My wife and I recently returned from a vacation to Maui. We flew nearly 5,800 airmiles round-trip. According to figures used by the World Bank the two of us generated just over 2 metric tons of carbon dioxide, almost 4,500 lbs!
Worldwide, commercial aviation produced 915 million metric tons of CO2 in 2019. If you liquified all that CO2and loaded it into typical 30-foot-long propane delivery trucks, at 13.45metric tons of CO2 per truck, it would fill 68,029,740 vehicles. Parked nose-to-bumper, the line would wrap around the Earth’s equator 15 ½ times!
Given these kinds of numbers, the airline industry is a significant contributor to atmospheric carbon dioxide and resultant climate change. Experts agree it accounts for about 2.4% of total annual carbon dioxide emissions. Established airplane manufacturers and a plethora of start-up companies are exploring no-carbon fuel options for commercial aviation.
There are three basic zero-carbon options available to power commercial flights: battery electric (BE), hydrogen fuel cell electric (HFE), and direct hydrogen combustion (HC). I’m omitting alternative carbon-based fuels only because of the difficulty of scaling up production in a meaningful way.
Like battery electric vehicles, BE planes rely on a lithium-based power pack to run an electric motor. Instead of turning a wheel, the electric motor turns a propeller.
BE designs are highly constrained by the power density of the batteries. Lithium-ion batteries store a large amount of power but discharge it slowly. This limits use of these batteries to small regional aircraft. With larger airframe designs the battery mass quickly exceeds the ability to generate lift, making them impractical. Research is focused on reducing battery weight while increasing power output.
So far, engineers have widened or lengthened the fuselage to account for battery storage, rather than place the batteries in the wings. Around 100 electric aircraft designs are currently under development worldwide.
Instead of using a battery for energy storage, HFC designs rely on fuel cells to run electric motor-driven propellers. In the reversal of water electrolysis (
October ’22 Just Over the Horizon). Hydrogen is catalytically combined with oxygen within the fuel cell to form water, releasing electrons in the process. Hydrogen has an energy density of 35,000 watts per kilogram, giving fuel cells a big advantage over batteries which release their stored energy more slowly.
The challenge for HFC is designing and incorporating bulky liquid hydrogen cryogenic storage tanks. Like BE designs, fuel will be stored within the fuselage rather than in the wings. But due to the lighter mass of liquid hydrogen, greater range and payloads can be realized.
Hydrogen combustion utilizes hydrogen rather than hydrocarbon fossil fuels to generate thrust. Liquid hydrogen, cooled to −253 °C, has an energy density 4.1 times lower than jet fuel (8.5 MJ/L verses 35 MJ/L). Just as the case when compared to batteries, the lighter mass of hydrogen ultimately allows for greater speed and range than fossil fuels. Like BE and HFC technology, HC airframes must be designed with larger volume fuselages to accommodate the higher volume of fuel required.
Here is a sampling of HC development leaders and the status of their tech. MagniX is developing electric motors for aviation. On May 28, 2020, a MagniX-powered nine-passenger Cessna 208B eCaravan flew on battery electric power. The company is working toward FAA certification for commercial operation for its 640 kW (850shp) Magni650 engine.
On September 7, 2022, BE startup Eviation flew a 9-passenger prototype electric plane driven by MagniX motors for 8 minutes to an altitude of 3,500 feet. The plane was designed and built to demonstrate the potential for an electric commercial commuter aircraft flying a few hundred miles between cities at an altitude of around 15,000 feet. It was powered by just over 21,500 small Tesla-style battery cells that, at just over4 tons, make up fully half the weight of the carbon composite airframe.
Another startup, ZeroAvia, builds a 600kW HFC powertrain for 10-20 seat plane. They installed two ZA600 hydrogen-electric powertrains aboard a twin-engine 19-seat Dornier 228 aircraft at its headquarters in Hollister, California. It will serve as the testbed for working with the FAA ahead of the ZA600 engine's planned certification in 2024.
In 2022, American Airlines signed an MoU for an order of as many as 100 of the H2-powered plane engines for eventual use in its regional fleet. Aircraft leasing company MONTE will purchase up to 100 ZA600 powertrains to be installed on existing and new Cessna Caravan, DHC-6 Twin Otter, Dornier228 and HAL-228 aircraft.
In September 2020, Airbus presented three ZEROe hydrogen-fueled concepts proposed for commercial service by 2035: a100-passenger turboprop, a 200-passenger turbofan, and a futuristic turbofan-powered blended wingbody. In February 2022, Airbus with partner CFM International, announced a demonstration of a liquid hydrogen-fueled GE Passport turbofan with modified combustor, fuel system and control system. Mounted on a fuselage pylon on a prototype A380, the first flight is expected within five years.
Rival Boeing has acknowledged the potential of the technology but recently cut research and development in an effort to return to profitability after its 737 Max and 787 production disruptions.
Pratt & Whitney, Rolls-Royce and GE are all developing hydrogen turbofan engines, with the expectation of incorporation into major passenger aircraft by 2035.
What obstacles remain to full adoption of zero-carbon aviation?
Battery weight. Heavier lithium-ion electrolyte tech is being replaced by lithium-polymer. In addition to lighter weight, L-P offers higher output. Even so, BE will probably always be restricted to regional commercial aviation, general aviation, and EVTOL flying taxis.
Ramping up production of green hydrogen will take time. By 2030 total annual production is projected at about 5.2 million metric tons. By 2050, when demand and production are expected to match, production will reach 500-680 million metric tons. The percentage of green hydrogen available to meet aviation demand in 2030 will be in the single digits.
Aviation will have to compete with cement, steel and fertilizer manufacturing, transportation, plus the general grid. It will have to be augmented by alternate green aviation fuels, de minimus use of gray/blue hydrogen and fossil fuels until green hydrogen production capacity catches up with demand some twenty years later.
Many of the hydrogen hub proposals submitted to the Department of Energy, seeking a share of the $7 billion program, will knit together green power hydrogen production sites with industrial sites, utilities and airports via pipelines, rail and shipping. Airports are already announcing commercial agreements with vendors to provide cryogenic storage and delivery to aircraft.
In the future look to see tanker trucks resembling propane delivery trucks fueling up HFC and HC aircraft.
Let's look about fifteen years into the future. At best, commercial zero-carbon aircraft manufacturers will be five years into production. Airlines may restrict the few hundred HC aircraft to specific hubs that can acquire and deliver the limited supplies of green hydrogen. BE and HFC regional aircraft, ten years into certification, could enjoy a greater market penetration than the larger aircraft. Smaller regional airports may offer deliveries by offsite vendors.
What if you live near an airport? HC aircraft won't necessarily be quieter than the most modern jet fuel powered craft, but they’ll be cleaner. Hydrogen combustion produces none of the soot, and a small fraction of the NOx pollution of jet fuel. Regional and general aviation craft will be noticeably quieter.
Will these first steps by commercial aviation reduce CO2 emissions and the worsening resultant climate changes by 2037? Probably not. Given the expected world-wide growth in commercial aviation, the carbon reductions will barely offset growth. But other sectors of the economy will have decarbonized farther. By 2037, we might realize diminishing annual global greenhouse gas emissions. We’ll finally beheading in the right direction.
For further Reading
https://en.wikipedia.org/wiki/Embraer_E-Jet_E2_familyhttps://en.wikipedia.org/wiki/Electric_aircrafthttps://en.wikipedia.org/wiki/Hydrogen-powered_aircrafthttps://nap.nationalacademies.org/catalog/26512/preparing-your-airport-for-electric-aircraft-and-hydrogen-technologieshttps://www.magnix.aero/serviceshttps://www.seattletimes.com/business/boeing-aerospace/first-u-s-all-electric-airplane-takes-flight-at-moses-lake/https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroehttps://www.energy.gov/oced/regional-clean-hydrogen-hubshttps://www.gminsights.com/industry-analysis/green-hydrogen-market
