And if EV planes become a reality, how much faster can it fly?
In: Engineering
Because batteries are heavier than Jet Fuel, and planes are all about being light.
As for speed, Electric planes wont fly any faster than current planes.
Weight and profitability for one, but frankly I’d say they need to learn how to build a door before dealing with lithium batteries.
The other comments pretty much nailed it, weight and the ability to quick-turn the planes. It underscores truly how incredibly powerful liquid fuels can be to get heavy airplanes up in the air.
As others said, energy density. Batteries are roughly four times less energy dense than fuel.
But that’s not all! As the fuel gets used, the plane becomes lighter, needing less fuel. Fuel requirements are calculated per flight and the planes are filled with just enough fuel, so that it doesn’t lug extra weight around. This is not something that’s possible with an empty battery.
As for speed, air resistance increases at a cubed rate. What this means is that for every doubling in speed, air resistance increases eight times. However, this applies even to smaller factors. So a plane flying at 800km/h, if it were to fly just 10% faster, the air resistance would increase by 33%. Now couple the increased energy demands with the lower energy density of batteries.
It’s not the fuel limiting the speed (for common use cases, at least), but rather the laws of physics making it unfeasible.
It doesn’t make financial nor economic sense.
Aero engines have always been designed to maximize efficiency at every step, because a more efficient engine burns less fuel, and less fuel means less cost to operate per hour. This is extremely lucrative and has pushed the current state of the art for aero engine efficiency to 40% with the latest engines in service on the airbus neo family and the boeing maxes. Fully 40% of the energy in the fuel becomes useful thrust.
For a car, it’s 5%.
For an EV car, the battery pack only needs to store that 5% of useful energy or so.
For an EV plane, the battery needs to be eight times more energy dense. Beyond this, the battery is a fixed weight – planes fuel as little as needed to minimize non-revenue weight and batteries would disallow this.
Air travel does have electrification coming to it – but not where you might think. Taxiing is a major contributor to aircraft emissions, and electric taxiing is expected to be the Next Big Thing. Instead of needing to run the main engines for twenty minutes on a long taxi out, a plane can just use the APU to power two electric motors in the main gear to taxi out with the big mills at a stop, only starting them in the last phase of taxiing and doing a runup right as they’re lined up for takeoff.
Current airliners fly a bit under the speed of sound because flying at or above the speed of sound requires different aerodynamic design and much more energy, and neither of those things would change with electric engines.
It is all about energy and weight.
A jet turbine has an Efficiency of 90%.
So from 1 kg of fuel (kerosin) it gets 9 kWh of energy to propel the aircraft.
The same amount of energy taken from a battery needs at last 60kg of akku.
On a flight to europe 26 tons of kerosin are needed.
A LiIo battery to store the amount of energy would weight 1.560 Tons.
Airlines don’t want to fly faster. If you look at 50 years old departure tables and flight times for the big airports it’s more or less the same.
This is because airliners typically cruise at mach 0.7-0.8. Any faster you would approach the speed of sound and as you get close to it you get a lot of drag, which costs tons of fuel.
Modern airlines are about flying lighter, not faster, to optimize fuel and costs. And batteries are heavy
Also batteries perform poorly in cold environments (the chemical reaction in the battery slows down) while the exterior of the aircraft is facing below -40 degrees. You would probably need to heat your battery for it to work at all.
Battery energy density is awful at the moment.
Take the use case of a car. Consider a 95kwh Tesla battery, which weighs over 600kg and takes up ~400L of space. If you have a gas engine that’s only 25% efficient, that’s about as much energy output as 36kg of gasoline. So a comparable battery will weigh around 20x more, and take up 10x more volume.
When a plane like a 737-800 can hold like 20,000kg of fuel…that energy density makes batteries a far worse option.
Additionally, there’s no opportunities for hybrid tech either. Hybrid cars harvest braking energy, and run engines at their most efficient rpm to get fuel savings. Planes don’t stop, and the engines are already designed to run at their most efficient rpm, so there’s no real opportunity for improvement.
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