...campus of Harvard University, where units currently installed on seven buildings use “standing-column wells,” about 6 in. dia and drilled as deep as 1,500 ft. These wells—an average of three per building—use a pump that extracts water from the bottom of the well where the water is cooler, exchanges it with heat pumps and returns it to the top of the well, where the water is warmer.
Like ground wells that are left to heat up after summer, COP drops when water wells heat up. Well-designed and -constructed wells promote heat dissipation, while poor ones require users to “pump and dump,” or bleed, water. Wells that are not drilled deep enough, or “short-drilled,” by an inexperienced contractor also may lack the capacity to allow heat to escape, warns Nathan Gauthier, assistant director of Harvard’s sustainability office and a member of the U.S. Green Building Council’s Energy and Atmosphere Technical Advisory Group.
Another environmental problem is the cross-linking of aquifers. Since drilling exposes soil strata, underground water that may not naturally come into contact with other sources suddenly is introduced. “If there is an aquifer at 100 ft and another one at 300 ft, a gas station may have contaminated the one at 100 ft,” Gauthier explains. While geotechnical consultants can help identify the risks, the construction industry as a whole has not solved this potential problem yet, experts say.
All heat pumps depend on compressors and refrigerants to transfer energy to and from the ground loop. Savings come from a lower head pressure that geothermal “recycling” affords. However, not all geothermal systems rely on heat pumps. One such system, popular in Europe and quickly gaining traction in North America, is deepwater source cooling (DWSC).
Cornell University uses DWSC to cool its classrooms in Ithaca, N.Y. A 2-milelong intake pipe located about 250 ft below the surface of nearby Lake Cayuga delivers 39°F water into onshore heat exchangers. A second, closed loop brings chilled water to campus at about 47°F. Warmer water is rejected through a 500-ft-long diffuser pipe close to the lake’s surface. The chilled water flows directly through the classrooms, eliminating the school’s previous chillers, which ran on ozone-depleting refrigerants.
Cornell’s system, designed for 20,000 tons, allows the $58-million investment to surge to a COP of 25 and reduce overall cooling energy by 86%. Cornell estimates savings are on track to repay the cost premium over conventional methods in 10 to 13 years from the 2000 completion date, a payback that now is approaching. “The savings is on the order of 25 million kWh,” says William S. “Lanny” Joyce, Cornell’s senior manager of engineering, planning and energy. “That is almost 10% of our campus electrical use.” He notes that the local community also benefits from reduced peak loading on the electrical grid.
Because the loops never mix, Cornell’s approach is a closed-loop geothermal system that does not rely on heat pumps. A more typical closed-loop system that uses heat pumps can be found in Elgin, Ill., where Sherman Health, the owner of a new, 255-bed hospital almost finished in the Chicago suburb, plans to save more than $1 million a year by tapping into a 15-acre, 17-ft-deep geothermal pond—one of the world’s largest—that is heating and cooling the 650,000-sq-ft facility. The owner knew that it needed to build a 5-acre stormwater-detention pond on its 154-acre site, so it tripled the size to accommodate a geothermal, closed water loop.
Shades of Green
All geothermal systems require electricity; heat pumps require even more. The “greenness” is determined by the efficiency of the overall design and the type of electricity going in. There is no shortage of strong opinions here.
Before designers even consider adding a geothermal system, it is critical to reduce building energy loads by upgrading the envelope and, if appropriate, HVAC ductwork. “The technology is fundamentally sound,” explains Joe Lstiburek, principal of forensic consultant Building Science Corp., Somerville, Mass. “But a lot of people are using them for really, really poor buildings.” For every dollar spent on conservation—better windows, insulation, ductwork—two dollars can be saved on the size of the geothermal system. But as many buildings are designed inefficiently, geothermal has become the “greenie-weenie technology du jour,” he says.
Another issue is whether or not GHPs are a renewable source of energy. Gauthier and Lstiburek say the “geothermal” moniker is nothing more than smart marketing. Others, such as Bererton, think it “should be considered the same as solar,” he says. “If you look at a COP of four for a heat pump in heating mode, one unit of electricity gives you four units of heat, so where did those three units of heat come from? They came from the sun, so you really are 75% solar.”
With these systems, energy use likely drops, but it never goes away. Nor does it return. As such, USGBC’s LEED rating system does not identify GHPs as renewable. The Internal Revenue Service does, and it provides special tax credits. The debate rages on, but experts unanimously agree: These systems are green so long as they are intelligently planned, executed and maintained. Says Dwyer, “I think it has got a long, healthy future.”