Arguably the antecedent of today's horizontal-axis wind turbine was the 1.25MW Smith-Putnam turbine. Built in the early 1940s in the US, the world's first wind turbine over 1MW operated for 1,100 hours before one of the two 20-metre blades broke off and was propelled over 200 metres away.
Shortly afterwards, the project - which had cost over $1 million - was cancelled. Among the reasons given for this decision was that its manufacturer, S Morgan Smith, did not believe wind turbines would be profitable.
Evidently, he was not alone. It took until the late 1970s for several larger experimental turbines to be built.
Many commonplace products were preceded by the experimental and impractical - and even failure. One only has to look at other sectors to find examples, such as the evolution of the late 1980s brick-sized cellphone to today's smartphone, of disruptive technology moving from the 'wacky' to the mainstream. With this in mind, there is no reason to suppose that even today's common horizontal-axis turbine will continue to look the same.
A bigger, more cost-effective design could be being trialled right now. Providing a generator can be hooked up, energy can be harvested from any type of material that is moved by the wind. This has been borne out by a number of designs that have been publicised in recent times - usually alongside extravagant claims about capacity. But there are often substantial challenges involved in making the concept a reality, as proven by the popularity of the horizontal axis turbine.
One company that has not been put off by this is Google. As part of its multi-million-dollar renewables investment, it has pumped $5 million into Makani Power, which is working on a high-altitude wind design reviewed on the following pages. Makani claims it can reduce the cost of energy by as much as half.
Some of the new ideas reviewed overleaf have taken a fresh look at old designs. The Aerogenerator X is a 10MW vertical axis offshore turbine, simpler than today's models and dispensing with the need for yaw control. The WindWing - an idea born while the inventor's elbow was sticking out of the car window - is nothing less than an attempt to replace the three-blade horizontal wind-turbine concept.
The problem for these new products is that current wind technology is tried and tested - and it has taken a long while to get there. A typical philosophy is that of Siemens, which manufactures the world's biggest-selling offshore turbine, the 3.6MW 120, and is currently working on the 6MW. Many of the advances by Siemens' CTO Henrik Stiesdals' team are based on evolving the previous models, despite a switch to direct-drive technology.
Advances are made slowly because the worst thing that can befall a turbine is catastrophic failure. Track record is arguably more important than capacity, because even a 20MW turbine is useless switched off.
Or, in the words of one commentator, it is simply about three things: track record, track record and track record.
But rightly or wrongly there is a high value in new ideas, if only to sharpen the minds of those working with conventional technology and open them to different ones. Then again, maybe one of the designs overleaf will produce the wind energy of the future.
Theo Bird, founder of UK-based Wind Power Limited, is leading development of the 10MW Aerogenerator X. The vertical-axis Darrieus-type design expands on the classic eggbeater and more recent H-shape rotors. Essentially, it features a giant 270-metre diameter V-shaped rotor with a double winglet at each blade tip.
Each blade from tip to centre measures over 160 metres, and installation total height measured from the sea's surface is about 130m. Both fixed and floating support structures are being considered. Wind Power says the first machines are due to be produced in 2020.
A key advantage in using vertical-axis rotors is the ability to capture wind from all directions without needing a yaw system. Because the blades do not rise and fall against gravity, they can be larger and are easier to maintain.
One of the big challenges linked to large vertical-axis turbines is effective fail-safe power-output control. Through even the most extreme weather conditions the Aerogenerator's huge rotor torque has to be controlled within tight margins.
Two possible output control alternatives are being considered. The most likely is both a variable speed and a combination of fixed-angle stall-type blades with electronic-output control - also known as rotor-speed control. But there is also a second option of active power-output control via pitchable blades.
The Aerogenerator X's product development is still at an early stage. The design combines a radical innovative high-risk approach with prospects of future competitive advantages - for example a simple design without a need for yaw - against established wind turbines. However, Darrieus-type wind systems face an inherent and substantial aerodynamic efficiency disadvantage compared with horizontal-axis turbines - in practice maximum values of around 38% set against 45-50% or higher.
Horizontal-axis designs, which dominate the offshore market, have the advantage of a solid track record. For the Aerogenerator X, making up for the lower efficiency and technology-related challenges represents only one side of the coin. Convincing risk-averse financial parties that choosing the wind turbine might offer superior cost-of-energy performance might be the more formidable task.
Chance of success: Fair
Selsam Sky Serpent
US engineer-inventor Doug Selsam is the founder of California-based Selsam Superturbine. He has designed several unusual wind-turbine models, each comprising two or more rotors equally spaced along a shaft that is as stiff as possible. The generator is located centrally or elsewhere.
With the generator at the centre, the outer-shaft ends are not supported but rotate freely and keep themselves aligned. The application of several small fast-turning rotors allows higher rotating speeds using a direct-drive generator. Another distinct and shared feature is the shaft angle, which is inclined either relative to the horizontal plane or the approaching wind. In both cases the design aims to enable undisturbed airflow through individual rotors. Selsam says he has sold 20 of the 2kW twin-rotor SuperTwin model.
Building on this, Selsam demonstrated a small-size prototype of a new radical design called Sky Serpent at a 2009 US high-altitude wind-power conference. A model built on a pickup truck comprised a central pole with a direct-drive generator attached at the top. One side of a highly flexible shaft is connected to the generator while the remaining shaft length is kept airborne by balloons.
Analogous to other Superturbine models, the Sky Serpent design again seeks to achieve undisturbed wind flow through all individual rotors. A key operational difference is that the relative position of the Sky Serpent's shaft changes continuously.
Explaining some key future challenges, Selsam said: "One main overall difficulty is power-output control, which initially resulted in several burned generators."
Selsam's long-term project goal is tapping into high jet-stream winds with large optimised installations. But the project development appears in a very early stage with many issues still unsolved.
Chance of success: Poor
According to its website, Atelier DNA won second prize in a 2010 "land art generator" competition organised by the United Arab Emirates as part of Madras city development.
The Windstalk design consists of 1,203 stalks that are 55 metres high and anchored with concrete bases ranging between ten and 20 metres in diameter. The stalks are carbon fibre reinforced composite poles, 300 millimetres in diameter at the base and 50 millimetres at the top. LED lamps light the final half-metre section, in between glowing and dimming, depending on the extent the poles are swaying. When there is no wind the poles go dark.
The stalk bases are shaped like non-identical vortices and form a multi-row array with each line "following a logarithmic spiral".
It is possible to walk in between. During rain, the water slides down the base slopes of the stalks and is collected and stored in between, allowing plants to grow.
Each hollow pole incorporates a stack of piezoelectric ceramic discs with electrodes in between. Piezoelectricity is a phenomenon that can be explained as a means to produce electricity from continuously changing mechanical pressure (compression and relief cycles) during motion. When wind force sways the poles a current flows through the electrodes and in turn powers generators housed in each stalk base.
To compensate for wind-power fluctuations, Atelier DNA envisages a hydro energy storage system, comprising large underground basins at different elevations and pumps with dual-mode motor-generator functionality.
The designers describe the project as conceptual yet based on a set of systems that already exist and work. This seems an optimistic statement. In reality there are multiple risks and uncertainties to consider, including the lack of wind-capture potential inside the array, materials and energy input, power conversion and energy storage.
It is difficult to imagine the Windstalk becoming a sustainable and viable clean energy-generating overall solution offering acceptable lifecycle costs.
Chance of success: Poor
Makani Airborne Wind Turbines
US-based Makani Power has developed a high-altitude power-generating device based on a tethered rigid wing with multiple turbines. Makani Airborne Wind Turbines (AWTs) will fly between 250 and 600 metres, where the wind is stronger and more consistent compared with lower altitudes. The company has received a $3 million award through the US Department of Energy's Advanced Research Project Agency-Energy programme. Makani says it is close to completing the first phase of its development, when all flight modes were demonstrated using a 30kW Wing 7 prototype.
The next stage is described as a 600kW AWT for utility-scale generation and at a cost below conventional solar and wind.
An AWT comprises three main elements. The first is a fixed carrier shaped like an airfoil that creates lift for keeping it airborne as well as enabling forward movement. The device travels on a circular path, controlled by integrated wing flaps operated by an on-board computer.
The second element involves pairs of wing-mounted rotors that capture wind flow across the wing. This drives rotors coupled to direct-drive generators similar to some types of conventional wind turbines. The final main element is a cable tether, made of a core of high-strength fibres surrounded by conductors. This flexible tether connects the wing during operation and transmits power to a ground-based collection/distribution station.
These airborne turbines have three distinct functional modes. Before launch and in unfavourable conditions the wing is stowed at the top of a fixed spar buoy. The wing launches and lands by hovering like a helicopter, and during flight it follows a circular path.
A possible operational benefit an AWT offers compared with an inflatable membrane wing with a ground-based generator is that it produces power through well-known continuous rotation. However, it presents potential disadvantages including a power-transport cable that can be struck by lightning, and concerns about safe landing in the event of system failure.
Despite being in an early development stage, Makani's brainchild does seem to offer considerable potential as a viable commercial wind technology for the future.
Chance of success: Good
Airborne Wind Energy
Since 2005, Delft University of Technology in the Netherlands has been developing Airborne Wind Energy concepts that can access altitudes of 150-400 metres and higher. The research has been driven by the fact that wind-power density is a function of wind speed cubed. Since the wind gets stronger and more persistent with increasing altitudes, these innovative concepts can potentially reach capacity factors up to 60%.
As altitudes above 150 metres are beyond the reach of fixed structures, the system uses tethered aerostats or rigid wings incorporating rotating power-generating devices, or tethered rigid or flexible wings. The energy is produced via a pumping cycle or up-and-down motion. The latter employs ground-based generators.
Roland Schmehl is an associate professor at Delft University and has headed the project team since 2009. His design utilises an inflatable membrane wing with a ground-based generator. Calling it a third-generation wind-power system, Schmehl explains that a 20kW demonstrator has been field tested since 2010.
Elaborating on specific features he adds: "The demonstrator uses a single non-conducting cable connecting a 25-square metre kite to the ground station, which houses a cable drum connected to a dual-mode electric generator motor unit plus battery storage. Each pumping cycle involves two phases. In the reel-out (going up) phase power is generated. A small amount of this energy is then stored in the battery and reused to reel in the cable in a second phase."
The control unit suspended below the kite incorporates two remote-controlled micro winches. These are used to steer and rotate the complete wing, which is functionally comparable to the rotor-blade pitch control of horizontal-axis turbines.
The biggest challenge is launching the kite and bringing it down safely. Current research is focused on optimising overall functioning and automated flight control, automatic launching and landing. Another challenge is optimising the power-conversion system.
The design could be a success if these challenges are overcome. And, showing a sign of progress, the group is collaborating with Hamburg-based company SkySails, which has a track record in marketing similar technology for ship propulsion.
Chance of success: Good
W2 Energy Development Corporation's WindWing design was formed when the inventor Gene Kelley put his arm out of the car window. When his thumb was tilted upward, wind force pushed the arm up. As it tilted downward, the wind pushed it down, creating a discontinuous up-and-down motion.
The WindWing design currently exists as a scale model prototype, which consists of a central tower with a pivotable lever attached in two opposing sections of different length.
The design closely resembles that of piston-type water pumps. The main difference is that the lever is moved up and down by wind force rather than by hand. Two or more vertically positioned interconnected pivotable airfoil-type wings are attached to the outer end of the longest section.
The shorter opposing side contains a counter-balancing weight and a connection for the energy transfer rod. This rod can be coupled to a reciprocating generator pump or to a compressor.
If wind flow hits the upward-tilted wings, they are thrust upward simultaneously pushing the transfer rod down. When the wings reach the top, a position sensor is tripped and the orientation of the wings tilts downward, creating an upward movement of the rod.
The Wind Wing contains several unfavourable design features. These include discontinuous operation, complex pivot points that are prone to fast wear versus rotation, and a likely need for uncommon and expensive linear generators.
Kelley claims WindWing has a 40-60% wind-power conversion efficiency describing it as a superior solution compared with conventional three-bladed wind turbines.
But Kelley wrongly states that conventional turbines can capture only 5% of the energy in the wind, arguing that the bulk escapes unused through the open three-bladed rotor plane.
It is difficult to think of any technical, operational or cost-of-energy-related advantages the WindWing would offer compared with conventional turbine designs.
Chance of success: Poor.