Solar panels and wind turbines are easy to spot, but you could be walking over savings and not even know it. A geothermal heat pump system, also known as geoexchange, may be tucked away under the ground, but your utility bill will surely notice its presence.
Renewable Energy Underfoot
Regardless of weather conditions, temperatures below the earth’s frost line remain constant, ranging from 45-70 degrees F. depending on the climate zone. A geoexchange system uses these temperatures as a heat source in the winter and a heat sink in the summer. It takes less energy to transfer this heat to and from a building than it does to condition outside air.
A geoexchange system depends on three components. The ground heat exchange, or loop field, is a series of pipes buried in soil or a body of water. Depending on your space limitations, these can be installed vertically or horizontally. The exchange is filled with antifreeze or water that conducts heat as the liquid circulates through the pipes.
In the winter, the refrigerant absorbs ground heat and carries it to a heat pump, which transfers the energy into the building. In the summer, heat is drawn from a building’s air and rejected back into the earth. A traditional air delivery system is used to carry the air into the work environment.
It may sound daunting to drill holes hundreds of feet into the earth or sink pipes into a lake, but a geoexchange system produces many benefits that directly contribute to energy savings and sustainability goals. Reap benefits in four key areas:
- Installation – Geoexchange systems can be installed in most climate zones. All you need is the space and soil conditions to support a loop field. Geothermal heat pumps can also be installed as you work on or build other parts of your building – it doesn’t need to be an isolated project. When finished, you can place landscaping, a parking lot, or your building on top and no one will be the wiser.
- Energy Savings and ROI – Geoexchange helps you lessen your dependence on the grid. Systems are estimated to cut your energy thermal load between 40-70%. The DOE rates a system’s ROI at 5-7 years and a 10% tax credit is also available.
For example, Doug Dougherty, president and CEO of the Geothermal Exchange Organization, had a bid for a conventional HVAC system that was $600,000 while the geothermal system was $900,000. “Because the geothermal system is eligible for a 10% federal income tax credit and 100% accelerated depreciation, the $300,000 difference would be made up in less than a year. The energy savings are projected to be $40-50,000 a year for a 20-year system,” Dougherty explains.
- Maintenance – Despite combining the function of furnace and air conditioning units, these systems are simple to operate. While the heat exchange is buried in the ground, all of the other elements – the heat pump, compressors, and blowers – are located inside and away from weather extremes or vandalism. This is an “out of sight, out of mind” solution that needs simple preventive maintenance, such as changing the filters and oil.
- Sustainability – Geoexchange systems are made of environmentally friendly materials, such as non-corrosive polyethylene piping and non-toxic antifreeze. While the system isn’t carbon neutral because of the use of electricity, emissions are greatly reduced by eliminating fossil fuels burned on-site. They contribute to occupant comfort because pollutants and mechanical white noise are reduced. The heat island effect is also minimized because the heat exchange associated with rooftop HVAC units has been eliminated.
Sound too good to be true? Geoexchange may not be for every facility. Apart from costs, space can be a major barrier to installation. Buildings in urban areas or on small properties may not have the luxury of adding a loop field.
A Boilerless Building
Great River Energy is no stranger to energy efficiency. As Minnesota’s second largest electricity provider, the company built its headquarters as a showcase for green technologies that its clients could also implement.
As a result, a lake geothermal heating and cooling system was selected to support the building’s LEED certification. With a closed loop exchange of over 35 miles of polyethylene tubing, the system has an average 8-year payback and contributes to the building using 40% less energy than a comparable property.
Previously a gravel pit, the man-made lake is over 30 feet deep, but installing the pipes under water was a surprisingly simple operation. “Because the coils have air in them initially, they float,” explains Doug Pierce, senior associate for the design firm Perkins + Will. “They were positioned on the lake and then the fluid was pumped in. Once the air was dispersed, they sank. What’s great is that if we need to work on them in the future, you just drain the system, pump air back into it, and the coils will rise up.”
The system also has a minimal impact on wildlife. Rejecting heat into the water only raises the lake temperature by less than a quarter of a degree. The exchange also provides an added habitat feature on the uniform lake bottom and the antifreeze is food-grade, Pierce specifies. Leaks are highly unlikely, but should one occur, no harmful chemicals would be dispelled into the water.
Unlike an earth system, lake temperatures fluctuate between 39 degrees F. in the winter and in the mid-50s during the summer. This requires a design change so the system can acclimate to lake conditions. “There is a variable frequency drive pump that is computer-controlled, pumping more or less fluid based on the temperature of the building and the lake,” says Pierce.
This is a departure from conventional geoexchange systems, which typically use a constant volume pump. With a smart system, however, energy consumption can be lowered even further. Particularly during the shoulder seasons, “we don’t have to turn the compressors on the heat pumps to get the air to be the right temperature,” Pierce says. “Whether it’s heating or cooling, it doesn’t really matter – the temperature of the fluid coming off the lake is more compatible with the air temperature that’s being delivered.”
To maximize the geoexchange system, it was paired with an underfloor, low-velocity displacement ventilation system. Unlike a conventional HVAC system, which delivers cool air at about 55 degrees F., an underfloor system moves conditioned air to a room at 67 degrees. By introducing air at a low velocity from the floor, body heat from workers uses convention currents to draw up the air.
One of the most remarkable results of implementing the geoexchange system was the ability to forgo boilers. It’s difficult, admits Pierce, to predict the absolute performance of the building, so space was left for boilers to be added. Though the headquarters has been open since 2008 and already experienced several harsh winters, the boiler room remains empty.
A $2.5 million geoexchange system is getting high grades at Cross Oaks Elementary School in Denton, TX. The recently opened building wasn’t originally designed to accommodate renewable energy, but the district’s desire to lower construction and operational costs prompted some significant design changes from the original prototypes.
The 88,231-square-foot school has a massive geoexchange system. A typical installation will include 1-2 loop fields, but Cross Oaks has 5, totaling 288 individual wells that are 300 feet deep. Because the system is so extensive, the payback is doubled.
However, the extended time is offset by reduced replacement costs. “The school felt comfortable paying more for the upfront costs of a geothermal system, knowing they would have an 18-year payback when they went to replace it,” says Terry Hoyle, principal for the SHW Group, which designed the project. “When you have a rooftop system, you’re replacing the whole thing. For geoexchange, you only replace the fan and the pumps.”
Utility and replacement costs weren’t the only area for savings. By switching to geothermal, the roof structure design was greatly simplified. Without HVAC units on the roof, penetrations, required access points, parapets, and internal drains were reduced in number. “Whenever you have equipment on your roof, you have the potential for oil and chemical leaks that can degrade the roof,” Hoyle points out. “You have people going up for maintenance, which means it’s more susceptible to wear and tear from cuts or splits. You also have to reinforce your structure for the equipment. By moving the system inside, the impact is minimized.”
To ensure maximum efficiency, the school also opted for a centralized pumping system, which reduces the number of moving parts. “With a centralized system, all of the equipment is in the mechanical room and the maintenance staff knows where it is,” says Mike Elmore, project manager for SHW Group. “In a localized system, the pumps can either be on the unit or on the well head out in the field.”
While geoexchange systems are easy to install, SHW ran into a dilemma when it discovered that the school’s daylighting goals complicated the ability to place pipes. Previously, ductwork ran unseen in the attic space, but an added clerestory in that area required it to be relocated. “The solution that we came up with was to tap into the agrarian design style of the building and use stainless steel piping,” Elmore explains. “We brought the ductwork through the clerestory, turning it 90 degrees and exposing it in the classroom.”
In addition to utility savings, the geoexchange system inspired several creative side projects. The open roof design allows rainwater flowing unrestricted off of the roof to be directed to a bioswale, which eliminated the need for underground storm collection.
“We also collected the condensate from the geoexchange system in underground storage and used it for drip irrigation. Typically you run your drainage pipes to the storm sewer – we just ran it to a holding tank and used a landscape irrigation pump,” says Hoyle.
While still in its first year of operation, the school has been pleased with the decision to include geoexchange. ”If you’re serving the public, geoexchange is a good investment of public funds and builds trust with the community,” Hoyle adds. Switching to this renewable energy source created opportunities to make the building design more efficient, which will save an average of $34,000 per year in operational costs.
A Well of Opportunities
While still a small portion of the HVAC market, the “no fuss, no muss” benefits of geoexchange systems are making a splash around the country. GSA now requires mandatory bids for them, the Idaho and Oklahoma state capitals have installations, and Louisville, KY, boasts the world’s largest system. Stop by Lincoln’s Tomb in Illinois or the Gettysburg National Park in Pennsylvania and find geothermal underfoot.
Wind and solar may be a flashier way to secure green power, but geoexchange systems offer firm ROIs and more reliable energy production. “Geoexchange systems have some very real efficiencies with good paybacks,” says Pierce. “With the right property and conditions, it’s a viable, reliable, and easy-to-manage system.”
Jennie Morton (firstname.lastname@example.org) is assistant editor of BUILDINGS.