Aerospace companies including Facebook, Boeing and Google have been trying for years to create a solar-powered plane that can fly at high altitudes for years at a time, and can provide broadband communication services. They could have many uses, for example acting as a relay station for communications in emergencies such as natural disasters that have knocked out ground-based telephones and internet links. Civil applications could include pipeline, crop and forestry fire monitoring, fisheries protection and border control. Military is also turning to solar powered stratospheric drones for communications and persistent, wide area, real time Intelligence, Surveillance, and Reconnaissance (ISR).
Near space has long been seen as a promising frontier for intelligence services, but has remained relatively untapped because it is too high for most aeroplanes to operate, and too low for satellites. Until now, the Northrop Grumman RQ-4 Global Hawk, limited to an altitude of about 19km, has been the highest flying drone in use, but it can only stay aloft for 32 hours at a time. The RQ-4 Global Hawk provided outstanding surveillance and reconnaissance in Afghanistan and Iraq, though the number of aircraft was insufficient to provide 24/7 coverage.
High-altitude communications platforms “such as Aquila and Titan Aerospace show tremendous promise,” Richard Whitt, director of Google’s strategic initiatives, and Yael Maguire, head of Facebook’s Connectivity Lab, wrote in a joint blog post. “Over 30 percent of the world lives beyond the range of mobile broadband. In rural areas, in which approximately half the world lives, that number is over 70 percent,” Whitt and Maguire wrote. “Recent improvements in composite materials, low-power computing, battery technology and solar panels have made (platforms such as Titan and Aquila) a new way to spur the growth of rural wireless networks.”
Challenges of Near Space Stratospheric Drones
According to USAF Chief of Staff; Peter Teets, defined near space as the altitudes between 20 and 300 km. The region of near-space starts where controlled airspace ends. Near-space extends up to the lowest altitude that a vehicle can maintain low earth orbit, defined as 490,000 ft or about 150 km. Near space, begins at about 20km above sea level, has until now been regarded a “death zone” for drones – thin air at this altitude makes it hard to generate lift, while extremely low temperatures mean electronic components like batteries are prone to fail.
Platforms operation from near-space operations faces many challenges the foremost being having to contend with an atmosphere that is less dense. At 65,000 ft, the atmospheric density is just 7.2% the density at sea level. Jet engines lose thrust as the operating altitude increases. This means for nearspace aircraft that wing area has to be larger to carry the same weight. The same is true for turboprop engines. Propellers in near-space require large diameters or many propellers.
Another challenge is Ozone concentration increase around the 65,000 ft region. Ozone is a known corrosive to some rubber and fabric materials. In general, ozone is not much of a corrosive threat to aircraft, but it could be to airships or balloons exposed a month at time. Higher in the near-space regime, ozone concentrations drop and monatomic oxygen dominates. The single atom of oxygen is very corrosive, so designs must protect the NSVs from it. In addition to protecting from monatomic oxygen, the designs should handle cosmic radiation that causes single event upsets in the electronics.
The final aspect of near-space to address is the wind. Once NSVs rise above the jet stream, they may encounter average winds between 10 and 20 knots (kts). The wind velocities become a factor when deciding the station keeping fuel requirements for airships.
“The problem is very, very difficult,” said Morgan, who worked earlier in Simi Valley-based AeroVironment. “It’s more difficult than it seems when you first start testing it.” An atmosat needs to be strong enough to climb through the troposphere, where weather happens, but light enough to cruise in the thinner air of the stratosphere. The biggest challenges are battery life and the efficiency of the solar array, Morgan said. The planes are also hard to operate outside the tropics, where their solar cells benefit from sustained sunlight near the equator.
Engineers at Boeing Phantom Works in California have filed a patent application for an electric-powered plane with solar cells covering its wings, to deliver Internet, phone and video – even in remote regions where cable can’t be laid. “For various reasons, it may be desirable to place a payload above a point on the earth’s surface for a relatively long time. Potential payloads include imaging systems such as a camera or radar; communication systems for use with radio signals, cell phone signals, microwave transmissions, earth-to-satellite links, television broadcasts, internet connections, payload-to-payload linkages, and the like; atmospheric sensing systems for measuring wind, temperature, humidity, gases present, sunlight, and other purposes. Maintaining a relatively fixed position above a point on the earth’s surface enables many such payload systems to operate in a simple and/or effective manner,” says Boeing.
Aside from maintaining a payload at a fixed position relative to the surface of the earth, it is also desirable to maintain the payload at a favorable altitude. If the payload is at too low an altitude, its utility may be reduced because its field of view is diminished, weather effects may be increased, obscuration by clouds may be more common, and other obstacles to efficient operation may be present. If the payload is at too high an altitude, its ability to resolve objects on the earth’s surface may be diminished, electronic transmission signals must be stronger to reach the payload with sufficient strength, and signals emitted from the payload must be stronger to reach the earth’s surface with sufficient strength. Accordingly, there is a range of altitudes that may be desirable for many payloads.
One potential way to maintain a payload at a fixed position above the earth’s surface and at a desirable altitude is with an airplane. However, a conventionally fueled airplane can only stay aloft for a matter of hours or days, based on its fuel capacity and rate of fuel usage. A nuclear powered airplane might feasibly remain aloft for a long period of time, but this approach faces opposition and certain risks. Therefore, solar power may be the one of the few feasible methods of providing long duration propulsion to an airplane. However, there are many difficulties associated with accomplishing this successfully.
“Solar power is very weak in terms of energy flux, providing at most about 100 Watts per square foot. For example, if a Boeing 747 were equipped with perfectly efficient solar cells on its entire upper wing surface, it would receive at most approximately 600 kilowatts, or about 800 horsepower from the solar cells. This compares with approximately 100,000 horsepower required for the 747 to maintain cruising speed and altitude. Thus, solar power can provide only 0.8% of the needed power to a conventional 747, even if the solar cells are 100% efficient and the sun is directly above the airplane. With typical very good cells, solar power can provide only 0.3% of the needed power to a 747. The conclusion is that a very special airplane is needed–one that can fly on very low power while gathering lots of solar energy,” says Boeing.
Even landing is a problem. Facebook has suspended its internet via Drone experiment Aquila in an announcement posted to Facebook Code. The Aquila drones lack the standard landing gear that aircraft typically have as part of an effort to reduce weight. Instead, they are intended to land on specially designed kevlar pads. Both Aquila test flights in Arizona faced difficulties during the landing phase, which caused varying degrees of damage to the drones. The first test flight in Arizona in June 2016 launched and flew as expected, though turbulence before touchdown resulted in the drone landing short of the runway and receiving damage to the right wing in the process. A second test flight of a different model drone in May 2017, did not crash, though apparently suffered dings during landing, according to a post by Gomez at the time.
While helium-filled balloons, such as those used by Google, are difficult to steer, aircraft, or one-wing solutions such as Facebook’s Aquila drone, frequently struggle with strong stratospheric winds, said Daniel Cracau, general manager of AlphaLink, which has just recently spun out from the University of Berlin.
The platforms need a very long wing to accomodate a sufficient amount of solar cells but at the same time need to be extremely lightweight. That makes them vulnerable to bending and breaking when subjected to strong wind gusts. Making the aircraft sturdier and heavier thus would reduce the amount of communications or observation payload the platform could carry, said Cracau.
AlphaLink approached the problem by designing the platform as a formation of several aircraft of smaller sizes connected at wing tips. “By combining several smaller aircraft, we achieve a long total wing length but because the aircraft are connected via joints at their wing tips, we can control the wind influences,” Cracau explained. “Because we can handle better these aero-elastical phenomena, we can in general build the aircraft lighter, which is why we can fly longer and also carry more payload.”
If any of the aircraft in the formation experiences technical problems, it can be disconnected from the rest and send to a ground station for repairs, Cracau said, while a replacement aircraft could be send up to join the formation. Cracau said AlphaLink estimates the platforms could provide internet connectivity and other communication services to remote areas at 10 percent of the cost of satellite technology.
According to DARPA energy management technologies – solar collection (photovoltaic) and fuel cells (nergy storage systems) are vital for enabling ultra-persistent high-altitude, long-endurance (HALE) flights lasting multiple years. Solid Oxide Fuel Cell (SOFC) based energy storage systems have the potential to provide unprecedented round trip energy efficiency as the storage application of the technology is further developed.
Alta Devices is a developer of solar cell technology that is designed for autonomous unmanned systems. It is extremely thin and lightweight and can be integrated directly into a wide variety of materials. Adding power without compromising the weight, size, or maneuverability of the aircraft is ideal for unmanned systems that require power for long endurance missions without returning to ground. Because of their thinness and flexibility, Alta Devices solar cells can easily be adhered directly to a wing or fuselage surface with negligible impact to aerodynamics. It is also possible to integrate the cells directly into carbon fiber or fiberglass resulting in a seamless structure.
NRL Increases UAV Endurance with Solar Soaring Technology
Researchers at the U.S. Naval Research Laboratory (NRL) are developing technology for unmanned aerial vehicles (UAV) that has given them the ability to fly for more than 12 hours by harvesting energy from the atmosphere and the sun. Solar-Soaring is a pair of endurance-enhancing technologies. They aid the warfighter by enabling a UAV to fly longer without carrying extra weight in batteries.
“One of the common complaints that we hear across industry and the warfighters is that they want aircraft to fly longer,” said Dr. Dan Edwards, senior aerospace engineer in NRL’s Tactical Electronic Warfare Division. “One great way to do this is to capture atmospheric wind energy or solar energy to extend the endurance.” Since 2005, Edwards has been exploring how to teach an autopilot how to soar using thermals in the atmosphere, much like how a bird flies. Using special sensing and guidance algorithms, the UAV flies a waypoint route until it senses a thermal updraft, then commands the aircraft to circle in the rising air.
“Sunlight heats up the surface of the Earth, which in turn heats the lowest layer of air,” said Edwards. “That warm air eventually bubbles up as a rising air mass, called a thermal, which the airplane can use to gain altitude. It’s indirectly solar-powered.”
Solar power is also used directly to power the UAV using solar cells, which are semiconductor devices that convert light into electricity. “For a long time, even though there has been solar aircraft since the 1990s, the efficiency of the solar cells wasn’t high enough to pay the mass penalty, meaning you weren’t getting enough energy to justify the additional mass,” said Phil Jenkins, head of the Photovoltaics Section in NRL’s Electronics Science and Technology Division. “But over the last 10 years, that has really changed. The cells have gotten more efficient and lighter.” The aircraft still carries a battery. However, the battery can be smaller because of the solar and soaring capabilities on board.
“With Solar-Soaring, the UAV doesn’t need a huge battery because it is getting energy from the environment,” said Edwards. “It just carries more intelligent software in the case of the autonomous soaring algorithms, or a lightweight, integrated solar array that captures much more energy from the sun compared to the amount of mass.” Bringing these two technologies together, NRL found the combination works better than either individually. While soaring, the motor is turned off and the solar array can recharge the on-board battery faster. This increases the mission availability of a UAV for warfighters.
“Between the two, you have the most robust energy-harvesting platform, because sometimes you’ll be able to soar and sometimes you won’t have the solar, and vice versa,” said Jenkins. The NRL-developed technologies are applicable to platforms that are already in use by the military, such as the Raven, a small hand-launched remote-controlled UAV or the Predator, a larger UAV. “In the case of Solar-Soaring, we’re demonstrating the techniques to fly aircraft with a higher endurance,” said Edwards. “These techniques are portable to a lot of the programs of record, like the small-size Raven or potentially the larger Predator, so it’s a pretty broad application space.”
Having a UAV with extended endurance capabilities is important for military information, surveillance, and reconnaissance missions, or a communications relay. The technology also has important uses for civilian applications, including monitoring and inspection of railways and oil pipelines, surveying crops, and search and rescue.
Both Edwards and Jenkins identified a hurdle they would eventually have to overcome with Solar Soaring – the ability to fly through the night. “We still can’t fly through the night because the batteries are just too heavy, but we currently get dawn to dusk-enhanced endurance,” said Jenkins. For Edwards, the next step in solving this problem is swapping out the battery for a hydrogen fuel cell. “Fuel cells have much more energy per unit mass than a battery,” said Edwards. “So we’re marrying the fuel cells, which are great for getting through the night, and the Solar-Soaring, which is great in the daytime for getting energy directly from the sun and wind.”
According to Titan Aerospace, their Salora platform maintains efficient energy harvesting even when the sun is low on the horizon, employing its high efficiency solar array positioned on horizontal wings and tail, slanted wing tips and vertical tail. To further conserve energy, the platforms use a distributed Maximum Point Power Tracking system, dynamically maintaining optimal voltage across the entire array at various solar insolation and incidence levels.
Aeroenvironment drone with gallium arsenide solar cells
California dronemaker AeroVironment unveiled a new version of its hand-thrown, 13-pound Puma AE—this one with gallium arsenide solar cells layering its wings and test-flown for more than 9 hours compared to air endurance of 2 hours of standard Puma E. The secret behind the new 9-hour endurance is the flexible, high-efficiency gallium arsenide cells, says Rich Kapusta, marketing VP at solar-power partner Alta Devices. Earlier attempts by other companies to build solar-powered microdrones, such as Bye Aerospace and its Silent Falcon, have been stymied by low-efficiency silicon, he contends.
GaAs solar cells hold the world record for the most practical type of solar cell (single-junction). The record solar efficiency is 28.8% (record held by Alta Devices). Most solar materials such as Silicon (Si) lose a lot of efficiency when the temperature rises. Gallium Arsenide has a low temperature coefficient and thus experiences very little efficiency loss at higher temperatures. Gallium Arsenide is naturally resistant to damage from moisture, radiation and ultra-violet light. These properties make GaAs an excellent choice for aerospace applications where there is increased UV and radiation.
Siemens New electric motor
Siemens researchers have developed a new type of 50 kg electric motor for aircraft that delivers a 260 kW continuous – five times more than comparable drive systems.
New simulation techniques and sophisticated lightweight construction enabled the drive system to achieve a unique weight-to-performance ratio of 5 kW/ kg compared to 2 kW/ kg performance of the drive systems used in electric vehicles. The new motor delivers its outstanding performance at only 2,500 revolutions per minute so it can drive propellers directly, without transmission. “This innovation will make it possible to build series hybrid-electric aircraft with four or more seats,” said Frank Anton, Head of eAircraft at Siemens Corporate Technology, the company’s central research unit.
According to ABI Research, the drone industry is going to be worth a whopping $8.4 billion by 2019. It’s not just the sales of the hardware, it’s mainly the applications and services where most growth is expected. And the most exciting part: solar energy is playing an increasingly important role in the development of UAV technology.
References and Resources also include: