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Borehole logging: GeoCube cuts energy uncertainty

Summary

An integrated approach to geothermal borehole conductivity testing and analysis

In Without Hot Air, a new book on climate change and energy by Dr David MacKay, a professor of physics at Cambridge University, concludes that heat pumps need to – and will – become the standard heating and cooling technology for buildings in the UK and beyond.

He writes so because heat pumps are extremely efficient and in a world of increasing CO2 emissions, this is critical. Heat pumps achieve maximum efficiency when incorporated into a geothermal ground heat exchanger system.

Vertical bores heat exchangers range in depth from 30m to more than 150m and a single project can have as few as one or as many as 5,000 boreholes.

Throughout the year, as heat moves into and out of the earth via these boreholes, the earth’s temperature changes. This change depends significantly on a building’s heating and cooling requirements as well as several soil parameters including thermal conductivity, thermal diffusivity and undisturbed/initial soil temperature.

These three values, which can vary significantly even across a single project’s geology, should be determined empirically. Once this is done, the figures are used in design software to calculate the proper number of bores as well as their depth. Getting these numbers right is important.

Undersized systems – with too few bores – may fail and oversized systems are excessively expensive to drill and install.

The most effective way of determining these soil parameters is via a thermal conductivity test, also known as a thermal response test. Performing a conductivity test requires the use of a drill rig, a thermal conductivity test unit and a power generator.

The GeoCube is a conductivity test unit manufactured by Precision Geothermal LLC, based in Minnesota, US.

The first step is to determine the test bore depth using design software such as Gaia Geothermal’s Ground Loop Design. Ideally, the test bore depth should be the same depth as the other bores in the anticipated geothermal loopfield. This maximizes design accuracy and also enables the inclusion of the test bore in the final installation.

The next step is to drill the test bore, install a single HDPE U-bend pipe as per standard geothermal installation methodologies and then to fill the pipe with fluid (water). At this point, the field needs to be allowed to rest for several days so that the soil can return to its natural steady temperature.

Several days later, the U-bend pipe should be attached to the GeoCube and insulated. A 240V power generator is subsequently hooked up to the GeoCube.

An easy and fairly accurate way to measure the undisturbed soil temperature is to power up the circulation pump in the GeoCube, place a thermometer in the standing column of water that sits inside the unit and then measure the temperature of the water as it circulates through the loopfield and the unit.

After a few minutes, the temperature will stabilise and be equivalent to the average undisturbed ground temperature.

The conductivity test can now be performed by initialising the GeoCube’s built-in data logger, turning on the heating elements and letting the test run for approximately 72 hours. The GeoCube, with 8,000W of heating power, can handle up to 160m-deep boreholes.

After the test is complete, the data can be transferred to the GeoCube’s analysis software to determine the soil conductivity and diffusivity. The empirically derived conductivity value, which reduces design uncertainty, can then be entered into the Ground Loop Design software to complete a geothermal design.


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