24 June 2014

A SIMPLE MODEL TO ACCOUNT FOR THERMAL CAPACITY IN BOREHOLES


A relatively simple borehole heat exchanger model that accounts for grout and fluid thermal capacity has been developed and is presented in this paper. The quasi 3D
model divides the borehole into successive layers. The cross-sections of each layer are modeled by the so-called ‘thermal resistance and capacity models’ (TRCM) approach. The
resulting TRCM model has been implemented as a component (TYPE) in the TRNSYS environment. This enables its use with other models (building, heat pumps, etc.) in annual
energy simulations. The model has been compared successfully against other models. Results of annual
simulations of a typical ground source heat pump system show that neglecting fluid and grout
thermal capacities leads to a  3% over prediction of the heat pump energy consumption. The overestimation increases when boreholes are undersized or when the heat pump COP
variation with the entering water temperature is high.