Impact of Surface Insulation Geometry on the Transient Performance of Borehole Thermal Energy Storage

The present paper is motivated by challenges in the design of the surface insulation in borehole thermal energy storage (BTES). A case study of a BTES with nine borehole heat exchangers (BHEs) in a cold climate is considered. Transient numerical modeling of the storage charging phase is performed by solving the three-dimensional heat equation using the finite difference method. Heat conduction through the insulation cover is simulated in accordance with Fourier’s law. A parametric study is conducted with respect to the prescribed heating setpoint temperatures in the BHEs and to the geometry of the insulation cover. The thermal analysis shows that the efficiency of the storage volume is strongly dependent on the heat transfer through the upper boundary. The insulation layer affects the minimum temperature reached within the BTES, with the influence of insulation thickness being most pronounced at thicknesses up to 10 cm. Furthermore, it is demonstrated that the lateral extension of the insulation cover has a greater impact on storage capacity gains than increasing its thickness, and that these energy gains expand progressively over time. Under cold ambient conditions, effective seasonal storage requires managing sharp ambient thermal gradients via a wider peripheral coverage of the insulation layer to offset vertical conductive losses.