Water Well Journal
Issue link: http://read.dmtmag.com/i/629532
D avid Henrich, CWD/PI, CVCLD, has a name for the work needing to be done when a company is called in to clean up a poorly designed geothermal system. "Geo janitorial." And these are jobs that happen more than people think. Henrich, of Bergerson-Caswell Inc. in Maple Plains, Min- nesota, was called in to monitor a geothermal system that had too many loops on a circuit, the energy loads weren't done well, and the manifolds weren't well designed. "The system was potentially undersized," says Henrich, who serves on the National Ground Water Association Board of Directors. "It hung on through a pretty tough winter, but just barely." That's risky when designing geothermal systems. You have only one shot to get it right. "Once they're in, adding more bores and capacity is pretty difficult," he says. "You're talking about cutting apart brand new buildings or brand new finished upgrades to a building. You generally don't get a second chance." That's why it's critical to follow a proper design process— including one with a thermal conductivity test. Thermal conductivity tests provide the designer with the scientific information necessary to properly size a closed loop geothermal system or standing column well. The thermal con- ductivity test gives you an actual average heat transfer rate for the soils for that specific site instead of relying on a broad set of values. "There's an assumed set of values of soil," says NGWA President Jeffrey Williams, MGWC, CVCLD, of Spafford & Sons Water Wells in Jericho, Vermont. "And with that as- sumed set of values in soil, there's also an assumed tempera- ture in different regions of the country because some soils give and take heat much better than other soils. If we install it and our assumptions were skewed by 10% or 20%, we're going to have a malfunctioning system." Thermal conductivity tests take the guesswork out of your system's design by providing an actual average heat transfer rate for the soils for that specific site. The tests work by injecting a known amount of heat at a known rate consistently through water during the duration of the test, which is usually 48 hours. The temperature data collected during the test is used to calculate the thermal conductivity of the soil. "The test itself is really quite simple," Henrich explains. "It's simply just moving a known amount of energy into the ground and then measuring how the ground is reacting to that." Why you need a thermal conductivity test Besides providing critical data to create a well-designed system, a thermal conductivity test could save your customers money. For example, say you're working on a job where you know the geology and can reference soil tables that give you a ther- mal conductivity range of 1.2 to 1.8. When you run your de- signs, you get two scenarios: one says you need 6000 feet of drilling and one says you need 7000 feet of drilling. "If you're using proper engineering, you always design to the worst case scenario," Henrich says. "The extra 1000 feet of loop field will cost you $15,000 to $20,000." Alternatively, a thermal conductivity test, which costs a few thousand dollars, could give you an accurate value that says you actually need 6500 feet of drilling. "It gives you the financial justification for doing the test," Henrich says. "By spending a few thousand dollars, you could save (your customer) about $15,000 just by knowing what the actual conductivity of the soil is." How to ensure an accurate test You need to make sure the test loop well is the same as the bores you plan to drill for the loop field. Drill it to the same depth and then you can incorporate the test bore into the field. "You have to make sure that loop well is a true indicator of what you're really going to do," Williams advises. "You've A thermal conductivity test can help you avoid pitfalls and have a well-designed system. By Jennifer Strawn CONDUCTIVITY continues on page 22 Thermal conductivity tests provide the designer with the scientific information necessary to properly size a closed loop geothermal system or standing column well. AVOIDING COMMON PITFALLS Twitter @WaterWellJournl WWJ February 2016 21