Numerical investigation of using a field of shallow boreholes with latent heat storage and solar injection in cold climates

Abstract

Modeling heat transfer within a field of boreholes (borefield) is essential to correctly assess the long-term performance of a solar-assisted ground source heat pump, which uses the ground to store the injected solar thermal heat. A novel model was developed that uses a hybrid approach, combining a numerical thermal resistance and capacitance model for the borehole with an analytical g-function for the ground. The developed model has the capability to simulate the long-term performance of a borefield with the phase change that is occurring in the water content of the grout. The model is used to assess the benefit of phase change within a saturated sand mixture, acting as filling material in the borehole and thermal energy storage for solar heat injected in the ground. The injected solar heat compensates the ground thermal load imbalance caused by substantially higher heating demand than cooling demand in cold climate regions. Through long-term hourly simulations, results show that water content in the borehole helps keep the temperature constant at the vicinity of the borehole, particularly during peak hours, and that solar injection permits the design of a more compact borefield by concentrating the stored heat in the vicinity of the boreholes.