NREL Model Cuts Through Complexity of Deploying Electric Buses at Airports
As the U.S. aviation industry generates 2% of the nation’s carbon dioxide emissions and consumes 10% of transportation energy, the National Renewable Energy Laboratory (NREL) is leading the way to find pathways to sustainability, including through sustainable aviation fuels, electrification, and new transportation options for moving small groups in dense city centers.
NREL researchers recently developed a simulation-based optimization modeling framework to help Dallas Fort Worth International Airport (DFW) transportation operators design effective electric bus deployment strategies. The model analyzed real-world conditions and simulated the existing DFW system to determine optimal battery capacity, charging power, and number of charging stations while minimizing capital cost and emissions.
“The model can help DFW simulate their shuttle bus fleet operations and help them make informed decisions on fleet electrification and operation strategies,” said NREL’s Zhaocai Liu, lead researcher on the study.
Stemming from a Department of Energy (DOE) push for national laboratories to use supercomputers for transportation projects, this effort is the latest to come out of Athena—a public-private partnership between NREL, Oak Ridge National Laboratory, and DFW to support the transformation of related energy systems. The work is also part of NREL’s larger sustainable aviation research, which seeks to lower the carbon intensity of flight and assist the aviation industry in meeting emissions-reduction goals across the aviation ecosystem.
Optimizing Shuttle System Design
With transportation accounting for 27% of U.S. emissions in 2020, transit electrification has great potential to decarbonize energy systems.
A key component of electrification is optimization of operational design. The simulation aims to accurately characterize bus operations for a given bus system design. Using high-performance computing (HPC), the team searched all possible designs using the simulation to find the design that would perform the best in practice. The simulation enabled researchers to minimize capital cost and greenhouse gas emissions by reducing miles traveled by nonelectric buses.
The simulations represented many configurations that took into account battery capacity, charging power, placement and number of charging stations, travel distance of nonelectric buses, passenger wait times, and other key factors. Results showed how these configurations affected cost, nonelectric bus travel distance, and emissions.
“We leveraged HPC to enumerate and explore hundreds of thousands of planning and operation scenarios,” Liu said. “We then looped through all those scenarios and compared them to generate a set of Pareto optimal solutions—‘Pareto’ in essence meaning there is no solution with a lower cost and lower emissions. Decision makers can choose solutions based on their preferences toward cost, performance, and emissions.”
The modeling framework provides options based on combinations of factors and allows shuttle operations managers to consider those factors in planning. In one scenario, with 10 buses converted to e-buses with medium battery capacity (150 kWh) and two charging stations, there is balance between the capital cost and total miles traveled by electric buses.
The researchers state that the current research’s key contribution is in its real-world modeling.
“Using real-world data, the simulation model can better capture the real-world operation of DFW’s airport shuttle operation,” Liu said. “Planning and operation strategies from real-world data-based simulations will be more robust and reliable.”
Toward Electrification of Airport Transportation
Further, NREL’s Qichao Wang developed the discrete-event simulator ASPIRES to simulate airport shuttle fleet operations and created a model to reduce energy consumption within airport shuttle operations by optimizing routes and schedules. Funded by DOE’s Vehicle Technologies Office (VTO), in partnership with DFW, ASPIRES is but one in a long line of achievements attributable to the Athena project since the collaborative multiyear research effort wrapped after a three-year stint.
“Thanks to VTO’s vision and foresight, the Athena project has been instrumental in helping DFW understand complexities, improve their decision-making, and inform the integration of transformative technologies,” said NREL’s John Farrell, laboratory program manager for vehicle technologies.
The framework’s job is not done: The team hopes to continue to evolve and shape it to better optimize system design and make the capabilities developed by the Athena team widely available to meet other airport systems’ needs.
“The team is currently looking at refining and extending the proposed modeling framework. It might be integrated with other systems, such as those focused on the power grid and buildings,” Liu said.
ASPIRES is now being integrated with NREL’s HELICS cosimulation platform and adopted in NREL’s Advanced Research on Integrated Energy Systems (ARIES). The team’s efforts all work to meet the goal of decarbonizing airport shuttle operations.
“Airport shuttles have heavy operation demands,” Liu said. “Their energy and fuel consumption are significant. Motivated by concerns about air quality impacts and regulations, many airports are transitioning vehicles and equipment to technologies that reduce emissions, such as battery electric vehicles. For instance, California has required 13 state airports to exclusively operate zero-emission vehicles by Dec. 31, 2035.”
NREL is also working with DFW and the Seattle-Tacoma International Airport on passenger behavior analysis, congestion mitigation, fleet planning and operation, airport traffic simulation, airport curbside management, and other issues.
“Athena set the bar for bold thinking and continues to be in a class by itself,” Farrell said. “I have no doubt its legacy will continue to inspire the creation of new opportunities for carbon-emission reduction and optimization, such as ASPIRES, for years to come.”
Partners in Sustainable Airports
Just as NREL partnered with DFW to simulate optimal e-bus deployment, researchers offer support in other NREL modeling for sustainable aviation fuels, energy generation, and electrified aviation. Developed solutions require industry partners for application and deployment.
Researchers are also looking at airborne transportation and advanced air mobility to move small groups of passengers in electric, autonomous air vehicles in high-density city centers through a collaboration with the city of Los Angeles. NREL’s capabilities and research staff can be useful to potential aviation partners looking to bring energy efficiency and renewable energy to their systems.
The need for more sustainable aviation continues. NREL is applying lessons from its research efforts forward into work on autonomous aircraft, demand modeling for new sustainable (hydrogen and electric) aircraft, and energy modeling. Past successes in sustainable aviation—including e-bus deployment modeling—position NREL well to provide research support to airports looking toward a clean energy future.
Learn more about NREL's sustainable aviation and sustainable transportation and mobility research. And sign up for NREL's quarterly transportation and mobility research newsletter, Sustainable Mobility Matters, to stay current on the latest news.