Energies, Vol. 19, Pages 1523: Coupled Heat–Moisture Effects of Initial Soil Water Content on Seasonal Underground Thermal Energy Storage with Coaxial Borehole Heat Exchangers
Energies doi: 10.3390/en19061523
Authors:
Haitao Wang
Dianli Ye
Jianjun Zhang
Bingyan Dong
Engineering sizing of seasonal underground thermal energy storage (SUTES) systems remains constrained by the complex coupling of heat and moisture transport in unsaturated porous media. Neglecting these coupling effects can lead to significant errors in the design of borehole length and spacing. This study presents a three-dimensional numerical investigation of a coaxial borehole heat exchanger (CBHE) field over a full annual cycle, including storage, transition, extraction, and recovery stages. A coupled heat–moisture transfer model for the soil–CBHE system is developed and validated against experimental data, yielding mean relative errors of 6.8% for temperature and 7.7% for volumetric water content. The model is then used to quantify the sensitivity of SUTES performance to the initial volumetric water content (θ0). Increasing θ0 from 0.20 to 0.40 m3·m−3 enhances the average heat injection rate per unit depth by 6.6% (from 53.84 to 57.39 W·m−1) and the heat extraction rate by 7.1% (from 23.73 to 25.41 W·m−1). This enhancement is primarily attributed to increased effective thermal conductivity and heat capacity, together with moisture migration and the associated latent-heat effects within the soil matrix. While the variations in seasonal energy and exergy efficiencies are within 1 percentage point, radial soil-temperature uniformity and effective heat diffusion are significantly improved in moister soils. These findings clarify the coupled transport mechanisms in borehole seasonal storage and provide engineering guidance for sizing CBHE fields in unsaturated formations.