Configurational Effects on Borehole Thermal Energy Storage Performance

Borehole thermal energy storage (BTES) systems are promising technologies to provide thermal energy demands of buildings since they offer very high efficiencies for ground source heat pumps. In this study, the effects of the configuration of boreholes on BTES performance are studied. 10 vertical and 50 m depth borehole heat exchangers (BHE) are considered for the local climate conditions of Uppsala, Sweden. A circular configuration is considered for BHE, one at the center and also three and six on the inner and outer circles respectively. The charge benefit, configurational benefit factor and the storage efficiency are defined and their variations with both time and the radius of the outer circle are examined for the first five years of operation. For the radius larger or shorter than an optimal distance, the collective behavior of boreholes weakens and individual borehole character dominates the total behavior, which causes decrements in performance parameters. Furthermore, these parameters exhibit asymptotic temporal behaviors. All the performance parameters strongly depend on the configuration radius and the results may help to design higher energy efficient BTES systems with low initial and operational costs.