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Evolving Trends in Wind Power Generation

Wind Turbine Under a Blue Sky
Wind power generation is expanding worldwide with the tower-mounted 3-bladed design being the most popular wind power conversion technology. However, the variation of designs and range of output level is expanding as researchers and developers continually seek innovative ways to reduce the cost per kilowatt. Small-scale designs that cover the range of 200-watts to 5-kw is the fastest growing segment of the wind power market. Much of the technology in this power range can be mass-produced, including the bladeless solid-state piezoelectric device where wind-induced vibration produces electric power.

Many homeowners worldwide are installing various small-scale wind power devices on their properties. Developers of tower-mounted and roof-mounted small-scale wind power systems have sought ingenious ways to raise output by improving the conversion efficiency. While many of these technologies are being mounted on towers of 30-feet to 60-feet in vertical height, many home and building owners are installing various small-scale wind power devices directly on to buildings. While some newer buildings have been purposefully designed to carry a wind power technology, the structure of most homes and buildings were never designed to withstand the static and dynamic stresses imposed by wind power technology.

Internationally, there are numerous developers of small-scale wind power who are seeking to access energy more powerful winds that blow at higher elevations of up to 3000-feet or 1000m. Research into high elevation wind power and the construction of various prototype technologies has been ongoing for many years. Most small-scale airborne wind power technology combines a kite that flies between 500-feet and 3000-feet elevation with ground-based electrical generating equipment. The kite converts wind energy to a cyclical force that repeatedly pulls a tether that rotates a generator while winding up its powerful return spring.

Ampyx of Holland has developed a wind-powered glider that continually flies in a figure-8 loop and produces a cyclical force that activates a spring-loaded, ground-based generator that produces up to 10kW output. Kitelab in the U.S. uses a kite that flies side to side and pulls on a tripod tether system that divides into 6-cables near ground level. The cables are routed around a circular arrangement of pulleys that are secured to the ground. Regardless of wind direction, the cables can almost continuously drive either a vertical crank lever or a series of one-way clutches to continuously rotate a generator capable of up to 20kW output in rural settings.

Large-Scale Wind Power:

In large-scale wind power conversion, the 3-bladed Enercon E-126 with the hub at 100m elevation and blade diameter of 126m (413-feet) presently provides the highest output at 7MW. The 3-bladed design is known to produce a low-frequency sound wave whenever a blade passes the tower and there is the danger of birds being struck and injured by a moving blade. Residents of coastal neighborhoods oppose the offshore installation of the turbines for aesthetic reasons.

Despite growing opposition from neighbors to tower-mounted 3-bladed wind turbines, there are plans to install many new such turbines at wind farms over the next several years. At many locations around the world, government programs encourage the development of renewable energy and the tower-mounted, 3-bladed turbine is often the only design of megawatt-size wind turbine that is readily available. Despite the popularity of the tower-mounted 3-bladed wind turbine, wind energy researchers are developing competing designs of wind power technologies.

Such technologies include large vertical-axis wind turbines and even include a transverse-axis turbine from BroadStar Wind Power. One British wind energy group has illustrated a tower-mounted vertical-axis wind turbine that can extend to a height of 450m above ground level. A research group in China has revealed a mega-scale vertical-axis turbine that is mounted on magnets and is believed to be capable of up to 100MW output in powerful winds. Other wind power technologies include airborne and various forms of hybrid concepts.

Large-Scale Airborne Wind Power:

While developers of small-scale wind power technology seek to access wind energy at elevations of 500-feet (150m) to 3000-feet (1000m), there is a growing group of researchers and developers seeking to access wind energy from much higher altitudes. Groups such as Makani Wind Power, Magenn Wind Power, SkyWindPower and JobyEnergy seek to generate electric power at high altitudes and transmit that power to ground level using specialized tether technology. The LadderMill group from Delft University in the Netherlands seeks to combine ground-level generation with kites attached to an airborne conveyor system.

SkyMill Wind Power (USA) is apparently exploring the relative merits of both systems. A research group in the southern U.S. seeks to develop a reciprocating kite-balloon that would tap energy from the jet stream and activate a ground-level electrical generating system. SkyMill, SkyWindPower and JobyEnergy seek to use rotors that will simultaneously keep the technology aloft while driving electrical generation equipment. Magenn's technology is an inflatable balloon that spins on a transverse axis while Makani Wind Power uses a kite to keep rotors and generating equipment aloft.

Several of the aforementioned groups have built prototypes of their technology that have flown and produced electric power. While much of this technology is still in the prototype and developmental stage, it does show potential for future power generation at competitive cost per megawatt. It will likely be several years before commercial development of large-scale installations of airborne wind power. Such technology would likely have to operate in designated "no-fly" zones so as to assure the safety of commercial air transport operations.

Terrain-Enhanced Hybrid Wind Power:

The nature of the terrain where wind power is installed or could possibly be installed can increase output while reducing cost per megawatt. The roof mounted wind turbine is an example of a terrain-enhanced form of hybrid wind power as the slope of a roof can accelerate wind velocity. On a larger scale, the nature of ravines and valleys in mountains can increase the output and cost effectiveness of tower-mounted and other possible designs of wind turbines.

There are coastal mountains where cable-suspended aerial tramways and/or cable cars carry passengers between stations at vastly different elevations. Powerful winds often blow from the ocean and up the many ravines that occur along the sides of coastal mountains. The nature of many ravines can accommodate the installation of the cable systems for an aerial tramway. There is scope to modify the design of an aerial tramway cable system to allow for the operation of the LadderMill wind power system developed by researchers at Delft University in the Netherlands.

The LadderMill is essentially a conveyor system of kites that keep the system aloft while capturing wind energy to generate electric power. When the LadderMill is positively secured to the top of the sidewalls of a ravine, the kites can be redesigned to transmit more energy through the cables of the conveyor system to the electrical generator. Turntable systems may be installed at upper and lower elevations to allow the modified LadderMill to be steered so as to capture wind energy as wind direction changes.

Coastal mountains often have valleys where powerful winds may blow. At some locations, the valleys may be very wide or of low height and allow for the installation of tower-mounted, 3-bladed turbines. A boundary layer effect at valley entrances can often steer prevailing winds into the valley. Those winds can accelerate to higher velocity as the valley cross-sectional area reduces. It is possible to install the suspension cable systems of bridges across valleys of sufficient height and appropriate width.

That cable suspension system could support the weight and operation of a modified LadderMill that has been tilted by 90-degrees to operate on a vertical axis across a valley, at an elevation of over 250m (800-feet) above the valley floor. The top of the 60m (200-ft) high airfoils could reach an elevation of over 300m (1000-feet) and convert energy from a wind stream of 4000-feet (1200m) width blowing at 45-mi/hr (66-ft/sec or 20m/s). At a conversion efficiency of 35 percent, the installation could generate up to 125MW on generators mounted into the valley walls.

The transverse-axis turbine from BroadStar Wind Systems is an existing wind power technology that can also be suspended by cables at high elevation across a valley. The transverse axis system allows multiple turbines to be coupled into a group of 10-turbines that drive into a single electrical generator mounted near the valley side. Several rows of turbine groups that operate at different elevations may be linked via a drive system to allow up to 30-turbines to drive into a single generator. There are many locations around the world where such technology could operate at competitive cost per megawatt of output.

Conclusions

While most wind power installations use the classical tower-mounted 3-bladed turbine, other designs that promise greater output at lower cost, that produce less noise and that are less harmful to birds are beginning to appear. There is expanding home-based application of small-scale wind turbines where homeowners seek to reduce dependency on often over-taxed power utilities. Within the next few years, that segment of the market would likely expand to include airborne kite-based technologies.

Commercial development of terrain enhanced wind power generation is a possibility as some of the technology that is required for such installations exists and is already proven. Commercial application of large-scale, low-altitude (1000m or 3000-feet) wind power conversion may be possible within the next few years. Similar application of high-altitude airborne wind power technology may be at least a decade into the future and will depend on the willingness of aviation regulators to declare designated no-fly zones to allow such technology to operate.

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