Temperature response factors at different boundary conditions for modelling the single borehole heat exchanger

Abstract

Design and simulation of borehole heat exchangers rely on the solution of the transient conduction equation. The typical approach for predicting the ground temperature variations in the short and long term is to recursively apply basic thermal response factors available as analytical functions or as pre-estimated tabulated values. In this paper a review of the existing response factor models for borehole heat exchangers (BHE) analysis is presented and a numerical model, built in Comsol environment is employed for calculating the temperature distribution in time and space around a single, finite length, vertical cylindrical heat source also taking into account its position with reference to the ground surface (effects of the adiabatic length or “buried depth” D). The temperature values are recast as dimensionless response factors in order to compare them with analytical solutions where available. Furthermore new temperature response factors suitable for describing the single Finite Cylindrical Source (FCS) under different operating modes (i.e. boundary conditions) are generated. Boundary conditions include imposed heat transfer rate, imposed temperature and a combination of both conditions, where spatially uniform temperature at the BHE interface is attained while also keeping constant the applied heat transfer rate.