ADVANCES IN GEOTHERMAL RESPONSE TESTING

Abstract

A Geothermal Response Test measures the temperature response to a thermal energy forcing of a borehole heat exchanger. The temperature response is related to the ground and borehole thermal parameters such as thermal conductivity, heat capacity and the conductivity of the borehole material and is therefore used to obtain estimates on these important parameters. Generally such data are analysed using a line source method. Although quick and, when certain conditions are met, accurate, the line source method has several drawbacks. First of all it is only valid for the constant heat flux case, secondly it only allows estimates of ground thermal conductivity and borehole resistance, other parameters, like heat capacity, cannot be estimated. Thirdly effects of ground water flow cannot be quantified. Moreover, selecting test parameters a-priory is not always easy and it is difficult to adjust test conditions due to the necessity of constant heat flux. Parameter estimation techniques have been developed to allow analysis of test results under varying heat flux conditions and to allow simultaneous estimates of different parameters to be obtained. In this paper we develop such a method based on the TRNSYS simulation package with the DST borehole model, using a generic optimisation package—GenOpt—to perform the calibration. Moreover we extend the GRT test protocol to use different heat extraction and injection energy levels within the same experiment. Apart from improving estimates of ground and borehole thermal parameters, it is our final goal to allow the characterisation of ground water flow using the in situ thermal response test.