Below the Surface

Geothermal experts share system specification tips for this ever-emerging, energy-efficient, sustainable technology.

CONSULTING-SPECIFYING ENGINEER: It’s been said that geothermal technology has been gaining popularity over the past few years at a rate of roughly 20% per year among building owners and design professionals alike. What’s driving this trend?

BRADFORD : Several factors have combined to create significant economic and environmental impacts. First, the availability of lower cost capital has helped spur institutional and commercial construction projects. Second, extreme price volatility in the energy markets has led many owners to evaluate investments in facility infrastructure that have longer paybacks.

Finally, environmental stewardship is a theme increasingly being adopted by business and industry leaders. Consequently, programs promoted by the U.S. Dept. of Energy and the U.S. Green Building Council are beginning to have an impact on the traditionally conservative HVAC industry.

TOWNSEND : Another significant factor is that geothermal systems require no noisy outdoor equipment, which the architect and owner have to figure out how to hide. Also, as a sustainable technology, these systems help building owners fulfill Leadership In Energy and Environmental Design (LEED) certification program requirements. But perhaps the greatest factor has been documented proof that when properly designed, installed, operated and maintained, these systems produce considerably lower utility, operating and maintenance costs. Owners can immediately see that a small increase in initial investment yields 20% to 30%—possibly even greater returns—from reduced overall operating costs.

CSE: For what building types, and under what specific conditions, is geothermal technology efficient and cost effective?

DOOLEY : Buildings such as public schools, colleges, office buildings and hotels, which require individual heating and cooling for many zones, are ideal candidates for heat pumps.

BRADFORD : I would add hospitals to that list, although the overall efficiency of the system is dictated by the application and varies with the design conditions being specified. Applications in which energy-recovery strategies are being employed benefit the most from geothermal heat pump systems.

PAMPLIN : Geography also makes the technology more effective and economical. For example, direct-exchange geothermal technology uses Mother Earth as the heat sink or heat source. Therefore, in extreme northern climates, the technology can extract heat from the earth for heating in winter but doesn’t have to depend on it for cooling in summer months because it isn’t as major an issue as it is in extreme southern climates, where the cooling season is long, and there’s not much need for heating.

CSE: What factors are inhibiting wider use of the technology?

DOOLEY : Lack of awareness.

BRADFORD : Also, lack of training and certification programs for design professionals and mechanical contractors.

As the availability of these programs has improved, along with the publication of successful long-term case studies that include empirical data, geothermal heat pump systems become more widely understood and accepted. This, I believe, has resulted in more systems being proposed to building owners.

TOWNSEND : But there’s also a number of technical reasons:

• The cost of installing the geothermal loop field.

• Unacceptable and unpredictable soil conditions, such as underground caverns or unstable earth strata.

• An unreliable water resource.

• Insufficient land area that can be used for a geothermal field.

• A shortage of competent loop contractors in a particular geographic area.

But perhaps the greatest barrier remains the ignorance and fear that results in the presentation of unrealistic installation cost figures to justify the need to go with a more conventional HVAC system.

CSE: For the record, what are the most common geothermal configurations, and under what conditions are they best employed?

TOWNSEND : Vertical loop, horizontal loop, groundwater source, and pond/lake/river. As far as vertical loops, ideal conditions include either solid rock or a stable earth strata that doesn’t require bore-hole casement. For horizontal loops, it’s important to have a large enough surface area with stable soil conditions for trenching purposes. For groundwater sources, it’s key to have a year-round supply of water with a temperature range of between 55

CSE: How about the loop technology itself—what’s out there and what are the pros and cons?

PAMPLIN : Furthermore, copper-fitted direct-exchange systems are less costly to install, because there is less digging and trenching associated with copper than plastic loops. For example, if an installer has to drill a water-source geothermal system, it requires 5-in. to 6-in. diameter holes that need to be 200 ft. to 400 ft. deep. On the other hand, some copper loops only need 3-in. diameter holes dug 50 ft. deep.

CSE: What are some basic design guidelines and strategies consultants should consider when getting involved with geothermal systems?

PAMPLIN : Since building professionals typically want to know about cost and return on investment, it’s important to know that it will typically cost $3,000 to $5,000 per ton to install. Typical return is three to seven years without any utility rebate, incentive programs or rate increases. However, in some states like New York, Pennsylvania, New Jersey and New Hampshire, utility incentives are phenomenal and building owners can see a return in just one to two years.

BRADFORD : Many professional organizations, such as AEE (Assn. of Energy Engineers), IGSHPA (International Ground Source Heat Pump Assn.) and ASHRAE, offer a number of publications that are helpful to designers. One of the most comprehensive guides is ASHRAE’s Commercial/Institutional Ground-Source Heat Pump Engineering Manual. Beyond good design practices, proper system commissioning is an essential key to optimizing these high-performance systems.

DOOLEY : It’s important to note that drilling in an area of rock is not a problem and does not greatly increase costs. Additionally, large areas of land are not needed for a vertical heat exchanger. In reality, approximately 250 to 500 tons of capacity can be installed in an acre of land. And the capacity of the field is dependent on building loads, ground temperatures and soil conductivity.

CSE: In what ways is the technology continuing to improve?

DOOLEY : Computer simulation tools have improved system performance. Also, with the advent of improved grouting materials, the length of loop has decreased 30% over the past five years.

BRADFORD : The risks associated with earth-coupled heat exchanger designs have also been identified and significantly reduced as a result of the research being sponsored by government and industry groups.

Participants

Ray Bradford , East Coast Region Manager, WaterFurnace International, Fort Wayne, Ind.

Robert Dooley , P.E., President, R.J. Dooley Construction, Inc., Poughkeepsie, N.Y.

Chris Pamplin , Manager, Marketing & Sales, American Geothermal DX, Murfreesboro, Tenn.

Terry E. Townsend , P.E., President, Townsend Engineering, Chattanooga, Tenn.

Getting Educated on the Basics of Geothermal Technology

When it comes to geothermal systems, the first thing would-be users have to overcome is the impression that geothermal heat pumps and systems are “new technologies.” Geo-based HVAC systems have been around for more than 50 years and have been very reliable in their operation and overall performance. Consultants who are considering a geothermal heat pump project should follow this sequence of activities:

Before becoming involved in a formal design effort, take a design course/seminar offered by a utility, college or a professional organization such as ASHRAE in order to learn the design criteria for a geothermal project.

Determine a source for conducting vertical and horizontal loop-sizing calculations. While hand calculations are really impractical for most applications, a computer program may be too costly to purchase for a single design application.

For a vertical loop field, have a sample bore-hole drilled in a location where it can become a part of the overall bore-field layout, and review a thermal conductivity report. For a horizontal loop configuration, it’s important to commission a soil scientist’s report for the area being considered. For a groundwater source system, check with state agencies to see if they have historical information on the underground water flows, depths and annual temperature ranges. Finally, for a pond/lake/river application, check with state and federal agencies, such as the U.S. Army Corps of Engineers or Parks &Recreation, for access to historical data on depths, temperatures and restrictions for systems being introduced into a waterway.

If there are no stumbling blocks at this point, the final size of the geothermal field/water source is dependent upon the actual heating and cooling loads that the heat sink/source will experience during the course of a year. The use of block loads with schedule diversity integration can help achieve a properly sized geo-source. Undersizing of the geo-source will create unacceptable loop temperatures and could possibly damage the geo-components. Oversizing will usually produce a high first cost and could possibly kill the project even though the overall operating efficiency may actually be increased with an oversized geo-source.

Other considerations:

Judiciously treat and control the introduction of outside air into the facility.

Avoid any corrosion in the geothermal loops by the proper selection of components and materials.

Install a water meter in the make-up water line to the geothermal loop(s). Make sure a monitoring capability is built in. When water is being made up, make sure it is occurring as the result of reasons known by the owner/operator, and that it’s not a system leak.

Try to locate all geothermal loop control valves and manifold headers inside the facility in an accessible area. If it’s not possible, have the loop control valves located in an accessible or lockable vault or manhole. These loop control valves may have to be used to balance the loop flows.

Seek out a good grout specification for installations where grouting is required. Make sure that the grout thermal conductivity is what is needed in order to make the geo-source as thermally effective as possible. Don’t rely upon the loop contractor for this information.

Secure or develop comprehensive specifications for high-density, polyethylene (HDPE) pipe for the anticipated performance parameters—temperature and pressure ranges, along with the installation, grouting, purging and welding—and testing activities required during the project. Separate specifications may be required for HDPE piping. One word of caution: HDPE piping has greater thermal expansion properties than other types of piping. This must be taken into consideration for any interior, above-ground loops.

Separate the loop contractor duties from the mechanical contractor duties and have each treated as a subcontractor by the general contractor.

Integrate the geothermal system components and controls into the overall facility’s computer control/energy management system.

Finally, before the design effort reaches the design-development phase, it would be beneficial to have someone with geothermal design/installation experience conduct an overall review of the design concept. Too many times designers become so entrenched in their design effort that they overlook some fundamental concept that come back to haunt them. Also, this overview in the early stages of the design process could avoid some unnecessary efforts on the part of the design team.

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