Columbia University Project Goes Underground for Renewable Energy
While the previous occupants of Knox Hall, located at Columbia University affiliate Union Theological Seminary, looked to the heavens for inspiration, its new tenants are looking in the opposite direction for a source of renewable energy to heat and cool the facility.
Work is underway to transform the former UTS residence hall, at 122nd Street and Broadway, into the new home for Columbia’s Sociology Department. Renovations, slated for completion this fall, feature four 1,800-ft deep geothermal wells that will deliver 95-tons of heating and cooling capacity and reduce the facility’s energy consumption by 50 to 60 percent.
Geothermal systems, or ground-source heat-pumps, use the thermal energy stored in the ground to heat or cool a building, replacing conventional boilers and air conditioning systems.
Unlike geothermal power which draws hot water or steam from deep within the earth to drive electrical turbines, GHPs use the thermal energy stored in the upper several hundred feet of the earth’s crust. “About 50 percent of the sunlight that falls on the earth is absorbed by the earth and stored,” explains John Rhyner, Senior Project Manager, P.W. Grosser Consulting, Bohemia, N.Y.. “That is what we are tapping.”
At shallow depths the underground temperatures are stable, warmer than the air in winter and cooler in summer. During the winter, GHPs transfer thermal energy from the ground to the building. In the summer, the ground absorbs heat from the structure.
Inside the building, the mechanical equipment is similar to that used for standard condenser water systems, says Chris McHugh, Partner AKF Engineers, New York. “From the outlet of the well you are designing the filters, heat exchangers, piping, pumps and the water-to-water or water-to-air source heat pumps, depending on the system.”
Three types of geothermal systems are commonly used in New York; standing column wells, closed loop systems and open loop systems. A subsurface analysis, investigating geological and hydrological conditions, underground structures and potential environmental issues, is essential for selecting the right system for a site, Rhyner says.
A building’s heating and cooling demand and the thermal capacity of a site, established by the subsurface analysis, will determine the number of wells or the size of the loop field. These requirements need to be reconciled with available land for drilling, Rhyner explains.
Standing Column Wells
SCWs are best suited to Manhattan and the Bronx, where bedrock is relatively close to the surface and space is limited. The systems employ vertical shafts, typically 1,500 to 2,000-ft deep. Fifty-three SCWs are currently active in the City.
The General Theological Seminary of the Episcopal Church recently installed seven SCWs, ranging from 1,500- to 1,800-ft. deep, and plans 15 more. The Seminary consists of 16 landmarked gothic revival buildings spanning 9th and 10th Avenues between West 20th and West 21st Streets.
The GHPs are replacing aging oil-fired boilers and providing centralized air conditioning to campus facilities for the first time. Wells will be linked to seven mechanical rooms located throughout the campus, each housing a 30-ton water-to-water heat pump.
A New York City water tunnel beneath the site required some wells to be eliminated or moved. The city does not permit drilling within 200 ft of a water tunnel, Rhyner explains. City drilling permits required drift monitoring to insure the wells did not encroach towards the tunnels or neighboring properties.
Typically construction of SCWs starts by drilling a rock socket 75-ft into bedrock and installing a ten-inch steel casing cemented in place with grout, explains Lenny Rexrode, president, Aquifer Drilling and Testing , New Hyde Park, N.Y. “This seals off the overburden and prevents any surface contamination of ground water.”
A rotary air hammer drills the remainder of the well, which decreases in diameter to eight-inches for the next 300-400-ft and then six-inches to the bottom.
Once completed, a four-inch PVC pipe, called a shroud, is installed in the well. Slots at the bottom 40-ft. allow water to enter, Rexrode says. A submersible pump installed several hundred feet down in the PVC pulls ground water, generally at 54-56 degrees Fahrenheit (F), from the bottom of the well. The water passes through a heat exchanger and is returned to the hole on the outside of the PVC pipe, where contact with the bedrock cools/heats it back to 54-56 degrees F.
Open & Closed Loop Systems
CLS and OLS work best where bedrock is deeper and more space is available for drilling.
With a CLS, building water containing a nontoxic anti-freeze is circulated through a series of pipes run vertically or horizontally beneath the ground. The water within the pipes transfers heat from the earth to the building during winter and vice versa during summer. In New York City, vertical CLSs are most common due to space limitations.
ADT drilled test wells earlier this year for a CLS at Solar 2, an environmental center at 34th Street and FDR Drive slated for construction this fall. The 8,000-sq.-ft. facility will feature 18, 400-ft. deep wells beneath the structure, delivering 25 tons of heating and cooling capacity.
Rhyner is working on a CLS system with 90, 500-ft. deep wells for two landmarked structures at the Staten Island Institute of Arts and Sciences. GHP provides an unobtrusive option for landmarked buildings that can’t install mechanical equipment on the roof, he explains.
Constructing a CLS involves drilling a series of bore holes 75- to 500-ft. deep. Supply and return lines of 1¼-inch diameter black polyethylene pipe are connected by a U-bend at the bottom of each hole to form a loop, Rexrode explains. The hole is then filled with thermo grout to maximize heat transfer from the ground and to protect the piping.
Unlike SCWs, there is no contact between the water and the earth with a CLS, explains Brian Blum, Associate, Langan Engineering and Environmental Services, Elmwood Park, N.J. All the equipment for the system, including the pumps, is located within the building. “The systems are extraordinarily efficient. Once the installation is done, it is turn the key and walk away.”
OLSs (also known as pump and dump systems) are composed of separate pumping and injections wells. Ground water pulled from the pumping well passes through a heat exchanger and is pumped back into the injection well. The systems are most efficient for transferring heat and the least expensive to install.
The systems are well suited to Brooklyn and Queens where the aquifers are very prolific water producers, Rhyner says.
The recently completed Queens Botanical Garden Visitor Center includes an OLS consisting of one supply and two return wells, each 300-ft. deep. The building uses 40 percent less energy than a comparably structure, largely due to the GHP system, explains Jennifer Souder, the Garden’s Director of Capital Projects.
Water quality testing is essential for SCWs and OLS. Ground water from deep wells may be highly mineralized or contain high chloride levels. “Water with high chloride levels tends to corrode pipes and highly mineralized water causes scaling,” Blum explains. “Over time, scaling and corrosion can make system operation difficult.”
MEP engineers equipped with water quality data can adapt their designs to be compatible with a well’s water quality, Blum says. At Columbia, excessively high chloride levels may necessitate changing the materials used in the well. “Instead of stainless we may need titanium.”
With OLSs, natural minerals in the groundwater, such as iron and magnesium, can also cause bacteria problems. When water is pumped from a well, changes in water temperature, pressure and dissolved oxygen cause minerals to precipitate, Blum explains. Naturally occurring bacteria feed upon the precipitated minerals, clogging the return wells and reducing performance. Periodically the wells must be cleaned and disinfected to remove the bacteria.
At the Queens Botanical Gardens, iron eating bacteria in the aquifer are clogging the screens of the return wells. To kill the bacteria, the Garden is installing UV filters where water enters the building, Souder explains.
Despite the startup problem, Souder is glad the Garden installed the system. “This is a demonstration project for the city and there is a learning curve for everyone.”
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