A versatile method for estimating grout and ground thermal properties in a thermal response test

Abstract

Ground-Source Heat Pumps (GSHPs) are efficient means of space heating and cooling. However, owing to their high installation costs, their development in societies has not grown as it should. Therefore, it is crucial to have a meticulous design of these systems. The first step in designing GSHPs is determining the ground and grout thermal properties. Thermal Response Tests (TRTs) are developed for this purpose. The previous methods of estimating the unknown thermal properties are either inefficient or yield inaccurate estimates for the ground volumetric heat capacity, to which the mean fluid temperature has the slightest sensitivity among all thermal properties. Among previous methods, some facilitate estimating ground volumetric heat capacity by assuming a low ground-to-grout thermal conductivity ratio. This paper aims to estimate the thermal conductivities and volumetric heat capacities of ground and grout, assuming a higher ground-to-grout thermal conductivity ratio. A TRT, in which the ground-to-grout thermal conductivity ratio is 3.86, is simulated using a sufficiently accurate model, and thermophysical properties are identified utilizing a parameter estimation technique. A novel approach is adopted to estimate the ground volumetric heat capacity. The estimation technique identifies the remaining thermophysical properties iteratively by segregating the TRT duration into three intervals. The results indicate that the ground thermal conductivity and volumetric heat capacity are estimated with errors of 2.16% and 16.09%. The estimated grout thermal conductivity and volumetric heat capacity errors are 1.10% and 1.66%, respectively. In addition, the effect of fitting a smooth curve to the temperature readings was investigated. It was demonstrated that adding the pre-fitting step reduces the uncertainties of the estimated parameters.