Thermal resistance and capacity models for borehole heat exchangers

The detailed design and energy analysis of ground source heat pump systems requires the ability to predict the short-term behavior of borehole heat exchangers (BHE). The application of fully discretized models leads to extensive computation times and a substantial effort in terms of pre-processing work. On the contrary, analytical models offer simple, parameter input-based modeling and short computation times, but they usually disregard the transient effects of heat and mass transport in the borehole and hence are not suitable for the prediction of the short-time behavior. In order to combine the advantages of both types of models, the authors developed two-dimensional thermal resistance and capacity models for different types of BHE. These models take the capacity of the grouting material with one capacity per tube into account and, therefore, the range of validity is extended to shorter times. The correct consideration of all thermal resistances between the fluid in the pipes, the grout capacities and the borehole wall is important because of the significant influence on the validity of the models. With the developed models, the modeling work and the computation time can be significantly reduced compared with fully discretized computations while precise results are still achieved. The validation of the suggested models against fully discretized FEM models shows a very good agreement. Copyright © 2010 John Wiley & Sons, Ltd.