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Best endurance under the sun - solar powered aircraft

George Marsh

A solar-powered aircraft has set a flight endurance record by remaining aloft for over two weeks. George Marsh reports.

The Zephyr unmanned air system (UAS), designed and built by research and technology organisation QinetiQ, landed after being in the air for 14 days and 21 minutes, powered entirely by solar radiation.

The previous official endurance record had been set by the Northrop-Grumman Global Hawk, a conventionally-powered unmanned aircraft, with a flight of 30 hours (h) 24 minutes (min), although an earlier version of the Zephyr had created an unofficial record of 82h 37 min in 2008. Zephyr's 2010 flight took place over the US Army's Yuma Proving Ground in Arizona.

So how was a two-week flight, using no fuel and a heavier-than-air-aircraft possible?

The answer is a combination of an ultra light airframe; a wide-span wing whose upper surface hosts an array of efficient solar cells; lithium-sulphur power storage technology; electric drive motors of unprecedented power-to-weight; and an advanced power management system.

After being hand-launched by a five-man team, Zephyr first climbed to about 60,000 feet (ft), where no clouds can obscure the sun. During daytime, electrical power from the solar cells was used both to drive the two propulsion motors and to charge the Li-S batteries. At night, propulsion was maintained using power drawn from the batteries. A small amount of altitude was allowed to be lost each night, and was made up again the following day.

Zephyr has a conventional platform of forward-located wing, fuselage and aft-mounted rudder and tail plane, but in every other respect it differs radically from other aircraft. In reality, it is nearly all wing, this single item spanning a whopping 22.5 metres (73.8 ft) and being ‘home’ to the solar array, battery and power management system.

The wing is connected to the tail plane by a minimal fuselage. To secure the required combination of high strength with low weight, the airframe structure is an assembly of thin-walled composite tubes made from high-modulus carbon fibre in epoxy resin. Wings and flight control surfaces are skinned with Mylar, resembling a high-tech cling film. A complete Zephyr weighs a mere 53 kg.

QinetiQ's lead designer for the Zephyr project, Chris Kelleher, notes that the aircraft's design owes as much to the organisation's background in space technology as to its aviation experience. Space expertise informed critical decisions over solar conversion technology, power storage management, weight minimisation of the aircraft and its payload, thermal tolerance (with temperature at altitude likely to range between -60°C and +40°C) and durability, especially in a high UV environment.

Virtually the entire upper surface of the wing is occupied by the solar array, a flexible thin-film laminated product developed by Michigan-headquartered United Solar Ovonic LLC (Uni-Solar). The laminate comprises strings of modules and Chris Kelleher, describes each module as being “paper-thin and roughly the size of an A4 paper sheet”. And he adds, “we have worked with Uni-Solar for about 8 years and they have developed these large, very thin solar panels especially for us. The cells are optimised for operation at high altitude and in conditions where the radiation is of a much more black-body character with high ultra-violet content.”

Each Uni-Solar module utilises triple-junction amorphous silicon solar cells in which red, green and blue light wavelengths are absorbed in different layers of the cell. By-pass diodes are connected across each cell, allowing the modules to deliver power even when partially soiled or shaded. Electrical energy yield is approximately 1.35 kW/m2. Uni-Solar claims that its cells suffer minimal decline in kWh output at high temperature unlike, it says, crystalline silicon modules.

An advanced power management system developed by QinetiQ's avionics team ensured optimised operation of the cells during the record-breaking flight. As Kelleher told Renewable Energy Focus, “it is important that the cells operate at or near their peak power point, so we have a peak power tracker that ensures this for all the cells in the strings. The power conditioning system optimises the operating point whether cells are working in propulsion or power storage mode.”

A proportion of the power developed each day is stored in laminar lithium-sulfur batteries that are installed as a thin-film sandwich within the wing. The batteries, custom-built by Arizona-based Sion Power Corporation, deliver some 350 Wh/kg, the highest power currently available for a rechargeable battery, according to Sion. Each individual cell delivers 2.1 V. It was important to select a battery chemistry that would permit recharging at temperatures as low as -60°C. Li-S meets this, as well as onerous power delivery requirements. Advanced electronic controls are used to maintain battery condition during flight.

Sion worked with materials specialist BASF to address an issue of limited cycle time that has been associated with Li-S technology. In particular, BASF protective layers enhance the durability of lithium foil and other electrode materials within the battery structure.

The aircraft is driven by two wing-mounted propulsion units, each comprising a brushless dc electric motor driving a two-blade propeller. The motors were developed by a team from Newcastle University's Centre for Advanced Electrical Drives. Team leader Barrie Mecrow, professor of electrical power engineering at the University, had previous experience with solar power, having designed motors for contenders in solar-powered races in Australia.

As Mecrow explained, “our brief was to produce an ultra-efficient motor having the minimum possible mass while still delivering enough power to drive the propeller. Every watt of lost energy matters so we had to develop new ideas to deliver one of the lowest-mass motors ever built.”

A key innovation was the use of a special new material developed in Japan for the motor laminations. (ED: further details were not being revealed at the time of writing.)

Zephyr is designed to carry an ultra-light payload enabling it to ‘stare’ continually at a large area of ground in an intelligence and surveillance role. Development and the recent flight were part-funded by the US Department of Defense through the Defense Advanced Research Projects Agency (DARPA), which seeks an alternative to expensive satellites as a means to meet intelligence, surveillance and reconnaissance (ISR) requirements, or as a communications relay platform. Chris Kelleher suggests that high-altitude solar planes like Zephyr will be able to fulfil these roles at about a hundredth the cost of the satellite option.

"Zephyr [...] is a combination of an ultra light airframe, a wide-span wing whose upper surface hosts and array of efficient solar cells, lithium-sulfur power storage technology, electric drive motors of unprecendented power-to-weight and an advanced power management system."

DARPA is aiming ultimately for a perpetual flying machine that can stay aloft for as long as five years, with a 450 kg, 5 kW payload. If this endurance could be achieved, maintenance and recovery costs could be made non-existent since, in one vision of the future, the aircraft would simply destroy itself at the end of service life, to be replaced by another.

Before DARPA's grand vision can be realised, though, battery technology, thin-film solar cells and electric motors must all show significant further improvement.

Vehicles like Zephyr are, nevertheless, showing the way ahead. Modest acquisition and operating costs, compared with satellites, are the lure. For Zephyr, whilst five people are needed for hand launching, when aloft the aircraft is operated by just two ground crew – although there would also be a mission controller and other staff dealing with mission data. The aircraft can be dismantled for stowing and shipping, along with a number of other Zephyrs, in a standard ISO 40ft container, a separate 20ft container being used for the ground equipment.

Project Vulture

Currently, the QinetiQ team has a less visionary but nevertheless challenging target in its sights, which will involve extending Zephyr's already impressive endurance. So far, QinetiQ has been working within Phase 1 of DARPA's Vulture project, the ultimate aim of which is to deliver a viable ‘atmospheric satellite’.

Last year DARPA announced Phase 2 of the project in which three teams that have developed independent solutions under Phase 1 are competing for the right to build, under DoD contract, a high-altitude unmanned aerial system (UAS) that can remain aloft for three months. Other teams are also being allowed to enter the bidding process at this stage so that no promising innovative solutions are overlooked.

QinetiQ is collaborating with Boeing Integrated Defense Systems as one of the three established teams. Team leader Boeing is likely to base its bid on a scaled-up Zephyr. Competitor Lockheed Martin's concept is a solar array-clad flying wing with 10 electric propulsion units and three tilting tail planes mounted on vestigial fuselages. This craft is intended to be carried aloft by balloon and then released.

The third contender, Aurora Flight Sciences, is proposing a Z-wing solution made up of three sections that will be launched independently but will link up at altitude to make the complete system. Each section is essentially a powered flat, oblong wing with a central canting tail plane attached via a strut system. The wing carries a solar array which, together with a power storage system, provides power to the section's three electric propulsors.

The novelty of the Z-wing concept is that, once joined, the three wing sections can alter their longitudinal alignment to each other so that the total system can conform to anything between a flat, continuous shape for minimum-drag flying at night, to a pronounced ‘Z’ so that surface area presented normal to the sun is maximised throughout the day.

Adjusting the ‘Z’ angles dynamically will, believes Aurora, enable Vulture to operate in low solar energy conditions, going some way towards meeting DARPA's wish for a vehicle that is operable even at the winter solstice at latitudes up to 45 degrees.

Sun or hybrid?

Solar power is, of course, not the only possible way to go. US pioneer AeroVironment Inc, for example, is relying on liquid hydrogen to power its Global Observer, a UAS designed to operate at up to 65,000 ft for five to 7 days at a time.

Visually resembling a glider but with an ultra-slim aft fuselage and four propulsors on its 175ft wing, it will relay communications or provide persistent surveillance over a 965 km (600 mile) diameter circular area. Other potential roles include storm tracking, crop management, disaster relief operations and GPS augmentation, plus maritime and border patrol.

"It was important to select a battery chemistry that would permit recharging at temperatures as low as -60°C. Li-S meets this, as well as onerous power delivery requirements."

The initial scale demonstrator, GO-1, is intended to carry a payload of 400lb but a planned successor, GO-2, will have a 250 ft span wing and carry a 1000 lb load. The scale version has been subjected to extensive ground testing and recently entered the flight test phase, having first flown on 5 August.

Initially, the flight test aircraft is being battery powered but Global Observer is designed to burn hydrogen gas in an internal combustion engine that drives an electrical generator supplying power to the four electric motors and payload. The hydrogen is stored as a liquid. Fuel cell power is envisaged as a longer-term possibility. Three Global Observers are being built for a US military operational assessment due to start once flight testing has been completed.

Affordable persistence is likewise Boeing's aim for its Phantom Eye UAS, publicly revealed in July. This unmanned plane is designed for operation at more than 65,000 ft with a 450 lb payload for some four days continuously, to be extended to 10 days in a successor version.

Like Global Observer, the glider-like aircraft is hydrogen powered but Boeing has chosen to burn the hydrogen directly in two converted 150 hp Ford petrol engines, mounted beneath the wing. The 150 ft span, 60-70% scale demonstrator is due to be taken to Edwards Air Force Base for a series of ground tests prior to an intended first flight in early 2011.

Boeing has gone even further in marrying conventional fossil fuel technology with alternative fuelling concepts in its Subsonic Ultra Green Aircraft Research (SUGAR-Volt) project, one of five designs it submitted to meet a NASA requirement for a 2030 aircraft designed to burn 70% less fuel and emit 75% less nitrogen oxide than today's airliners. Essentially a hybrid vehicle, like a number of contemporary road cars, Sugar Volt is a twin-engine 154-passenger aircraft powered by a combination of jet engines and electric motors.

The aircraft takes off under conventional fanjet engine power, though with take-off thrust augmented by application of electric drive to the fan. Once cruising altitude is reached, the jet engines are turned off and momentum is maintained by electric motors, which draw their energy from powerful batteries.

Although such an aircraft could in principle emit 65% less carbon dioxide and reduce Knox emissions by 85% compared with today's passenger jets, Boeing needs a quantum step in battery technology before the hybrid vision can become reality. To this end it has challenged the battery community to develop a system offering at least 750 Whrs/kg, several times what the best batteries today can deliver.

Boeing hopes that later this year NASA will select it to take part in the second phase of its N+3 (three generations beyond today's aircraft) programme. NASA is expected to select the recipient for a three-year contract, under Phase 2 of the programme, to mature the hybrid design. Boeing will work with fellow team member General Electric if it is selected.

Both AeroVironment and Boeing have experimented with solar-powered flight, so it may seem surprising to witness their apparent shift way from pure solar concepts to hybrid technologies. This is especially so for AeroVironment which is well known for having produced the Solar Challenger, Pathfinder, Centurion and Helios series of solar powered aircraft.

Helios, an enormously long (247 ft) flying wing with solar array, fuel cell and battery-based power arrangements, first flew in 1999. Initial flights were battery powered, but the aircraft gained its solar array the following year. The SunPower array featured a rear-contact cell design in which the wires were located beneath the cells so as not to obstruct the solar radiation.

In 2001, Helios set a world record by flying at nearly 97,000 ft, some two miles higher than any other winged aircraft had achieved, but was unfortunately lost in 2003 when it broke up over the Pacific.

Yet solar aviation is still a beguiling concept especially, as Zephyr has convincingly demonstrated, for the high-altitude long-endurance (HALE) role. Defence and other official agencies like NASA are actively pursuing both these and other technologies at present, with no certainty as to which concepts will prove viable in the long term.

Nor do solar aircraft have to be unmanned. Solar Impulse, produced by a European collaboration and based at the Ecole Polytechnique Federale de Lausanne, has become the first manned solar plane to have flown throughout a night as well as by day, powered solely by the sun. This happened recently when pilot Andre Borschberg, co-leader of the project team, flew the prototype for 26 hours, 11 minutes, averaging 23 knots and climbing to 8700 m (28,500 ft).

Some twelve thousand solar cells on the upper surfaces of the wing and tail plane provide power, via lithium-ion batteries, for the four 7.5 kW (10 hp) wing-mounted electric engines. Over 24 hours in the best conditions, the power train can deliver an average 6kW (8hp), about the same power as used in the Wright brothers' Wright Flyer of 1903.

Solar Impulse has a 64 m wing span and weighs approximately 1600kg. Construction of a larger version, with an 80 m wingspan – greater than that of an A380 'super-jumbo' passenger jet – was due to start this summer. This aircraft is to have a pressurised cockpit and avionics consistent with the team's next aim of achieving a manned solar trans-Atlantic flight and then a circumnavigation of the world.

About the author

George Marsh is a technology correspondent for Renewable Energy Focus magazine.

This article first appeared in Renewable Energy Focus, September-October 2010.

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