In the war to cut down on our energy usage, a frontline has been drawn in the field of temperature control. The U.S. Department of Energy estimates that roughly 40 percent of the nation’s overall power consumption goes toward heating and cooling our homes, offices, and institutions. Whether they draw on the electric grid to beat the heat during summer or burn fossil fuels on site to fight the cold of winter, buildings, on the whole, make SUVs look environmentally friendly.
While significant strides have been made to change this picture by designing more efficient mechanical systems, increasing thermal performance, and developing on-site generation of renewable energy—all important and admirable advances—perhaps the most promising resource available to architecture may be found within the very terra firma upon which structures sit, in the form of geothermal systems.
The first thing that any expert on the technology will tell you is that geothermal systems for buildings, also known as geothermal heat pumps or ground-source heat pumps (GHPs), are not the same thing as geothermal power plants. Geothermal power plants—known in the industry as hot rock geothermal—are large installations built (in this country) mostly around the Rocky Mountains and the Sierra Nevada range, where extremely high temperatures from the Earth’s mantle can be found relatively close to the surface. They tap into these reserves of heat and use them to produce steam, which then drives a turbine, thus producing electricity.
GHPs, on the other hand, use the thermal energy stored in the upper portion of the earth’s crust to heat or cool a building, replacing conventional boilers and air-conditioning systems. “The temperature of the Earth down 20 or 30 feet is a relatively constant number year-round, somewhere between 50 and 60 degrees” F, says John Kelly, the COO of the Geothermal Exchange Organization, a nonprofit trade organization in Washington, D.C., that lobbies for wider adoption of the technology. “A geothermal heat pump moves heat to and from the Earth by circulating water through a well.”
“For example,” Kelly offers, “let’s say in Kansas City, the underground temperature is 55 degrees. In summer, the air temperature is 100 degrees, and in winter it’s 20, but underground it’s still 55. It’s not that hard to get that constant temperature out of the ground so you can heat in winter and cool in summer.”
In other words, in winter, a GHP moves the thermal energy from under the earth into a building, and in summer it reverses that process, moving the heat in a building down into the earth. These systems incorporate a piping loop buried in the ground through which water is circulated, and the heat pump removes the temperature from the water and distributes it through the building, much in the same way that central air conditioning works. Alternatively, groundwater is directly circulated through a series of wells.
Either way, GHPs are significantly cheaper to operate than conventional heating and cooling systems. “The cost savings occur because the ground offers starting temperatures closer to what is desired for heating and cooling than the seasonal temperature extremes upon which many conventional air-source HVAC systems rely,” says John Rhyner, a senior project manager at P.W. Grosser Consulting in Bohemia, N.Y., a civil engineering firm that specializes in geothermal and is currently authoring a book of guidelines on the technology for the New York City Department of Design and Construction. “It takes less energy to make up that smaller difference in temperature,” Rhyner says.
While the theory and technology behind GHP are simple, implementing a GHP system can be a more complex matter. There are several different types of GHP systems, and choosing the best one for a specific project can require a good deal of study and tailoring. “It’s not a cookie-cutter approach,” Rhyner says. “Some level of up-front feasibility analysis is needed to pick the right system for a particular site. For a medium-to-large commercial system, due diligence and feasibility analysis are critical, and shouldn’t bust the budget.”
“The objective,” Rhyner adds, “is to get everyone on the same page at the start and provide clear direction on approach. The type of system that’s most suited varies geographically, all depending on the geologic conditions, how the building gets coupled to the ground, and what drilling method is appropriate.”
The three most common types of GHP systems are closed-loop, open-loop, and standing column well. Closed-loop systems circulate water through a sealed network of pipes buried underground. The water within the pipes transfers heat from the earth to the building during the winter, and vice versa during the summer, by way of a heat exchanger. Since the water flows in a closed loop, it does not exchange all of its temperature; it can get as warm as 80 to 90 degrees F in summer and as cold as 40 to 30 degrees F in winter. For this reason, the water is usually combined with a 30-percent mixture of food-grade antifreeze (for example, propylene glycol) to keep the fluid from gelling during the winter months.
Closed-loop systems can be laid out either horizontally in fields, buried just beneath the frost line, or vertically in wells, bored typically 200 to 500 feet deep. Horizontal systems are generally used for smaller or residential projects. They are cheaper to install, but are affected by outdoor air temperatures, meaning that they can become less efficient as a season progresses and as the soil takes on the characteristics of the air temperature.
Vertically drilled closed-loop systems are more efficient than horizontal systems, as more of the pipe is in contact with the stable, cool earth materials. They are most efficient if they can be drilled into groundwater rather than dry ground, since water is a good conductor of heat. Closed-loop systems typically require large amounts of land. “For a closed-loop system, it’s all a function of how much pipe you can get in the ground with the open land area you have available to work with,” Rhyner says. “You get a certain number of tons per linear footage [a ton of heat is 12,000 British thermal units per hour], and can get more pipe in the ground going vertically than horizontally.”
Open-loop systems draw actual groundwater from a well, move it through a heat exchanger, and then return the water to separate wells, where it is allowed to percolate back to the aquifer. The supply and return wells (the latter are also known as “injection wells”) must be placed far enough apart to ensure that the thermally altered water, (i.e., the water that is heated or cooled) is not sucked back into the system through the supply wells until it regains ground temperature. The number of injection wells needed depends entirely on the rate of flow generated from the supply wells.
Open-loop systems are generally more efficient than closed-loop systems because they are better coupled with ground temperatures, with no heat transfer occurring across the plastic closed-loop pipe or the grout used to seal the borehole. However, these systems do present challenges, most notably water chemistry, which can corrode heat-pump equipment or may foul the system over time, requiring extra cleaning. If groundwater contains high levels of salt, minerals, or iron, closed-loop systems are generally preferable.
Standing column wells are a specialized type of open-loop system that is well suited where bedrock is not too deep below the surface. Standing column wells are drilled to depths of 1,500 to 2,000 feet. The shallow portion of the well through the soil zone has steel casing installed, while the remaining depth is drilled and left as an open rock borehole. In these systems, the groundwater is pumped up from the bottom of the well, passed through a heat pump or heat exchanger, and then returned to the top of the well, where it filters slowly downward, exchanging heat with the surrounding bedrock.
According to Rhyner, “Standing column wells provide the most thermal capacity per installation, thus are popular in urban locales like New York City with limited real estate to drill.” Where bedrock is deeper than 100 to 125 feet it can get too expensive to install these wells, because of the amount of steel casing you would need to seal off the soil zone.
Choosing which of these systems is right for a specific project requires calculating a building’s heating and cooling demand and conducting a subsurface analysis to determine the thermal capacity of the site, and how many wells or how large of a loop field will be needed. If the calculations are done correctly and the system is properly designed, GHPs can handle all of a building’s heating and cooling loads, no matter what climatic conditions prevail.
“Heat pumps work anywhere in world,” Kelly says. “They certainly work well throughout North America. They are widely used in Canada, and likewise in Mexico.” When designed and installed right, GHPs drastically reduce the amount of energy needed to heat and cool a building. According to the U.S. Environmental Protection Agency, GHPs are 48 percent more efficient than the best gas furnace and 75 percent more efficient than the best oil furnace. They require 25 to 50 percent less energy than other HVAC systems and bring down operation and maintenance costs by as much as 40 percent.
The main inhibitor to the wide-scale adoption of GHPs today is the relatively high up-front cost of installation, most of which goes toward the drilling involved in constructing wells and loop fields, and the design and analysis needed to tailor a system to a building. The mechanical equipment itself—the heat pumps and heat exchangers—is no more expensive than conventional heating and cooling systems. Annual savings on energy bills, however, offset the up-front cost. Payback periods for commercial GHP systems are generally calculated in the 10-to-20-year range but often can be shorter, for example, if the system is replacing an aging, inefficient HVAC system. GHP systems can be cost-competitive against many conventional systems in new construction. As a result, GHPs have primarily been popular with municipal and institutional clients, building owners who plan to inhabit and operate their facilities over the long term, and those who are simply more interested in environmental stewardship than the bottom line.
The City of Chicago is currently in the midst of constructing five new branch libraries that incorporate a number of sustainable design features. Designed by Lohan Anderson, the libraries rely on GHP systems. Among them is the Richard M. Daley Branch Library, a 16,300-square-foot facility slated for LEED Silver certification. The branch is served by 24 closed-loop geothermal wells drilled to 395 feet beneath the parking lot. These will provide hot or chilled water to an air-handling unit, and also to an underfloor radiant system. While the GHP system’s thermal capacity is sufficient to provide all of the library’s heating and cooling needs, the client also requested a backup boiler.
“There’s not a lot of experience on the city’s part with these systems, so they didn’t feel comfortable relying solely on geothermal,” explains Stephen Novak, a project engineer at Chicago’s Henneman Engineering, which designed the library’s GHP. “After a period of time, once there is more documentation and they see that these systems work, the boilers may be eliminated from the city’s branch library prototype.”
GHPs don’t necessarily have to be custom-designed for individual projects. Novak sees the construction of large district geothermal fields that investors could tap into much like a utility as one way that the technology could become more prevalent in the future. Boise, Idaho, for example, has a district geothermal system that heats buildings in its downtown core. But the system relies on geologic hot springs, a natural resource not available to most cities.
Ball State University in Muncie, Ind., on the other hand, is currently working on a GHP system of a similar scale. When completed, it will be the largest closed-loop GHP system in the nation. The project will replace four aging coal-fired boilers with 3,600 boreholes in fields located throughout the 660-acre campus. These fields will provide heating and cooling for more than 45 buildings, cutting the university’s carbon footprint roughly in half and saving it $2 million per year in operating costs. The entire system will run through two energy stations, where heat pulled from the ground or returned to the ground will be exchanged by way of heat pumps connected to two distinct loops running through the campus. One loop will carry cold water at a constant 42 degrees; the other will carry hot water at a constant 150 degrees. These loops will run through heat exchangers in each building, where fans will deliver the desired temperature to occupants.
While GHP systems present obvious environmental incentives, they also offer architects an opportunity to free themselves from the burden of hiding unsightly mechanical equipment, since most components of the systems are underground, well out of view. This can be especially welcome when retrofitting historic structures.
One such project is Helpern Architects’ design for the restoration of Knox Hall at Columbia University. Completed in 1909, Knox Hall is a 50,000-square-foot, seven-story Neo-Gothic stone building with pitched copper roofs. “It’s not a landmark structure, but we were treating it as if it was because it’s occupied by Columbia University at the Union Theological Seminary campus,” says Margaret Castillo, AIA, a principal at Helpern. “We studied five different HVAC systems, including geothermal. In the end, while the architectural concerns were a factor—it wouldn’t have been easy to put chillers on the roof—geothermal proved to be the most cost effective on an operational level.”
Knox Hall relies on four standing column wells drilled to 1,800 feet deep for all of its heating and cooling needs. The system helped the project earn LEED Gold. While the up-front cost was relatively high, Columbia estimates the payback to be about six or seven years. “They are very pleased this project is reducing energy use and greenhouse gas emissions,” Castillo says.
