Solar Impulse

A leap into the future

In a world depending on fossil energies, the Solar Impulse project is a paradox, almost a provocation: it aims to have an airplane take off and fly autonomously, day and night, propelled uniquely by solar energy, right round the world without fuel or pollution - an unachievable goal without pushing back the current technological limits in all fields.

Two men, both pioneers and innovators, both pilots, are driving force behind Solar Impulse. Bertrand Piccard, an aeronaut who made the first non-stop round-the-world balloon flight, is the initiator and chairman. André Borschberg, an engineer and graduate in management science, a fighter pilot and a professional airplane and helicopter pilot, is the CEO. The former’s avant-garde vision and the latter’s entrepreneurial and managerial experience are an ideal combination.

By Michael Weickhmann, Head of Applications Engineering, Permanent Magnets Division, Vacuumschmelze GmbH & Co. KG, Hanau, Germany

Michael Weickhmann, Vacuumschmelze

Is it possible to fly around the world in an electric-powered aircraft? Does an aircraft that could do this even exist?

This sounds like the kind of question that children would ask their parents! But, unlikely as it may sound, this is set to become a reality. The aircraft is called "Solar Impulse“and the maiden voyage will take place this summer.

The Challenge!

In a world dependent on fossil fuels, the Solar Impulse project is a paradox – indeed, almost a provocation. An aircraft powered entirely by solar cells, which can take off autonomously and continue to fly day and night until it has circumnavigated the globe – with no fuel or harmful emissions whatsoever. A goal that can only be achieved if the boundaries of today's technologies can be pushed further than ever before...

This is the claim of the Solar Impulse project team.

Bertrand Piccard, Aeronaut

André Borschberg, Engineer

Where did the idea of Solar Impulse originate?

Bertrand Piccard hatched the idea after the success of the Orbiter 3 project, a non-stop, round the world balloon flight which in 1999 unleashed a wave of enthusiasm within the aviation community. This enthusiasm is precisely what is needed to enhance awareness to one of the main challenges of the 21st century: to harmonize the interests of commerce and ecology. With the idea of promoting new technologies, we aim both to reveal the opportunities that are possible by harnessing alternative energy sources and to promote the realisation that existing fossil fuel resources must be used sparingly.

Why is VAC involved in the project?

When we first heard about Solar Impulse in 2000, we were fascinated and enthusiastic, we were also convinced that with our product portfolio and know how, we could make a positive contribution to the project. We contacted André Borschberg, the CEO and one of the pilots of Solar Impulse and offered to support the project by supplying our neodymium-iron-boron (Nd-Fe-B) permanent magnets for the construction of the electric driveline systems. The resulting partnership has endured to the present day, we currently supply magnet systems made of VACODYM^® 655 HR with a protective coating developed by our own engineers, VACCOAT^® 20011. These magnets are assembled into systems using special adhesives and integrated into permanent magnet synchronous machines in Switzerland.

How will the goal of flying around the world without using any fossil fuel actually be achieved?

Solar Impulse is a completely new concept which will expand our knowledge of materials, energy management and man/machine interfaces. It is a solar powered glider with a wingspan of 63 meters, similar to an Airbus A340 and totally disproportionate to the very low weight of just 1,500kg. The initial prototype demonstrates state of the art aerodynamics combined with a highly optimized power system, from the solar cells to the propellers and the electric drive units.

The drive unit comprises four permanent magnet synchronous motors, each with a nominal output of six kilowatts (eight HP) and powered by a total of 12,000 solar cells on the glider’s 200 sqm of wing surface. We know that the midday sun delivers the energy equivalent of up to 1,000 watts, or 1.3HP of luminous intensity, per square meter. Over a period of 24 hours, this is equal to an average of 250 watts per square meter. To enable continued flight during the night, the excess solar energy collected during the day is stored in special lithium polymer batteries. These batteries have an energy density of 200 watt hours per kilogram and a total mass of 400kg – more than one-quarter of the entire weight of the aircraft! Because the aircraft is designed to operate at an altitude of up to 8,500 meters or around 28,000 feet, the engines must operate at external temperatures of –40°C. Each engine has a maximum output of nine kW (twelve hp) and drives a twin-bladed propeller three-and-a-half meters in diameter via a 2:1 gearbox at up to 400 rpm. This enables the aircraft to reach an average speed of around 70km/h (44mph).

Why does the aircraft have such a huge wingspan?

First, a large wingspan improves aerodynamic efficiency by reducing induced resistance, which lowers the aircraft’s sink rate and minimizes the engine power necessary for horizontal flight. The second benefit of a broad wingspan is the large surface area which is available for mounting the solar cells.

What factors currently limit the performance of the aircraft?

The energy density of the batteries is the primary factor. Their storage capacity is still limited and their weight impacts significantly on the total weight of the aircraft. If this storage capacity could be doubled, a second pilot could be taken on board and far longer flights could be undertaken.

What happens if something goes wrong? What risks do the pilots face?

If the pilot exhausts their energy reserves during the night, a night landing must be performed.

How efficient are the solar cells?

The efficiency of the solar cells selected for the project is around 20%. Two criteria were considered in the selection of the cells: efficiency and weight. The cells finally selected are thin walled cells of monocrystalline silicon. Although more powerful cells with efficiency levels of up to 30% are available today, they are heavier. It is obvious that a significant improvement in solar cell efficiency would also enable improvements to be made in the aircraft’s performance.

Why can the round-the-world flight not take place uninterrupted?

This is currently impossible for manned flight. Even if we were technically in a position to achieve perpetual flight, the human factor poses limitations. The aircraft cockpit is tiny and only has space for one pilot, who is physically incapable of enduring these conditions for more than four or five days at a stretch. A round-the-world flight without interruption would only be possible if there were two pilots on board. Don’t forget that it took around 60 years for conventionally powered aircraft to develop from the first round-the-world flight with breaks, to the first non-stop round-the-world flight!

What technological developments could result from a successful circumnavigation of the globe?

It’s hard to say. But we can certainly expect some advancement in reduction of weight and energy, component efficiency and improvements in the reliability of the electric engines, solar cell efficiency, energy storage or the cabin air pressure equalization system. But before we can pass on the corresponding expertise, we have to develop it first.

To shape the future, we need people who are willing to explore farther horizons and strike out along new paths. We need visions similar to those of Prof. Bertrand Piccard, André Borschberg and their 50-strong team. At VAC, we try to enable and promote these visions through our development and application of leading edge materials and products.

Our thanks go to André Borschberg and Rachel Bros de Puechredon for providing us with the information that made this article for Vacuumschmelze possible. And our best wishes go to the two pilots, Bertrand Piccard and André Borschberg, for every success in their mission!