Utility Participation and Iterative Design over Rules of Thumb

Ground Loop Precision for Commercial Applications

Blog #82 CaliforniaGeo 9-7-21

 IGSHPA’s latest Town Hall-

In keeping with an effort to stimulate, enlighten, and provide education on topics aligned with geothermal heat pump technology, the International Ground Source Heat Pump Association produced its most recent Town Hall session on August 26th.  Even though the option for your live attendance has passed, the 61 minute presentation resides at IGSHPA’s YouTube Channel.

Otter Tail-

OtterTail Power of Fergus Falls, Minnesota serves a territory with electricity in western Minnesota and eastern edges of the Dakotas.  They are strategically retiring coal generation in favor of solar and wind resources.  With less fossil base load supplying their grid, the cutting demand peaks is needed.  With greater overall electric load addition through promotion of carbon-free heat pumps, there is an even greater need to keep those larger electric loads balanced.

In OtterTail’s region, the winter and summer seasons require plenty of heating and cooling.  And with an eye towar serving customers as well as keeping demand low, they are running a program called Integrated Building Design Plus, (IBD) where three kinds of professionals join with commercial building owners to build from scratch or to retrofit with cost-effective measures that will bring the best returns in energy and cost reduction. 

For the benefit of architects and engineers’  consideration of energy efficiency into their designs on behalf of customers, OtterTail sponsored training sessions on the ground loop design process.  Ed Lohrenz led this effort.  His dba is appropriately named GeOptimize, and that is what he focuses on to yield geo heat pump loop field performance that handles the building loads in perpetuity with thermal stability in the loop field underground. 

The expression “Rule of Thumb” (ROT) has some applicability across much of our society, including the building trades.  But, in the calculations and design of geo heat exchangers to match their building’s needs, they can be as disruptive as they are simplistic.  They can sometimes be successful, but if we’re talking about a sizeable commercial building, following ROT can quickly boost a geo heat exchanger’s cost, knocking geo out of the picture.  One antidote to this outcome is to walk away from ROT.

Iterative Modeling- 

If you’ve worked with spreadsheets to test outcomes with different variables, you have an idea why and how multiple iterations (fresh calculations) can lead a designer to a preferred outcome.  And since this process is in an office before architects and engineers ever lock down a final building design with their wet stamps—it’s less expensive—but it must start early in the process.

This process is familiar to those utilities who have “coached” their customers toward energy efficiency measures based on cost versus savings.  The same principle applies here (although working with thousands of cubic feet of underground soil mass is more difficult).


OtterTail Power is pleased with the results.  Their leadership has unified professionals to seek minimal first development costs with solid savings over the long-term for their customers.  This expands the use and notoriety of geo heat pumps in their territory.

The Design Challenge Underground-

How do we achieve the greatest benefit from the least amount of underground pipe to cut down on construction costs?  We have to get the design right so that ground loop area does  not grow warmer or cooler over time, resulting in equipment inefficiency.

If the underground formation at the project site were to not have adequate conductivity (for importing heat) or adequate diffusivity (for rejecting heat) that formation could get cooler or hotter over time.

The greatest zone of action here is close to the vertical loop field in the illustration.  But this chunk of formation also interacts with the surrounding zones, and how we lay out a loop field also affects its ability to interact within an acceptable thermal range.

Also, if we design just for anticipated peak heating or cooling loads, our underground temperature range may be fine, but the total cost of installation will climb.  The better course is to analyze a working system in advance with iteration technology.  (That’s a glorified spreadsheet program, to you.)  We can look at the value of better windows, insulation, lighting controls, etc. to see if their costs for better performance could markedly reduce drilling needs.  We can also look at controls, scheduling, or set backs.  For example, a better insulated building would not require as much energy to return to a set point—but will that avoided cost be more (or less) than the reduced loop field that would have been necessary?

 Site Review-

When you know how much territory there is for installing a loop field, you might choose to make the spacing of individual bores further apart.  This means 25 feet instead of 15.  More formation between bores means you have more thermal storage to access, down there.

The Fergus Falls Library expansion (at right) was reviewed for the maximum potential bore hole spacing.  Once the wider future building footprint took up more of the property’s surface area, what remained for design purposes became critical information.  We should also note that in a “from scratch” project, vertical bores could have been placed under the building itself, expanding the heat exchanger’a potential capacity. 

Crunching Numbers with Computerized Iteration-

In a project that is not the above library, here are a couple of examples of this process.  Once the building’s dimentions, climate location, and occupancy schedule are known, the amount of peak cooling and peak heating required can be identified.  With geo heat pump technology, we always know that we must serve the highest load.

However, if either one of the two loads is vastly higher than the other, we are forced to 1) add far more ground loop or 2) accept migrating temperatures in our loop field over time.  Answering 1 would cost more money.  Answering 2 would require adding a small amount of mechanical equipment to either raise (boiler) or lower (chiller) the loop temperature when necessary.  There’s nothing inherently wrong with ending up with a “hybrid” ground loop system, but is there another way?  Yes.

What’s below is an out-of-whack situation, where the cooling load is much higher than the heating load during the entire year.  Note the area of red on the left compared to the area of blue.  This imbalance forecasts loopfield thermal migration.

After a number of iterations involving different glazing, insulation and elimination of a planned 15-degree cooling set-back, a much improved balance was predicted.  Once this was achieved, further calculation for an optimally sized geo heat exchanger (at less cost) was possible.  This one actually shrunk by 40% and $227,000.  And with the renewable use of earth’s thermal battery, the operating costs remained lower than conventional HVAC would. Sample List of Measures that could end up in the Iteration Process-

Glazing options                                                             Window shade structures (outside)

Insulation                                                                       Window coverings (inside)

Daylighting techniques for less wattage                      Indirect vs Task lighting

Building orientation                                                     Interior thermal mass

Heat recovery ventilation                                             Dual outside air system (DOAS)

Cool roof treatment                                                       Off-peak cooling and/or ice storage

High conductivity grout                                                  Grout additives

[These features can be modeled singly or in combination with each other]

Comparative quantitative answers are available because the number crunching is done by computer.  You get to see results with single or multiple adjustments of the above factors.  Experience with this approach instead of ROT can shorten a mechanical engineer’s learning curve.  

Computer modeling with the iterative process can be the next best thing to building the project and then monitoring it closely.  Clients want good information for decision making.  We are better positioned to provide it with an expansion of iteration-based design. Summary-

Experience is better than guessing and use of predictive modeling is even better.  We want a dependable estimate of what it will take for one of the largest costs in a geo heat pump project—the underground heat exchanger.  Rules-of-thumb are no way for accuracy, and drilling is too expensive to overdo.  Computer-based iteration can provide everyone connected with a project greater confidence.  Customers are more likely to choose Geo for their commercial buildings as a result.

—Bill Martin