Airports around the world are transforming. They're not just adding flight and passenger capacity, but improving the experience: more natural light, better Wi-Fi, and if we're lucky, maybe even quicker trips through security.

But most impactful of all are arguably the changes we can't see. From Amsterdam to Auckland, Beijing to Brussels, and here in the Pacific Northwest too, airport terminals and concourses are undertaking efforts to decarbonize and use energy more efficiently. In most cases, it means upgrading their heating and cooling systems and eliminating the use of fossil fuels.

In recent projects at our region's two largest international airports, Seattle-Tacoma International Airport (SEA) and Portland International Airport (PDX), PAE and its partners have turned to two different kinds of heat pumps.

By the shores of Puget Sound, SEA's C Concourse Expansion utilized an air source heat pump that's decentralized from the airport's central steam plant, as part of the Port of Seattle's first all-electric project. Meanwhile, along the Columbia River at PDX, as part of a transformative main terminal renovation and expansion, a new centralized groundwater heat pump was chosen in order to reduce per-square-foot energy intensity usage by half.

Just as it takes powerful thrust to lift a passenger jet into the air, these airport HVAC projects needed careful planning, analysis, and investment. But the final destination is worth it.

C Tactics at SEA

The expansion of Seattle-Tacoma International Airport's C Concourse, overseen by our architect partners at Miller Hull and Woods Bagot, is impressive to behold. A grand staircase clad in locally-sourced Western Hemlock leads passengers to a collection of eateries modeled on Seattle's iconic Pike Place Market, as well as an interfaith prayer and meditation room, a nursing room, and a new Alaska Airlines lounge. Walls of glass to the west and south offer views of the airfield and the Olympic Mountains.

At the same time, SEA's new C Concourse gets an A-plus for its sustainable design features. Electrochromic glass enables the glass walls to tint as needed in order to prevent glare and unwanted heat gain. Rooftop photovoltaic panels deliver renewable energy, while restaurant kitchens utilize all-electric equipment, all contributing to a dramatically reduced carbon footprint. But central to these efforts is the decentralized air source heat pump.

SEA Airport utilizes a central steam plant powered by natural gas, which the client chose to keep in place due to the prohibitive cost of replacement. But for the C Concourse Expansion, an air source heat pump could not only reduce SEA's overall use of fossil fuels but also take pressure off the central plant. Keeping the centralized steam system also meant it could serve as a backup for the new concourse expansion.

Our clients at the Port of Seattle approached the C Concourse Expansion project with ambitious goals to reduce fossil-fuel dependence and carbon emissions. But it was also important that the airport's operations team could deliver consistent comfort to passengers, which meant that changing systems had to be underscored by engineering experience: providing examples of how past projects have realized these goals, how the transition can be accomplished with minimal impact on day-to-day maintenance, and how this will make for a better future.

PAE joined the C Concourse Expansion project early in its conceptual stage to help evaluate a range of sustainability measures and calculate their various efficiencies and costs, as part of a holistic design. The team knew that this heat pump could work in tandem with photovoltaic panels across the roof, as well as a highly energy-efficient array of glass and façade insulation.

For building projects with all-electric systems, heat pumps are generally the HVAC technology of choice. They're highly efficient, with every kilowatt of power that goes in yielding 3 to 5 kilowatts of warm air. By comparison, using electric resistance heating, every kilowatt of power utilized in generation produces a maximum of one kilowatt's worth of output heat.

In this case, the team decided to utilize the heat pump solely for heating and not for cooling the building, because the airport's central plant has a much higher efficiency than could be achieved by the air source heat pump. Also, the opportunity for heat recovery was very little due to minimal overlapping heating and cooling loads in the building. Even so, the heat pump's efficiency made it an easy choice.

Keeping the centralized steam system also meant it could serve as a backup for the new concourse expansion.

The team evaluated a variety of types. A ground source heat pump takes advantage of stable ground or groundwater temperatures. In winter, water circulating through an underground pipe loop absorbs heat from the ground, and an indoor compressor concentrates this thermal energy to warm the building. In summer, the process reverses as heat is pulled from the building and safely deposited into the cooler earth.

But at SEA, it wasn't the best choice because the project was being constructed within an existing building footprint: a single-story structure that was demolished and two stories below ground that stayed. A subterranean ground loop for a ground source heat pump would have been outside this scope and extremely cost-prohibitive.

Instead, for the C Concourse Expansion, the team chose an air source heat pump which extracts heat from outside air to warm a building in winter. The Pacific Northwest's relatively mild climate is actually quite well-suited for air source heat pumps, traditionally not seeing consistently cold temperatures in winter months or extended summer heat and humidity that exist in other regions.

The C Concourse Expansion is currently pursuing a top-level Platinum rating from the US Green Building Council's LEED rating system, thanks to how the air source heat pump works in tandem with a robust thermal building envelope and photovoltaic power. It took careful planning as well as fine-tuning for the new heat pump to tie into the airport's existing centralized steam heating. The project benefited from a client willing to pursue ambitious goals for decarbonization and energy efficiency.

PDX’s Groundwater System

PAE has been fortunate to work at Portland International Airport for decades on numerous expansion and renovation projects. Even before the Port of Portland began planning in 2015 for the expansion and transformation of PDX's main terminal (completed in 2026), the airport was routinely rated among the best in America. But to accommodate an estimated 95% passenger increase by 2045, and to meet the Port's ambitious goals of reducing greenhouse gases by 80% and achieving net-zero energy by 2040, the airport would inevitably grow and change.

The project also presented a tremendous opportunity to create a breathtaking new terminal designed by ZGF Architects teeming with natural light thanks to dozens of skylights, resplendent with natural materials and plants (including a dramatic wood ceiling fabricated from local and sustainably sourced timber), and a streamlined passenger journey from the moment of entering the airport to arriving at the gate.

Like SEA, PDX relied on a central steam and chilled-water plant for heating and cooling, which had become aged and inefficient. In particular, the steam heating system suffered from substantial distribution losses, generated heat using natural gas, and had become increasingly difficult to maintain. Unlike its northern metropolitan neighbor where a decentralized heat pump system was the best fit for a new concourse, PDX's complete main terminal reconstruction favored a centralized system. Even so, the new heat pump could supplement rather than wholly replace the existing HVAC infrastructure.

Because PAE was involved in planning and design from its earliest stages, the team could extensively evaluate costs, performance metrics, and overall merits of different systems. A heat pump could be air source, ground source, or utilize groundwater. An open-loop heat pump would use groundwater directly as a heat exchange fluid before discharging it, while a closed-loop system would recirculate a sealed mixture of water and antifreeze through underground pipes. A hybrid of the two could also be considered. There was the potential to integrate the system into structural piles or rely on separate boreholes.

The new heat pump could supplement rather than wholly replace the existing HVAC infrastructure.

PAE collaborated with the Port of Portland to identify the evaluation criteria and then ranked each option against those criteria, including:

• Safety
• Utility costs
• First costs
• Occupant comfort
• Maintenance
• Redundancy
• Operational resilience
• Indoor air quality
• Potential for escalation of maintenance or utility costs.

After evaluating multiple alternatives, the client and building team decided on an open-loop groundwater exchange system, which ranked near the top across all major criteria, including first cost, emissions, energy use, lifecycle cost, and long-term ownership cost.

The new system extracts water from the Troutdale aquifer (an underground water system spanning roughly 300 square miles across the Portland metro area), exchanges heat, and returns the water to the aquifer at slightly warmer or cooler temperatures while still protecting fish and wildlife. Its efficiency stems from the groundwater's stable temperature, which enables more effective heating and cooling than air source heat pumps.

The groundwater exchange system was projected to reduce building energy-use intensity by 47% and heating energy use by 90% compared to the airport's existing system. Relative to expanding the original steam system, it would save an estimated $723,000 annually in energy costs while reducing carbon dioxide emissions by 6,700 metric tons per year. The groundwater exchange system ultimately became the single largest contributor to achieving the Port of Portland's energy and carbon reduction goals.

Primed for Heat Pumps

Heat pumps are not merely an engineering technology but a strategic tool for transforming large, complicated airport infrastructure. Airports don't necessarily need to replace their entire existing HVAC infrastructure to decarbonize. They can strategically introduce heat pumps in ways that work with what they already have. The key? Use the most appropriate technology for each task and combine systems strategically.

After all, there isn't one airport decarbonization playbook. The right solution depends on the building, the existing infrastructure, the site, the climate, the economics, and the operational requirements. Luckily, PAE is here to help.