Modeling and Optimization On Underground Thermal Storage For Large Scale Borehole Heat Exchangers Cluster

Borehole heat exchangers (BHE) have great potential in seasonal thermal storage by taking advantages of large storage capacity underground, especially in cold climate areas where the buildings are heating dominated. Moreover, due to the load imbalance between heating and cooling, surplus cold or heat accumulation near the borehole would appear in actual project operation, which might negatively affect the thermal performance of ground heat pump system (GSHP). In order to maintain sustainable and high efficiency operation of GSHP, proper modeling and optimization is important in practice so as to fully explore the potential of BHEs in thermal storage. Particularly, there is an absence of available methods and guidance for the swift simulation of complex heat transfer between large BHEs cluster, which is essential for on-line control to implement optimal thermal storage strategy. In this paper a numerical model adopting convolution kernel function is presented. The model is created for efficient simulation of both pure heat conduction and advection with ground water in saturated porous medium. Typical operation of the large BHEs cluster is simulated for two years, which are installed with shallow and deep BHEs operating in a shifted storage cycle. Dynamic temperature field underground is shown and discussed. Moreover, thermal interaction characteristics is depicted that may occur between adjacent BHEs, and the corresponding reduction in available heat extraction or storage is also quantified. Finally, the energy injected into and extracted from the ground is compared with the case without optimization, it was found that overall operation efficiency of BHEs cluster is significantly improved.