A thermal conductivity test at a geothermal system job site.
By Jennifer Strawn
David 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, Minnesota, 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 conductivity 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 assumed set of values in soil, there’s also an assumed temperature 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 thermal conductivity range of 1.2 to 1.8. When you run your designs, you get two scenarios: one says you need 6000 feet of drilling and one says you need 7000 feet of drilling.
Thermal conductivity tests provide the designer with the scientific information necessary to properly size a closed loop geothermal system or standing column well.
“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 got to make sure your loop well is grouted really well and it’s a true representation of what this field is going to be. If you didn’t get it grouted the way you wanted it or if there is something else wrong with it, it will skew the data for the whole field.”
If the data is wrong, the field may be oversized or undersized. That’s why it’s important to pay attention to the construction of the test bore.
If you want extra accuracy, Williams suggests taking soil samples about every 10 feet.
“What you do is you take good samples of the material you’re drilling through and that will go to confirm your test as well,” he notes. “You can really never have too much information.”
If you’re working on a large loop field—about 50 loop wells or more—it’s a good idea to do more than one test borehole. Williams prefers to set up his tests at the far ends of the field so he can get a good average across the entire field.
“Anybody who drills can tell you you can move 10 feet away from one borehole and drill and find a completely different set of geology,” Henrich points out. “So, the thermal conductivity of that bore is going to change slightly as well.”
On most jobs with tests on multiple bores, Henrich sees results within a few hundredths of a point or a tenth or two of a point different.
“So they’re not the same by any means,” he contends. “So getting an average when you’re doing a bigger field is a very good idea. You’re dealing with more energy, so you have more opportunity to have bigger mistakes.”
When you’re ready to test, read all of the instructions before starting. Although it’s a simple test, following the steps in the instructions is one of the best ways to ensure an accurate test.
“There’s a pretty simple set of steps you need to do in order to get a good TC (thermal conductivity)—but you need to do those steps,” Henrich emphasizes. “There’s not a lot of room for creativity in testing, so read the steps and follow the steps.”
In general, the steps are:
- Set up your equipment in the field
- Get the native ground temperature
- Purge the air from the loop
- Start your equipment—including the heating element and data logger
- Run your test
- Collect and analyze your data and submit your report.
Common errors in testing
Even if all the steps are followed, there are common pitfalls that could skew your data results. For an accurate test, you need three things: stable power, consistent flow rate, and proper insulation.
“Your goal is consistency,” Henrich says. “It’s about making the test as steady state as possible. Anything that gets in the way of that goal is something that needs to be mitigated or needs fixed.”
Inconsistent power generation is the biggest problem in the field. Generators without good voltage regulators or generators that can’t run for 48 hours straight are often to blame.
“You need good, clean power,” Williams says. “I prefer to use a diesel generator because the power is nice and clean and they’re consistent. I also use a pretty good sized one, so I’m actually over-powered most of the time.”
A consistent flow rate is also important. To ensure a consistent flow rate, you should make sure all of the air is purged out of the unit before starting the test.
“It’s basically the last thing you do before you engage the heating elements and inject heat into the loops,” Henrich says. “If you don’t have all of the air out, you can cause a flow rate to change or have inconsistent flow throughout the test which again get into that consistency when you’re trying to pump the heat in.”
Properly insulating your setup is always important but is especially critical in the heat of summer or in areas with harsh winters.
“We’ve seen tests where people have not insulated the piping where it comes out of the borehole into the units and it rains,” Henrich recalls. “The rainwater just running down the side of the loops saps a lot of the energy, and you’ll notice the temperature creep level off while it’s raining.”
Keeping everything covered and insulated helps keep the temperature curve consistent. Adding more insulation by covering the unit with concrete blankets or other specialized insulating blankets also can keep the highs and lows out of your data stream.
During the test, Williams performs real-time monitoring where he can look at actual temperatures and make sure his data profiles are good.
“When I get to the end of that test I’ve got even more confidence,” he maintains. “That’s because I’ve had some handson during that 48-hour period of time.”
After collecting the data, you’re looking for slope stability and temperature stability during the data collection to make sure there’s not a lot of highs and lows or aberrations in your data.
You can also compare your data to the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) standards, which is the criteria used in the United States to determine acceptable data.
If there were problems with the data collection, let the unit sit long enough for the temperatures to reset in the earth, probably five to seven days, and retest.
“Pay attention when you’re doing the testing,” Williams insists. “You want to be sure your unit is performing properly and you don’t have any hiccups or issues that could question the test.”
Also, don’t be afraid to learn more with more research.
“If you’re going to do thermal conductivity testing, be curious and learn a little more about it,” Henrich suggests. “Become a little more well-rounded. There’s several different methods of doing thermal conductivity testing and data analysis, and learning more about those gives you a better understanding of why we’re doing a test.”
By making sure you have stable power, consistent flow rate, and proper insulation—you can be sure your system is well designed.
And won’t need “geo janitorial” work after the fact.
Jennifer Strawn was the associate editor of Water Well Journal from 2004 to 2007. She is currently in the internal communications department at Nationwide in Columbus, Ohio. She can be reached at firstname.lastname@example.org.