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.
You will not see, any time soon, electric power plants in every day workhorses general aviation class aircraft just because there are still too many limitations to overcome. The main problem is power storage: liquid fuel (AVgas or Jet fuel) has the highest energy density per weight, which is very hard to beat, except for nuclear power plants.
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. Or use a small efficient internal combustion engine generating electrical power. We can then use that energy to recharge small batteries and drive the propeller(s) through electric motors.
This is 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. There remains only the problem of weight of the whole installation (see the next page). Even more so of a problem are the batteries, they are the culprit at the moment (have been for decades) and will remain so the next decade or three if not forever.
Liquid fuels, be that AVgas or JET, have the highest amount of energy per weight and this property is called energy density. 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
OXIS energy is developing a Lithium Sulfur Li-S battery which promises a higher energy density than Li-Ion while being safer at the same time. Its still not even close to liquid fuels but progress is slowly. More information in the Solar Storage Application Note from OXIS.
Lets see how much storage is possible: the annual output of Tesla's battery factory is enough for three (3) minutes electricity of the U.S. 1000 year production would store two whole days worth (source: Manhattan Institute). To think that batteries can be used as a buffer for renewable energy is believing in an Unicorn (or fairy tales).
Another option is by using fuel cells to generate electricity to drive the electric motor and the aircraft systems. 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.
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.
We see remarkable developments in electric powered flight but the current low battery energy density and low yield solar cells limits these solutions to small ultralight and glider type aircraft. These have low power requirements and a large wing possibly suitable for solar cells.
General aviation and larger aircraft have much higher energy requirements which electric technology is unable to fit into the wings of the aircraft and is easily done with liquid fuels. All in all, electric propulsion is in its infancy with no real solution in the near and far future.
Hydrogen powered is another possibility but we need large amounts of electricity to convert water to obtain this fuel. 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.