Paris (AFP) - In its round-the-world tour, the plane Solar Impulse 2 has become a showcase for Sun-powered technology, featuring innovations which could have a bright commercial future.
What's new about it?
Solar Impulse 2 is not the first solar-powered plane, but unlike its predecessors it can store enough energy in its batteries to fly through the night.
"The plane has the wingspan of a 747 but only the weight of a car -- it had to be designed with the principle of efficiency first and foremost," said Adnan Amin, head of the International Renewable Energy Agency (IRENA).
The breakthrough has been made possible by several technological advances by industry partners in the project.
Belgian chemical group Solvay, for instance, developed batteries which store more energy but weigh less, and composite material that contributed to making the aircraft lighter.
US solar panel maker Sunpower tweaked photovoltaic cells to enhance their electrical yield.
By putting these elements together, engineers produced an unprecedented combination of "what generates electricity, what stores it and materials which allow it to carry passengers," said Vincent Jacques Le Seigneur, head of the Paris-based Renewable Energy Observatory.
The plane has "the greatest power possible from a reduced surface area," said Cedric Philibert, an expert in renewal energy at the International Energy Agency (IEA).
What are its future uses?
The next step will be to harness the technical gains of Solar Impulse to improve power-to-weight efficiency in related fields.
In terms of maritime transport, there are already solar-powered craft: in 2012, the catamaran PlanetSolar made the first round-the-world trip by a vessel powered by solar energy. It was the biggest such boat ever built.
Solar energy is also used to power military drones, which can be remarkably similar in design to Solar Impulse 2.
Recent progress in electricity storage mean it is now feasible to have solar energy plants a long way from traditional power grids.
Solar passenger aircraft?
Solar Impulse 2 is nothing like a commercial airliner.
Its wings are as wide as those of a jumbo jet, the carbon fibre fuselage is much lighter, but the plane flies at a fraction of the speed of an airliner and can only carry one person.
"The surface area of (solar) cells needed to fly with only two people is the size of an Airbus or a Boeing. So for a commercial airliner it's unthinkable," acknowledged Vincent Jacques Le Seigneur.
The project's great merit has been to act as a test for lighter and cleaner 21st-century technology, said Philibert.
"It is possible, albeit extremely difficult, to do without" fossil fuels for flying, he said.
The aircraft industry is working more towards electric planes powered by batteries that can be recharged on the ground, such as the E-Fan, a small twin-prop developed by Airbus.
Efforts are also being made to develop planes which use less carbon-based fuel, or use biokerosene made from vegetable material. Air transport represents nearly three percent of global CO2 emissions.
How much solar energy in future?
At the end of 2015, solar power generated nearly 224 gigawatts of the world's electrical capacity, up 21 percent on 2014 thanks to improvements in solar panel technology and lowered costs.
Even so, solar energy only constitutes less than one percent of the world's output, dominated by fossil fuels of oil, coal and gas.
The IEA forecasts that solar capacity will grow to between 430-515 gigawatts by 2020.
Currently solar power is the cheapest form of energy in numerous countries situated on or near the equator, and by some yardsticks is profitable without subsidies. The costs of solar cells are expected to fall 59 percent by 2025, according to IRENA.