation condition by comparing simulation results with measured data. Different soil saturation conditions were also used in the numerical simulation to consider the effect of soil moisture on the system performance. The results show that if the turbulent flow of the circulating air is fully developed, the difference of the exchanged energy could reach more than 40%. (C) 2018 Elsevier Ltd. All rights reserved.