Extending capabilities of Thermal Response Tests in vertical ground heat exchangers: an experiment-based local short-time temperature response factor.

Abstract

This work presents a fast and practical method to determine a characteristic function that predicts a local ground response of vertical ground heat exchangers (GHE) in a short term, which is mandatory when intermittent operation modes are modelled. It expands the use of a Thermal Response Test (TRT) that is recommended previous to the design of a borehole field into a vertical Ground Source Heat Pump (GSHP). These tests are used to provide some ground thermal characteristics, such as ground conductivity or borehole effective thermal resistance. The same device can also be used to measure the undisturbed ground temperature along depth. From the measurements, it has been determined a temperature response factor function that characterizes the ground behavior in a short term as a consequence of a heat pulse. This function is then included into a finite line-source model to simulate the average temperature of the fluid that flows into the borehole pipes along time. In order to validate this method, several additional tests have been performed by the same device used for TRT’s, with intermittent operation modes. For each test, both experimental and simulated average fluid temperatures have been compared. Results present an excellent accuracy; thus, they demonstrate de effectiveness of the method, as well as other advantages: i) fast and accurate; ii) prevents high precomputing times; iii) several uncertainties from measurements disappear when it is used.