Solar Impulse 2 made history on Tuesday as the first airplane to circle the globe powered only by the sun, opening up new possibilities for the future of renewable energy.

Cheers and applause broke out as it touched down before dawn in Abu Dhabi after the final leg of a marathon trip which began on March 9 last year.

Swiss explorer and project director Bertrand Piccard was in the cockpit during the more than 48-hour flight from Cairo, crossing the Red Sea, the vast Saudi desert and the Gulf.

It capped a remarkable 43,000-kilometre (26,700-mile) journey across four continents, two oceans and three seas, accomplished in 23 days of flying without using a drop of fuel.

"The future is clean, the future is you, the future is now, let's take it further," Piccard said after landing.

"One thing I would like for you to remember: More than an achievement in the history of aviation, Solar Impulse has made an achievement in (the) history of energy.

"We have enough solutions, enough technologies. We should never accept the world to be polluted only because people are scared to think in another way."

Hours earlier, UN Secretary General Ban Ki-moon lavished praise on the team in a live-streamed conversation.

"My deepest admiration and respect for your courage," he said. "This is a historic day, not only for you but for humanity."

Swiss vice president Doris Leuthard said Si2's success "comes at a moment where the world needs optimism" and "gives hope... there's reason for optimism, reason to work for a better life".

Abu Dhabi Crown Prince Sheikh Mohammed bin Zayed Al-Nahyan tweeted: "We hope the success of @solarimpulse helps to deliver Abu Dhabi's message about the need to invest in clean energy and encourage innovation."

- Food research -

Dubbed the "paper plane", Solar Impulse 2 circumnavigated the globe in 17 stages, with 58-year-old Piccard and his compatriot Andre Borschberg taking turns at the controls of the single-seater.

Last year Borschberg, 63, smashed the record for the longest uninterrupted solo journey in aviation history between Nagoya, Japan and Hawaii -- nearly 118 hours and 8,924 kilometres.

No heavier than a car but with the wingspan of a Boeing 747, the four-engine, battery-powered aircraft relies on around 17,000 solar cells in its wings.

Its average speed was 80 kilometres an hour (50 miles per hour).

The pilots breathed oxygen at high altitude and wore specially designed suits to cope with extreme conditions -- temperatures ranging from minus 20 degrees to plus 35 degrees C (minus 4 degrees to plus 95 degrees F).

Nestle Health Science, which provided their tailor-made meals, said its research could help develop "convenient, highly-nutritious food" for elderly people.

Piccard has said he launched the project in 2003 to demonstrate that renewable energy "can achieve the impossible".

His dream took much longer than planned. The attempt was initially expected to last five months, including 25 days of actual flying.

But Si2 was grounded in July last year when its batteries suffered problems halfway through the trip.

- 'Solar drones' -

The project was also beset by bad weather and illness, which delayed the final leg.

In the air, the pilot was constantly in contact with mission control in Monaco, where weathermen, mathematicians and engineers monitored the route and prepared flight strategies.

A psychiatrist who made the first non-stop balloon flight around the world in 1999, Piccard had warned the last leg would be difficult because of the high temperatures.

But he showed little sign of fatigue after landing.

"It was a project that was very difficult, a lot of people doubted we could do it, so of course for the team it's fantastic but also for all the people who believe in clean technologies," Piccard told reporters.

"The biggest challenge is to have an airplane that can fly perpetually, days and nights without refuelling, because there is no fuel."

While the pilots do not expect commercial solar-powered planes soon, they hope the project will help spur wider progress in clean energy.

On the Solar Impulse blog, Borschberg voices hope that "electric propulsion will increasingly become the norm".

"I am very happy to see that large groups such as Airbus and NASA are starting to work on electric propulsion. The ball is rolling!"

With Si2 demonstrating that sunlight can be a continual source of energy in perpetual flight, "we will soon see solar drones flying in the stratosphere", Borschberg said.

"We can hardly believe that we made it. It's still a little bit like in a dream. We have to realise that it's the reality," Piccard said.

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