22 May 2008

IMPROVEMENTS OF THERMAL RESPONSE TESTS FOR GEOTHERMAL HEAT PUMPS


At present, the ground thermal conductivity at borehole heat exchanger (BHE)
array locations is usually derived from thermal response test (TRT) data. Thermal response
tests record the temperature variation at the outlet of a BHE due to fluid circulation. The fluid
outlet temperature is directly related to the rock thermal conductivity around the well. The
TRT method only provides the average value of thermal conductivity over the entire length of
the borehole. If environmental and experimental conditions satisfy the usual experimental
standards, a TRT can predict effective ground thermal conductivity within an error of
approximately ±10%. However, the line source approach (on which the analysis of the TRT
experiment is based) is restricted to ideal conditions such as homogeneous ground
temperature and homogeneous rock thermal properties. GEOWATT developed a new TRT
concept in order to overcome many of the TRT constraints and to improve the accuracy of
the results. The new concept is based on temperature measurement over the entire length of
the BHE using NIMO-T, a wireless temperature probe, which was developed at GEOWATT.
The temperature data is evaluated by numerical simulation, using a detailed finite element
mesh, which maps the BHE and ground geometry as detailed as possible. This concept
allows to calculate the vertical variation of ground thermal properties and groundwater flow
and thus provides a better estimate of BHE power.