Fast action on climate means picking the right mitigations
May 2024
For the aviation sector, shifting to sustainable biofuels and new battery technologies will bring positive climate impacts sooner than hydrogen. Former NASA scientist Dennis M. Bushnell provides his analysis.
Correction: The original version of this article incorrectly stated that all steels are vulnerable to weakening by hydrogen embrittlement. In reality, steels show various levels of resistance. The piece has been updated to make the broader point that embrittlement must be considered when choosing metals.
In 1995, the year I became the chief scientist at NASA’s Langley Research Center in Virginia, the United Nations Intergovernmental Panel on Climate Change issued its second assessment of the climate. In it, IPCC cautioned that “unambiguous detection” of human-induced warming will be “extremely difficult” to make in the coming decades.
For the next 28 years, I and many others worked on the climate issue and assessed the power and energy needs of the nation. Over that span, something dramatic happened: The ambiguity in climate science fell away. This was partly due to the work of scientists, but mostly because of what our planet is telling us.
Last year was the warmest in NOAA’s records dating back to 1850. The rate of sea level rise, which varies depending on location, has doubled globally to 3.6 millimeters a year this century compared to the 20th, according to NOAA’s Climate.gov website. Once-in-500-year floods now come regularly, as do droughts. The warming is increasing the likelihood that endangered species will go extinct, according to the International Union for Conservation of Nature. Ocean currents, crucial to Earth as we’ve known it in modern times, are slowing down.
Climate change was always a serious issue, but now it is an existential one.
At this juncture, mitigation efforts must be timely, realistic and bold. Use of renewable energy sources, mainly solar and wind, is increasing at the scale of the climate threat. But the threat is now so dire that, in addition to targeting net-zero emissions of carbon dioxide and methane to mitigate the many and large projected adverse climate changes, we must actively remove them from the atmosphere and also undertake an effort to increase the planet’s albedo by reflecting a greater portion of sunlight to space. The timeframes for requisite serious mitigation efforts have shifted nearer term, as climate change has become more severe and we approach climate tipping points and an increasing number of climate-change-enhancing feedbacks.
This means no sector, including air transportation, can shirk its responsibility to minimize its carbon dioxide emissions and address other warming influences it might be responsible for. Jet airliners produce some 2.5% of global CO2 emissions. Aerosols in their engines also exacerbate climate change in another way: When conditions are right, water molecules in the exhaust and in the lower stratosphere condense around these particles and freeze, creating contrails that merge with others and spread into blankets of cirrus clouds that reflect the outgoing infrared energy back to the planet.
Looking specifically at the CO2 emissions, it should now be axiomatic that these emissions need to be affordably reduced in years, not in the decades envisaged by the net-zero by 2050 movement. In reality, 2050 is going to be too late. The current approaches to seriously mitigate aero CO2 emissions include electric propulsion at ever-increasing ranges, shifting to sustainable aviation fuels made from renewables including agricultural sources, or hydrogen. Some or all of those would be combined with improvements in aerodynamic performance (a doubling of lift-to-drag ratio looks feasible); reductions in weight, notably via 3D printing of parts and 3D printing at the nanoscale to improve material microstructure; along with greater use of composites.
Consider first hydrogen and its many challenges.
On the ground, hydrogen will require new and costly infrastructures for supply, transport and fueling of aircraft. Storage would require large ground facilities, given hydrogen’s low volumetric density. Then there is the emissions problem. When hydrogen gas is combusted, the emissions are water and nitrogen oxides, or NOx, the water at rates 4.3 times higher than when jet propellant is burned. At altitude, the water will under certain conditions create thin cirrus clouds that will reflect the outgoing infrared energy back to the planet, exacerbating warming. The NOx would damage the ozone layer. Those facts stand whether the hydrogen is conventionally produced or whether it’s “green” hydrogen created by applying clean electricity to an electrolyzer that splits water into hydrogen and oxygen.
Then there is the problem of low energy efficiency through the entire chain, from production through combustion. Energy is lost when electricity is applied to water to create hydrogen gas, more is lost when the gas is cooled to liquefy it for storage. Then there are more losses when the hydrogen is converted back to gas and combusted to produce electricity or thrust. The overall initial green electricity to aircraft thrust efficiency is some 40%. By comparison, the efficiency of electric propulsion driven by batteries is some 80%.
Yet another issue is that hydrogen is several times easier to ignite than jet fuel, and any leaks would be difficult to detect because there is no untoward smell. Speaking of leaks, even if they do not lead to an explosion, those would be counterproductive to climate change mitigation for the following reason: Once in the atmosphere, hydrogen has an average global warming potential 11 times higher than that of CO2, according to the U.S. Environmental Protection Agency.
As for the aircraft engineering challenges, hydrogen fuel must be cryogenically stored aboard the plane for reasons of volume and structural weight, and that would complicate certification of such an aircraft. Designers would need to show that the aircraft can safely dump fuel before attempting an emergency landing. The issue there is that hydrogen requires noncylindrical tanks that won’t fit in conventional wings, so the tanks would probably need to be placed at the back of the aircraft. As they empty, the aircraft could possibly become uncontrollable. Also, hydrogen’s atomic structure makes it corrosive. Metals for aircraft structures and airport equipment must be carefully chosen to consider hydrogen embrittlement, which is when hydrogen atoms are absorbed, weakening a material. What’s more, the lower volumetric density of hydrogen compared to jet propellant translates to a larger wetted area (the parts of the vehicle exposed to the airflow) and, therefore, greater drag.
The novel nature of hydrogen-fueled aircraft would increase an already yearslong certification process by a factor of two.
Simply put, for long-haul transport, the engineering and cost challenges on the ground and in the air would take decades to solve and certify, creating enormous business challenges to match. Here’s what could be done to improve the situation:
- Efficiency: Improve electrolyzer efficiency to reduce creation losses. Improve fuel cells if used in lieu of combustion to reduce conversion losses and their weight and cost.
- Safety: Develop superb containment processes for ground and airborne applications that are leakless with no boil off; develop advanced leak detectors.
- Corrosion: Create coatings and select the right materials.
- Cost: Generate end-to-end hydrogen economies of scale so that utilization increases and cost per unit drops; improve electrolyzers; consider drilling for naturally occurring “white” hydrogen to reduce hydrogen creation costs.
- Storage: Develop ultra-high pressure, advanced materials to reduce weight and cost.
- Aircraft design: Consider channeling cold hydrogen through a smooth aircraft fuselage skin to cool it and maintain low-drag laminar flow.
- NOx and water emissions: Continue research to detect and fly around atmospheric regions where thin cirrus clouds would be formed due to exhaust.
The nearer-term, lower-cost option for serious reductions in air transportation emissions are SAF biofuels and green hydrocarbon jet fuel produced from atmospheric CO2 and renewable energy. The two common criticisms of SAF biofuels are, first, that they are expensive and, second, that producing them competes with food for arable land and fresh water.
Let’s look at the second criticism first. There is a strategy that could create the feedstocks for SAF and more arable land for food agriculture and also provide more drinking water. It’s an idea I raised in an article I wrote in a January special issue of the journal Water, “Halophytes/SalineWater/Deserts/Wastelands Nexus as a Scalable Climate Mitigation including Freshwater impacts.” Consider this: Some 6,000 varieties of halophytes, plants capable of growing in a salty environment, could be grown on land irrigated with saline or seawater, a massive, largely unused resource that comprises 97% of Earth’s water. Much of Earth’s saltwater is in the ocean, but a lot is underground, including in aquifers under many of Earth’s deserts and wastelands — areas that are largely unused and that comprise 44% of Earth’s land. So, we could pump this water up and grow salt plants — halophytes — in it. This way, we could green the planet soon, profitably and with existing technology. Massive amounts of biomass, including foods, could be produced. On top of that, right now, 70% percent of Earth’s fresh water is dedicated to growing food, and a shift to halophytes for food would make that water available for direct human use. In a direct climate benefit, the halophytes would sequester some 18% of their CO2 uptake in their deep desert roots. At scale, this would amount to pulling some 4 gigatons of CO2 per year out of the atmosphere.
For aviation, the halophytes would be a ready supply of feedstock for green biofuels. In 2014, researchers from Boeing and the United Arab Emirates grew halophytes for SAF fuels. Experiments with these fuels indicate they are a drop-in, meaning no changes need to be made to the aircraft’s engine or tanks. Also, flight experiments show that their emissions contain fewer particulates for water to coalesce onto to create contrails. Thus far, cost and availability are the major SAF fuel issues, and halophytes could solve those faster and be less expensive. Also, halophyte biomass could replace petroleum as the feedstocks for manufacturing of synthetic rubber and a variety of plastics.
Research is also progressing in the area of electrification, which is of course emission-free, provided the electricity is produced cleanly. The issue is the weight of the batteries. Existing lithium-ion batteries store some 250 watt-hours of electricity per kilogram of battery weight. Coming soon are solid state, metal and other technologies that promise two to three times the Wh/kg of lithium-ion technology. Beyond those, lithium-air technology promises some six times improvement. The li-ion viable range today, depending on the aerodynamic efficiency of the aircraft, is some 640 kilometers, a range that accounts for nearly half of today’s air trips. The coming battery technologies could increase range out to some 3,800 kilometers, a range that would begin to compete with longer conventional trips and therefore reduce the emissions of such travel. The battery wild card is the ongoing development of a scalable weak force nuclear battery by Clean Planet, a Japanese company that reports measuring 10,000 times more energy density than is possible with conventional fuels, with no radiation emissions and using only a small amount of hydrogen. This weak force nuclear battery would further extend the range of all electric flight, obviating the water and NOx emissions of SAF fuels.
So, here is where matters stand: Regarding SAF, long-haul demands for it are increasing rapidly, and cost remains an issue, although prices will drop with higher-volume production. For more capacity and ever lower prices, halophytes can be used as the SAF feedstock. As for net emissions, SAF fuels promise to reduce the amount of new carbon deposited in the atmosphere over time, by at least 80%, as the emitted CO2 is taken up by new plants to produce more SAF fuels, although the exhaust still contains water and NOx. Turning to hydrogen fuel, the required infrastructure and aircraft alterations are costly and will take far longer to develop and establish than the transition to SAF, and the emissions will consist of water and NOx. Batteries will provide increasing range capability with nearly zero emissions, reducing the aircraft water and NOx emissions, but the development timelines for the most advanced versions remain a question mark.
DENNIS M. BUSHNELL
retired from NASA last year after a 60-year career at the agency, the last 28 as chief scientist of Langley Research Center in Virginia. Bushnell is a futurist and AIAA honorary fellow.
