Optimal operating strategies for borehole thermal energy storage

Borehole thermal energy storage (BTES) provides an approach to decarbonizing space and water heating through seasonal storage of waste heat. Different operating strategies, resulting in different thermal load distributions among boreholes, can be employed for the BTES. This work determines the optimal operating strategy, defined by the flow arrangement and distribution scheme, to employ for maximal storage performance, depending on the size of the available waste heat. The assessment is carried out using a newly developed, efficient 2-D finite volume heat transfer model that enables mathematical optimization of flow distribution in zoned BTES. It is shown that when the available waste heat is less than 70 % of the BTES design load, a serial flow arrangement with radial distribution yields the best storage performance, with up to 6.8 % higher utilization efficiency than the best-performing parallel arrangement strategy. For higher load scenarios, equal flow distribution in a parallel arrangement is shown to provide optimal storage performance, up to 2.9 % higher than operation under a serial arrangement. The results also highlight that maximizing heat transfer within boreholes is more critical to effective BTES performance than minimizing conductive heat loss from the system.