The world trip of the Solar Impulse 2 airplane is not the dawn of sun-powered commercial air travel, but it has fuelled innovations in aviation and other areas, according to engineering experts.
"Setting up this project and seeing it through was a heroic feat," said Josef Kallo, an energy systems engineer at the German Aerospace Center (DLR) in Stuttgart.
Since launching a first feasibility study in 2002, the team led by Swiss pilots Bertrand Piccard and Andre Borschberg has worked on an airplane that can be flown around the world day and night with the help of solar panels and batteries.
The lightweight construction of the Solar Impulse 2 and of all its components is one of its most important innovations, said German aviation engineer Karl Michael Kaeser, who has been involved in this and other alternative energy aviation projects.
"It is pretty much a technological masterpiece," he said of the aircraft that weighs only 2.3 tonnes.
It is made of light carbon fibres, and Kaeser noted that its development coincided with similar successful efforts by Boeing and Airbus to use carbon materials to create fuel-effient aircraft.
"One can debate whether Solar Impulse looked at what they were doing or they copied Solar Impulse," he said.
"I think that both sides spurred each other," he said of the relationship between the team that worked on the Swiss experimental plane and the commercial aircraft companies.
In addition, the solar airplane consists of components that can be used in other fields, especially in electric cars.
The airplane uses batteries that are lighter and last longer than other models, and its polycarbonate windows weigh less than glass. These innovations could help make lighter and more efficient cars, the Solar Impulse team says.
However, it is not realistic that airlines will use sun-powered planes, because the solar panels cannot generate enough energy.
The single-seat Solar Impulse 2 has 200 square metres of solar panels on its wings, which are as wide as those of a large commercial airliner.
To keep 100 passengers in the air, several square kilometres would be needed.
However, companies such as Airbus, Facebook or Google, which sponsors Solar Impulse, have been working on solar-powered drones that fly for long periods to relay mobile phone or data signals, at much lower cost than satellites.
"They could replace satellites or complement what they do in terms of observation and communication," pilot Borschberg has said.