posted on 17 February 2016
from The Conversation
The British military is reportedly set to purchase two planes that can fly for months on end without needing to land. These large solar-powered "Zephyr" drones would likely be sent to carry out long-term surveillance missions and could constantly monitor an area with high-quality imagery. They could also be used to provide mobile and internet communication signals in remote areas, to support ground missions, and even conduct long-term research projects.
Alongside efforts by Facebook and Google to develop similar technology, the launch of the Zephyr aircraft by European aerospace firm Airbus could mark the start of a new era of continuous flight. What made this possible was a series of breakthroughs in lightweight materials, solar power and batteries, and autonomous navigation. These advances have come together to create planes that can fly day and night without intervention, potentially for months at a time.
Self-guided and self-powered planes started with NASA, which began working with the team behind the the manned Solar Challenger plane that flew across the English Channel in 1981. By 1994, NASA's Pathfinder aircraft had demonstrated solar panels were able to power aircraft to high altitude. But the planes still needed a power source at night. Batteries at the time were too heavy so the NASA engineers turned to hydrogen fuel cells, which they integrated into their Helios prototype, aiming to demonstrate round-the-clock operation.
Helios in flight. NASA
Unfortunately, the Helios proved structurally fragile and broke up dramatically on a test flight in 2003 after encountering turbulence, marking the end of NASA's pursuit of solar powered drones. Just two years later, however, AC Propulsion's SoLong plane proved it was possible to integrate lightweight batteries into a solar aircraft and flew for 48 hours, controlled remotely by a team of six pilots.
Today the Zephyr, originally developed by UK firm QinetiQ, has a 23m wingspan and yet only weighs 55kg (compared to the Helios's 726kg). It cruises at 20km, high above commercial aircraft and the fast-flowing atmospheric winds of the jet stream.
More importantly, it can fly for potentially months on end without the need for refuelling. So far it has only flown for 14 days straight but, theoretically, the only limit is how many times the battery can charge and discharge before it degrades. To enable this, the craft has overcome the crucial challenge of generating and storing enough power to both keep it continuously aloft and run its cameras and communication equipment.
The efficiency of the solar panels used on the planes today is not significantly different from those used for the first continuous flights. What's improved considerably are the weight and the robustness of the panels, as well as the cost. In fact, the Zephyr mark 8 uses amorphous silicon cells that are actually less efficient than the mono-crystalline cells used by the SoLong craft ten years ago. But because today's cells are lighter and more flexible, they contribute to a more reliable structure that needs less power to propel it.
Another significant development that has helped make these planes viable is the improvement in energy storage technologies, enabling them to save power generated by the sun in the day for use at night. Modern lithium sulphur batteries are able to store 60% more energy per kilogramme than the lithium polymer batteries available ten years ago. About 40% of the weight of the Zephyr 8 is the battery array. This means that improving the energy density (how much energy it can store without adding to weight or volume) can have a dramatic impact on the performance of the whole craft, ultimately enabling it to carry more equipment.
Building the Zephyr. Airbus
Other advances include the artificial intelligence that guides the craft, the sensors that gather data on the surrounding and continually changing weather, and the carbon fibre composites used to build the plane. Although the raw materials used are the same, new manufacturing processes that better control the direction of the carbon fibres and use less plastic resin to hold them together have made the overall structure lighter.
The result of these technologies is a plane that can do things previously only possible with satellites, but that can fly continuously over one area rather than having to orbit the entire globe. Although the UK will reportedly pay £10.5m for its two Zephyrs, this is a fraction of the hundreds of millions needed to launch and run a satellite. Plus, unlike with satellites, it's possible to land and repair the craft if something goes wrong.
The challenge now for the engineers working on solar-powered drones is to increase the amount of power they can collect and store and validate how long the batteries can stand constant charging and discharging. This will enable them to provide higher bandwidth communication and sustain flights in higher latitudes and during the winter months, when the incoming solar radiation is weaker. If this can be achieved, it could allow the likes of Google and Facebook to provide internet services not through cables but via drones.
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