Below the Surface

By Barbara Horwitz-Bennett, Contributing Editor -- Consulting-Specifying Engineer, 3/1/2004

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 55ºF and 75ºF. For ponds, lakes or rivers, a year-round, constant-depth water source is critical in order to minimize the potential for adverse factors such as anchors, pollution and fishermen.

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

PAMPLIN: There's direct-exchange, indirect exchange or water-source geothermal systems. Direct-exchange, in my opinion, is the more efficient of the three. Also, almost no one would argue that copper is a better conductor. [Editor's note: According to Dan Geremia with the Copper Development Assn., the reason is that when buried underground, copper comes into direct contact with the earth's heat source for immediate thermal transfer. Water-based systems, he says, that use plastic pipe need two transfer stages, which require up to twice as much energy to operate. And unlike systems that draw or release heat into the water or air, DX systems draw and release heat into the ground instead. They do not emit combustion gas, so he claims they don't need to be vented. And unlike conventional, forced-air heating systems, which provide heat in short blasts and can rob moisture from the air, Geremia says DX systems provide a cooler, more constant flow of air for greater comfort.]

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.