A TRNSYS assisting tool for the estimation of ground thermal properties applied to TRT (thermal response test) data: B2G model

Abstract

[EN] The determination of ground thermal properties is essential to design competitive commercial ground source heat pump systems. A thermal response test (TRT) carried out on site allows the determination of both the conductivity of the ground and the borehole thermal resistance by means of analytical approaches, which require several simplifying assumptions as well as TRT durations of at least fifty hours and constant heat injection. In this context, a detailed dynamic numerical model of the borehole can help reducing both the uncertainties, associated with these simplifying assumptions, and the required test duration. This paper presents a TRNSYS tool to obtain the grout and ground thermal properties by means of a parameter estimation technique in conjunction with a two-dimensional dynamic numerical model, the B2G model, which is able to provide accurate results with a much shorter testing time and without the necessity of a constant heat injection.

The methodology to estimate the borehole and ground characteristics has been validated thanks to the analysis of a set of experimental TRTs for U-pipe vertical boreholes carried out in different type of soils and borehole geometries. Results show that, for the analysed tests, it is possible to obtain an accurate and fast estimation of the ground thermal conductivity with reductions of the necessary TRT duration of up to a 70%, with a total uncertainty between +/- 10% and +/- 18%, considering not only the uncertainty introduced by the reduction of the test duration, but also the main sources of uncertainty.