BRYAN ROBERTS has great plans for outback Australia. Somewhere in a vast swathe of land stretching northwest from Sydney, he dreams of building massive power stations churning out hundreds of megawatts of electricity. Although each of these stations would be huge-up to 28 kilometres across-you probably wouldn’t object if you lived nearby. In fact, you probably wouldn’t notice it at all.
Roberts is an engineer at the University of Western Sydney, and he’s hoping to build wind farms in the sky. He has worked out how to extract energy from the atmosphere with a fleet of whirring “gyromills” tethered more than 4 kilometres above the ground. Here, rivers of air called jet streams race around the globe. Compared to the fickle winds that waft about closer to the Earth’s surface, this air flow is a torrent, rocketing along at a top speed of 500 kilometres per hour. Over a thousand times more power is available in the jet streams than can be collected from even the windiest hilltop.
Sure, admits Roberts, running wind farms at this altitude won’t be simple. But this is more than just a wild dream. Roberts and his team have already built and flown a number of prototype gyromills-a cross between a helicopter and a kite. And after thirty years of research, Roberts is finally ready to send his fleet aloft.
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Roberts first got hooked on wind power during the oil crisis in the late 1970s. He soon realised that winds near the ground were far too fickle for a regular power supply. “With the Earth’s friction and all the ground turbulence down here, the surface is the worst place to put a wind farm,” he says. The perfect answer would be to put the generators in the jet streams which blast around the planet day after day.
These winds occur mainly at altitudes of between 8 and 12 kilometres. There are two main jet streams: one races around near the poles, the other circles the planet from west to east at latitudes between about 30 and 40 degrees, both north and south of the equator. This takes it over China, Japan and the US in the northern hemisphere, and southern Australia, South Africa and Argentina in the south. Jet streams pack in huge amounts of energy: an average of 17 kilowatts per square metre is available in the skies over upstate New York, for example, compared to an average of 0.4 kilowatts per square metre at ground level. “At altitude, the power available is up to 100 times the power available from any other energy source,” says Roberts, “and one of the best sites in the world is in western New South Wales. That really makes us the lucky country.”
The snag is getting the equipment aloft in the first place. A balloon large enough to raise rotor blades and generators many kilometres into the sky would be hideously expensive. Gliders or kites would also need to be huge. “To carry turbines with a total output of one megawatt, you’d need something roughly the size of a jumbo jet,” says Roberts.
But not if you use a gyromill, he realised, a type of unpowered helicopter that rides the wind like a kite. Ideally, you could use a lightweight machine with a skeletal fuselage, a small tailplane, and a streamlined cross-member resembling a pair of “wings”, all built from thin but strong metal tubing. Mount a rotor and a generator on each wing tip, set the blades at the right angle, then let the winds spin the rotors. For maximum strength and simplicity, the rotors should be single-bladed with a counterweight, says Roberts (see Diagram).
As they spin, the rotors create enough lift to keep the whole thing airborne. To stop it flying away altogether, the craft must be tethered to the ground by a set of strong cables. And as the rotors spin, they generate power that can be fed down aluminium wires embedded in the tethers to a ground station and on into the electricity grid. A machine with a pair of rotors the size of a large helicopter should easily churn out 10 megawatts, says Roberts.
He built his first gyromill in the early 1980s and tested it in the wind tunnel at the University of Sydney where he was based. Although the craft was fairly crude, his choice of generator-scavenged from electric drills-for the small rotors was fortuitous. The motors had excellent power-to-weight ratios and each generated a few kilowatts of electricity when the rotors were spinning.
Without a constant source of funding Roberts had to wait a few more years before he could finally get his craft off the ground. He and his team mounted a larger gyromill with a pair of 4-metre rotors on a trailer behind Roberts’s car and drove it to a disused airport. Then they zoomed up and down the runway to simulate strong winds and the tethered machine lifted a metre or so into the air. These tests proved that a gyromill of this size could generate enough lift to fly, as well as the power to provide electricity. But Roberts also learned a lot about the craft’s lack of stability. Because the machine swung around unpredictably, the researchers had to install metal shields on the back of the car to protect themselves.
The next step was to test their prototype on windy farmland in Goulburn, New South Wales, where it flew to a height of 30 metres. At this stage they decided to use five tethers to try to control the gyromill like a kite. “The device was always on the point of instability, always ready to move from side to side,” he says. More tests took place in the mid-1990s at the University of Western Sydney, where Roberts was appointed professor of mechanical automation engineering. He and his students installed a gyroscope on the craft to sense attitude, and a processor and software to automatically control the attitude and position of the tethered gyromill. “So we had lift, power, and now control and stability, too,” he says.
Since then, Roberts has concentrated on perfecting his designs, calculating the optimum power-to-weight ratios and constructing a business plan to convince energy-generating companies that his scheme is not just pie in the sky. His team has also analysed 30 years’ worth of wind data from the Australian Bureau of Meteorology which shows that over northern New South Wales, energy in the jet stream reaches 19 kilowatts per square metre 9 kilometres up. But he also realised that to get the best from his gyromills, they shouldn’t be flown too high.
Strongest winds
Wind power is greatest closest to the core of the jet stream, dropping off closer to the ground. But harvesting the wind power in the jet’s core would mean unmanageably long tethers that are expensive to manufacture and maintain. Considering this, and the costs of competing energy sources in Australia, Roberts has calculated that the best return on investment is to fly the craft at an altitude of about 4.5 kilometres.
He’s planning a fleet of ten 10-megawatt devices tethered above a field 28 kilometres in diameter. This would provide ample space to keep the machines more than a kilometre apart, a decent safety margin for emergency landings. Down below would be maintenance sheds and converters to connect the flying generators to the electricity grid. A wind farm of 10 machines would be equivalent to the energy output of a medium-sized power station, says Roberts.
Running a wind farm at this altitude has important advantages. People living close to the site wouldn’t hear the rotors spinning-a major problem with existing wind farms-and the gyromills themselves would be seen only as specks in the sky. If the wind farm had to be moved somewhere else for any reason, it would be fairly simple to shift the entire system. Try doing that with a traditional power station, says Roberts. This portability even allows the wind farm to “follow” the jet stream as it weaves from north to south during the year. “The devices could be mounted on a railway in outback Australia or tethered from a ship south of Japan to enable them to always be directly below the jet stream,” he says.
Despite the power of the jet streams, gyromills don’t need to be as robust as ground-based wind turbines. Wind farms sited on exposed hillsides or islands are often hit by gusty, turbulent winds that wear out their mechanical systems. However, high-altitude winds are far steadier, and even when they are turbulent a gyromill can ride the gusts, rising and descending on the wind like a kite rather than standing rigidly against it. This motion reduces the forces on the rotors and other mechanical parts. Metal fatigue will be less severe, so Roberts believes he can make his gyromills lighter and more reliable than ground-based wind turbines.
Even when the gyromills require a spot of maintenance-or when violent storms threaten-they can simply be winched back to Earth. Landing the craft and sending them up again afterwards will be easy, says Roberts. The generators can work in reverse: feed mains power up the cables and the electric generators will turn the rotors. Now the gyromill can be flown and controlled like a powered helicopter. This trick could even be used to keep the fleet aloft during short calm periods. During longer lulls, which Roberts’s team calculates will occur for a period averaging about 30 hours once every week, the gyromills can be landed.
Despite the potential for limitless green power, not everyone in Australia is so keen on Roberts’s idea. Pilots, for example, may not like his devices encroaching on their airspace. Even though an altitude of 4.5 kilometres is below the cruising height for passenger jets, Australia’s Civil Aviation Safety Authority (CASA) has expressed concern. Under current regulations, tethered objects can’t fly more than 100 metres above ground level without special permits. “Any equipment tethered to the height he is proposing would require adequate lighting at the very least,” says a CASA spokesperson. “Once this issue is addressed, CASA would then begin to explore airworthiness considerations for the craft and its moorings.” Australia is also one of the world’s biggest coal exporters, so Roberts expects some objections from industries with vested interests in maintaining the status quo.
Over the years, Roberts’s research has been funded piecemeal, but support for a large field trial for his gyromills has proved elusive. The Australian Greenhouse Office, the government department dealing with greenhouse matters, has rejected his applications for grants. Yet, to Roberts’s frustration, CASA won’t make his requests a priority before he has a finished craft. “It’s a catch-22 situation,” he says.
When funds finally materialise, Roberts envisages a craft with rotors up to 35 metres across, capable of producing 20 megawatts of electricity. He suggests a trial near Australia’s rocket launch site at Woomera in South Australia, where aircraft are prohibited. If successful, this kind of craft could supply communities across vast areas of the outback with power. But he’d settle for enough money to fund a small trial of three 50 kilowatt craft at a height of l kilometre-about A$1.6 million (£600 000). He even has an agreement from a local power company to connect these units to the electricity grid, should they fly.
Even if Roberts gets to test his machines up there, there are other problems that may shatter his dream. Peter Coppin, director of the Wind Energy Research Unit at Australia’s national research organisation, CSIRO, says there’s more to wind energy than wind speed and size of the rotors. “There’s a law of maximum yield that limits the amount of energy you can generate from a spinning propeller,” he says. “On top of this, there’s a question of reliability. But it’s the ratio of the cost of construction to the yield that is the real killer for this type of project.”
Still, Roberts is confident. He has completed a cost-benefit analysis showing that while the return on investment for a 50-kilowatt machine would be small, larger craft generating up to 20 megawatts would pay for themselves in a few years. “At current borrowing rates, the potential profit margin is very attractive,” he says. Others agree: “I think it could become cost-effective,” says David Eccles, an engineer formerly with the Australian energy company North Power. “This is tapping into a very large renewable energy resource. It’s worth a trial.”
Although Roberts’s main aim is to use gyromills to generate energy, once they are operational he believes these machines could be harnessed for communications or surveillance. “They would have a 200-kilometre line of sight, so they could be used to eliminate 95 per cent of mobile phone towers in these areas.” And if his craft succeed on Earth, why shouldn’t they work on other planets? He has suggested to NASA that his machines would be ideal for drawing energy from the Martian jet stream during a mission to Mars.
With most of his research now complete, he’s simply concentrating on proving the scheme is financially viable. “I know we can do it,” he says. “At this stage, the pen is mightier than the sword.”