By Paul Grad, engineering writer
“There is enough energy in high-altitude winds to power civilization 100-times over,” professor Ken Caldeira says, an atmospheric scientist with the Department of Global Ecology of the Carnegie Institution for Science at Stanford, California.
Power available from the wind, harnessed by means of wind turbines, is proportional to the area swept by the turbine blades, the air’s density and the cube of the wind speed. The power density, i.e. power per unit area, usually expressed in kW/sq m, is proportional to ½ρV3, where ρ is the air density and V is the wind speed. It is therefore very sensitive to the wind speed and wind turbines are installed where wind speeds are highest.
All wind turbines now in use operate close to the ground. However, at high-altitudes, from hundreds of metres to several kilometres, wind speeds are far higher than close to the ground and the winds at high-altitude are, therefore, far more powerful.
In the past few decades, scientists and engineers around the world have developed many technologies to capture the kinetic energy from those high-altitude winds and generate electricity from them. Those technologies include covered tethered balloons, tethered fixed winged craft, tethered kites in simple or cross-wind flight, climbing and descending devices and rotorcraft.
One of the pioneers in the development of some of those technologies, professor Bryan W Roberts, director of Altitude Energy and formerly a professor of mechanical engineering at the University of Sydney and the University of Western Sydney, said: “In Australia and elsewhere, the winds at altitude are about 80-times more powerful and three-times more persistent than the winds available to ground-based wind turbines.”
Two major jet streams, the sub-tropical jet and the polar front jet exist in both hemispheres. These streams are a result of the combined effects of sunlight and the earth’s rotation. These winds are available in northern India, China, Japan, Africa, the Mediterranean and elsewhere. In the US, Caldeira and others have shown average power densities of about 17kW/sq m are available. Average densities can be as high as 20kW/sq m in a 1000km wide band around latitude 30° in both hemispheres.
However, there is, as yet, no commercial exploitation of that energy and no high-altitude wind power technology has, so far, come up with a prototype that has provided a solid record of electricity generation and associated costs.
Dr Nykolai Bilaniuk, CEO of LTA Windpower of Ottawa, Canada, said the fact no manufacturer is yet shipping product on a commercial basis makes it difficult to say how close we are to commercialisation. This is partly because almost every manufacturer is proposing a radically different technological approach.
“By contrast, the world of terrestrial wind turbines has long since converged on just one design with only very minor variations: the tower mounted, axial-flow propeller turbine rules the industry completely. Of the many possible wind turbine technologies, many were tried, but only that which is most economically-efficient survives,” he says.
Regarding high-altitude wind energy, the equivalent of the axial-flow propeller turbine has not yet been identified. Two basic approaches have been proposed. Mechanical energy can be transmitted from altitude to the Earth’s surface, where generators produce electricity on the ground. Alternatively, electricity could be generated aloft and transmitted to the ground with a tether.
The PowerShip from LTA Windpower Inc (ltawind.com) is an airship with wings providing additional lift. The wings also provide mounting places for turbines, flight control surfaces and landing gear. There are two generator nacelles, one on each wing. The propellers face the rear and are, therefore, on the wings’ trailing edges.
The tether is attached to the front centre of the wing. There is no need for a winch on the ground to deploy or retrieve the PowerShip since it operates close to neutral buoyancy. Units up to 50kW primarily for off-grid use will employ non-grid-synchronous permanent magnet generators, storage batteries and inverters. Larger grid-connectable units may use AC synchronous generators and blade-pitch adjustment.
Instead of winches or vehicles on the ground, the PowerShip only needs a solid tether point that can withstand the load, mostly wind drag, and a rotatable joint that prevents excess flexure of the tether. The anchoring system for the tether post may be embedded in a mass of concrete on the ground.
Bilaniuk says by using the minimum buoyant lift required to achieve neutral buoyancy and additional lift to offset wind loading, we require a lifting body a half to a third the size of that in a system that uses buoyant lift alone.
“Besides, the neutral-buoyancy approach eliminates the need for a ground-based winch and turntable, which is otherwise needed in all other designs, kite or buoyant. Also, our design produces constant power output in constant wind, like terrestrial wind turbines. Thus, we are clearly less expensive per unit output than other buoyant lift systems,” he says.
Roberts, of Altitude Energy said: “Our preferred option, for a variety of reasons, is a tethered rotorcraft – a variant of the gyroplane. Here, conventional twin or quad-rotors generate power from the on-coming wind, windmill style, while simultaneously producing sufficient lift to keep the entire system aloft, gyroplane style. An electro-mechanical tether is used to secure the craft to a ground point, while electrical energy is conducted at high voltage down the tether to the ground station.”
Roberts has proposed a quad-rotor assembly that gives superior flight control and reduced fatigue loads on the rotor assemblies, compared with a twin-rotor assembly. He says the four rotors are synchronised, using a cross-shafting previously proven to be satisfactory on a twin-rotor prototype.
“In the quad-rotor craft, differential collective pitch action is applied to the blades on each of the four rotors. This action allows rotor thrust variations on each of the rotors, to automatically control the craft’s four basic functions, power output, pitch, roll and yaw,” he says.
Altitude Energy’s business plan for commercialisation of the rotorcraft covers four steps. The initial step involves the detailed design, construction and demonstration of a quad-rotor craft, with rotors of about 2m in diameter. It would produce about 4kW of electrical power in a wind of at least 13m/s. This system would be demonstrated at an altitude of 0.5km.
Step two would involve operating with higher voltages at an altitude of about 4km. Subsequent steps would increase the scale of the units to give a single unit of electrical output of 20MW. A wind farm with 10 of those units would need airspace of about 10km by 10km and have a power output roughly equivalent to a modern electrical power station.
Another company harnessing high altitude wind energy by means of an airborne turbine is Altaeros Energy (altaerosenergies.com) of Boston, Massachusetts. The company was founded in 2010 at the Massachusetts Institute of Technology. The Altaeros buoyant airborne turbine (BAT) lifting platform is adapted to tethered aerostats. It integrates four main components. The shell, made of gas-tight and durable fabric, is inflated with helium. Aerodynamic lift combines with buoyant lift to keep the BAT aloft in both strong and weak winds.
The lightweight, horizontal-axis turbine generates electricity both when airborne and while docked on the ground. Tethers are used to connect the shell and turbine to winches on the portable ground station. Tether control is automated to adjust turbine altitude, stabilize the turbine in the air, and provide an electrical connection to send power to the ground. The ground station is built onto a trailer platform for easy transport. Winches on the ground station control tether speed and length, and align with the shell to prevent tethers from dangling.
Altaeros Energies has just announced an agreement with SoftBank Corp, a Japanese provider of mobile communications, fixed-line communications and internet services, under which SoftBank will invest
US$7 million in Altaeros to support the development and commercialisation of its BAT technology.
Makani Power (google.com/makani) of San Francisco, California, has developed a kite that is a tethered airfoil outfitted with turbines. The kite flies across the wind in vertical loops, fixed to the ground by a flexible tether. Air moving across the rotors forces them to rotate, driving a generator to produce electricity, which travels down the tether to the grid. The kite harnesses the wind energy by pulling against the tether while the kite travels in a circle, guided by the flight computer.
Makani Power is sometimes seen as the world leader in the development of airborne wind power systems, partly because it has received enough funding for extensive prototyping. On May 2013, Makani Power was acquired by Google, which has supplied much of Makani’s funding. However, according to Bilaniuk, this does not mean they are closer to a commercially-viable product, because they may be barking up the wrong tree.
NTS Energie- und Transportsysteme GmbH (x-wind.de) of Berlin, Germany, is developing the X-Wind (Cross-Wind) technology, combining automatically steered kites and generators on a rail system, to produce electricity. The company chose cable car systems to extract energy from the system. The grounded vehicles are connected via a steel cable laid vertically to the rolling stock. The energy generated by the kites is transmitted via the tow lines into the NTS grounded system.
The company’s CEO Uwe Ahrens says: “We have proof of concept for fully automated flying and production of electricity. Because we are only using standard components like tracks, bogies, electric motors, ropes and kites, we have been able to build a first serial plant. But most investors want to see a demonstration plant with a closed loop first”.
Ampyx Power BV (ampyxpower.com) of The Hague, The Netherlands, has developed the PowerPlane system to harness wind power by having an autopilot-controlled glider plane creating pull on a tether by flying repetitive cross-wind patterns at an altitude of 300m-600m. Conversion to electrical power takes place in a ground-mounted generator while the tether is extended. Once the tether is extended to full length, the glider dives to a lower-altitude and the tether is retracted. During this reel-in phase, tether tension is minimal and power consumption is only a fraction of the power produced during the reel-out phase.
To address low wind or extreme weather conditions, fully automated launch and land capabilities are integrated into the system, based on weather information fed into the system controller.
A system with the generator on the ground has been developed by KiteGen Research S.r.l. of Torino, Italy (kitegen.com). The wind is captured using Power Wing profiles whose movements are controlled by computer. Through cables the kites are anchored to a structure that rotates, generating electricity. The structure constitutes the turbine, and the kites function like blades of the turbine.
Some 20 kites can rotate a turbine of 1600m diameter at a speed of 15 revolutions per hour, generating 1GW of power. Thus, one cubic kilometre of sky can provide 1GW of power during 80 per cent of the time.
As with all of the many forms of renewable energy, the quest to harness high altitude wind energy has already led to significant advances in science and engineering, regardless of which of the many designs proposed and developed will ultimately see the light of day.