Finding the breakeven on batteries
If you’ve ever tried to push an electric car you know how heavy they are.
Today the best aircraft batteries you can get off the shelf today are about 225 watt hours per kilogram (W⋅h/kg). This is better than a Tesla or Nissan Leaf at around 175W⋅h/kg.
The chemistry between the two battery types is identical. But the weight difference comes from reengineering the casings and structures outside of the battery chemistry. If you could eliminate everything but the battery chemistry, you could achieve around 350W⋅h/kg. This is good but far less than jet fuel at 12,000 W⋅h/kg.
But today batteries are expensive because the technology is immature in a number of areas including: thermal issues (they don’t perform as well in really hot and cold environments), energy density, cycle life, discharge and charging rates, reliability, manufacturability, size and second life.
Manufacturers hope that the cost can be made up for in two ways: first, by cutting maintenance costs and second by using batteries many times. The second is the biggest challenge.
Looking at longevity, the life of aircraft propulsion energy storage batteries is measured in recharge cycles. This lifespan can vary significantly depending on how these batteries are recharged and in what environment they operate, making this a consideration for OEMs, especially those planning to operate Part 135.
Guy Gratton, test pilot and associate professor, Cranfield University, tells Revolution.Aero: “If you routinely charge your battery up to 100% and then discharge it down to zero again, you’re probably going to get around 1,000 cycles out of it. If you instead recharge by routinely going between around 90% and 40%, which in reality is what most EVs are doing, that will improve your battery life massively – probably to around 3,000 recharge cycles.”
Given that batteries are so expensive, they are the most expensive part of the eKub (the electric light aircraft Gatton flies) and of most EVs out there, anything you can do to extend the battery life will mean saving significant sums of money.
Jeff Belt, battery technical fellow, EP Systems tells Revolution.Aero: “While battery cost ($800-$200/kWh) is higher than automotive ($100/kWh), increasing volume will continue to bring costs down and aviation can absorb higher costs than automotive. Our cycle life tests show ‘economically viable’ cycle life capability for fixed wing mission that can reduce the cost of pilot training by 40%.
“Both cycle life coupled with fast charge (ability to do multiple trips per day cost effectively) and specific energy (range) will be the key for a viable eVTOL industry,” adds Belt.
Once an aircraft’s purchase costs and an indication of the use cases are understood, it is possible to begin modelling the through-life-per-hour cost of your battery amortisation.
“Which looks not unlike the current costs of conventional airplane, where the engine has to be rebuilt at enormous cost every few thousand hours,” says Gratton. “Right now, the batteries are very expensive and the cost has been going up, essentially because of supply and demand. Now I think that is a temporary situation because a lot of new factories are being built or are coming online.”
If you look at typical 1,000 hour battery life, the current conventional fuel-plus-engine model is significantly cheaper – on a through-life cost basis potentially a third cheaper per flying hour or more. According to Gratton’s models the breakeven seems to occur when you hit the 2,000 recharge cycles mark for a battery’s life. “Then if you get to 4,000 cycles, battery aircraft start to look financially much more viable and better through-life than a conventional aircraft,” he adds.
OEMs betting on better battery tech emerging by the time they are ready to certify is a story we’ve heard before. So, whilst achieving 4,000 cycles remains some way off, could hybridisation offer a potential stop-gap? John Cooley, chief of Products, Nanoramic Laboratories believes it is highly application dependent because it adds a degree of complexity to any system. “Generally, mixing energy storage technologies, e.g. batteries, super-capacitors, fossil fuels, hydrogen fuel, makes sense from a high level technical perspective to exploit the energy and power attributes of each technology in one system. Many electrified aviation products are being developed to simplify that consideration and focus on use-cases that allow for a single energy storage technology (batteries) to become sufficient,” says Cooley.
For long-range aviation, there will be other considerations and hybridisation may make sense. This will likely be the case especially if hydrogen fuel cells win out for those applications because hydrogen fuel cells provide high energy but suffer in power and their ability to ramp power levels up and down, Cooley explains.
Batteries in an electric aircraft typically come in at about 20-35% of the overall weight. “You’re not going to want to remove those [batteries] on a regular basis,” he says. “Therefore, you are going to be constrained to charging and care cycles that are compatible with leaving the batteries in the aircraft. That is fundamentally expensive, because it is going to shorten the battery life.” If batteries could be taken out between flights, stored in a temperature controlled chamber and charged in an optimal way that would extend the life, but is also very expensive. And that is as long as you don’t drop or damage one when removing it. “I don’t see civilian flying environments that are going to perform that well,” concludes Gratton.
So, just how far off is that 4,000 cycles mark? Gratton thinks the arrival of that technology is going to depend upon both the battery management and recharging systems. “We’re only just starting to understand this. The way in which you recharge batteries, how rapidly, what temperatures you maintain them at in and out of storage, state of charge – this all has got huge impact over the life of a battery. It is very hard to get this right [and financially viable].”
With the first certification timelines for eVTOLs set to conclude from next year onwards, that doesn’t give the technology long to mature. Community spirit was alive and well when two kind people stopped to help push my EV the last 200 yards to the charger, but its going to be harder rounding up numbers for a six-man eVTOL carry.
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