Geothermal Response Test
The response test, also referred to as geothermal response test, is an accepted field method to determine the effective thermal conductance of the ground. The thermal conductance is one of the essential parameters when measuring geothermal probes, energy piles, components with ground contact and geothermal doublet plants (conveyer and injection wells). In-situ measurements of the thermal conductance allow planners to carry out their measurements without safety margins. This enables remarkable savings of investment costs, particularly with medium-size and large geothermal plants. The response test is superior to laboratory measurements as lithologic and ground physical parameters are not corrupted with this methodology. The Senate committee of Berlin demands the execution of response tests with installed capacity of 30 kW and higher.
Apart from determining the thermal conductance, the so-called thermal borehole resistance can be determined regarding geothermal probes and energy piles. This parameter displays how good the installation of a geothermal probe is or whether the pressing operation material has a high thermal resistance.
In general, a response test is carried out at test bores (extending a test bore with a geothermal probe) or at the first bore of an intended geothermal probe field. With most of the response tests, a constant amount of heat is applied to the subsoil and the temperature response is measured. The failure rate can be reduced to a minimum by using modern SPS-technology which makes the response tests quick, reliable and cost-effective.
The conventional evaluation of a response test is mostly done according to Kelvin’s line source theory [e.g. Hellström, G. (1991-1994), Gehlin, S. (1996-2000)]. The evaluation is based on the mean fluid-temperature-course of the geothermal probe field during the response test.
A geothermal test sensor is often applied at representative spots within the construction area during the preparation of energy pile projects. Thus, the effective thermal conductivity is determined by means of response test - which is an essential design parameter for planned energy pile plants.
Response tests at energy piles occur quite seldom as the efforts for a test energy pile production and a construction site installation of the technical devices (pile boring rigs or pile driving machine) are quite high. Yet, it is essential to determine the thermal pile resistance of an energy pile type, as this parameter has significant influence on the removal capacity of the relevant energy pile – independent from the effective thermal conductivity of the soil. The thermal pile resistance depends mainly on the design of the energy pile and its heat exchanger configuration. Some energy pile manufacturers have already determined this important design parameter for selected energy pile types.
For further information on geothermal response tests can be found on our Joint venture website: www.response-test.de