A number of companies are developing electric power plants for aircraft, basically for powered gliders and small ultralight aircraft. In most of these designs the power plant is of secondary importance and the aircraft can fly, or better, glide considerable distances without an engine purely by its basic aerodynamic properties.
Developments are in progress to electrify some every day general aviation class aircraft or even new designs. Because of many limitations to overcome, large scale applications will not come any time soon. The main problem is energy storage: liquid fuel (Mogas, AVgas or Jet fuel) has the highest energy density per weight, which is very hard to beat, except for nuclear fission or fusion. This will limit the range of any aircraft trying to fly any considerable amount of time or distance with a worthwhile payload.
To be able to operate its engines, aircraft need energy in a compact stored form. And we need a lot of it in as little room and weight as possible (high density). Or use a small efficient internal combustion engine generating electrical power. We can then use that power to recharge small batteries and drive the propeller(s) through electric motors in series or parallel with the IC engine.
This is called a hybrid drive and this can run potentially more economical than driving the propeller by the combustion engine alone. Using this method we can optimize both engines to do what they do best, see the next page for more on this. There remains only the problem of weight of the whole installation (see next page also).
Even more so of a problem are the batteries, they are the culprit at the moment (have been for decades) and will remain so if not forever as we are reaching the limitations of battery physics. With these devices there is a trade-off between specific power vs life span, specific energy vs performance and safety vs cost. Its shown in a spider diagram and you can't have it all at the same time.
Liquid fuels, be that AVgas or JET, have the highest amount of energy per weight and this property is called energy density. For example: 1 kg of diesel fuel contains about 43000 kJ which is equivalent of one metric ton (1000 kg) raised to 4300 meters (potential energy).
Storage of energy in chemical form (batteries, lead-acid, NiCad, Li-Ion, LiPo or LiFePO4) is not in the same category as with ordinary fuel. And this is the main problem with electric driven aircraft or any hybrid vehicle. For a comparison, see image to the right. Reprinted with permission from Wattsupwiththat
A 50 kWh Tesla battery is equal to 4 kg of diesel fuel. But lets see how much storage is possible: the annual output of Tesla's battery factory is enough for three (3) minutes electricity of the US (2019). 1000 year production would store two whole days worth (source: Manhattan Institute), even doubling that production will not kick a dent anywhere soon.
To think that batteries can be used as a buffer for renewable energy is almost like believing in an unicorn (or fairy tales for that matter), more in the article The Truth Behind Renewable Energy or Battery Basics from WattsUpWithThat. For more information on battery storage, read on in our power generation articles or have a look at the article on Energy Storage and its difficulties.
With all the physics limitations that exists on battery technology we can conclude that using these devices for large scale storage of electrical energy is quite impossible. For more detailed information follow these links about "Battery Storage – An Infinite Small Part of Electrical Power" at WattsUpWithThat and Energy Central.
Even James Hansen (NASA) stated in 2016: "The notion that renewable energy & batteries alone will provide all needed energy is fantastical. It's also a grotesque idea, because of the staggering environmental pollution from mining & material disposal, if all energy was derived from renewables & batteries"
An image showing the amount of energy we use with electricity compared to several storage solutions in operation today can be seen here: Global Energy Storage vs. Usage, source: Sandia Labs.
This can be stored in a number of forms: under high pressure, cryogenics and with chemical compounds that release the hydrogen when heated. Liquid storage of hydrogen requires a cooled vessel (20 K or -250 °C) which must be well insulated resulting in added weight and cost. This might not be ideal for aircraft. Compressed hydrogen has a nice energy density per weight but very low energy density by volume. You will need a larger tank compared to normal liquid fuels.
SkySpark performed a test in a Pioneer 300 with a 75 liter fuel tank at 350 bar containing some 26000 liters (!) of hydrogen (H2). The aircraft has a fuel cell in the copilot seat, a 65 kW motor and an auxiliary Lithium type backup battery.
Oct 2020: ZeroAvia is testing a hydrogen fuel cell powered with a Piper six place aircraft flying from Cranfield in the UK. Read more here in the article Zero Emission Hydrogen by CNN. Skip the alarmist part about the CO2 emitted by aviation, as this comes down to: 3% (aviation) of 3% (total human) of 0,04% (total = 415ppm, 2020) resulting in a total of 0.4 ppm or 0.00003735% for aviation as a whole.
This 'fuel' can also be used in internal combustion engines and the only byproduct coming from the exhaust is water (H2O). This would be the solution for our cars on the road if a transition is required from ordinary petroleum fuels. Laboratory test have proven that combining H2O and CO2 makes it possible in combination with the catalyst Ruthenium to store and release the hydrogen much quicker.
Hydrogen can be used in a fuel cell where electricity is produced and heat and water results as a byproduct. This technology has been around since 1959 in several cars and used in during manned space missions. The cell consists of an anode (where hydrogen is fed in) and a cathode where oxygen is fed in. At the anode side, hydrogen molecules are split into electrons and protons. These protons pass through an electrolyte membrane and the electrons are generating current. At the cathode the protons, electrons and oxygen combine resulting in water molecules.
We see remarkable developments in electric powered flight but the current low battery energy density and low yield solar cells limits these solutions to two seat/training and glider type aircraft. These have low power requirements and a large wing possibly suitable for solar cells partially recharging the battery.
General aviation and larger aircraft have much higher energy requirements which electric technology is unable to fit into the wings of the aircraft and which is easily done with liquid fuels. All in all, pure electric propulsion is in its infancy with no real solution in the near and far future for aircraft larger than small two seat or gliders.
Hydrogen powered is another possibility but we need large amounts of electricity to convert water (not sea water) to obtain this energy carrier, pure hydrogen is not as abundant on the planet as hydrocarbons. As long as we need oil or gas to do that, instead of clean nuclear power like a Thorium MSR reactor (or LFTR, not Uranium), we would better use ordinary liquid petroleum based fuels and concentrate on building higher efficiency combustion engines with hybrid drives.