An Evaluation of Ground Thermal Properties Measure Accuracy by Thermal Response Test of Horizontal Ground Heat Exchangers

Abstract

This study aims to compare temperature measurements in horizontal ground heat exchangers to predicted values using thermal response tests. A scale 1 test facility of horizontal ground heat exchangers has been implemented in BRGM (Orleans - France) to test performance in real conditions. The heat exchanger is divided in four parts of 100 m² each with different characteristic locations:
- sunny grass
- shaded grass
- sunny car-park
- shaded car-park

These different configurations have been chosen to compare the performance of ground heat exchangers in different environments (surface state, boundary conditions). Furthermore, the temperature in the soil is measured continuously at 3 different levels (-0.5 m, -1 m, -1.5 m). To map the temperature field at these 3 depths, optical fibers are distributed underground, and the use of a distributed temperature sensor (DTS) allows accurate measurement of the temperature in the soil surrounding the ground heat exchanger.

A thermal response test was carried out on the horizontal ground heat exchanger. A monitored constant heating power was injected at a constant mass flow, and the temperature was measured at the 3 different levels of a three-dimensional numerical model of ground heat exchanger.
In particular, the thermal response test gives the noticeable differences between the 4 different conditions of each part of the ground heat exchanger. The indicated uncertainty with respect to the effects of soil thermal properties on sizing are discussed.