Effects of Atmospheric Conditions on Energy Pile Performance-Paper

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The study examines how rainfall affects the heat-exchange performance of energy piles operating in unsaturated soil. Energy piles are foundation elements containing closed-loop pipes that transfer heat between a building and the ground. Because greater soil moisture generally increases thermal conductivity, rainfall may improve a ground-source heat pump’s capacity, although the benefit depends on how quickly water infiltrates the soil.<br /><br />Researchers investigated two full-scale piles at the U.S. Air Force Academy’s Energy Pile Education and Research Facility. They applied a steady thermal load using an 11 kW geothermal unit and monitored fluid inlet and outlet temperatures alongside rainfall and other weather conditions. Laboratory tests on site soil also measured thermal conductivity across a range of moisture contents.<br /><br />The lab results showed an approximately linear relationship between moisture and thermal conductivity, with a 30% increase in conductivity between the tested moisture limits. During the field study, light-to-moderate rain fell intermittently over four days. The piles’ heat-exchange performance began increasing several days later, consistent with the time needed for water to infiltrate to deeper soil. Compared with pre-rain conditions, the maximum outlet–inlet temperature difference increased by 22% for one pile and 23% for the other. Enhanced performance persisted for days after the rain.<br /><br />The findings indicate that ordinary rainfall—not just extreme storms—can temporarily improve energy-pile performance in permeable, unsaturated soils. The timing and duration of this benefit depend on soil properties and moisture movement. The authors recommend further research combining water-flow and heat-exchange models to predict the magnitude and persistence of these gains. The results are most directly applicable to the site’s silty sand and may extend to soils with similar hydraulic and thermal properties.

Keywords

rainfall

energy piles

heat exchange

unsaturated soil

soil moisture

thermal conductivity

ground-source heat pump

geothermal system

silty sand

infiltration