Solar Impulse 2 Takes Wing

Ever since the first photovoltaic cells were developed some 40 years ago, engineers have worked to develop a solar-powered airplane that doesn’t require any fuel. The latest project, Solar Impulse, has a lofty goal: to fly around the world in March 2015.

L’avion solaire défi  Solar Impulse II
Bertrand Piccard and André Borschberg's Solar Impulse I plane. © E. CABANIS / AFP

Bertrand Piccard and André Borschberg, the two experienced Swiss pilots who designed the Solar Impulse, have mapped out the details of their global flight. The journey will be split into five stages of five consecutive days and nights, with each pilot taking turns in the cockpit. It is expected to take about five months1.

The starting point for their adventure will be the Persian Gulf, in March 2015. The aircraft, the Solar Impulse 2, has a wingspan of 72 meters, larger than that of a Boeing 747, and weighs just 2.3 tons, the same as a big car. The prototype Solar Impulse 1 set several records, including the first solar night flight and first solar intercontinental flight.

The first solar airplane tests were conducted 40 years ago. 

Solar Impulse 2 has more than 17,000 photovoltaic cells designed by SunPower built into its wings. They supply powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... to four 17.5-horsepower engines. The energy is stored in lithium batteries weighing more than 600 kilograms. During the flight, the pilots will use heated insoles and gloves to protect them from the cold and a specially formulated sunscreen to block out harmful rays. They intend to sleep for periods of 20 minutes.

40 Years of Solar Aviation

The Solar Impulse is by no means the first solar-powered airplane. In 1974, the California Sunrise I achieved a flight of 20 minutes at an altitude of 100 meters. Five years later, the Solar RiserIn offshore oil production, risers link the seabed to the surface and are primarily used to transfer oil... was the first manned aircraft to fly on solar power, at an altitude of 800 meters. In 1990, the manned solar aircraft Sunseeker I crossed the United States in several stages, including one that was 400 kilometers long. Another remarkable solar aircraft was the remotely piloted NASA Helios, which set an altitude record of 30,000 meters in 2013.

The Solar Impulse 2 has a wingspan of 72 meters, larger than that of a Boeing 747. 

A solar airplane works by capturing the light of the sun and converting it into electrical energy. This is done using photovoltaic panels comprised of semiconductor cells, which are mounted mainly along the wings. The energy produced by the panels is used to power the aircraft and operate the flight instruments.

Surplus power can be stored for use at night, with the energy capture cycle beginning again at daybreak. The aircraft can keep flying without having to land to refuel, making “perpetual flight” a reality.

Challenges to Overcome

Although solar airplanes can in theory achieve 100% energy efficiencyIn economic terms, energy efficiency refers to the efforts made to reduce the energy consumption of a system... , they need to have the right irradianceThe power of electromagnetic radiation per unit area, expressed in watts per square meter (W/m²)... and weather conditions, in other words a cloudless sky, which doesn't happen very often. This means that efficiency can be as low as 12%. The fragile nature of the photovoltaic cells is also hindering the development of solar aviation.

Passenger transportation is another challenge. Engineers need to come up with innovative ways for solar airplanes to carry the weight of passengers. It will be a long time before solar aviation becomes commercially viable.

Solar aircraft also perform poorly in terms of speed: 80 kilometers per hour for a small model and 65 kilometers per hour for a manned flight. Nevertheless, the latest solar airplane prototypes have made great strides, particularly in the area of range and human-machine interface.



(1) Solar Impulse