Batteries & Biofuels, Not Aluminum & Hydrogen, Will Fuel The Airlines Of The Near Future (Part 2)

In the first of three parts of this series on refueling aviation, the challenges of replacing jet fuel that is cheap, convenient, and effective — as long as we ignore global warming and air pollution — and getting alternative fuel airplanes certified to carry passengers were covered. In summary, it’s hard to find a good alternative to jet fuel, and hydrogen is very expensive and hence a poor alternative. If hydrogen were the only alternative, then there would be an argument for it, but no.

The section of Wright Electric’s white paper dealing with hydrogen and ZeroAvia’s choices as well, have been plumbed. But the second half of the white paper strays even further from anything sensible, into a proposal to use aluminum air fuel cells.

Aluminum Air Fuel Cells For Airplanes Make No Sense

This is an old technology that some people hope to dust off for electric vehicles, without any success to date. They were proposed in the 1960s, have been working since at least the early 1980s, but they aren’t commonly used today, outside of off-grid military applications. The lack of use should be a red flag for this technology, but it keeps being proposed for electric vehicles nonetheless.

Why isn’t it used? Well, it’s not a battery, for starters. You have to get specially prepared aluminum electrolytes delivered. Once they oxidize in the fuel cell, delivering their energy, you have to send them back to an aluminum smelter for reprocessing, repackaging as an electrolyte and get them shipped back. The fuel supply chain would be doubled, with masses of ‘charged’ and ‘discharged’ aluminum going back and forth between wherever you need the fuel and smelters.

Energy density of aluminum air batteries is high, but there is exactly zero supply chain for aluminum electrolytes at the scale of airline energy requirements. If Wright goes down this path, the entire fuel supply will be a bespoke one-off at first, with very high costs. There are other technical problems that are somewhat being resolved with the technology as well, such as their tendency to self-corrode.

Another issue is that they are aren’t scaled to anywhere near aircraft requirements yet. The biggest commercial product I could find was an Alibaba listing for a $16,000 32 kWh emergency power supply for remote areas. As Wright’s whitepaper points out, the requirement is about 10 MWh, about 300 times as much.

The white paper’s projected future energy density of aluminum air batteries is 2,000 Wh/kg or 0.002 MWh/kg. 10 MWh would require about 5,000 kg of aluminum at that future date. However, the highest demonstrated is 1,350 Wh/kg, so in the next few years, that’s actually going to be 7,400 kg or 7.4 tons of aluminum electrolytes for every 400 mile flight.

Aluminum Smelters Canada

Aluminum smelters in Canada courtesy Government of Canada

As this map of Canada shows, aluminum smelters are not evenly distributed. They require huge amounts of electricity and are typically located near hydroelectric dams. Since aluminum is a durable product, shipping it once a long distance is fine, but imagine that every single 400-mile airplane flight requires you to use trucks and trains to ship 7.4 tons of aluminum hundreds of miles in each direction. The fuel will end up traveling three to five times as far as every plane flight, with every mile traveled accruing additional costs and carbon debts.

The Wright white paper also seems to think that loading aluminum catalyst cartridges is the same as loading luggage. There are innumerable mechanical concerns with that thinking that are elided completely, but suffice it to say, a few tons of catalyst that has to get into and out of a fuel cell is not the same as a container of suitcases that just sit in the hold for the duration.

Then there’s the final problem with aluminum fuel cells. They are high density, but low efficiency. They are projected to have a fuel-cell to motor efficiency of around 20%. But as soon as you are shipping tons of aluminum back and forth for potentially hundreds of miles for every flight, the efficiency drops further. Well-to-wheel numbers don’t seem to have been created by anyone for this technology, but I can’t imagine it getting above 10%.

Aluminum air batteries don’t seem remotely viable.

Battery Aviation Haters Do Very Bad Analyses

As I mentioned, someone reacted to my discussion with Heart Aerospace’s Forslund by pointing me at a critique by a self-proclaimed aviation industry analyst. Remarkably, this analyst found hydrogen to be the real answer despite the obvious economics I outlined in part 1, and specifically critiqued Heart Aerospace’s range calculations for its ES-19. Reading through their analysis I found two glaring errors immediately.

The first is that they used the fuel efficiency of modern jet turbofan engines at cruise as the basis for the critique. There are two problems with this. Heart Aerospace’s target is not 30,000-foot operating altitudes for a thousand miles, but much lower altitude, maximum 250-mile hops, and mostly shorter. The turbofan efficiency, as Forslund points out, is very poor on the ground, and ground taxiing can be easily 10% of the flight time for short flights. That’s why the industry has dropped most short hop flights. It’s just too expensive to run modern turbofan planes on them. That turns into an error of over 80% in the analyst’s calculations.

The second failure is in their assertion of 160 Wh/kg as the maximum energy density of lithium-ion batteries that will work in airplanes, with the requirement being 400 Wh/kg. The Tesla 4680 cell is already at 380 Wh/kh, and Tesla’s older batteries were at 260 Wh/kg. Tesla’s older batteries are completely certifiable for aviation, so even assuming that earlier energy density, that’s another 63% error at minimum, and more likely 138% with 4680-level technology and of course worse with even the simplest projections of ongoing improvements in batteries. The analyst claims, with exactly zero evidence or supporting material, that higher density lithium-ion batteries can’t be certified for aviation, when Teslas drive over 12,000-foot high mountain passes regularly, above the operating altitude of Heart’s target market.

The third failure is the analyst’s cycle time for lithium-ion. Both Heart Aerospace and the analyst suggest 1,500 cycles, the analyst as a maximum, and Forslund et al., as a minimum, but as the analyst artificially triples the battery, it becomes uneconomic in their analysis, but is completely reasonable in Heart’s. Unsurprisingly, with the addition of 3x the mass of lithium-ion battery theoretically required for the ES-19 design parameters, it turns out to be unworkable.

By comparison to hydrogen or aluminum, rechargeable lithium-ion batteries stay in the plane, get recharged with ubiquitously available electricity and have 80% well-to-wheel — wind-turbine-to-propeller? — efficiencies, 8 times as high. This means that the 19% of operating expenses that aviation spends on fuel goes way down, instead of going way up, leaving lots of money for battery replacements every few years. As a case in point, one early adopter of a Pipistrel electric flight trainer say their fuel costs dropped by 95%.

The analyst is clearly so deeply biased that they will pick completely unreasonable numbers for their comparison, and then claim justification for their opinion. As I said, “self-proclaimed.”

And so, part 2. Aluminum air batteries don’t get within an intercontinental flight of being a viable choice, and certainly not by Wright’s 2024 target date for first flight with one engine running on the new fuel. And those biased toward fuels other than batteries and biofuels use completely inappropriate assumptions to make their cases. In part 3, I’ll deal with why I’m bullish on biofuels for long-haul aviation.


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