A few months back we had a guest post from NavalDrones on the site discussing power needs for drones, focusing on the advantages of batteries compared to today’s combustion engines. Engines are noisy, limiting drones’ stealthiness, and both engines and batteries require refueling/recharging. Thus, lengthy, days-long on-station operations aren’t in the cards for today’s drones. (For example, the Global Hawk can fly continuously for about 28 hours.) A balloon or dirigible could stay aloft for longer periods, but at the expense of maneuverability and speed. For reasons like these, harvesting solar power during flight has captured the attention of many aerospace engineers.

One challenge terrestrial solar-powered vehicles face is the variability of cloud cover. In contrast with its grounded brethren, solar aircraft can often negate a cloudy day by just climbing to a sufficient altitude. However, night is, of course, still an obstacle to long-term flight (or short-term missions not in the daytime).

Nevertheless, with the aid of batteries, today’s solar drones and UAVs can fly non-stop for weeks. The British-US aerospace and defense company QinetiQ developed the drone Zephyr, which stayed aloft for 14 days in July 2010 (h/t to Solar Impulse). Zephyr is not small (12-m [39-ft] wingspan), as one can see in the following video, but it is light—only 27 kg, or ~60 lbs, hence the hand-launch. It reached an altitude of 21.6 km (13.4 mi) on that first flight, boosting its observational capabilities.

[youtube http://www.youtube.com/watch?v=ejXaAwsIDoI&w=560&h=315]

Meanwhile, the goals of the Solar Impulse team might be even more audacious: a solar-powered flight around the world in 2015— with a pilot. While it’s perhaps not the most agile, the HB-SIA has already demonstrated 24-hr flight in the past year (with a battery system) from Switzerland to Morocco. And the team has strong backing; it was launched by Bertrand Piccard, who made his name in aviation by circumnavigating the world in the Breitling Orbiter balloon in 1999. Industrial partners include Solvay, Décision, and Bayer MaterialScience, who increased their funding for the project in October [h/t to Flightglobal]. In contrast to Zephyr, HB-SIA’s mass is 1600 kg (3500 lb), about as much as a car, and its 63-m (208-ft) wingspan is about 60% longer than Global Hawk’s – necessary to fit enough solar cells to lift that mass.

So what’s next for solar aircraft? A higher-density storage system than batteries would help by extending flight time. NASA tested a series of solar UAVs in the early ’00s, including Helios, which included an “experimental fuel cell system” that used solar power to regenerate its fuel, storing more energy per pound than batteries. Unfortunately, a crash in 2003 destroyed Helios, but a fuel-cell system remains a possible avenue of advancement. Surface-based lasers can also offer additional illumination for a power boost (also covered in Naval Drones’ post).

Increasing the efficiency of solar cells is another route. Aircraft using solar cells require large wings whose size and shape are driven in part by demands for enough surface area to power the aircraft. These designs limit maneuverability and high-performance (i.e. high-power-demand) attributes like sudden acceleration and changes in direction. Unfortunately, physics principles constrain just how much efficiency can increase. Solar Impulse uses cells with an efficiency of 22.7% — higher than most commercial modules in solar farms. But using only one kind of material in the cell to absorb light means it can harvest only part of the sun’s light, at maximum about 33% (something called the Shockley-Quiesser limit).

Multi-junction cells can capture more slices of the solar spectrum, but in practice their complex assembly limits them to two or three absorber materials. So far they are mostly used in spaceflight, where low weight is a bigger driver than low cost. Still, according to the U.S. National Renewable Energy Lab, the record triple-junction cell (without concentrators, which are another topic) has 35.8% efficiency. So assuming for the sake of estimation that these triple-junction cells weigh about the same per unit surface area (not true at present, according to Solar Impulse), they could reduce wing area by about 37%.  Or, depending on the requirements, they could produce 58% more power.

And power is the big difference between a solar airplane like HB-SIA and a fuel-burner like Global Hawk. HB-SIA’s electric engines produce a maximum of 30 kW (40 hp), whereas Global Hawk’s engine produces at peak 7600 lbs of thrust at a top speed of 357 mph, which works out to 5.4 MW (7200 hp). In part we could say that HB-SIA is more efficient, so it doesn’t need as much power, but on the other hand, Global Hawk can carry a 1360-kg (3000-lb) payload, whereas HB-SIA can carry… one human.

Doing the math shows the upper limit of improving power capture. The sun provides, at midday, 1.3 hp per square meter (of land surface). This handy figure gives you an idea of the maximum solar power wings of a given size could produce (with magical 100% efficient cells). Thus, performance improvements may come from vehicle lightweighting, rather than ratcheting up solar cell efficiency. For example, batteries make up one-quarter the total mass of HB-SIA (400 kg, or 800 lb). And while modern aircraft bodies are increasingly made of carbon fiber (instead of aluminum), companies such as Nanocomp and TE Connectivity are also beginning to manufacture data and power cables made of carbon nanotubes (CNTs) on the scale of miles. CNTs can match the conductivity of copper while saving ~70% of the weight.

Even if it doesn’t displace the combustion-engine in aviation when speed and heavy lift are required, solar power’s promise of nearly indefinite sustained flight is likely to expand its role in aeronautics in the near future.

Dr. Joel Abrahamson holds a PhD in chemical engineering from the Massachusetts Institute of Technology (MIT), where he created nanomaterials for lightweight, high-power electricity generators. He currently researches materials for thin-film, flexible solar cells at the University of Minnesota. The opinions and views expressed in this post are his alone and are presented in his personal capacity. They do not necessarily represent the views of the University of Minnesota.