Underground solar energy storage via energy piles: An experimental study

Energy storage needs to account for the intermittence of solar radiation if solar energy is to be used to answer the heat demands of buildings. Energy piles, which embed thermal loops into the pile body, have been used as heat exchangers in ground source heat pump systems to replace traditional boreholes. Therefore, it is proposed to store solar thermal energy underground via energy piles. To investigate the performance of such systems, a laboratory-scale coupled energy pile-solar collector system was built for this study. Experiments were performed to evaluate the effects of various controlling parameters on the short-term performance of the system. These include the degree of saturation of the soil, the flowrate of the heat-carrying fluid, the intensity of solar radiation, and their interaction. The results showed that under abundant solar radiation, the daily average rate of energy storage per unit pile length increases by about 150 W/m when the soil condition changes from being dry to saturated, with a maximum value of about 200 W/m. As the intensity of solar radiation drops, it becomes the dominant factor. Compared to the laminar flow, the turbulent flow contributes more to the underground solar energy storage as the soil is more saturated. This suggests a technique to minimise the electricity consumption by the system and thus optimise its performance through regulating the flowrate. In addition, a mathematical model of the coupled energy pile-solar collector system was validated against the measurements. Long-term simulations in prototype using the validated model further confirm the above conclusions.