New Thermal Response Testing Facilities

Knowledge of the thermal conductivity of the strata into which a 'closed loop' borehole heat exchanger system is to be installed is important during the design stage of the system.

All sites have varying underlying stratas with differing levels of saturation, rock types and mineral content - all factors that can effect the thermal conductivity over the length of the proposed vertical penetration beneath the site into which the system is to be installed.

It therefore follows on that the design of each system is 'site specific'.

A 'Thermal Response Test' is an in-situ scientific test to determine the thermal conductivity of a particular location. The results of this test assist in determining the number of closed loop boreholes needed to service the heating/cooling demands of the project.

The test is generally carried out in the following steps:

Test Rig

 

1.    Drill a borehole and install a heat loop, grouting throughout with a thermally enhanced grout.

2.    Carry out flow and pressure tests on the loop to ensure good working order.

3.    Connect a Thermal Response Testing Rig to each of the installed loop tails and start the test.

During the test, the rig circulates liquid around the loop. This liquid is heated by the rig to a known temperature which is recorded by a data logger on entering the loop. During it's journey through the loop down the borehole and back up to the surface, there is a heat transfer to the surrounding strata and the temperature of the liquid is once again recorded using another data logger on re-entry into the test rig. On passing back through the rig, the liquid is reheated and so the circulation is continued for usually 3 - 4 days.  During this period large volumes of data is continuously collected relating to the liquid temperature on leaving and re-entering the rig.

The data is presented on a thermal conductivity curve and demonstrates the amount of heat that can be released or absorbed per metre of a known length of borehole through a known sequence of strata beneath the site. This is known as the 'thermal conductivity' and once this is known, then the design engineer can decide on the amount of boreholes needed for the property.

Although there is a cost implication in carrying out the Thermal Conductivity Test, the results can give the design engineer confidence that they are not drilling too many boreholes and therefore costing more than necessary and also not drilling too few boreholes resulting in a system that underperforms - either way saving on unnecessary costs. The borehole used for the test is also incorporated into the system.