Researchers Explore New Batteries To Power Electric Planes (technologyreview.com)

Can researchers built a new kind of battery powerful enough to fuel an electric airplane? MIT's Technology Review profiles a company co-founded by MIT materials science professor Yet-Ming Chiang: He and his colleague, Venkat Viswanathan, are taking a different approach to reach their next goal, altering not the composition of the batteries but the alignment of the compounds within them. By applying magnetic forces to straighten the tortuous path that lithium ions navigate through the electrodes, the scientists believe, they could significantly boost the rate at which the device discharges electricity. That shot of power could open up a use that has long eluded batteries: meeting the huge demands of a passenger aircraft at liftoff. If it works as hoped, it would enable regional commuter flights that don't burn fuel or produce direct climate emissions...

The initial plan is to develop a battery that could power a 12-person plane with 400 miles (644 kilometers) of range -- enough to make trips from, say, San Francisco to Los Angeles, or New York to Washington. In a second phase, they hope to enable an electric plane capable of carrying 50 people the same distance.... Last year, the company announced plans to deliver a line of "hybrid to electric" aircraft with room for 12 passengers in 2022. At launch, the company intends to offer a hybrid plane with a gas turbine and two battery packs capable of flying around 700 miles (1,127 kilometers), as well as an all-electric version with three battery packs and a range of less than 200 miles....But crucially, the plane itself is expected to feature an open architecture that allows owners to switch out these modules over time, enabling them to upgrade to better batteries developed in the future or shift from hybrid to all-electric operation.
About 2% of the world's CO2 emissions come from air travel, and it's one of the fastest-growing sources of greenhouse-gas pollution. "More than a dozen companies, including Uber, Airbus, and Boeing, are already exploring the potential to electrify small aircraft," the article points out, "creating the equivalent of flying taxis that can cover around 100 miles (161 kilometers) on a charge. The hope is that these one- or two-passenger vehicles -- in most cases envisioned as autonomous vertical takeoff and landing aircraft -- could shorten commutes, ease congestion, and reduce vehicle emissions."

But with less ambitious batteries, "these would largely replace car rides for the rich, not displace air travel."

Comments

Slightly significant

[raymorris]

This could be slightly more significant than one might think. While these small planes aren't the main type most people think of when they think of "airplanes", they happen to be roughly the least efficient transportation available. Less efficient per passenger than large airliners.

To give one well-known example, when Al Gore and his wife go to dinner, his G-11 B burns 578 gallons per hour. ( 0.8 mpg).

Replacing transportation that gets 0.8 MPG with potentially renewable energy is an easy win.

Re:Slightly significant

[avandesande]

Actually small propeller planes get about 25mpg.

Re: Slightly significant

[Rei]

Power really is not the limit; energy is.

The model 3 battery pack - which also includes not just modules, but the charger, DC-DC converter, cooling, etc etc - can deliver 370kW and weighs 478kg, or 774 W/kg. It also has 77,8kWh usable, so 163Wh/kg Note that the modules (which also include an integrated BMS) are just over 3/4ths of the mass, so there's some real potential for weight reduction when you go to larger scales and use lighter materials (as aircraft do vs. cars). Let's say that in an aircraft pack you get 1kW/kg and 200Wh/kg (the modules are 223Wh/kg, and the cells even higher).

(Note: Even this may be somewhat pessimistic with respect to power. The Model 3 pack is rated for 1200A at 402V when fully charged (aka at takeoff) - that's 482kW rather than 370kW. But we'll stick with the amount you can actually use in current Model 3s)

Now let's look at a case where you try to push range and power to their limits - half the aircraft's mass as batteries. So the net power and energy densities of the plane are 0,5kW/kg and 100Wh/kg, respectively.

Now let's say that we're trying to reach a cruising height FL330 (10km) and a velocity of 600mph / 965 kph (268m/s). The energy required for the altitude is 9,81*h*m, so per unit mass, 9,81*10000 = 98100J/kg (27,5Wh/kg). Kinetic energy is 1/2 mv^2, so per unit mass, 0.5*268^2 = 35912J/kg (10Wh/kg). So 3/8ths of your energy is just required by fundamental physics - ignoring all losses - to get up to your flight level and velocity. If you get 80% net propulsive efficiency (between the drive units and propellers), now you're at half your energy just to reach cruising altitude and velocity. Now factor in the drag losses during your climb, particularly at low altitudes... and remember that we're talking about an aircraft where half its mass is batteries...

Clearly, electric aircraft are highly energy limited. You get more of your range during the glide down than you do cruising at altitude.

Now let's look at power. To accelerate up to... oh, let's say 70m/s... that's 1/2* m *70^2, or 2540J/kg (2540 W/s / kg). Our batteries can provide power at a rate of 500W/kg. Thus it could reach 70m/s in just over 5 seconds, or an average acceleration of 14m/s^2 (1,42 lateral g forces). Even factoring in the above assumed 80% drive unit / prop efficiency, you're still at an average of 1,1 lateral gs. Commercial aircraft are normally only 0,2-0,3 lateral gs. So clearly we are not power limited; indeed, with such extreme power possiblities, electric aircraft would be prime candidates for VTOL.

Summary: focusing on power is focusing on the wrong problem.

A side note: energy density improvements in batteries have a much more significant impact to range than one might think. With an electric car, if you double the energy, you double the range. But with an electric aircraft, you far more than double the range. Not simply due to the fact that the first half-or-more of your energy is needed just to get up to cruising altitudes and velocities, either. Electric aircraft have a much higher altitude theoretical operating envelope than combustion-based aircraft, due to the lack of need to maintain sufficient pressure inside an engine to sustain combustion, and avoiding the problems that occur with trying to maintain combustion at ever-increasing airspeeds. Higher altitudes come with lower air densities; optimal speeds increase and energy consumption per unit distance drops significantly.

There's another factor at play that combines that those high accelerations that we calculated previously with the energy density issue: if you can have full - or even just partial - VTOL, then you don't have to have the wetted area for takeoff and flight at lower velocities. Aka, you can make a more stub-winged aircraft. This makes your aircraft lighter (lower lift required, aka less drag via the L/D ratio) and more efficient at higher altitudes. In short, there's a significant virtuous cycle at work. You use the high power provided b