The Analysis of Expansion Thermal Response Test (TRT) for Borehole Heat Exchangers (BHE)

Abstract

Rational use of big borehole heat exchanger (BHE) installations necessitates performing thermal response tests (TRT) at the initial stage of works. As a result, thermal conductivity λ and borehole thermal resistance Rb are determined, i.e. parameters characterizing the rock mass and BHE construction. They are required for calculating the number of borehole heat exchangers so that heat assumptions for the entire installation can be reached. The classic TRT is usually realized with constant heating power of 50 W/m at the heat exchanger, and time 40 to 100 hours. This solution does not provide real abilities to optimize TRT and design a complex installation of borehole heat exchangers. In this paper there was given a TRT modification, where average temperature of fluid in- and outflowing from the heat exchanger at the current analysis of regression equation used for determining the coefficient of directional coefficient of straight line k, on the basis of which thermal conductivity λ and borehole thermal resistance Rb can be calculated. It was assumed that TRT should be stopped after the time in which k does not differ more than by assumed Δk, e.g. 5%. To optimize technology of borehole heat exchangers, an extended TRT was suggested, where thermal parameters were measured at three different volume flow rate of heat carrier Q and three different thermal powers N. To standardize the time of the extended test, 7 measurement tests were established to determine the influence of volume flow rate of heat carrier Q and thermal powers N on thermal conductivity λ and borehole thermal resistance Rb. Numerous measurements were performed on borehole heat exchangers owned by the Geoenergetics Laboratory of Faculty of Drilling, Oil and Gas AGH University of Science and Technology to prove the theoretical and practical usability of the test, and some of the results have been presented in this paper.