Thermal performance improvement of a horizontal ground-coupled heat exchanger by rainwater harvest

•Rain gardens benefit GCHE heat transfer apart from direct rainwater harvest.•Heat and water transfer were coupled under HP heat rejection condition.•Water immigration was more evidently to occur when sandy soil had low moisture content.•GCHE thermal performance predication needs coupled model when soil has low moisture.•For high moisture soil, moisture-dependent thermal conductivity variation is necessary.

Increased moisture content leads to increased thermal conductivity and is beneficial for heat exchange of ground-coupled heat exchangers (GCHE). In this study, a horizontal GCHE was combined with a rain garden, characterized by the beneficial supply of rainwater to groundwater, increased soil moisture content and favorable conditions for GCHE. Sandy soil container experiment results showed that water immigration was possible under thermal action of the horizontal GCHE. Compared with little water transfer under the heat evacuation condition, with a relatively small temperature difference between fluid and adjacent soils, heat and water transfer were coupled under the heat rejection condition and a drying region appeared at regions adjacent to the tube wall because of the relatively large temperature difference. Water immigration was more likely to occur when sandy soil had low moisture content, for example 0.1 m3/m3. The coupled heat and water transfer model was necessary for prediction of horizontal GCHE thermal performance. Under high moisture content conditions, the coupled model was dispensable but it was necessary to consider moisture-dependent variation in thermal conductivity.