Quick pit stops: A challenge for zero-emission planes?

Zero-emission flights are not yet commercially available, but it’s time to think about the infrastructures that will support them. Aircraft powered by batteries, hydrogen fuel cells, and hydrogen combustion are under development, but the charging and hydrogen refueling technologies needed to support these aircraft have yet to be designed. Here we calculate a key dimension of the economic viability of zero-emission aviation: the charging and refueling speeds needed for zero-emission aircraft to match the turnaround times of conventional aircraft.

Turnaround refers to the time an aircraft is parked between flights, the time between arrival at a gate and the start of its taxi for the next flight. Airlines work hard to minimize turnaround time because parked aircraft generate no revenue; instead, they incur costs in the form of airport parking fees. Ensuring turnaround times equivalent to those of conventional aircraft is necessary to make the economic case for zero-emission planes.

Turnaround varies across airlines and airports but is related to aircraft size. Commuter and regional aircraft can be ready for departure within 30 minutes of parking because of their lower passenger capacities. They typically operate over shorter distances and offer no food service, which eliminates the need to refresh the aircraft’s pantry. Narrow-body aircraft have turnarounds of closer to 45 to 60 minutes, because they need to unload and clean a bigger aircraft and often require pantry reloads. Long-haul wide-body aircraft can require more than 90 minutes to turn around; however, because zero-emission aircraft are not expected to replace long-haul flights by 2035, long-haul zero-emission aircraft are excluded from this analysis.

For commuter and regional aircraft, batteries and hydrogen fuel cells are plausible alternative power sources. ICCT research indicates that a 9-passenger battery aircraft would have an operational range (after deducting the energy reserve required for safe operation) of approximately 280 km using batteries with a pack-level specific energy of 350 Wh/kg (the projected battery technology in 2035). The energy use for such a mission would be around 500 kilowatt-hours. Supplying that energy in 30 minutes would require a 1 megawatt or higher charger. The Combined Charging System (CCS), an electric vehicle charging technology popular in Europe and North America, can deliver up to 350 kW, short of the need for a small electric aircraft. However, chargers of megawatt capacity are being developed through the Megawatt Charging System, a product of the CharIN initiative.

Ongoing ICCT research suggests that a hydrogen fuel cell aircraft using gaseous hydrogen could carry 50 passengers 650 km. This would require around 270 kilograms (kg) of gaseous hydrogen stored at 700 bar. Filling that amount in 30 minutes would require refueling at 9 kg per minute (kg/min) or 240 liters per minute (l/min). As a reference, current roadside hydrogen fueling stations using the SAE J2601 protocol allow a maximum flow rate of 3.6 kg/min. Thus, fuel cell aircraft would require 2.5 times the current refueling speed.

While liquid hydrogen combustion aircraft are expected to enter the market later than fuel cell or battery-electric aircraft, they likely have the greatest emission mitigation potential. A narrow-body aircraft with a hydrogen combustion turbofan engine would use about 5000 kg of liquid hydrogen to carry 165 passengers 3400 km. Filling that amount within 45 minutes would require a refueling speed of 111 kg/min or 1560 l/min. There are no accepted fueling protocols for liquid hydrogen, so no direct comparison to existing technology can be made. However, a flow rate of 1560 l/min is substantially greater than the 900 l/min rate achieved with conventional jet refueling hoses.

Table 1 summarizes the recharging and refueling speeds required for different aircraft types.

Table 1. Recharging and refueling requirements for zero-emission aircraft

A few other concerns bubble to the surface around hydrogen refueling. Safety procedures surrounding hydrogen refueling may prevent other turnaround operations from happening in parallel while the aircraft is refueled. This would increase the turnaround time for the aircraft. Additionally, liquid hydrogen refueling stations would need insulated hoses to keep the hydrogen in liquid form and at cryogenic temperatures. This could result in bigger and heavier hoses, making them difficult to maneuver without machines and automation to do the heavy lifting. Using multiple, smaller hoses could solve this problem and help achieve the faster refueling speeds, but that would displace complications onto the tank design, which would need to accommodate multiple fueling ports.

Since development of megawatt-class charging infrastructure is already underway, the recharging requirement for electric commuter aircraft will likely be met easily. Refueling regional fuel cell aircraft with compressed hydrogen gas will require nearly tripling the existing hydrogen refueling speeds used for road vehicles. However, researchers at the National Renewable Energy Laboratory have demonstrated refueling speeds greater than 13 kg/min, which would be more than sufficient to meet the 30-minute turnaround time requirement. Liquid hydrogen refueling, however, will be a challenge. Without any commercialized solutions, this option first requires investment in research and development of new technology, infrastructure, and standard refueling procedures. Fortunately, liquid hydrogen powered aircraft are unlikely to fly before the end of the decade, so there is time to build the technology.

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