NOA’s experience in geotechnical engineering has led to development in geothermal design and testing. We currently have a thermal conductivity test unit that is portable and ready to be used on any geothermal project requiring thermal conductivity values for testing. This page gives some background information on the testing methodology and analysis. Contact us today for any geothermal thermal conductivity testing needs.
Ground Loop Thermal Conductivity Testing Background
To measure the thermal conductivity of the material surrounding a vertical ground loop, a transient line source model is used. This model employs an infinitely long power source with constant power per unit length. A temperature profile versus time is measured and using an inverse relationship model the thermal conductivity can be calculated.
In the field, a vertical ground loop is installed to the desired depth, typically 200ft, and our thermal conductivity test unit is attached to the loop. By carefully measuring the power input, temperatures, and flow rate with a datalogger a temperature response curve for the test location is created. Because a vertical ground loop has finite depth and width, the initial 5 to 10 hours of test data is not used to allow for the borehole to thermally equalize. Using the transient line source model the thermal conductivity is calculated.
Ground Loop Thermal Conductivity Testing Specifications
The recommended specifications for testing the thermal conductivity are published by the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) and the International Ground Source Heat Pump Association (IGSHPA). The following specifications are recommended for a valid thermal conductivity test result. All specifications are met or exceeded when we perform a thermal conductivity test.
- Test duration is to be 36 to 48 hours
- The linear heat source is to be 15 to 25 watts per foot of vertical bore hole
- The coefficient of variance of input power is to be less than 1.5%
- Peak power is to be less than + 10%
- Temperature accuracy is to be + 0.5 °F
- The accuracy of the power transducer and recording device is to be +2% of the reading
- A waiting period of five days is recommended for low-conductivity soils after the ground loop has been installed and grouted (or filled) before the thermal conductivity test is initiated. A delay of three days is recommended for higher conductivity formations.
- Data collection should be at least once every 10 minutes.
- All above ground piping is to be insulated with a minimum of 0.5 inch closed cell insulation or equivalent. Test rigs are to be enclosed in a sealed cabinet that is insulated with a minimum of 1.0-inch fiberglass insulation or equivalent.
- If retesting a bore is necessary, the loop temperature should be allowed to return to within 0.5°F of the pretest initial ground temperature. This typically corresponds to a wait period of 10 to 14 days.