Denver Int’l Takes Deep Dive into Power System to Ensure Reliability & Resilience

Like many other airports throughout the industry, Denver International (DEN) was unnerved when Hartsfield-Jackson Atlanta International (ATL) experienced a massive power outage in December 2017. In addition to causing an 11-hour blackout at the nation’s busiest airport, the outage impacted air travel throughout the country and prompted airports around the globe to take a second look at their own power systems.

Cullen Choi, manager of systems engineering at DEN, notes that he and other industry insiders know what happened at ATL was not unique, that every airport’s power system has potential vulnerabilities that could have a ripple effect throughout the air transportation system. “That’s something that can’t be understated,” he emphasizes. “When we improve our own reliability and resiliency, that not only has the benefit of making sure we can maintain our operations, but that every other airport can also maintain their operations.” 

Evaluating Needs

Choi and his team have consequently taken the opportunity to evaluate DEN from top to bottom, identifying opportunities for improvement. Like ATL’s, the energy supply system at DEN includes a high degree of redundancy, but airport officials nevertheless want to eliminate as many vulnerabilities as possible, he explains.

When considering solutions for current and future power/utility needs, the team focuses on four goals: low-cost, low-carbon, enhancing reliability and improving resiliency. “Those are not always in harmony—sometimes they can be in conflict a little bit,” he notes. 

For instance: Batteries are a relatively expensive backup option with a comparatively short life; but they are reliable, resilient and low-carbon when used in partnership with a solar photovoltaic array. Diesel generators are low-cost in terms of capital investment and operations, but not low-carbon. They are also not a resilient solution if diesel fuel is in short supply when the power goes out. “So, we always want to look at potential solutions through all four lenses,” says Choi.  

At the same time, it’s important to accommodate various stakeholders. “Everyone has specific needs that we as an airport, landlord and service provider want to meet in a cost-effective way,” he adds. Airlines, tenants and other business partners need the ability to serve passengers in the event of a power disruption. And, as the event at ATL showed, disruptions at one location can have a domino effect. 

Multiple Redundancies 

DEN has two substations through the local utility, each capable of supporting the airport’s entire power load. “We have redundancy on top of redundancy at the airport boundary,” Choi comments. 

If the power supply at one substation is interrupted, transfer switches automatically change the system over to the other substation. In addition, the substations are located about 10 miles apart, so problems that affect one would not likely affect the other. 

Between the airport boundary and the interior wall plugs, the system transitions from redundancy to a single feed. That’s where DEN really scrutinized its strategy, Choi explains. The primary service voltage from the local utility provider, Xcel Energy, is not widely used throughout the utility’s territory. DEN runs on a 25kV distribution system, so the voltage it receives from the utility is too high for equipment and needs to be stepped down to a lower voltage before it transitions from the utility’s system to the distribution system owned and maintained by the airport. 

During its facility-wide assessment, DEN found opportunities to inject additional redundancy and reliability for mission-critical loads at the lower voltage side to ensure the transition occurs in appropriate places and represents appropriate risk versus the cost of carrying the redundancy all the way down to point of use.

“It’s just not cost-effective for any airport to carry redundancy down to every single end load,” Choi explains. 

The team consequently focuses on finding the right tool for the right job. For example, critical equipment like the airport data centers have large racks of uninterruptible power supplies where power comes in from the grid and essentially charges large batteries that subsequently power the network infrastructure. Elsewhere, a diesel generator is at the ready with emergency backup power to ensure continuous operations in the air traffic control tower. “We want the right kind of backup, reliability and redundancy for the right application,” Choi stresses. 

Metered Response

After the high-profile outage at ATL, some airports pressured their local utilities to guarantee a more reliable feed and spent millions purchasing generators for every potential load. “We didn’t do that,” says Choi. “We’re taking a slow, metered approach.” 

DEN’s excitement to help move the aviation industry forward is tempered with patience to deploy the most appropriate technology, Choi explains. Support from executive leadership has been integral to exploring innovative energy solutions, he adds.

Choi predicts that ultimately, the best solution will likely be a mix of technologies—traditional and innovative measures working together to provide a high level of reliability in a cost-effective manner. “There are different tools for different applications, and different solutions lend themselves well to specific needs,” he says.

For example, emergency lighting is served well by batteries or generators. However, diesel generators are not intended to run continuously for long periods of time, so electrical storage batteries or fuel cells make more sense for such applications. Because diesel generators are relatively expensive to run and maintain, Choi notes that they can become “stranded assets.”

DEN is exploring a combination of solutions that include generation and storage technologies to create a more cost-effective solution. By interfacing the generation side with the load side, the airport could improve power quality, reliability and resiliency on day-to-day operations and beyond, Choi explains. 

Microgrids

One potential solution to improve power reliability and resiliency is creating a microgrid, a local energy network designed to separate from the larger electrical grid during power outages. Burns Engineering helped DEN assess a microgrid as part of a larger Energy Master Plan that includes potential zero-net-carbon strategies. 

Microgrids incorporate smart technologies with diverse power generation and storage systems to provide a level of power continuity that exceeds what backup generators typically can offer. At the same time, microgrids can also enhance power quality and provide energy savings and environmental benefits. The customized electric distribution system is designed to provide added reliability and resilience, explains David Smith, Burns’ director of Energy Services. And while a microgrid works in coordination with a larger grid, it can also operate independently. 

In the event of an outage on the larger grid, a microgrid can provide power to an airport through alternative energy resources such as diesel generators, natural gas generators, solar arrays, wind systems, battery storage or a hybrid approach that combines several.

Smith explains that under normal conditions, a microgrid can take power from the grid and make some of its own power. If power from the main grid goes out, the microgrid is equipped to take on the responsibility of the full load. “It’s like a big backup system, but it’s not just for backup purposes,” he remarks. “It operates day-to-day in ‘blue sky’ to the benefit of the owner.” 

Ideally, a microgrid is designed and configured to allow the owner to make power cheaper, more reliably and/or cleaner than power from the outside grid. Microgrids can serve various purposes, depending on a user’s challenges and needs. “There are no two microgrids exactly alike,” says Smith. “It really depends on the goals, objectives and needs of the client.” 

What makes sense for one airport might not be possible at another for a variety of reasons, including physical space, climate or budget. A microgrid can be modular and adapt incrementally as an airport grows or its needs change, Smith adds. 

Typically, a microgrid includes a number of complementary resources, depending on the objectives, to provide a predetermined level of reliability and resilience for a collection of interconnected loads, he explains. Each technology has different cost, performance and operating characteristics and serves different purposes. For example, solar is renewable, but as an intermittent resource dependent on sunshine, it can’t be dispatched in the way natural gas engines can. “Diesel generators are good for what they do, but they are not rated for continuous duty and also have limited amounts of stored fuel, so you don’t want to develop a use-case that says you need to be able to operate on diesel generators alone for more than 24 to 72 hours,” Smith says. 

In many cases, microgrids could reduce an airport’s energy expenses. Cogeneration, whereby electricity is generated on-site and the heat produced in the process is recovered and put to use, can provide efficiencies and cost savings. “There’s the potential to save money, the potential for resilience and the potential for making money,” Smith summarizes. 

Possible Changes

DEN has an 11.5-megawatt solar photovoltaic system with five separate arrays, but the airport is not able to use all of the energy it produces with the infrastructure currently in place. That could change if the airport expands the array and combines it with other technologies in a coordinated airport microgrid, says Choi. “That improves our return on investment by reducing the overall blended utility costs, but also has benefit to the utility, where it is not having to address our peak loads,” he explains. If that demand responsibility is no longer on the utility, energy costs could be reduced for all ratepayers, he adds. 

The airport’s large footprint can be both a challenge and an opportunity. Not all 53 square miles of land carry critical loads or even need to be served by utilities. Because the current infrastructure is underground, with many buried interconnection points from the two substations, it’s difficult for maintenance technicians to access connections when the region receives significant snowfall. 

At the same time, the land offers DEN significant opportunities to expand its solar arrays or install other microgrid components. Specifically, it could incorporate a large solar farm with utility-scale battery storage and reciprocating engine technology that would provide resilience and carbon reduction. Leveraging strengths is important to energy planning, Smith notes. 

Keeping airport tenants, employees and passengers safe and comfortable is a top priority when DEN explores power backup systems. Choi and his team consequently focus on evaluating each facility and its critical functions, and take an integrated systems approach to providing solutions that improve reliability and resiliency while also being cost-effective and low-carbon. 

However, electrical service and infrastructure can be very site-specific for airports, as can issues regarding power reliability, resilience and supply. Finding solutions that take a systems approach, where every component’s benefits and drawbacks are evaluated in the context of other components, is key. That said, Smith says that over time, microgrids combining on-site generation, energy storage and smart, responsive energy loads, could prove to be an increasingly viable means to address the growing challenges at any airport.