Thermal Conductivity Testing, also known as Formation Thermal Conductivity, is a testing method that provides the geothermal system designer site-specific information on the thermal conductivity and diffusivity of the area in which the system will be installed. The benefits of using this method include:
-More accurate installation and material cost estimates
-Test parameters that can be set to simulate loop response under cooling design conditions
-Test results that enable ground loops to be sized to match existing formation conditions
-Multiple tests that allow comparison of different grouts
-System designs that operate more economically
-Test results that can be easily input into software programs
In recent years, the installation of geothermal heat pumps has gained popularity due to advances in system design and overall performance. Geothermal systems benefit from the earth’s ability to maintain a relatively constant temperature by circulating a water/anti-freeze solution through subterranean loops of pipe to transfer heat to or from the structures they service.
Typically, the design of these systems relies on empirical information based largely on conservative estimates to determine the overall size of the system. However, when geothermal or formation thermal conductivity testing is used, a more accurate determination of the capacity of the soil to transfer heat can be provided. This allows designers to develop tighter tolerances when sizing loops and heat pumps, often reducing the cost of equipment, material, installation, and operating cost as well as the size of the field required. This can equate to substantial savings for the project both in time and money. A basic rule of thumb is that thermal conductivity testing is economically justified for geothermal systems that have more than 10 tons of cooling capacity.
Geothermal conductivity testing typically takes around three weeks from equipment mobilization to data analysis. The test involves installing a polyethylene loop in a 6-inch borehole which is routinely installed to depths in excess of 300 feet. The loop and borehole are sealed with grout, and pressure tested. Fluid is then circulated through the loop and the resulting change in temperature over time is calculated. With this information, a detailed description of the geothermal conductivity, diffusivity, and natural earth temperature can be provided.
