Thermal performance analysis of multiple borehole heat exchangers

Effect of borehole spacing on Heat Transfer Rate per unit borehole length (unit HTR) is computationally investigated for multiple Borehole Heat Exchangers (BHE). Experimentally verified computational model is used to analyze different configurations consisting of various number of BHE. To determine the performance loss due to mutual thermal interactions of BHE, the averaged unit HTR value of the most critical borehole in each configuration is compared with that of a single borehole alone for various borehole spacing and operation durations of 1800 and 2400 h. Effect of thermal conductivity of ground on the relation between performance loss and borehole spacing is also examined. Furthermore, variations of total HTR values of a borehole field with borehole spacing are compared with each other for different configurations. It is seen that 4.5 m spacing is enough to keep the total performance losses less than 10% even for 2400 h non-stop operation. An analytical formulation is proposed for total HTR value which depends on thermal interaction coefficient (δ) as well as spacing and number of BHE (N). Dependence of δ on N is also examined for different operation durations. Variations of dimensionless HTR value with borehole spacing are analyzed for different values of N. The results can be used during the engineering design stages of a BHE field to predict the variation of total HTR value of the field with borehole spacing and number of boreholes.