Improvements on a Numerical Model of Borehole Heat Exchangers

In the mathematical simulation of bore fields for ground coupled heat pump (GCHP) systems, it has become a common practice to consider the boreholes as having a uniform temperature. As a rule the boreholes are hydraulically connected in parallel and the small temperature difference between incoming and outgoing heat carrier fluid justifies the assumption that all boreholes have the same uniform temperature in operation. Two simultaneous boundary conditions usually apply: All borehole walls should have a uniform temperature and the heat flow from the bore field should equal the energy needed by the heat pump. This paper describes improvements applied to a previous numerical approach that employs the concept of a highly conductive material (HCM) embedded in the boreholes and connected to a HCM bar above the ground surface to impose a uniform temperature boundary condition at the borehole wall. The original boundary condition with the uniform fluid temperature comes in conflict with the concept of the uniform borehole wall temperature. Between the fluid and the borehole wall there is a thermal borehole resistance. The heat flux increases at the borehole ends and thus also the temperature changes between borehole wall and the fluid. The borehole wall temperature deviates from the uniform assumption and will cause an error in the simulations. This paper presents a correction to that error. Firstly, the improvements to the HCM model are validated for g-function generation, which presents a good agreement with reference solutions. Secondly, the improvements to the HCM model are illustrated to predict fluid temperatures for measured variable daily loads of a monitored GCHP installation. The predicted fluid temperatures are compared with monitored data for about four years. The predicted fluid temperatures deviate from the measured data by less than 1 K during the last monitored year. QC 20160928