The combination is irresistible: photovoltaic cells covering the long, wide wings of an airplane, making the electricity to drive the propellor. If the solar cells could also charge batteries to run the plane at night, it would be possible to keep the plane in the air as close to perpetually as the weather allows. But heavyweight batteries and inefficient solar cells made such an accomplishment difficult.
Difficult -- but not impossible. Last week, AC Propulsion, an organization specializing in electric vehicle engineering, demonstrated a solar-battery unmanned air vehicle (PDF) that flew for just over 48 hours straight.
Remaining aloft for two nights is the milestone for sustainable flight. One night is possible just by discharging the batteries, but two or more nights means that the plane has to fully recoup and store the energy used at night while flying in the sunlight the following day. Once that is achieved, the cycle can repeat continually, and keep the plane airborne indefinitely.
The 76 photovoltaic cells used were only 20% efficient -- decent, but by no means cutting-edge -- but they produced enough power (around 225 watts) to charge the Lithium-Ion batteries and power the communication and control system. The UAV was controlled entirely from the ground, and never strayed more than 8km from the launch area; at 30-50 miles per hour, that restriction wasn't particularly difficult to stick to. But what makes the SoLong notable is the size: as the picture above shows, the wingspan is only about 4.75m, and the entire aircraft weighs about 12kg. It's not quite a microflyer, but it's definitely down in the hobbyist/NGO camera plane range. (More images can be found in this PDF document.)
Further up the size scale will be the Mercator solar UAV. Built by QinetiQ for a project run by the Flemish Institute of Technology, the aircraft is designated a "High Altitude Long Endurance" vehicle (HALE), and is intended to fly for months at a time at a height of 60,000 feet. The size difference is notable: the Mercator will have a wingspan of 16m -- nearly four times that of the SoLong -- and weigh 27kg. The difference is wingspan is related, in part, to the height at which it will fly; at so high up, the air is much thinner, and the wings have to be larger to keep the vehicle aloft.
Compare this to the Solar Impulse, a planned round-the-world manned solar aircraft. We posted about the Solar Impulse project in February; this week, at the Le Bourget Air Show, the Solar Impulse team revealed some of the specifications for the aircraft's design.
A team of 60 specialists has been working for 15 months to reach the present stage of this project, reflected in the model unveiled at Le Bourget: an impressive ultra-light structure with a wing span of 80 metres – similar in size to the brand new Airbus A380 – and a weight of just two tons. This airplane has the capability to fly day and night powered entirely by solar energy and to fly completely around the globe in fiveday stages, with the goal of promoting sustainable development and renewable forms of energy.
The "fiveday stages" aspect of the trip is almost certainly related to passenger comfort and health; as the SoLong excerpt notes, if the craft can make it for 48 hours, it can stay up indefinitely. Regardless, an 80m wingspan, 2000kg weight solar aircraft will be impressive, if it gets built.
Without substantially more efficient photovoltaic cells and batteries, solar powered flight will remain in the realm of robots and adventurers. But even if Boeing and Airbus never put solar planes on the runway, the research and development that goes into making these the frames, skin, solar and battery systems for these aircraft as light and efficient as possible will have real-world applications. If your gas-optional hybrid car of 2012 has a photovoltaic layer to help keep the batteries topped up, part of its genesis will have come from the air.
