Short-term experiments with borehole heat exchangers and model validation in TRNSYS

• A 2 h test sequence has been applied to double-U-tube borehole heat exchangers (BHE).
• Three common TRNSYS models (SBM, DST and EWS) have been validated.
• Two models neglecting fluid and grout thermal capacity underperform by >50%.
• A simple method with a pipe model in front of the BHE model is tested successfully.
• A correct transient behavior is strongly influencing system simulations.

The short-term behavior of borehole heat exchangers (BHE) is investigated using three 2 h test sequences each with constant inlet temperature and constant mass flow rate. Three models of the transient systems simulation program (TRNSYS) have been compared to these measurements. In order to assess the deviations a 3D-FEM model, built in COMSOL Multiphysics®, is compared to the measurement as well.The “Erdwärmesonden” model (EWS) shows a very good accuracy with a deviation less than 10%. The superposition borehole model (SBM) and duct ground heat storage model (DST) neglect the internal borehole heat capacity of fluid and grout. In order to improve their short-term behavior, the models are modified with an adiabatic pipe model with wall capacity in front of the BHE model. Different parameter sets for the pre-pipe are investigated. The optimum parameter set reduces the error between simulation and measurement of the injected heat from 50% to about 5%. With this model modification, all investigated models SBM, DST, and EWS perform as accurate as the COMSOL model.