Numerical evaluation of the Ground Response to a Thermal Response Test experiment

A prerequisite for the correct design of vertical ground heat exchangers (or Borehole Heat Exchangers, BHE) for heat pump applications is the knowledge of the ground thermal properties, in particular the thermal conductivity. The Thermal Response Test is a well known experimental procedure that allows the ground thermal and the BHE thermal resistance to be evaluated. A TRT is performed by providing a known and constant thermal power to a fluid (usually water) that circulates through a BHE buried in the site of interest; the water temperature measurements, which varies over time, represent the data to be analyzed in order to solve an inverse conduction problem. The standard analysis method addressed to parameter estimation is based on the Infinite Line Source (ILS) model. In the present paper different 3D numerical models, are developed in order to numerically describe a TRT experiment. The calculation environment is Comsol Multiphysics® and either the thermal conduction inside the ground and grout or the fluid to pipes interactions are taken into account. The results of the simulations have been employed for a back evaluation of the ground thermal conductivity according to the standard ILS approach, to infer useful information on the errors in parameter calculation and to check the estimation capabilities of a new method based on temporal superposition and optimum search. The proposed method is in particular able to cope with situations where the TRT experiments are related to highly variable heat transfer rates to the carrier fluid. The present results show that the proposed approach is very reliable alternative to the standard ILS approach and that BHE parameters can be estim percent error with respect to reference values.