A number of companies are developing electric power plants for aircraft, basically 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.
Hybrid propulsion means a combination of electric and internal combustion motors. The idea is that you use the best of both worlds to come to the same or even better result. In the case of aviation this means: lower fuel consumption, lower emissions and as such a cleaner flight with less impact on the environment. Keep in mind that this must mean the whole picture from fabricating of the engines, maybe batteries (chemicals) and all that is needed for the installation and final removal.
Pipistrel from Slovenia is also experimenting with electric powered flight. Their former WATTsUP project is a 2 seat Alpha Trainer, equipped with a 18 kWh battery and 85 kW engine and can fly one hour with 30 minute reserves. This should be enough for a flying school when they have a number of batteries fully recharged on standby. They also sell a quickcharger able to recharge within an hour, but this method will kill any battery prematurely.
On AERO 2015 the company explained that they had no policy on what to do with batteries from customers that were worn out or failed. One could argue that electric aircraft could potentially become a large source of non-recyclable chemical waste.
Pipistrel (meaning Bat) is also experimenting with an hybrid drive where a gas engine supplies the power to an electric motor in combination with a small battery so that the aircraft can take off on electric power. This should reduce the noise foot print of the aircraft considerably. Move the mouse over the image.
At AERO 2016 Pipistrel showed the Hypstair (www.hypstair.eu). This is an hybrid drive with an Rotax 914 driving a generator charging batteries in the wing. The propeller is driven by an 200 kW electric motor from Siemens, of course the combination has electronics to charge the batteries and control the three(!) engines and propeller.
We all know how complex a Rotax engine is, let alone the turbo version added with two electric engines. This hybrid drive redefines the word complexity to the third power, take a look here. One wonders about the cost and if any fuel savings are worth it.
Siemens in cooperation with EADS and Diamond Aircraft developed an electric hybrid drive which consists of a 30 kW Wankel engine with generator, a battery developed by EADS and electric motor of 60 kW. This configuration is tested in a Diamond DA-36 motor glider and is expected to reduce fuel consumption by 25%.
A Siemens converter supplies the electric motor with power from the battery and the generator. Fuel consumption is very low since the internal combustion engine always runs with an efficient constant low output of 30 kW. The battery system from EADS provides the increased power required during take-off and climb. The accumulator is recharged during the cruising phase. (Source: September 2011, instrumentation.co.za and phys.org July 2013)
Update 2015: Siemens developed an electric motor of 260 kW (348 hp) for use with small regional aircraft. AVweb has the story: http://www.avweb.com/avwebflash/news/New-Electric-Aircraft-Motor-From-Siemens-223995-1.html.
Raptor Aircraft is also promoting a hybrid driven aircraft: "The Raptor Hybrid will be powered by a 250 hp modern, automotive derived, diesel engine in combination with an 62 hp electric motor and battery pack. This combination will provide the full 312 hp for takeoff and climb continuing all the way to FL250. Once there you will throttle back to run on just the diesel engine." (source: http://www.raptor-aircraft.com/models/hybrid.html).
Update 2016: it seems that Raptor dropped development of the hybrid drive as their is no mention at all on their website and the page is gone.
Hybrid drives are commonly used in diesel electric locomotives and generator sets since the beginning of the last century and one wonders why it is not used more often in cars and aviation. As this seems the best solution up until now as battery technology is still light years away from a good power density. And running a gas or diesel engine supplemented with a small battery and electric motor could mean more fuel savings while not generating a huge pile of wasted, worn out or failed chemical batteries in the environment.