Future paths
Is the dream of solar aviation now within our grasp?
18/10/2011
Ever since the first photovoltaic cells were developed, humankind has sought to use the sun’s energy to power airplanes. Overview of the innovations that led to fuel-free flight.
@ AFP PHOTO/NASA/HO
40 years of testing
The United States, 1974. The first solar airplane, California Sunrise I, achieved a 20-minute flight, at an altitude of 100 m. Five years later, Solar Riser flew over 800 m in the first manned solar-powered flight.
In 1990, the manned Sunseeker aircraft crossed the United States in a number of legs, the longest of which was a full 400 km!
Another remarkable airplane is Nasa’s remotely piloted Helios, which in 2001 achieved a record altitude of 30,000 m.
Moving closer to the sun in order to better capture its energy
The principle of the solar airplane is to capture the light of the sun and convert it into electrical energy. To do this, photovoltaic panels made of semi-conductor cells are put in place, mainly along the wings. The energy harnessed is then used to power the aircraft and operate the flight instruments.
Surplus energy can be stored for use at night, and the energy recovery cycle starts over again at daybreak. This is what is known as eternal flight, as in theory, the airplane does not need to land to refuel and can keep flying.
The solar photovoltaic airplane also stands out in terms of its design. To maximize aerodynamic performance, particularly at low speed, innovation has concentrated on:
• A very long shape and a large surface area to improve the way the plane lifts in the air (like a glider),
• Extremely lightweight materials: with a wing span of about 60 m (the same as an Airbus A340), the latest prototypes weigh less than 1.6 tons.
@ AFP PHOTO PIERRE VERDY
Challenges to be overcome
While solar planes can in theory achieve 100% energy efficiency, the right irradiance and weather conditions (i.e. a cloudless sky) are rarely achieved. In this case, performance can be as low as 12%.
The fragility of photovoltaic cells is also hindering the development of solar aviation.
Another challenge is the issue of passenger transportation. More innovation is required to build engines capable of bearing the weight of passengers. And we are still many years away from commercial operation.
Finally, solar models perform poorly in terms of speed: just 80 kph for reduced models and as low as 65 kph for manned flights. Nevertheless, the latest solar airplane prototypes show what huge progress has been made, in particular in terms of autonomy and human-machine interface.
For more information: www.solarimpulse.com
Sources
National Renewable Energy Laboratory – Avril 2011
Wiley Interscience: www.interscience.wiley.com
www.flightglobal.com/pdfarchive/view/1979/1979%20-%202438.html
www.nasa.gov/centers/dryden/history/pastprojects/Erast/centurion2.html
spaceplace.nasa.gov/helios/en/
www.sky-sailor.ethz.ch/docs/Thesis_Noth_2008.pdf
