Updated: Sep 16, 2020
We're in the beginnings of a period of global vehicle electrification. Each class of vehicles has seen the rise of various electric options from e-scooters and e-bikes to electric cars, planes, and passenger drones. A combination of range anxiety and curiosity is pushing more people to explore using on-board solar panels to achieve a sort of ‘solar perpetuum mobile’.
Although no possible combination of solar panels, lithium batteries and electric motors will allow us to break the first law of thermodynamics (energy cannot be created or destroyed), systems can be designed such that the solar power produced meets or exceeds the vehicle’s power requirement at cruise conditions. This condition is called Infinity Cruise.
The critical factor to consider when integrating solar power onto an aircraft is the ratio of power required to maintain cruise speed and altitude versus flat surface area available on the airframe.
For rotary wing aircraft, such as helicopters and multicopters, solar integration is unpractical and challenging. On the one hand, large, the flat surfaces of the solar panels produce aerodynamic forces which interfere with flight and actually decrease efficiency. On the other hand, as a rotary-wing craft, a consistent power draw is required just to stay aloft, maintain speed and maintain position regardless of the flight conditions.
Success has been had in the integration of solar into fixed-wings, but these solar airplanes, even at a minimum viable size, have all had long, flimsy wings and a low useable payload.
In 2003, the Helios Prototype, a collaboration between Aerovironment and NASA, ripped itself apart in gusts and crashed into the Pacific Ocean.
Solar Impulse is a cutting-edge Swiss solar airplane flown around the world by Bertrand Piccard, a real-world Fileas Fogg, in 2015. Although it has a crew of only 1, it's wingspan of 80m, almost as large as an Airbus A380 and it cruises at only 58mph. For perspective, a Cessna 172, common trainer aircraft has a wingspan of 11m and cruises at 180mph.
Getting enough solar power on your wings to power your propulsion system at infinity cruise requires very large wings. Large wings produce large drag, and therefore require more power to fly. The problem cascades this way forcing many expensive design decisions and compromises, including a relatively slow cruise speed.
Powered LTA (Airships)
Firstly, because Airships lift passively using the buoyant forces of its lifting gasses, the power required to remain airborne is substantially less than for any other airframe type. Low power requirements mean that the minimum solar panel area required for infinity cruise can be also smaller than for other aircraft types of comparable size in comparable conditions.
Secondly, the gas envelope sized to hold lifting gas provides ample space in or on which to install solar panels without greatly affecting the aerodynamic quality of the craft. This makes the airship-type craft uniquely suited among airframe types for effective low-cost solar-power integration.
Solar power presents an interesting opportunity for aerial intelligence craft, by decreasing the minimum viable size for a long-range intelligence mission, and increasing the maximum range and endurance envelopes for small, low-cost intelligence craft.
In November 2016, Mothership Aeronautics demonstrated the Scout, a prototype Solar Airship in Infinity cruise in Silicon Valley. The Scout was 6.5m long and powered by 400W of solar panels, consisting of 127 cells. It performed maneuvers in wind for 90 minutes at 10:00am without reducing battery voltage.
While technological improvements in efficiency of solar photovoltaic technology, batteries and motors will certainly make solar integration possible for a growing variety of hybrid aircraft. Only Airships present a practical, viable solution today for high endurance long range solar aircraft.