Wright Electric in the United States has announced a 100-seat electric short-haul aircraft that is expected to enter service in 2026. It can be powered by the company's "aluminum fuel cell" derived from recyclable metals. Wright also has a number of large electric aircraft projects underway, including the larger 186-seater aircraft it has developed in conjunction with European easyJet and BAE Systems.
It would be a "low-emission" electric aircraft that could use fossil fuel range extenders to supplement its batteries and extend its flight range to about 1,290 kilometers (800 miles). The partnership will use it as a "path" to clean aviation, like the Toyota Prius in the sky, which will prove that electric powertrains are viable while waiting for energy storage technology to develop to a certain level.
Wright's latest project, however, will be completely emission-free and will use high-density energy storage to tackle flights lasting up to an hour — enough to make about 1,000 kilometers (620 miles) of flights between Sydney and Melbourne, London-Geneva, Tokyo-Osaka or Los Angeles-San Francisco.
Once in production, the Wright Spirit, based on BAE 146, will be a simple 100-seater electric option that carriers can use on a variety of very popular routes.
Wright's business is focused on its areas of expertise: megawatt-class motors and frequency converters. In fact, the company doesn't seem to have identified an energy storage solution at this stage and is evaluating the pros and cons of hydrogen fuel cell systems, as we now see from a few different companies, and an "aluminum fuel cell" system that really fascinates us.
Why not use hydrogen directly, as most other fair-scale clean airliner projects do? An important factor is volume. In terms of weight, liquid hydrogen is an excellent lightweight energy storage medium – its specific energy is 33313.9Wh/kg, almost three times that of jet fuel (about 12,000Wh/kg). But in terms of volume, it sucks. At just 2358.6 watts/liter, a certain amount of liquid hydrogen takes up almost four times the same energy (about 9,000 watts/liter) in jet fuel.
For commercial aircraft operators, volume was a big issue; most of these early projects were retrofits of fuselages that were not designed to carry additional volumes of hydrogen. Every seat that needs to be removed from the cabin to make way for fuel is a direct blow to practicality. That's what's interesting about aluminum. By weight, aluminum fuel cells carry less energy than jet fuel or liquid hydrogen; but it has a specific energy of 8611.1Wh/kg, 33 times better than today's leading lithium-ion batteries. What's more, it's also excellent in terms of volume, loading 23,277.9 watts per liter. That would be a boon for airlines. Aluminum acts as an anode, opposite the carbon cathode and catalyst behind the porous polymer separator. Between the two is an electrolyte, usually an alkaline liquid. Aluminum reacts with oxygen in the atmosphere at the cathode, forming hydrated alumina and releasing energy.
Cathodes and electrolytes do increase the weight of the entire system to some extent, limiting the specific energy cap of aluminum to 60%-70% that a hydrogen system can reach. But Wright's rationale: "Since half of the single-aisle narrow-body airliner market is shorter than 800 miles, the conundrum of the voyage may not be as important as it initially seems." "
Wright called it a fuel cell, not an aluminum-air battery, to avoid confusion because it couldn't be charged like a battery; instead, it needed to add fuel like a fuel cell, while also taking the alumina scrap out and recycling it in a smelter. But this will not be much more difficult than processing liquid hydrogen tanks, which also need to be sent to an external facility for filling. But while hydrogen infrastructure needs to be built, there are aluminum smelters everywhere that can turn alumina back into pure metal, ready to be loaded back into tanks for cramming into aircraft, or for other uses.
In terms of logistics, it's easy: tanks can be transported around in standard trucks and loaded onto planes like cargo. If desired, the metal can be fed into the pipe in the form of particles.
However, challenges remain. Thin, cold, low-oxygen air at cruising altitude means that aluminum-fueled aircraft will need to run compressors and heat exchangers, which has the potential to run out of control of weight budgets. Aluminum fuel cells as a whole need to evolve further from their current state to achieve useful specific energy figures, and today's aluminum-air batteries are typically not designed for the need to run aircraft engines. To obtain a higher reaction rate, more aluminum needs to be exposed, possibly by using powders or granules instead of aluminum plates.
Wright conducted a rough "first round" simulation of the airline's operating costs, and with the expectation that the use of hydrogen fuel cells could increase costs by about 25 percent, while biofuels could increase costs by about 32 percent, aluminum systems would actually be a little cheaper than jet fuel operations today.
Between the cost advantage and the fact that these aircraft may run more seats than hydrogen aircraft in a modified situation, the power generated by the aluminum-air reaction process has the potential to present a compelling case for short-haul commercial flights. But to make such a system a green option, airlines will need to source aluminum from green smelters, using clean energy, clean heat and carbon neutral smelting anodes. Note that these technologies are being developed, and hydrogen has its own challenges in achieving zero emissions. Wright summed up the hydrogen vs. aluminum debate on this point:
"Hydrogen fuel cells: longer range, smaller payload, more difficult to operate, and more expensive. Aluminum fuel cells: shorter range, larger payload, easier operation and lower cost"