Electric Planes: Big names target test flight in 2020 – A Response

Electric plane

We applaud the initiatives being shown by some of the world’s leading aviation technology companies in the E-Fan X project, and if they do manage to power a commercial-sized aircraft using electric technology by 2020, it will be a major achievement which should be celebrated.

It is important to understand that E-Fan X will be a hybrid aircraft in a similar way to hybrid cars – which are electric some of the time but much of the time they use conventional combustion engines, especially when range and speed are needed.

Turning to fully electric aircraft, we must not think that they are just around the corner. Liquid fuel-powered aircraft will be with us for many decades to come.

Time to do some arithmetic.

If we assume that the best commercially-available mass-produced battery technology today is represented by the Tesla car battery pack (100 kWh), let’s compare the weight of the batteries which would be required to fly an aircraft over a typical long haul flight with the weight of liquid fuel required (long-haul flights are more fuel-efficient than short-haul flights because much more time is spent in the cruise, so if the numbers do not work for long-haul, they will not work for short haul flights).

London to Hong Kong is 9600 km and a Boeing 747 will use approx. 150 tonnes of jet fuel. Jet fuel has a minimum energy content of 42.8 MJ/kg, so 150 tonnes equates to 1.78 million kWh of energy required for the flight. Whether an aircraft is powered by jet fuel or electricity, it will still require more or less the same amount of energy to fly between the same two places.

So, if we need the same 1.78 million kWh from the battery for an electric aircraft, the battery pack needs to be 1.78 million ¸ 100 times the capacity of the Tesla battery pack, or 17,800 times its capacity. The Tesla battery weighs something like 500 kg. To scale this up 17,800 times, the battery for the Boeing 747 would weigh over 8,000 tonnes.

In order to compete with jet fuel, battery technology therefore needs to improve so that the same battery capacity can be squeezed into a 150 tonne battery pack – in other words, something like a 50 fold improvement in energy density is required. Even if our numbers are somewhat approximate, that is a quantum leap in technology. It is why we strongly believe that unlike land transport, commercial aviation will not shift to battery power in the foreseeable future.

We are passionate about renewables and the greening of aviation. We will continue to watch developments of the E-Fan X project with interest and wish it all success. But for the time being we remain committed to liquid fuels for aircraft, and champion sustainable liquid aviation fuels.

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