Because the Batteries Alone Would Weigh 2X Allowable Takeoff/Landing Weight
After accounting for battery weight, there is no weight left over for the pilot, passengers, luggage, or even for the airplane itself!
People can fantasize about flying an electric airplane as much as they want. The problem is that airplanes are meant as practical tools, for transporting people and objects from one place to another in an affordable manner. If a specific airplane design cannot even get off the ground due to the weight of its batteries, it can no longer be considered practical — and the word “affordable” loses any meaning if it cannot do the job.
The planes now being touted as “electric airplanes” are nothing more than toys. Here is why that is not likely to change soon:
Why Do the Batteries Weigh So Much?
Batteries for electric vehicles weigh so much because they contain very little energy per kg (low specific energy). Being fair, we allow for the fact that electric motors and drive systems are lighter than liquid fuel engines and drives. We also allow for the higher mechanical efficiency of electric motors vs. aviation fuel powered piston driven engines. But it doesn’t matter, because the very low specific energy of lithium ion batteries (the best battery available for this purpose) forces us to pile on too much total battery weight in order to achieve useful range. And by “too much total battery weight” I mean too much weight for the plane to get off the ground.
Here are some calculations done at the Aviation Stack Exchange Q & A website using a 90% efficiency for electrical systems and a 35% efficiency for the liquid aviation fuel system:
The specific energy of avgas and jet fuel is about 43 MJ/kg. The best lithium-ion batteries top out at about 0.9 MJ/kg (the batteries in a Tesla are about 0.7 MJ/kg), so they have a fraction of the storage of liquid fuels.
Assuming that all other factors are equal (propeller efficiency, etc) we can extrapolate the actual efficiency of the systems to get an approximate figure of an internal combustion on avgas: 35% of 43 MJ/kg = 15 MJ/kg of actual benefit. We can use that figure to determine what specific energy we would need from batteries to get the same amount by dividing by electric motor efficiency: 15 MJ/kg / 0.9 we get 16.7 MJ/kg.
Right now commercially available battery technology is about 0.9 MJ/kg, so it would need 18.56 times more storage capacity (16.7 / 0.9 = 18.56) to supply the same amount of energy.
Here are some specifications for the Cessna 150, a small but useful airplane:
The Cessna 150 comes with either a standard 26 gallon fuel system or an optional 38 gallon long range system. Burning fuel at about 6 gallons per hour, one might be able to fly between four and six hours at about 110 mph (wind and weather permitting). Let’s estimate the range at 400 miles.
Looking at the standard C 150 fuel tank, 26 gallons is almost 100 liters, and there is 34.2 MJ of energy in a liter of avgas. A tankful of avgas provides about 3366 MJ of energy to fly about 400 miles. Compensating for the difference in efficiency, how heavy would your battery system have to be to provide an equivalent energy and range?
3366 MJ X (0.35/0.9) = 1309 MJ of electric energy required. We will be generous and allow a specific energy for the battery of 1 MJ/kg, which requires us to provide a battery pack weighing about 1300 kg, or about 2880 lbs. Looking at the C150 specs, we see that takeoff weight is about 1500 pounds. Our battery pack alone is roughly twice the allowable takeoff weight for the whole plane/pilot/passengers/luggage!
It is true that a battery system would cost less for the equivalent energy: It takes $130 to top off an empty C150 fuel tank (26 gal.) with avgas, while it would cost just under $45 to charge 1309 MJ of batteries (0.28 kwh per MJ, and $0.12/kwh). Electric energy is indeed cheaper, all other things being equal. But they are not equal at all. While you were flying your load of passengers or freight from Los Angeles to San Francisco, your friend “the electric pilot” would be spending all his charge just trying to get off the ground!
It is Worse Than That
Electric batteries do not perform well when they are either hot or cold. In fact you can lose over 40% of your range under cold battery conditions — such as those you find when flying an airplane over much of the year.
Some EV drivers — including this correspondent — recently found that range can drop by half when the mercury tumbles into negative territory. The AAA study appears to be the first to have used standard, repeatable methodology to confirm the problem and compare the effect of winter temperatures on different models.
Different factors can affect the loss of range, he and other experts have noted. Simply turning on the electric vehicles AAA studied in 20 degree weather revealed a 12 percent loss in range. On a vehicle like the Chevy Bolt, with an EPA rating of 238 miles per charge, that would drop range to 209 miles. But that part of the test assumed operating the vehicle with cabin heat and seat heaters turned off.
Brannon said using climate control revealed an even bigger surprise: Range dipped by an average 41 percent — which would bring an EV like the Bolt down to just 140 miles per charge.
The problem is that unlike a car with an internal combustion engine that can warm the cabin with waste heat, EVs have to tap into their batteries to power the climate control system. __ https://www.cnbc.com/2019/02/06/aaa-confirms-what-tesla-bmw-nissan-ev-owners-suspected-of-cold-weather.html
It gets cold up there, and if you are using up your battery range just keeping yourself from freezing, you may not get where you planned to go. If you only plan to use your airplane in the summer, perhaps it is more of a toy than a practical tool?
Airplanes are Highly Affected by Weight Considerations
The very low energy density of the best lithium ion batteries makes the idea of an electric piloted airplane impractical. If you want a toy plane to fly around for a few minutes, impressing your green-minded friends, then you probably have nothing important to do with your money or your time.
Electric cars are less affected by weight considerations than are airplanes, but they suffer from many of the same drawbacks as the electric airplane. Most EVs suffer in considerations of cost, weight, performance, range, time to refuel/recharge, cold weather/hot weather performance, decay of battery performance over lifetime, tendency of batteries to explode and burn, and other problems waiting to be discovered by unwary owners.
Not all of an EV’s faults are due to low energy density/specific energy. But the truth remains that the current generation of electric storage batteries is not ready to power useful (not toy) airplanes, and does not do so very well with automobiles other than for the short commute. As for using these batteries to backup a power grid or to level a fluctuating grid load & an intermittent supply — forget about it.
“Unless there’s a cosmic change in the battery, it’s just not going to work for bigger, faster airplanes,” he said. “It’s going to be a really long time before batteries weigh less than liquid fuel.” __ A Steeper Challenge
Good Intentions Are Not Enough
There are a lot of good books that look at the problems of the “energy transition” from the use of fossil fuels to an energy regime based upon electric power. But of all the people who vote for the politicians who devise energy policies, almost none of them have read anything about how energy systems work and how they fail — and even fewer of the politicians themselves know what they are doing.
The transition from fossil fuels to the long-shot alternatives will be very rocky, expensive, and bloody.
In terms of batteries, the resources are not there to back up those bold green projections.
The same is true for other forms of energy storage. The resources are not available. The green visionaries have dreamed up a moronic plan. Society will pay an ugly price if these dreams are pushed forward.
An abundant and expansive future is apparently a frightening prospect to most modern humans. Hence the appeal of the “Green New Deal,” a policy that would doom everyone but the political insiders who would insulate themselves from all the fallout.
Without the Hot Air by David MacKay is a book that is sympathetic to “green energy” but sternly realistic at the same time.
More on electric flight:
Virtually all of today’s electric airplanes are examples of battery assisted gliders. By thermal-hopping these “electric motor-powered gliders” can reach altitude and over time and in good weather can move a few hundred miles. As a stunt. Soaring is a wonderful sport, but it doesn’t get the work done.
Beautiful examples of electric fantasies not ready for the sky. Artists and computer graphics techs can portray amazing looking “electric airplanes” but they cannot make them do useful work.
A more viable solution is a hybrid system with at least two motors: an electric motor that turns the propeller and a gas engine that drives another generator for power. __ https://www.heraldnet.com/news/electric-planes-are-a-steeper-challenge-than-electric-cars/