How a solar-powered plane flew around the world
Having left Abu Dhabi in March 2015, the Solar Impulse 2 – the world’s first solar-powered plane to circumnavigate the Earth – has finally completed its 35,000km round-the-world journey by touching down in the UAE capital today.
The privately financed project was led by Swiss engineer and businessman André Borschberg and Swiss psychiatrist and aeronaut Bertrand Piccard, the latter also having co-piloted the Breitling Orbiter 3, the first hot-air balloon to circle the world non-stop.
The Solar Impulse 2’s journey has been years in the making, with Piccard having first initiated the project all the way back in 2003 in an attempt to achieve a historic first for the world of renewable energy.
Due to the nature of its clean-energy fuel, the Solar Impulse 2’s journey was, unlike Piccard’s previous hot-air balloon venture, not nonstop – having had many scheduled stops along the way in diverse locations such as Oman, India, Myanmar, China, Japan, USA, Spain, and Cairo before arriving back in its starting location of Abu Dhabi. This was a mammoth undertaking considering that Borschberg and Piccard, despite both being experienced pilots, were only able to use a simulation model of a virtual plane for practice.
The entire trip took well over a year to finish after having been delayed at the end of May 2015 when the plane's batteries were damaged by an overheating issue caused by being packed in too much insulation, making the completion of the journey even more triumphant.
The very construction of the plane is itself an accomplishment, with the Solar Impulse 2 being a single-seater aircraft that contains a whopping 17,248 solar cells built into its 72-meter wingspan – only slightly smaller than that of the wingspan belonging to the Airbus A380.
Though the plane lacks the power of the Airbus and only travels at a maximum speed of 140 km/h due to the limited capacity of its engines. The Solar Impulse 2 also weighs in at just 2,300 kg (roughly the weight of an Asian bull elephant), with only enough room for a single pilot in its 3.8 m3 cockpit, making it a far cry from the large commercial liners we are used to seeing.
Because of this restricted space, Borschberg and Piccard had to take turns flying the plane during the round trip, with support coming from a 60-man strong team on the ground who were tasked with ensuring the plane’s utmost safety.
Supplemental oxygen and various other environmental support systems allowed the pilots to cruise up to an altitude of 12,000 metres (39,000 ft), however, except for 20-minute power naps, the pilots hardly got more than two or three hours of sleep per 24-hour period. André dealt with this irregular sleeping pattern using techniques he learnt from yoga whilst Betrand relied upon self-hypnosis.
With a daily allowance of 2.4kg of food and 2.5 litres of water combined with a single toilet built into the seat and a volatile cockpit temperature that ranged from -40°C to 40°C – this really wasn’t luxury flying. Pilot Piccard himself noted this, saying that “Solar Impulse was not built to carry passengers, but to carry a message”.
And what a clear message the success of the Solar Impulse 2 has made. Technically, if the weather were to hold up, the plane could happily remain in flight forever, purely powered by the sun’s beams. Though this would realistically be far too taxing on both the pilot and the crew in reality.
The longest flight the Solar Impulse 2 took in its travels was the leg from Japan to Hawaii, in which it covered 8,924km in an impressive four days, 21 hours and 52 minutes. This was not only a world record for a solar plane with a pilot, but the flight’s duration was also the record for the longest solo flight, by time, for any aircraft ever.
In terms of the actual science behind it all, 17,248 photovoltaic cells coat the top of the wings, the fuselage, and the tailplane (amassing a total area of 269.5 m2) in order to power the plane’s lithium polymer batteries. A photovoltaic cell – otherwise known as a solar cell – is an electrical device that converts the energy of light, provided in this case by the sun, directly into serviceable electricity by a physical and chemical phenomenon known as ‘the photovoltaic effect’.
The photovoltaic effect is similar to ‘the photoelectric effect’ studied by the likes of Albert Einstein and involves the absorption of light causing the excitation of an electron or charge carrier to a higher-energy state. In the case of the Solar Impulse 2 the light energy from the sun is being used to power the four electric motors and four 41 kWh lithium-ion batteries it holds. These batteries in turn aid the Solar Impulse 2 in generating and storing enough energy to power its motors to allow it to fly for long periods of time.
During the day, the solar cells recharge the Solar Impulse 2’s lithium batteries, allowing the plane to fly at night using these batteries. That is, as long as the weather has been favourable enough during the day for them to have stored up enough energy to last the night.
Extremely lightweight materials also help the aircraft to remain as energy efficient as possible. However, due to the fragile nature of the design it can only fly in favourable weather and is particularly susceptible to be buffeted in strong winds. Because of this, the Solar Impulse team often had to accommodate weather and temperature windows with flight-times, hence the arrival of the Solar Impulse 2 in Abu Dhabi during the middle of the night.
Departures could also be delayed for prolonged periods of time due to poor weather, as was also the case when the final Cairo-to-Abu Dhabi leg of the trip (expected to occur on July 17) was delayed because of unruly winds and a stomach upset on the part of pilot Bertrand Piccard.
Strong winds and stomach upsets aside (the two mishaps bearing no relation), we might not see solar-powered airplanes becoming the travel norm within the near-future, though the Solar Impulse 2 has shown that clean, renewable energy is far more realistic than many naysayers would like to make out.