Aura Aerospace pitches Powercell ultracapacitor powertrains for eVTOL

San Francisco company Aura Aerospace is working on a remarkably odd idea: replacing batteries in electric VTOL air taxis with high-powered "Powercell" ultracapacitors capable of massive power output and 10-minute charging.

The Powercell, says Aura, offers twice the power density of a Tesla battery pack. It can charge up in just 10 minutes, and it'll handle being blast-charged like that some 250,000 times, where lithium ion batteries start losing their mojo within a couple of thousand cycles. It's safe, with no chance of thermal runaway, and uses no cobalt, and since it's primarily made from aluminum foil and graphene, it can be recycled when it finally reaches its use-by date.

By way of proving its capabilities, Aura furnishes a very unique video.

It costs roughly 10 times what you'll pay for a lithium battery, but it'll do so many more cycles that Aura says the lifespan cost will be vastly cheaper. Indeed, the company's ready to sell them to you right now, with deliveries in 6 to 8 weeks. A 600-Wh Powercell 100kF 25V unit will cost you US$7,599, with a sky-high peak output of 21.6 kW.

Of course, there's a but. A big, juicy but in this case. Ultracapacitors offer superb power density, but terrible energy density. They can charge and discharge at amazing rates, but they can't hold much energy, even compared to lithium ion batteries, which themselves hold so little energy that electric VTOL aircraft are waiting on next-gen capacity leaps to become commercially viable on battery power.

The Powercell's energy density is around 60 Wh/kg, where a Tesla Model 3 battery currently sits around 260 Wh/kg. So an eVTOL powered purely by these ultracapacitors would have less than a quarter the range of one running on lithium. To take a particularly silly example, the VoloCity's range would drop from a meagre 35 km (22 mi) to just 8 km (5 miles). You could just about pick yourself up and throw yourself further.

But pure ultracapacitor powertrains are not the intention here. Aura seeks to use the Powercell as a high-powered, super-responsive buffer, primarily for the most energy-intensive phases of flight: vertical takeoff and landing. A transitioning eVTOL with a wing can fly just about as efficiently as a regular plane once it's moving forward fast enough to use the wing for lift, and Aura sees an opportunity for a hybrid system in which the Powercells handle the big outputs of VTOL, while smallish gasoline engines or hydrogen fuel cells provide a slow, steady flow of charging power, enough to cover horizontal flight and top up the ultracapacitors for a high-powered landing maneuver.

And here, the company might have a good point. Gasoline and hydrogen offer energy density that far exceeds what even the most optimistic of laboratory batteries are promising. Running an efficient petrol generator or a consistent fuel cell for endurance and an ultracap for quick response and high power loads might prove a great combo in the early days of eVTOLs.

Let's see what the math might look like on the back of an envelope. Archer provides us with some energy use figures that may prove instructive. Archer's Maker eVTOL claims a range of 96 km (60 mi) from a 143-kWh lithium battery pack, breaking that capacity down into 26 kWh for VTOL and hovering, 80 kWh for cruising, and 37 kWh for reserve, and leaving room for capacity fade and inaccessible energy. Archer's figures don't add up, but I think they're saying that whole battery pack will weigh about 1,000 kg (2204 lb), it's honestly a little hard to tell.

So let's say we were to use a Powercell setup for that 26-kWh lift and hover capacity. At 10 kg (22 lb) per 600 Wh, that'd be 430.3 kg (949 lb) worth of ultracapacitor, leaving us 570-odd kg (1,257 lb) to fill up with a generator and fuel. That seems very doable, given that gasoline stores a huge 12,500 Wh/kg of energy. Even taking the inefficiency of combustion motors into account, you could replace the Maker's entire 143 kWh of energy storage and have plenty left over with 30-odd liters (8 gal) of standard pump gas, weighing about 22.5 kg (50 lb), and have more than 500 kg (1,100 lb) of weight spare to use for the engine, the fuel tank, exhausts, radiators and other bits and pieces. Heck, you'd be mad not to throw in triple that fuel and get yourself a much better range.

So maybe the idea's got legs – certainly with lithium batteries in their current state. It'd be interesting to see what a range extender system like this and the cost of liquid fuel might do to the per-mile price of riding in an eVTOL air taxi, which will need to be super affordable if it's to be the transport revolution everyone seems to be expecting. And that's ignoring the higher upfront cost of using Powercells as opposed to batteries, as well as the increased maintenance overheads that come with combustion engines.

But if batteries don't evolve in leaps and bounds over the next five years, the air taxi market may well have to start out with powertrains that aren't pure, clean and fully electric. And if that eventuates, then perhaps there's a place for high-powered ultracapacitor hybrid eVTOLs in the air over our cities.

To show the concept off, Aura is working on its own "eVTOL." It's not a transitioning eVTOL, so it doesn't really fit the lift-n-cruise hybrid model the Powercell espouses. Instead, it's a fairly conventional quadcopter design, the kind that needs to put out high power to keep itself aloft at all times. The team has flown a 1/8th scale model, but let's be honest, it's not going to set many hearts on fire, as the video below will demonstrate.

This is, of course, primarily a tech demo platform to show that the Powercell can keep a bird in the air. More interesting, perhaps, would be an endurance video showing how long it can keep a bird in the air.

Aura isn't the only company selling high-power ultracapacitors with much better energy density than traditional supercapacitors. Indeed, there are several companies working in the strange zone in between batteries and capacitors, each with its own balance of energy and power densities. But this is the first company we've seen pitching their use first and foremost for hybrid eVTOL powertrains, and that's a noteworthy idea that might just work.

Source: Aura Aero