Numerical Simulations of Vertical Underground Heat Exchangers for Heat Pump Applications

Abstract

Ground heat pumps use the ground as heat source/sink reservoir for space heating/cooling of buildings. The key components in these systems are the ground heat exchangers, their design is critical to achieve an optimal thermal performance not only for the short term but also for the long term exploitation. The present paper presents a thermal analysis of domestic multi-boreholes heat exchangers connected in series at shallow depth. Numerical simulations have been carried out using TRNSYS software taking into account local meteorological data. Flow direction of the water entering the underground secondary loop is inversed at regular interval in order to equilibrate the ground temperature nearby the different boreholes. This method is shown to be efficient to keep almost constant ground reservoir temperature by permitting a good heat recharge especially for the long term system exploitation. The conventional method (flow in one direction only) might cause in some cases performance degradation, due to the formation of an unbalanced ground temperature surrounding the different boreholes particularly after several years of intensive utilization and when the boreholes separation is smaller than recommended minimum distance. This study also illustrate the importance of taking into account a daily and annual schedule for system operation, the pump off’s periods are crucial for the natural heat recovery of ground reservoir resulting on higher performance and longer system life.