Blazing a trail: Solar Impulse 1 became the first solar-powered craft to fly at night
Next March could see the start of an unprecedented aeronautical venture – the first round-the-world flight by an aircraft with no liquid fuel on board.
Solar Impulse 2, which was unveiled to the public in Payerne, Switzerland, will rely on the energy of the sun to drive it forward. It will be powered by four sets of propellers, each driven by an electric motor with an associated lithium battery pack, that will be recharged in flight by 17,000 solar cells on the upper surfaces of the aircraft.
In fact, says André Borschberg, one of the project founders, the ability of the aircraft to recharge its batteries continually during daylight means it could theoretically remain aloft almost indefinitely – the only limit being the endurance of its pilot.
That last fact is of particular relevance to him. Not only is Borschberg, a mechanical and thermodynamics engineer, chief executive of the project, but he is also one of the two men who will fly the aircraft on its journey around the globe. The other is the initiative’s co-founder, Bertrand Piccard.
The two have been engaged on the enterprise for just over a decade. The venture is not just a technical challenge for its own sake but also an attempt to highlight the potential of renewable energy to solve a global problem, says Borschberg. The project’s previous plane, Solar Impulse 1, took to the air in 2010. Before its retirement last year, it became the first solar-powered craft to fly at night, when it stayed in the air for more than 26 hours.
The new aircraft represents a refinement of the design concept and technologies to meet a much more demanding set of challenges. The most extreme will, arguably, be the need to remain airborne for five days and nights when the plane flies across the Pacific.
The aircraft has a wingspan of 72m and weighs 2,300kg, of which 633kg is contributed by the batteries. The 3.8m3 cockpit is unpressurised and unheated, despite the fact that the external temperature will range from -40ºC to +40ºC.
Elsewhere, the plane will push some technologies to the limits of existing capabilities. The carbon-fibre sheets that provide much of the structure weigh only 25g/m2, while the solar cells are only 135µm thick. The electric motors will suffer only 6% energy loss in flight compared with an industry standard of 70%. The energy density of the batteries will be unusually high, at 260Wh/kg.
Borschberg says the round-the-world flight will comprise 25 days and nights of flying in, perhaps, 10 stages spread over three months. The plane will travel at altitudes of 1,500-8,500m at an average speed of 70km/h, although the following wind provided by the west-to-east route should make for a ground-speed equivalent of around 100km/h.
The flight will begin from the Gulf area and proceed via the Arabian Sea, India, Burma, China, the Pacific, the US, the Atlantic, and southern Europe or North Africa, before arriving back at the starting point. The total project cost, from its start in 2003, will come to £95 million.
The design and assembly operation has employed 80 staff at Dübendorf in Switzerland, with non-technical activities being carried from offices in Lausanne. The development of Solar Impulse 2 has had to focus on enhancing three aspects of the plane’s capabilities compared with its predecessor, says Borschberg: increasing the on-board energy resource; improving the craft’s robustness to cope with the extended flying periods; and providing a cockpit environment that is tolerable for the pilot.
The plane has been designed as a ‘flying laboratory’ to use a series of state-of-the-art technologies developed in cooperation with industrial partners. These include SunPower for the solar cells and Kokam for the batteries. The specially designed brushless and sensorless 13.5kW electric motors have been made by Etel. They turn the 4m-diameter propellers at a maximum of 525rpm with an overall system efficiency of 94%.
Pushing the envelope: Lightweight carbon-fibre sheets provide much of the structure
One of the companies involved in the project is multinational materials supplier Bayer. Up to 30 of its staff have been working on Solar Impulse over the past three years, led by Bernd Rothe, a chemical engineer with a PhD in polymer processing.
Although some Bayer materials were used in the first plane, they were standard products supplied from stock, says Rothe. But for the second plane, intensive research and development was required to meet much more demanding performance targets. “Providing materials that were as light as possible was the overwhelming criterion,” he says.
Bayer has supplied several categories of new materials for the aircraft, as well as carrying out primary design work for the cockpit shell. Polyurethane and polycarbonate materials were used for the cockpit, as well as a novel composite formulation.
Two sorts of polyurethane materials are used in the cockpit structure – one for the main shell and the other for the door. The former, says Rothe, was developed with the primary target of low weight in mind and has achieved a significant advance. The previous comparable material available from the company weighed 33kg/m3, while that provided for Solar Impulse 2 is 15% lighter at 28kg/m3. A change in the raw material formulation is key to this achievement, says Rothe.
But he says the material used for the door is even more innovative. The target for this element was not just low weight. It also had to combine high thermal insulation with extreme robustness to guard against the possibility that it could suffer any deformation that might inhibit ease of opening in an emergency. It is a safety-critical feature.
The material is a foam – “like a cheese with holes”, he says – with the voids, known as cells, filled with pentane gas, which has a low thermal transfer rate. The team succeeded in shrinking the volume of these cells by 40%, to produce a material with insulating properties comparable to that used in the rest of the cockpit structure at the same thickness, that could nevertheless provide the required extra degree of physical resilience.
An innovative material formulation was also used for the hinges of the doors. They are composites in which a carbon-fibre reinforcement is embedded in a polyurethane matrix rather than a more conventional, but heavier, epoxy material. “That is something new for us,” he says.
The transparent canopy for the cockpit is made from two sheets of polycarbonate material just 1mm in thickness that are glued together. The sheets have a special anti-fogging coating to ensure they remain transparent, and are fabricated into a rigid structure by 3D thermoforming, says Rothe.
From a commercial perspective, Bayer can envisage more down-to-earth applications for some of the materials developed for the project, he says. The properties of the door material, for instance, mean it could lend itself to use in domestic fridges, while the polyurethane-carbon fibre composite could have applicability in the automotive field.
Bayer’s involvement in the project has also left the company with a more intangible legacy, says Rothe. This is a “Solar Impulse spirit” – a willingness to attempt, and succeed in, challenges that would previously have been thought impracticable. He reiterates a key message from Borschberg: that despite the technological innovations the project will rely on, it is fundamentally an exercise of the human spirit.
Preparing for take-off: Outdoor tests on Solar Impulse 2