Hydrogen-power electric flying vehicle: Long road to liftoff

Hydrogen-power electric flying vehicle: Long road to liftoff
This Tuesday, May 28, 2019, photo shows the Skai vehicle, developed by Alaka'i Technologies in Newbury Park, Calif. The transportation company is betting its hydrogen-powered electric flying vehicles will someday serve as taxis, cargo carriers and ambulances of the sky. (AP Photo/Marcio Jose Sanchez)

A transportation company is betting its sleek new hydrogen-powered electric flying vehicles will someday serve as taxis, cargo carriers and ambulances of the sky, but experts say they will have to clear a number of regulatory hurdles before being approved for takeoff years in the future.

With six rotors on the roof and seats inside for five people, a passenger model of the Skai (pronounced "sky") unveiled Wednesday near Los Angeles resembles an oversized drone crossed with a luxury SUV.

Like a drone, the from Alaka'i Technologies takes off and lands vertically. It's one of many similar electric flying crafts in production, including prototypes from Boeing and Airbus that made successful test flights this year, according to Vertical Flight Society, an industry group.

Most are powered by batteries, which can add a lot of weight. The Skai instead uses very light hydrogen fuel cells to run its rotors, giving it a range of 400 miles (644 kilometers) and the capacity to carry 1,000 pounds (454 kilograms) in people or freight, the company says.

"We just couldn't get to the point where we could have enough batteries to get to the payload that we knew we needed," CEO Stephan Hanvey said of the choice to switch to hydrogen power.

Alaka'i says it's planning a test flight near its Massachusetts headquarters.

It would be flown by an on-board pilot using a pair of joysticks, but the technology exists to eventually fly it remotely and even autonomously, Hanvey said.

It will be years before the Federal Aviation Administration allows the autonomous flight of passenger vehicles, said attorney Thaddeus Lightfoot, a partner with the firm Dorsey & Whitney who helps companies navigate FAA rules.

Regulators are still grappling with the proliferation of smaller drones—those under about 50 pounds flown by hobbyists and filmmakers. The FAA just this year eased restrictions on flying small drones over crowds and at night.

Drone-like vehicles such as the Skai must first simply prove their airworthiness, like any common plane, Lightfoot said. After that, getting commercial certification is another convoluted process, he said.

"The technology is interesting, but the regulatory road will be very long," said Lightfoot, adding that the idea of allowing people to fly in a large aircraft without a pilot is "well outside the current regulatory regime."

Hanvey conceded that it could take at least a decade before the company realizes his vision of a fleet of electric flying vehicles ferrying passengers over major cities at nearly 120 mph.

Before that, he hopes to see Skai aircraft used by first responders to send in food or water following disasters like hurricanes or wildfires that can destroy infrastructure.

It could be used as an ambulance and to evacuate people, Hanvey said. Outfitted with a transmitter, one could serve as a mobile cell tower, hovering for up to 10 hours over a neighborhood to provide communication services, he said.

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More information: alakai.com/

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May 31, 2019
Battery tech will increase in capacity at least twofold within the next 5 years. Hydrogen is a dead end as mobile energy storage for small mass market vehicles. Nobody is going to invest in the infrastructure when wires do the same job far better.

May 31, 2019
Battery tech will increase in capacity at least twofold within the next 5 years.

Battery tech today: 250 Wh/kg
Hydrogen energy density: 33,300 Wh/kg, or about 15,000 Wh/kg after conversion to electricity.

The biggest problem with hydrogen is that the fuel tank tends to weigh more than the fuel it can contain. If you don't actually use hydrogen, but some hydrocarbon like methane, you can pack a whole lot more energy (and hydrogen) in a smaller volume and a lighter fuel tank, achieving practical energy densities that are more than 10x compared to any practical battery.

Regular gasoline has about 9 kWh/kg and even after losing most of the energy in the engine, you still have about 2,500 Wh/kg which beats batteries 10x in energy per mass.

Jun 01, 2019
The Hindenburg - LZ 129, a German made zeppelin

"Despite a U.S. ban on the export of helium under the Helium Control Act of 1927,[14] the Germans designed the airship to use the far safer gas in the belief that they could convince the US government to license its export. When the designers learned that the National Munitions Control Board would refuse to lift the export ban, they were forced to re-engineer Hindenburg to use hydrogen for lift.[11] Despite the danger of using flammable hydrogen, no alternative lighter-than-air gases could provide sufficient lift. One beneficial side effect of employing hydrogen was that more passenger cabins could be added. The Germans' long history of flying hydrogen-filled passenger airships without a single injury or fatality engendered a widely held belief they had mastered the safe use of hydrogen. Hindenburg's first season performance appeared to demonstrate this."

Jun 03, 2019
Battery tech will increase in capacity at least twofold within the next 5 years.

More precisely, about 10 years, but that's assuming an exponential trend which isn't the case with batteries. If battery tech followed a Moore-like development, we'd already had electric cars and airplanes in the 70's.


It seems to go up exponentially when you count different battery chemistries over the years, but comparing lithium to lithium, it's not going particularly fast. Furthermore:

It is not feasible to get much more energy into a material than 1 eV per atom. Most solids have atomic weights of 30 GeV/c^2 [*] which yields E/m = 3x10^-11 c^2. When you convert this into the more human Watt-hour/kg, you get 850 W-hr/kg.

Jun 03, 2019
So it seems that without some extraordinary chemical breakthrough, batteries are limited to about 850 Wh/kg which is about 1/3 the practical energy density of gasoline or similar liquid hydrocarbon.

For electric cars this might be acceptable, but for electric planes it's still going to be a dealbreaker if there is any other option available that can be put through a fuel cell, such as hydrogen, methane, methanol, ethanol, butanol...

The competition in airplanes is jet engines, which are actually very efficient at cruising - up to 50% efficient - so they go a whole lot further on a kg of jet fuel than any battery could, also considering that a jet plane gets considerably lighter as it burns the fuel. Charging the electric plane to half full won't make it any lighter, but fueling a conventional jet with less fuel makes it lighter, which makes it consume less fuel.

There is of course metal-air batteries, but those are still long in the making.

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