A Field Experiment to Study the Influence of Heat and Mass Transfer at the Soil Surface on Shallow Ground Heat Exchanger Performances

Abstract

Soil temperature is governed by two kinds of processes: (a) energy exchange/transfer at earth-atmosphere boundary (b) heat propagation within the soil which can vary substantially with moisture content. Near-surface and moisture transport effects on soil temperature distribution are usually neglected or have been addressed by relying on extreme simplifications during the design and sizing of shallow ground heat exchangers such as horizontal ground heat exchangers. The present study aims to investigate near-surface processes related to ground temperature variation during soil wetness and dryness to allow a better design of shallow ground heat exchangers. A field experiment was performed at the BRGM geothermal energy test facility in France (Orléans). The soil, excavated up to a depth of 2 m, has been homogenized and uniformly compacted. Continuous measurements of parameters needed to fully evaluate energy and moisture balance were carried out in situ. Energy balance components at the soil surface was monitored using a meteorological station, that included a pyrgeometer and a pyranometer to measure short and far infrared radiation, and using two heat flux plates installed in the soil at a depth of 0.08 m to measure ground heat fluxes. A 2.5 m deep trench has been dug in order to (i) characterize soil hydraulic and thermal properties at different depths and (ii) install tensiometers and thermocouples allowing continuous measurements of soil water tension and soil temperature. Ten temperature probes were buried at 0.03, 0.06, 0.14, 0.2, 0.3, 0.5, 0.1 and 1.5 m. Measurements were recorded every ten minutes. The monitoring data were used to investigate the interactions between heat transfer and saturated/unsaturated moisture transport caused by solar radiation, rainfall, evapotranspiration and water table so as to determine the key processes controlling ground temperature and moisture distribution. From the results of our study, recommendations are proposed to improve the design of shallow ground heat exchangers: modification of the soil cover, installation in a more or less sunny area, optimization of the exchanger geometry, adaptation exchanger implantation depth.