When temperatures rose to sweltering heights this summer, Nashville International Airport (BNA) kept visitors cool with an energy-saving geothermal system which leverages naturally low temperatures in an onsite quarry lake. Shrinking utility bills help airport management also stay cool as a cucumber.
Ever since mid-February, BNA has not used a traditional air conditioning system. Instead, it cools the entire terminal by tapping into a thermal layer of an onsite lake to cool water for use in its chiller plant. The new system proved especially valuable-and cost-efficient-during this summer's heat waves, but BNA officials expect its appeal to last well into winter. Even when Nashville's typically moderate climate dips to 20 degrees (Fahrenheit), the airport still needs to cool air for the terminal, because people, equipment, lighting and movement prove to be great heat inductors.
Project: Geothermal Cooling System
Location: Nashville Int'l Airport
Owner/Operator: Metropolitan Nashville Airport Authority
Cost: $10.4 million
Funding: 40.8% Energy Performance Guarantee; 34.2% federal; 12.5% state; 12.5% airport authority; $300,000 grant from Tennessee Valley Authority
Design/Build Team: Blakley Construction Services; Energy Systems Group; Garver; Nashville Machine; Smith Seckman Reid
How System Works: Water piped from airport's central plant is cooled by plates submerged in a quarry lake. Cooled water is then pumped back to chillers, which consume less energy to cool terminal air because they received pre-chilled water.
Key Elements: 11 plates submerged 50 ft. below lake surface, where water temperature hovers near 50 degrees year-round; 10,000 ft. of 20-in. pipe buried 3 ft. below ground surface
Est. Anticipated Annual Savings: 6,000 kw of peak demand electricity; 1.3 million kw-hours; 30 million gal. potable water; $430,000 utility costs
Documented Savings: 50% reduction in energy consumption (to 0.525 kw/ton), Feb. to mid-May 2016
Accolades: 2016 Environmental Achievement Award, Airports Council Int'l-North America; 2015 Governor's Environmental Stewardship Award, TN Dept. of Environment & Conservation
The new geothermal system at BNA is the very definition of sustainability, says Christine Vitt, vice president of Strategic Planning and Sustainability for the Metropolitan Nashville Airport Authority. It's also a significant asset for the airport, she adds, noting that the system is poised to far exceed the $430,000 projected annual energy savings for its first year in operation.
"I think we're getting the most out of this system we possibly could," Vitt comments. "If you think about every drop of water in a closed loop system doing the cooling work over and over again, there are so many benefits-from potable water reduction, energy savings, stormwater collection, even air emission reductions on the power plant side. There are many good things that have come from this program."
Interestingly, the airport doesn't pull cool water from the lake for the system. Instead, it uses cool lake water to chill other water, which is then used to condition air throughout the terminal. By doing so, it preserves the lake's naturally clean, cool water for repeated reuse.
Inspiration to Operation
Inspiration for the geothermal system occurred several years ago, when the airport authority and its team of designers, engineers and contractors were discussing what to do with an abandoned quarry the airport acquired when it purchased land for a runway project more than three decades prior. A 2008 feasibility study lead the airport authority to believe that the only viable way the airport could benefit from the quarry was to find a use for the water in its lake. In 2010, a flood overflowed the lake for the first time since BNA owned the property, further inspiring the airport to put its water to use.
The idea to use the quarry lake for a geothermal system was suggested and developed by Nashville Machine and Energy Systems Group, which had performed previous energy-saving work on the airport's chillers, lighting and heating/venting/air conditioning system.
In a nutshell, the contractors developed a system that uses water from 50 feet below the lake's surface, where temperatures hover around 50 degrees throughout the year, to pre-cool water for the chillers that condition air for the terminal.
"We're not drawing any water out of the quarry for cooling," specifies James Hurt, project manager at Nashville Machine. "We're flowing water through heat exchangers that are in the quarry, and that allows the transfer of heat from the exchangers to the water."
Scott Terry, project director from prime contractor Blakley Construction Services, provides more details: Eleven plates (essentially enormous car radiators) that are submerged in the lake chill water that comes in from the airport's central plant via 20-inch piping. After the water circulates through the lake plates and cools down to 53 degrees, it is pumped back to the airport for use in the air conditioning system. Because the chillers receive water that is pre-chilled in the quarry lake, they consume less electricity, explains Terry.
At no time in the process does the water inside the pipes touch the quarry water, he notes: "It's an entirely closed system, a recirculation system." Because the quarry lake collects stormwater runoff from the surrounding 140 acres, it acts as a retention basin and provides very clean water, adds Vitt.
Over several years, the idea of using the quarry lake for geothermal cooling gathered grassroots support all the way up to and including the board of directors, notes John Waddle, project manager at Energy Systems Group.
"By the end of 2013, we had a preliminary design in place that validated this would be a viable project," recalls Vitt. By Feb. 11, 2016, BNA was fully operating on its new geothermal cooling system.
Two other major players for the project were Smith Seckman Reid, the firm that designed the overall system, and Garver, which designed the pipeline between the quarry lake and the airport's central plant.
Ryan Sisemore, vice president and Aviation East Region Director at Garver, notes that the design/build approach helped to facilitate the project's timeline and push the unique design forward. "We really had to step outside of the industry to gather the right knowledge and people to do this," says Sisemore.
Nashville Machine installed components at both ends of the project: the 11 stainless-steel lake plates and the pumps and piping that connect to existing chillers for the condenser water. Blakley Construction installed the piping that connects the two ends. Energy Systems Group oversaw the energy and thermal operations.
Siemens optimized controls in the chiller plant to maximize the system's efficiency. Variable frequency drives installed on the new motors vary the water flow and adjust the temperature of the water entering the chillers. "During the peak days of cooling, when it's above 95 degrees, they will increase the flow to improve the ability of the chillers to meet the load," Hurt explains.
Considering the system as a whole, Waddle credits the airport for maintaining its commitment to the project. "It took six years to bring this to fruition from original concept," he comments. "We just didn't let it die. We kept putting the ball in play, and I think it's noteworthy that we all had the tenacity to stick with it. We believed in it."
Another huge hurdle was a lack of similarly sized projects. According to the airport authority, its new system is the largest geothermal lake plate cooling system in North America. "We were pioneering a lot of this because there wasn't a 'last time' to refer to," Waddle notes.
Ron Holdaway from Smith Seckman Reid had experience designing a similar, but smaller, geothermal system for Walt Disney Word-experience Sisemore called "invaluable." But the project at BNA was the first of its kind for most of the team. The project also included a unique water source. The airport's quarry lake, which was once considered a liability, is now "basically a battery pack that recharges itself," he quips.
Like Sisemore and other team members, Waddle considers the BNA geothermal project a once-in-a-lifetime experience. "So many things had to come together at the same time," he says. "We knew it would be difficult to have an opportunity like this again in our careers. This will definitely be the project I'll remember the rest of my life and one that has had the biggest impact on me personally."
Installing roughly 5,000 feet of piping was another formidable test. The pipeline corridor includes two 20-foot high-density polyethylene sections for supply and return as well as a 4-foot high-density polyethylene irrigation pipeline. Not only did the pipeline need to run underneath Runway 2R-20L and multiple taxiways, it also had to pass under a state highway. Crews completed the task during a 60-day shutdown the airport had already scheduled for airfield maintenance.
The team had to make open cuts, remove and replace runway and taxiway concrete pavement, cut electrical circuits, penetrate drainage systems and then put everything back together again after installing steel casings and piping throughout the area. Around the highway, crews bored beneath the pavements.
"We knew we were going to find a lot of unknowns [obstacles]," Sisemore notes. "After completing as much research on the existing infrastructure as we could, we still knew there would be a significant amount of field engineering." In the end, collaboration allowed the team to beat the original deadline by about a week, he adds.
In Hurt's opinion, the most challenging aspect of the project was floating 6,000-pound heat exchangers/cooling plates into the quarry and then sinking them 50 feet without compromising the piping that connects them to shore. Crews used cranes to lift 11 cooling plates into place, one at a time. Each was equipped with a special flotation device that gradually lost buoyancy as water flowed into the heat exchanger, eventually allowing the equipment to sink to the desired depth. Divers inspected the plate locations to verify conditions.
Weather was a major factor during the fall 2015/winter 2016 installation. Strong winds made it difficult for barges to deliver the exchangers to the correct location. "It seemed like [every] day we were going to put the heat exchangers in the water, the wind would pick up," Hurt recalls. "You could almost set your watch by it."
Waddle agrees that the quarry was not a work-friendly environment. The strategy to install the cooling plates required hours of calculations and counsel from multiple experts. Key questions included: How many barges are needed? How many cranes? What's the best way to maneuver the heat exchangers? How are we going to sink them in the right spots? "The wind wreaked havoc on us and there wasn't anything we could anchor to, so the barges were constantly being pushed out of position," he recounts.
Hurt explains that the heat exchanger is connected to a manifold in a "reverse return" configuration, with the first exchanger receiving the last water and vice versa, so equipment can self-balance for the system's flow rate. Vaults with a control valve regulate the exchange and allow personnel to turn off any specific heat exchanger if problems arise.
Regardless of multiple challenges, Hurt notes that the project team was well-prepared for each stage of installation. "We had done a lot of coordination and everybody knew what they were supposed to do," he says.
Terry notes that partnering was especially important for the fast-paced project. "It let us sit down as a group ahead of time," he relates. "Everyone was able to lay out their concerns, expectations as well as their commitment in front of the others."
Hurt considers BNA's geothermal system the height of sustainability, and is amused that all of its key components are buried deep below water. There's nothing to show people, he muses: "It works wonderfully, and you don't see any of it."
The transition from BNA's traditional cooling system occurred over a 24-hour period that ended on Feb. 11. Siemens continues to monitor the new system's performance, and Vitt estimates the optimization the company performed, tweaking every control that operates the cooling system, will likely save the airport another $70,000 per year. The first post-installation analysis in May indicates an overall efficiency improvement greater than 50%, with energy consumption decreasing to 0.525 kilowatts per ton. "There was a lot of relief when the numbers started showing what they did," Vitt notes.
Annual projected savings for the project include: 6,000 kilowatts of peak demand electricity usage, 1.3 million kilowatt-hours, 30 million gallons of potable water and $430,000 in utility savings.
To date, the project has already garnered multiple awards: a 2016 Environmental Achievement Award (special/innovative projects category) from Airports Council International-North America and the 2015 Governor's Environmental Stewardship Award (sustainable performance category) from the Tennessee Department of Environment and Conservation.
In retrospect, Terry says that the project offered a valuable opportunity to put corporate social responsibility in action by doing good things for the environment and the airport. "A lot of times we do projects because there's a need, but every once in a while, a project has a social consciousness, it's for the greater good," he notes. "This was one of those."
Sisemore credits Vitt for thinking out of the box and balancing risk with potential reward. "This project was very unique, hard to model and know exactly how it would function," he reflects. "Honestly, the system is running more efficiently than what the design team had targeted."
The project's performance-guaranteed contract undoubtedly multiplies that excitement.
Vitt considers the geothermal project a gratifying accomplishment that was a long time coming. "Everyone is happy with the system," she reports. "Even the maintenance crew is touting how easy it is to monitor and maintain. There's just nothing negative about it, and that in itself is pretty rewarding!"