Application of Soil Physics to Improve Efficiency of Ground-Source Heat Pumps in Fractured Saprolite.

The efficiency of ground-source heat pump systems (GHPs) is determined by the effective thermal properties of soil or rock and their ability to store and remove thermal energy for heating and cooling buildings. In temperate climates these systems typically result in energy savings up to 70 percent, effectively reducing greenhouse gas emissions and energy costs. This study used the Mobile-Immobile Model (MIM) to quantify the effective thermal properties of saturated, fractured saprolite from the Piedmont Geomorphic Province in Southeast Pennsylvania, and to evaluate how GHPs could be improved to enhance the benefits of fractured soils. Laboratory thermal experiments were conducted using large (20-cm diameter and 30-cm length) undisturbed columns of saprolite collected from a depth of 2 m. Iron-stained fractures were visible in the samples with a mean fracture spacing of 5 cm. Soil water was passed through the columns under a unit hydraulic gradient with a constant input temperature of 30 °C. Temperature was monitored by thermocouples at five locations in each column. The MIM was fit to thermal “breakthrough” curves, providing estimates of hydraulic conductivity, thermal conductivity, and specific heat capacity of 0.003 cm/s, 2.6 W/m °K, and 920 J/kg °K, respectively. Measurements of thermal flux using the MIM demonstrated that mobile pores (fractures) accounted for 80 percent of the heat transport under the conditions established in the laboratory. The net benefit of the fractures is to increase the efficiency of the GHPs by transferring heat to and from numerous immobile blocks. We conclude that a) the MIM is effective at simulating thermal transport in these systems, and b) the effect of fractures in saprolite may be enhanced to improve the efficiency of GHPs by increasing heat transport in soil water.