Heating and Cooling Hallowed Halls of Higher Education with Geothermal
"We've been interested in energy conservation since the early 1970s," explained Dr. Lynn F. Stiles, professor of physics at Richard Stockton College in Pomona, N.J. "We studied geothermal designs in the 1980s and actually began using geothermal systems on our own campus in the early 1990s." Dr. Stiles was filling in the background to one of the world's largest single closed-loop geothermal syst...
"We've been interested in energy conservation since the early 1970s," explained Dr. Lynn F. Stiles, professor of physics at Richard Stockton College in Pomona, N.J. "We studied geothermal designs in the 1980s and actually began using geothermal systems on our own campus in the early 1990s."
Dr. Stiles was filling in the background to one of the world's largest single closed-loop geothermal systems. Located in the pinelands 12 miles from Atlantic City, Stockton boasts a system with a total of 1,741 tons of installed HVAC capacity.
The story actually begins in 1990, when administrators decided to reduce overhead expenses—including heating and cooling costs. It just so happened that the school was already planning replacement of its aging fleet of multi-zone rooftop HVAC units—most of which dated to the school's original construction in the 1970s. It was also a time when the college was building new classrooms and dormitories to keep up with growing enrollments.
At the time, Stiles had already been researching geothermal applications. Particularly interested in heat-exchange wells and water-source heat pump (WSHP) technology, he had been urging administrators to seriously consider this option.
The result was a design that featured 400 heat exchange wells located in 4-in.-dia. boreholes on a grid, spaced about 15 ft. apart and to a depth of 425 ft. Within each borehole, installers placed two 1.25-in. high-density polyethylene pipes with a U-shaped close-return coupling at the bottom. Installation of the pipes was complicated by the fact that the boreholes filled with groundwater and the pipes were buoyed upward, but installers overcame the problem by attaching weights to each loop and filling the heat exchange pipes with water. After the pipes were installed in the boreholes, the holes were backfilled with a bentonite clay slurry to seal them and to enhance heat exchange.
In total, the loop system comprises 64 miles of heat exchange pipe. In addition, 18 observation wells are located throughout and around the well field for long-term observation of ground water conditions. The individual wells are connected to 20 4-in.-dia. lateral supply and reverse return pipes using a thermal butt fusion technique. These laterals, in turn, run to a building at the edge of the field where they are manifolded into 16-in. primary supply and return lines. The primary lines go to a pump house containing two 125-hp variable-speed pumps that pressurize the supply and return systems in the manifold house. In heating mode, the loop serves as a heat source, and in cooling mode, as a heat sink. The borehole field has a volume of 1.2 million cu. meters or, en masse, is equivalent to the heat capacity of about the same volume of water.
The boreholes were to be installed in a 3.5-acre area that included all of one parking lot plus some adjacent open space. Because of the protected environmental status of New Jersey's Pine Barrens area, the college was required to get special permits from the state's Pinelands Commission. Using the parking lot for a large part of the geothermal field reduced the disturbance of undeveloped land on the campus.
But the commission was also concerned about protection of three underground aquifers that would be crossed by the wells. The decision to use only pure water without glycol as the heat exchange medium helped assuage the commission's concerns.
From Parking Lot to Rooftop
Water is distributed from the pump house through six secondary loops to rooftop water-source heat pump units, located throughout the campus and ranging in size from 10 to 35 tons—a total of 1,480 tons capacity. Because the existing system had used multi-zone units, it was necessary to add 500 variable air volume boxes at the conditioned air distribution points to meet zone-level requirements with the new system. All of the rooftop units are equipped for air economizer operation. Pumps, rooftop units, economizers and VAV boxes are controlled by a building automation system using 3,500 data points.
In the first few years of operation, the average temperature of the well field drifted upward by several degrees but appears to have stabilized. This occurred because the cooling load annually releases more thermal energy to the ground than the heating load requires. Even in the cooler winter months, the system often operates with some units heating while others are in the cooling mode.
Savings Meet Expectations
The original estimate was that the geothermal system would reduce the school's electric consumption by 25% and natural gas consumption by 70%. "Because of the constant changes and additions to the system and other energy conservation steps, it is difficult to verify energy savings exactly," said Stiles. "[But] based on an extensive monitoring study, [the estimates] turned out to be quite accurate."
Stiles is frequently asked for advice by others contemplating geothermal solutions. "Sizing the system right is very important," he said. "For that reason, you need buildings with well-executed shells, and the heat loss and gain estimates need to be accurate." He also points to the importance of having access to the loop water and heat pump systems.
And he offers yet another suggestion: Owners should consider incorporating a cooling tower—either a wet or dry model—to precondition the well field in the winter months for cooling later in the summer. "For situations like ours, where cooling hours significantly outweigh heating hours, a cooling tower could add even more to system efficiency or capacity. That's something we didn't include in our original plans, but today I would."
Finally, he notes that there are numerous technical support assets that should be used. "It is critically important to have a good design and good implementation. Make sure that the engineer gets all the support needed to understand geothermal technology and design." said Stiles.