WHY THIS MATTERS IN BRIEF
Aluminium Air batteries have way much more energy density than even Hydrogen, and now one startup aircraft manufacturer’s ready to bet on the technology.
Future aircraft will be electric, and up until now the race to electrify aviation has been dominated by talks of ammonia, batteries, biofuels, friction, and hydrogen. But now US based Wright Electric has announced a 100-seat electric short-hop aircraft slated to go into service by 2026. It’ll either be powered by hydrogen, or it’ll use recyclable metal in what the company calls an “Aluminum Fuel Cell.”
Wright is working on a number of large electric aircraft projects, including an even bigger 186 seater it’s developing in conjunction with European airline EasyJet and BAE Systems. This would be a “low-emissions” electric, presumably using a fossil fuelled range extender to top up its batteries and extend its flight range to around 1,290 km (800 miles). The partnership is pitching it as a “path” towards clean aviation, a kind of Prius of the skies, that will prove the electric powertrain while waiting for energy storage to come up to scratch.
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Wright’s latest project, however, will be totally zero emissions, and will use high density energy storage to tackle flights up to an hour in duration – that’s enough for the ~1,000 km (620 mile) hop between Sydney and Melbourne, or London-Geneva, or Tokyo-Osaka, or LA-San Francisco.
Essentially, once in production, the Wright Spirit, based on the BAE 146, will be a simple 100 seat electric option that carriers can use on a wide variety of very popular flight routes.
The new concept electric aircraft. Courtesy: Wright Electric
Wright is concentrating mainly on its specialities namely the megawatt scale motors and inverters needed to pull a big ol’ Bessie like this through the air. Indeed, the company appears not to have settled on an energy storage solution at this stage hence the two options, and is evaluating the pros and cons of both a hydrogen fuel cell system, like what we’re seeing from a number of different companies now, and an Aluminum fuel cell system that’s really got us fascinated because ironically Aluminium fuel cells are not only an old technology but they’re much better than hydrogen alternatives – even though hydrogen is winning today.
Why not run straight at hydrogen like most of the other decent sized clean airliner programs are doing? One motivating factor is volume. Liquid hydrogen is an excellent lightweight energy storage medium by weight – heck, its specific energy of 33,313.9 Wh/kg is nearly three times that of jet fuel (~12,000 Wh/kg). But volumetrically, it’s terrible. At just 2,358.6 Wh/liter, a given amount of energy in the form of liquid hydrogen will take up nearly four times the space of the same energy in jet fuel (~9,000 Wh/l).
And volume is a big deal for commercial aircraft operators; most of these early projects will be retrofits to airframes that weren’t designed to carry the extra volume of hydrogen. Every seat that needs to be turfed out of the cabin to make way for fuel is a direct punch in the bottom line. And that’s what makes aluminum so interesting.
Aluminum doesn’t carry as much energy by weight as jet fuel or liquid hydrogen; at a specific energy of 8,611.1 Wh/kg, though, it’s about 33 times better than today’s leading Lithium ion (LiON) batteries. And it knocks it out of the park on volume, packing in 23,277.9 Wh/l. That’ll be music to the ears of every airline company.
How does it work? Well, effectively it’s an Aluminum-Air battery. The aluminum acts as an anode, opposed by a carbon cathode with catalysts behind a porous polymer separator. Between the two is an electrolyte, typically a basic liquid. The aluminum reacts with atmospheric oxygen at the cathode, forming hydrated aluminum oxide and releasing energy.
The cathode and electrolyte do increase the weight of the overall system somewhat, limiting aluminum’s specific energy ceiling to 60-70 percent of what a hydrogen system might achieve. But Wright reasons that “since half of the single aisle market is flights shorter than 800 miles, the range penalty might not be as consequential as it might initially seem.”
Wright calls it a fuel cell, rather than an Aluminum-Air battery, to save on confusion. It can’t be recharged like a battery; instead it’ll need to be refuelled more like a fuel cell, with the added task of taking the aluminum oxide sludge off for recycling at a smelting plant.
Wright says this won’t be much harder than dealing with liquid hydrogen tanks, which it says will also need to be sent off to an external facility for refilling. But while the hydrogen infrastructure all needs to be built out, there are aluminum smelters all over the place already that can turn the aluminum oxide back into fresh metal ready to be loaded back into a canister and stuck in a plane, or used for other purposes.
Logistically, it’d be easy; canisters can be carted around in standard trucks and loaded onto the plane much like cargo. In pellet form, the metal can be sent down pipes if need be.
Challenges remain though. Thin, cold, low oxygen air at cruise altitudes mean that aluminum fuelled aircraft would need to run compressors and heat exchangers that threaten to blow out the weight budget. Entire aluminum cells need to be developed further from their current state to realize useful specific energy figures, and today’s Aluminum-Air batteries are typically designed for low rates of discharge, as opposed to the demands of running aircraft engines. To get higher reaction rates, you’d need to expose more aluminum, potentially by using powders or pellets instead of plates. So there’s a way to go.
Perhaps the most interesting kicker here is the bottom line. Running a rough, “first pass” simulation of an airline’s operating costs, Wright projects that where hydrogen fuel cells are likely to raise costs by around 25 percent, and biofuels are likely to add around 32 percent, the aluminum system would actually be a hair cheaper than today’s jet fuel operations.
Between the cost advantages and the fact that these planes could potentially run more seats than a hydrogen plane in a retrofit scenario, Aluminum-Air could potentially put forward a compelling case for shorter range commercial flights.
But for this kind of system to become a green option, carriers will need to source their aluminum from green smelters, using clean energy, clean heat, and carbon neutral smelting anodes. Mind you, these technologies are under development, and hydrogen has its own challenges in getting to zero emissions.
Wright is agnostic at this point, summing up the hydrogen vs aluminum debate like so: “Hydrogen fuel cell: longer range, smaller payload, harder operations, higher cost. Aluminum fuel cell: shorter range, larger payload, easier operations, lower cost.”
If the technology makes the necessary strides, it could end up being a matter of horses for courses. But it’s certainly good to see that hydrogen might not be the sole viable way to decarbonize commercial air travel – even if it still looks like the only realistic path to clean long-haul flights.
Source: Wright Electric