The dark side of green
By Keith Button|June 2023
Researchers are beginning to weigh the environmental trade-offs associated with decarbonizing air travel by switching to hydrogen combustion, hydrogen fuel cells and lithium batteries. Keith Button tells the story.
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A glider soars at 33,000 feet carrying a jet engine that burns hydrogen and spews a trail of frozen ice crystals, while an aircraft flying behind collects samples of the contrail. At the bottom of the Pacific Ocean, a device the size of a recreational vehicle churns across the muddy floor, sucking up delicate potato-sized nodules.
These tests — the first one planned by Airbus for September, the other a demonstration late last year of seabed mining by The Metals Company based in Vancouver — might seem unrelated, but they are indirectly linked in that they are among the first attempts to assess the environmental impacts of two highly touted means of achieving net-zero carbon emissions for air travel.
Airbus is undertaking the “Blue Condor” glider project because it believes hydrogen-burning jet engines could power airliners with capacities of about 200 people, similar to the jetliners of today, and it is targeting 2035 to have such aircraft ready for sale. The exhaust from these aircraft won’t contain soot or carbon dioxide, but it will contain nitrogen oxides, including the greenhouse gas nitrous oxide, and water vapor that can turn into icy contrails that may or may not trap heat in the atmosphere — the effect is still unknown.
As for the mining demonstration, the poly-metallic nodules collected in October and November by The Metals Co. contained key ingredients for lithium-ion batteries, although not the lithium. There was nickel and cobalt, essential for battery cathodes, plus copper for the foil in their cells, and manganese, another ingredient in the cathodes but one that’s relatively low cost so of less concern. The demand for these metals is driven largely by the electrification of cars and trucks, but drones flown by consumers and delivery companies are already powered by lithium batteries, and vast fleets of air taxis and other small electric aircraft will someday be powered by them too, if the ambitions of entrepreneurs in the advanced air mobility sector are achieved.
All told, Benchmark Mineral Intelligence of London, which has done work for The Metals Co., projects that by 2030, the demand for cobalt for electric vehicles of all kinds will increase by 200% and the demand for nickel by 300%. That equates to 5.6 million metric tons of nickel and 240,000 metric tons of cobalt. Projections like these, and the desire among nations including the United States to shed dependence on China, explain the motivation for undertaking the technical challenge of hauling nodules up from a depths of 4,000 to 6,000 meters in the Pacific Ocean’s Clipperton-Clarion Zone west of Mexico. For scientists, it also underscores the need to determine the effects such mining could have on the marine ecosystem.
Glenn McDonald, a consultant for aerospace industry investors and manufacturers at AeroDynamic Advisory in Ann Arbor, Michigan, says most of his clients know they should be concerned about where the metals in their batteries will come from. “But I don’t think there’s a lot of knowledge in the aviation industry specifically about how to measure that or what to be concerned about. I think aviation will come along for the ride with automotive and with other industries.” In fact, he expects aviation to comprise only a low single-digit percentage of the global demand for lithium batteries. Benchmark Mineral Intelligence, which looked at the aviation industry’s demand for battery ingredients through 2030, placed aviation’s shares even lower: Aviation comprises 0.08% of the total demand for lithium, 0.12% of the demand for nickel and 0.06% of the demand for cobalt.
Turning again to hydrogen, “it’s a little bit shocking, but we don’t know in aviation what the relative contribution of the different drivers like contrails or contrail cirrus clouds [are],” McDonald says. “There’s a chance that contrails and water vapor emissions are actually more important than CO2, which would drive us away from hydrogen. We just don’t know yet.”
To try to answer the contrail questions, Airbus, with help from the German aerospace agency DLR, plans to attach a hydrogen-fueled jet engine on one glider and a conventional kerosene jet engine on another and compare the contrails. Gliders were chosen to avoid contaminating the contrail samples. Each glider will be towed to the test altitude, where the engine will be turned on. Sensors and gauges from DLR on the turboprop chase plane will measure the size, distribution and density per unit of air of the ice crystals, levels of nitrogen oxides and any particulates in the exhaust of the engines, along with sampling the background aerosols in the atmosphere, plus temperature, air pressure and water vapor.
If the results of the glider tests are unfavorable, it’s unclear what could be done. The water vapor exhaust can’t be condensed or otherwise managed because it is part of the thrust that’s pushing the plane forward, says James McMicking, vice president of strategy for ZeroAvia, a U.S. and U.K. company that is developing fuel cell propulsion for regional turboprops. Electricity-generating fuel cells are another option for carbon-free air travel, but probably for smaller aircraft. They do emit water as exhaust, but they aren’t expected to generate contrails as easily as hydrogen combustion engines, if at all. Their exhaust is cooler at 100 to 180 degrees Celsius compared to 1,800 degrees for the combustion jet engine, and the water is concentrated into larger droplets.
Airbus is studying fuel cells too, and researchers at its ZEROe Aircraft unit think they perhaps can alleviate the risk of contrails by figuring out the conditions under which contrails form. They would then determine whether cooling, warming or altering the pressure of the air flow will prevent contrails from forming when the droplets are released into the flow, says Hauke-Peer Lüdders, head of fuel cell propulsion for ZEROe Aircraft.
Fuel-cell-powered flight at up to 27,000 feet will only rarely produce contrails under unique conditions, predicts Josef Kallo, chief executive of H2FLY. The Stuttgart, Germany-based company is developing fuel cell propulsion for a 40-seat turboprop, which would not fly above 27,000 feet. When a contrail does form, it will be much smaller with denser ice crystals than a kerosene-combustion contrail, so it lasts about 20% as long in the air, Kallo explains.
Meanwhile, on the ocean bottom are the nodules that formed over millions of years, accumulating metals around fossilized bone fragments or other solids. The Metals Co. predicts that when the carbon footprint of the collecting methods are factored in, mining the seabed will prove to be less environmentally damaging than collecting the metals on land, an exception being the warming footprint of cobalt. Mining on land “tends to be carbon intensive, polluting and associated with controversial social issues such as displacement of indigenous peoples,” the company says in its “Life Cycle Assessment” report on its website. The company wants the go-ahead from the International Seabed Authority, a United Nations agency, to embark on its plan to harvest 240 million tons of the nodules over 20 years.
To test the harvesting method and assess its environmental impacts, a crew lowered a remote-controlled, tracked vehicle to the ocean floor, where it sucked the nodules into pipes that carried them up to a ship, the Hidden Gem. The harvesting vehicle is “kind of a cross between a combine harvester and an underwater vacuum cleaner, but on the scale of a combine harvester,” says Jon Copley, a deep sea ecologist and professor at the University of Southampton, who is unaffiliated with The Metals Co. Copley and other ocean researchers are studying how marine life in surrounding areas might be affected by the plume of silt left as the vehicle rolls along the seafloor. A second plume results when the nodules are sifted from the mud aboard the surface ship, and the mud is released at mid-depth through a pipe.
Researchers are also studying whether the mining-free zones of the ocean floor that have already been set aside by the International Seabed Authority will be adequate to maintain the biodiversity of animal and microbe species that live in the sediment and in some cases on the nodules, which are the only hard surfaces at that depth, Copley says.
The environmental nonprofit Greenpeace is advocating for a deep sea mining moratorium, and BMW, Volkswagen and Volvo are among the automakers that have signed an online petition promising not to allow metals collected this way into their supply chains. So far, no aerospace companies have signed on.
“These deep sea ecosystems cannot be recovered in human time scales,” says Arlo Hemphill, Greenpeace USA’s lead for deep sea mining and ocean sanctuaries. “They take literally millions of years to evolve. So once it’s done, any growth of deep sea organisms is just not at the scale that we’ll ever see it again.”
As for the mining issues on dry land, the practices for collecting cobalt and nickel are under particular scrutiny, says Laurent Pilon, a program director at the Advanced Research Projects Agency-Energy focused on electrical energy storage. Seventy percent of the world’s cobalt is extracted from mines in the Democratic Republic of the Congo, where child labor and worker safety issues are rampant. As for nickel, critics charge that Indonesia, the world’s largest producer, processes the metal with carbon-intensive, coal-produced electricity, while its mining practices pollute water, destroy forests and disrupt indigenous people. In March, Indonesia President Joko Widodo pledged to improve environmental monitoring of nickel mining, Reuters reported.
Research is underway on a potential new class of lithium batteries that would need fewer, if any metals. [See “A battery technology with fewer trade-offs”] Even advocates caution that market-ready versions of these lithium-air batteries, (called that because they cull oxygen from the air), are still years away.
But pressure from environmental groups could spur greater work on lithium-air batteries or other technologies. “In the future, there [will be] a big push to reduce the reliance on cobalt and possibly nickel,” predicts Pilon, the ARPA-E project director. As for eliminating the need to mine lithium, that’s probably not possible. “Looks like we’re going to need lithium for a lot longer.”
Lithium mining comes with its own challenges. In one method, lithium salts are extracted from pools of brine water, which can contaminate fresh water; in another, lithium is dug up, but doing so generates lots of carbon emissions. Also, China reportedly controls a growing share of the lithium mines around the world. The Institute for Energy Research, a conservative Washington, D.C., think tank, predicts that by 2025 China will control 32% of the lithium mines compared to 24% in 2022.
Regarding the environmental implications, uncomfortable trade-offs could be inevitable: “Is it better to emit CO2 or contaminate water? I don’t know. I mean, is malaria better than cholera?” Pilon asks.
What is the likely mix of aircraft that will dominate in the future? The Mission Possible Project, a consortium of companies advocating for decarbonization, expects sustainable aviation fuels made from renewable carbon sources to dominate the aviation market by 2050, measured by the percent of energy demand. Specifically, battery-powered aircraft are projected to comprise 2% of the demand, followed by hydrogen-fueled aircraft at somewhere between 13% and 32%, and SAF-fueled aircraft at about two-thirds. In this view, battery propulsion will be confined to smaller aircraft, while SAFs can be dropped into today’s kerosene-fueled planes and existing supply chains with little to no hardware modification. Hydrogen-fueled aircraft will need new airframe designs, new propulsion, and new fuel delivery and storage setups.
An open question is whether the aviation industry could, if it wanted to, affect where the metals for batteries come from. If the 2% figure is correct, the supply chain will likely remain set up to feed the demands of the automotive sector.
Perhaps for this reason, most of McDonald’s clients are focused on solving the technical and economic problems associated with future aviation concepts — like whether future batteries will be powerful enough, light enough and cheap enough. They’re not worrying about the sourcing of battery materials.
Robin Riedel, an aerospace sector consultant at McKinsey and Co., sees things somewhat differently. Some aircraft builders and investors in the electrification space are tuned in to the environmental issues. “It’s a pretty meaningful number, and yes, people are worried about it. But at the same time, people are saying, ‘Well, the automotive industry, which had so much more demand — orders of magnitude higher demand — is going to lead the way on this stuff,’” Riedel says. [Read our advanced air mobility Q&A with Riedel]
Hydrogen combustion is a different matter, given that there are no automotive coattails to ride. Any warming impact from contrails would belong solely to the aviation industry. “If I were investing in hydrogen combustion at this point, I’d be a little bit cautious, or I would want to know where regulation is headed before anyone sinks tens of billions of dollars into a new airplane,” says McDonald.
In his view, environmental considerations are beyond the expertise of aircraft builders, suppliers and investors, so therefore the industry needs standards or regulations that spell out how to weigh those concerns. Absent measures like a carbon tax or conflict mineral clauses restricting transactions involving metals or other minerals from war-torn countries, it’s too difficult to weigh the relative environmental or human rights costs of obtaining metals from one source over another, or whether a change in the supply chain is warranted.
“If there’s no regulation about how to consider those other environmental concerns today, they just won’t be considered,” he says.
A battery technology with fewer trade-offs
An experimental class of batteries that largely employ no metals other than lithium could make moot the question of where to get the nickel, cobalt and other metals for lithium batteries.
Though researchers are still in the early stages of developing them, lithium-air batteries are a candidate to power aircraft in a decade or so because each is capable of producing five times the power per kilogram of a Tesla battery. One promising lithium-air design developed by an Illinois Institute of Technology team has only lithium in its anode and a cathode containing molybdenum, which is mined in several countries including the U.S. and China, or created as a byproduct of copper mining. But the researchers believe they can make the cathode out of carbon and nitrogen-based materials, along with an electrolyte free of any metals, says Larry Curtiss, a researcher on the team and head of the molecular materials group at Argonne National Laboratory in Chicago.
If you wonder about hydrogen fuel cells, these do contain platinum, a rare metal that fosters the conversion of exhaust to less harmful gases in the catalytic converters of cars, mining platinum will not need to be done on nearly the scale of mining for the metals in batteries. That’s because 99% of platinum in spent fuel cells and catalytic converters can be recycled. Also, cars will be replaced eventually by electric vehicles, so demand for platinum won’t increase significantly as demand grows for fuel cells, says Hauke-Peer Lüdders, head of fuel cell propulsion for Airbus’ ZEROe initiative to build a hydrogen airliner by 2035. — Keith Button
