What's the real limit for solar powered aircraft?

This week saw the first successful intercontinental solar-powered manned flight with Swiss pilot Bertrand Piccard flying the €90m Solar Impulse HB-SIA straight into the record books. But if you scan the skies for a second you’ll soon find that some solar powered planes are flying that extra bit further.

Solar Impulse’s carbon fibre-bodied aircraft flew a distance of 830km while travelling from the Spanish capital of Madrid to the Moroccan capital of Rabat, with the flight path taking in the 39km Gibraltar Strait. The power generated by the 12,000 individual cells on the upper surface of the aircraft’s gigantic 208ft wingspan was just about enough to power the plane’s motors, which in turn charge the 400kg lithium polymer batteries that account for a quarter of the aircraft’s overall mass.

The Solar Impulse team now have their eyes set on a round the world solar flight next year. However, with an average flying speed of 70kmh (44mph), Solar Impulse poses no immediate threat to commercial passenger jets, which fly at more than 10 times the speed. A flight from Madrid to Rabat can be completed in just over an hour.

Furthermore, while this manned solar flight is certainly a landmark; the distance it achieved, or is looking to achieve, is dwarfed when one looks at solar powered unmanned aerial vehicles (UAVs).

The UK-built QinetiQ Zephyr solar powered UAV managed to linger in the air for 336 hours and 22 minutes (or the equivalent of two weeks), albeit just above a US military base in Arizona. Charlotte Pichon from Solar Impulse’s press department told The Engineer: ‘Of course 336 hours is very impressive, but QinetiQ’s UAV is an unmanned aircraft, while Solar Impulse is piloted by a human for 19 hours. It’s not only a technological challenge, but a human one.’

Looking ahead, the Defence Advanced Research Projects Agency (DARPA) have designed their future solar powered ‘Vulture’ UAV so that it can stay in the air for a staggering five years above areas that need to be kept under constant surveillance.

There is clearly a need for green aircraft technology, but solar power may not necessarily be the way to go.

AeroVironment’s carbon neutral Global Observer UAV – an endurance aircraft said to be capable of flying 3-4 times further than any other fixed wing aircraft – uses hydrogen to power four highly efficient electric motors. A fact sheet on AeroVironment’s website states: ‘From the Stratosphere, the GO will act like a mobile, 12-mile high tower, covering an area of 600 miles in diameter.’ The Global Observer is capable of flying for 5-7 days at a time, at heights well above conventional aircraft, which fly at altitudes up to 65,000ft.

While the aforementioned UAV spy planes may not be quite as evangelical as Solar Impulse – which simply wants to demonstrate that progress is possible using clean forms of energy – they are certainly picking up a few extra air miles. But why? Well the reality is that people have their needs and these needs all add extra weight to an aircraft. Build in a toilet, a kitchen, some storage and window or two, and you’re soon racking up those all-important kilograms that solar powered vehicles can’t afford to accommodate.

If Solar Impulse are successful in their round the world attempt then aerospace engineers must then continue to look at improving the power output of renewable energy sources. Current solutions are just about capable of lifting the equivalent of a small car off a runway. As a result, I don’t think we’re in any immediate danger of the four jet engines on 747s being replaced with the four batteries on the Solar Impulse. However, I can certainly see renewables working for sligtly more lethargic UAVs.