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.
The aircraft successfully completed the first fuel-free flight around the world on Tuesday, returning to Abu Dhabi after an epic 16-month voyage that demonstrated the potential of renewable energy.
Solar Impulse 2 first took off from Abu Dhabi on March 9, 2015, beginning a journey of about 40,000km (24,500 miles) and nearly 500 hours of flying time.
What’s new about the plane? 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.
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.
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 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.
The project’s great merit has been to act as a test for lighter and cleaner 21st-century technology.
At the moment it is extremely difficult to do without fossil fuels for flying.
The aircraft industry is working more towards electric planes powered by batteries that can be recharged on the ground. 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 3% of global CO2 emissions.
At the end of 2015, solar power generated nearly 224 gigawatts of the world’s electrical capacity, up 21% on 2014 thanks to improvements in solar panel technology and lowered costs.
Even so, solar energy only constitutes less than 1% of the world’s output, dominated by fossil fuels of oil, coal and gas.
“More than an achievement in the history of aviation, Solar Impulse has made history in energy,” said Bertrand Piccard, one of the pilots, on landing.
He said the technologies used on Solar Impulse 2 could be used on the ground in daily life to halve emissions of carbon dioxide, the main greenhouse gas blamed for climate change.
For the two pilots, landing back where they started is only “the beginning of the continuation” of a longer journey, said Piccard.