A few days ago the Solar Impulse 2 made its maiden flight, the Swiss aircraft with electric propulsion powered by solar energy built to be the first electric aircraft to go around the world, a flight that includes about ten stages and a duration of 5 months (from March to July 2015), even if the flight will require 500 'net' flight hours. The first test flight lasted two hours and 17 minutes and an altitude of 5.500 feet (1.670 meters) was reached. Several hours of testing will be required but the aircraft has all it takes to overcome the 8 established world records from his younger brother Solar Impulse (record of permanence in flight, record of altitude reached, distance…).
The Solar Impulse 2 weighs 2300 kg, of which 633 kg are the weight of the lithium batteries with a capacity of 165 kWh, necessary to store the energy generated by the more than 17.000 ultra-thin solar cells that cover the upper part of the huge wings of the aircraft 72 meters long, 4 meters longer than those of a Boeing 747. The four electric motors with which the aircraft is equipped have a power of 13 kW each. The batteries have innovative electrolytes and allow to reach an energy density of 260 Wh/kg. The aircraft is designed to go around the world and is undoubtedly capable of successfully overcoming this technological challenge, even if at speeds and with load capacities incompatible with the needs of modern commercial passenger/cargo flights as the motor power is very low and is commensurate with the electrical power that can be generated by a surface of 270 square meters covered with photovoltaic cells.
Another electric aircraft compatible only with "recreational" flying has just completed its first public flight, but it is a project by the aviation giant Airbus. It's called E-Fan, it's an ultralight demonstrator aircraft built entirely with composite materials weighing 500 kg and equipped with two electric motors connected to turbines that generate a power of 60 kW. In this case, integration with energy self-supply systems such as photovoltaic cells was not sought, the energy necessary to power the engines is supplied exclusively by the batteries which are distributed in the wings of the aircraft, 120 cells (4 V) with a capacity of 40 Ah which gives an autonomy of about 1 hour of flight. To be able to count on greater power at take-off, the E-Fan uses a small trick, a small wheel equipped with a 6 kW electric motor capable of helping to reach the speed necessary to get off the ground.
But Airbus, in collaboration with Rolls Royce (the division that builds aeronautical turbines) wants to get serious about electric flight, much larger hybrid electric planes are being designed and equipped with electric propulsion such as the E-Thrust, 6 electric motors powered by an advanced gas turbine. For electric motors with power levels of the order of megawatts which must have an extremely low weight, systems are being studied which use highly efficient superconducting coils, superconductivity is reached at extremely low temperatures and is necessary in order to be able to do without both conductors of copper (which involve losses due to the Joule effect) and of the ferromagnetic material of the motors (also site of losses) and even the permanent magnets used today in most of the advanced electric motors will not be necessary.
Airbus researchers think that over the next 20 years batteries with a capacity exceeding 1000 Wh/kg will be available, more than double the best results obtained today, and that this technological advance could pave the way, or rather the sky, to new electric aircraft.
