Thermal response testing of a multiple borehole ground heat exchanger

Abstract

This paper presents the methodology, results and a discussion of thermal response tests (TRTs) performed on a multiple borehole ground heat exchanger. The ground heat exchanger consisted of nine 80-m deep boreholes. TRTs with durations between 48 and 260 h were performed on individual boreholes. Tests were analysed using common evaluation methods, and ground conductivity and borehole resistance values were determined for all nine boreholes. In addition to these parameters, the undisturbed ground temperatures were also determined for individual boreholes using multiple approaches. A sensitivity analysis was performed to analyse the effects of various test and parameter uncertainties on the ground thermal conductivity and borehole thermal resistance estimations.

Comparison of two different models for pile thermal response test interpretation

“…Surprisingly R b , which is generally considered to be determined with less accuracy from thermal response tests [14,22], shows only a small percentage variation, generally within ±15% for the line source case. For the G-function analysis, the results are very stable with variation of less than 1% after the first few hours The preceding sections present a number of analyses, more than could be conducted on a routine basis.…”

“…It tests a larger volume of soil than laboratory methods and gives a lumped value of thermal conductivity of the soils that will be activated by the heat exchanger. Overall accuracies are quoted at 5 to 10% for thermal conductivity and 10 to 15% for thermal resistance [13,14,22,23]. …”

“…These equate to around 4% and 10% of the expected values. Larger intervals were used for the thermal resistance as it was expected to be able to determine this parameter to a lower level of accuracy [14,22]. Table 3 summarises the thermal conductivity and thermal resistance values determined from the thermal response test data using the line source model and the G-functions with a constant applied power ( Table 1).…”

A comparison of laboratory and in situ methods to determine soil thermal conductivity for energy foundations and other ground heat exchanger applications

“…Various sources of uncertainty effect thermal response tests, those relevant for this test will include variability in the applied power, the larger diameter and relatively short length of the heat exchanger and any variability in the initial undisturbed temperature condition. Nevertheless, studies of errors in thermal response tests suggest that well conducted tests should be accurate to within 10% [28,30,23]. However, the error may be a little larger in this case as the pile is of greater diameter than usually recommended [5].…”

Analysis of rock mass borehole temperatures with vertical heat exchanger

“…They propose hybrid experiments and simulations for quantifying the effects of temperature changes in the rock mass on the operation and efficiency of the whole energy system. Javed and Fahlén (2011) describe the results of heat reaction tests in nine 80-m deep boreholes. They documented the effects of heat characteristics of the rock mass in the boreholes on the temperature distribution.…”