A numerical study on the intermittent operation of u-tube and coaxial borehole heat exchangers

This study provides a comprehensive investigation into the impact of intermittent operation on two borehole designs: coaxial and u-tube heat exchangers. Previous studies have compared borehole heat exchanger (BHE) designs and others have investigated the impact of intermittent parameters; this study provides a systematic analysis of the intersection between intermittency and design. Using detailed numerical models of a coaxial and u-tube heat exchanger, intermittent operating duration, recovery time, and duty cycle were explored. These simulations demonstrated the importance of operating time scales. Findings showed that operating for durations smaller than the transit time (the time needed for fluid to travel the full length of the heat exchanger) provided performance benefits in the u-tube over the coaxial design, due to the limited thermal interaction between inlet and outlet flows in the u-tube. At operating scales above its transit time, however, the coaxial heat exchanger performed up to 12.9% better than the u-tube, with most benefit observed for operation durations close to the transit time. Additionally, the results showed little difference during operation between the two heat exchangers when varying recovery times. Furthermore, an assessment of the duty cycle demonstrated that a high ratio of recovery to operating time at low operating durations provided the greatest performance benefit. These findings highlight the need to model heat exchanger response over all time scales in order to accurately predict performance under intermittent operating conditions.