… Especially once you understand the different loop types. If I had a choice of fields from which to choose in hindsight, it would be well drilling. Drillers are in high demand; especially those that are in the know on geo-exchange technologies. It’s only going to get better, too. Read on for the basics on what’s what with geo-exchange loop types.
By far, the most common geo-exchange is vertical closed loop. It’s likely that most who have looked into geothermal heating and cooling have seen illustrations and animations of how the system works. Vertical boreholes are drilled, a pair of high-density polyethylene (HDPE) pipes are fused together at the end with a special “U” bend fitting, and that is inserted to the base of the borehole, typically between 200 and 500 feet deep. The borehole is filled from the bottom up using a tremie pipe to pump thermally enhanced grout to seal the borehole and promote heat exchange with the surrounding earth.
These boreholes are situated typically in a 20-foot on-center grid pattern, utilizing available space such as adjacent fields, under parking lots or even under the building itself in new construction scenarios. The pipe ends are interconnected between dozens and sometimes hundreds of boreholes in commercial applications.
Sometimes, when space becomes a limiting factor, such as in retrofits, the geothermal systems may be excluded from consideration due to the amount of real estate needed, infrastructure concerns (i.e., parking lot and egress concerns) and first cost. There are a considerable number of alternatives. Here is the geo-exchange 101 on loop types:
- Closed Loop: Pond loop, horizontal loop, vertical loop
- Open Loop: Aquifer-based (class V thermal exchange), water body (extraction and return)
- Standing Column
Much of the time, the geo-exchange project cannot be completed without the drilling of boreholes; the only exceptions being pond and horizontal loops. There are many considerations that go into the decision to depart from the normal vertical closed loop system. Properly applied, vertical closed loop systems are exact science, leaving little room for error while performing their functions with unwavering effectiveness. So what are the reasons for which the open loop or standing column geo-exchange source would be chosen?
- Limitations of available space
- Larger cooling dominant loads
- Scalable reduction of first cost
For a primer, take a look at the geological cross-section at right.
The graphic cross-section is an accurate representation of the lithology west of Washington, D.C. Following closely the path of Interstate 95 through that area, to the west you would find lithology similar to that shown on the left side of the graphic, and to the east of I-95 you will find plentiful aquifers as indicated on the right side of the graphic. If you drill geothermal wells in the Piedmont Plateau, you would be indeed fortunate to find well yields over 5 gallons per minute, because the lithology is such that water is found only in fractures in the dense rock (granite). Alternatively, if you are drilling in the Coastal Plane, you would find plenty of water resources.
While vertical closed loop geothermal will work in both examples above, the scenario represents a perfect opportunity to consider an alternate geo-exchange for one of the three reasons listed above. Let’s cover the Open Loop first.
An open loop geo-exchange system (sometimes called a groundwater exchange or class V thermal exchange) comprises at least a supply borehole and a return borehole. Water is withdrawn from one borehole utilizing either a submersible or above-ground turbine pump, and then injected into the other borehole. Since geothermal heating and cooling requires about 3 gallons per minute of source water per ton, it’s important to have a relatively permeable aquifer.
A standing column geo-exchange system involves extraction and injection into the same borehole. This may seem counterintuitive; however, properly engineered standing column geo-exchange wells perform remarkably well in certain lithology.
The most practical conditions under which the standing column will work involve geological conditions in which most of the borehole is comprised of solid rock. The borehole is drilled and cased through the extent of the overburden, then the walls are left bare the remaining distance or depth. Depth is determined much like closed loop, only with a new set of parameters, because their borehole walls provide three types of heat transfer. Conduction, convection and advective heat transfer.
Carl Orio, co-writer of “Modern Geothermal HVAC Engineering and Control Applications” (McGraw-Hill Professional, 2013), has been engineering standing column wells for over 30 years with 15,000 installations in the New England area. Many of these installations are in New York City operating flawlessly to heat and cool commercial buildings. He states in the book that one 1,500-foot well may yield up to 50 tons of heat transfer available for heating and cooling purposes. This is six to seven times the heat transfer per linear foot of bore as compared to closed loop. It makes sense then that a project that might need 100 boreholes closed loop, but doesn’t have the real estate, may be more feasible when the footprint is reduced to 16 boreholes.
Referring back to the open loop type of system, the footprint can be reduced further to perhaps four boreholes; two for supply and two for injection (two pairs are listed as typical for redundancy).
With standing column and open loop geo-exchange systems, front-end engineering is critical, but drilling costs and, hence, overall costs for the geothermal project may be reduced by a good margin.
In the National Driller column Geothermal Drilling; Your Roadmap to Profits from July 2014, I recommended that drillers might find success in partnering with MEP (mechanical engineering firms), mechanical contractors and manufacturers. It is comforting for design entities to know they have a driller that will understand what they’re talking about. It makes the choice easy when it comes to invitations to bid.
So go onto the GEO or IGSHPA websites and list your company, then get trained and keep it up yearly by attending a Geothermal Innovations Workshop held in your area. You’ll be glad you did!
To find all this and a lot more information on geothermal HVAC, pick up a copy of “Modern Geothermal HVAC Engineering and Control Applications” (McGraw-Hill Professional, 2013).
Jay Egg is a licensed geothermal consultant, columnist, and the owner of EggGeothermal. He has co-authored two textbooks on geothermal HVAC systems published by McGraw Hill. He can be reached at firstname.lastname@example.org.