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The blade's the thing

As demand for renewable, clean energy grows, blade design is receiving greater attention as researchers attempt to improve efficiency and reduce costs, writes Anne Feltus

AT THE growing number of wind farms around the US, gigantic rotating blades atop tall steel towers dominate the landscape. Although the technology for harvesting the energy of wind dates back several centuries, these sleek, modern structures, which are clustered and connected to produce electricity on a commercial scale, are exponentially more efficient than the ancient windmills that were used to pump water or grind grain.

Efforts continue to reduce the cost and increase the generating capacity of these wind-conversion systems. Much of that focus has been on improving their blades – the only component of wind turbines that, according to the Sandia National Laboratories, is designed and manufactured uniquely for wind energy applications.

Better blades

Typically, increasing the size of a wind turbine's blades also increases the amount of energy that can be harnessed from the wind. However, as blades become bigger, their weight can put a strain on the gearbox. One of the largest blades on the market is almost 61.5 metres long; the three blades of the wind turbine cover an area almost the size of two football fields and can generate enough power for almost 5,000 households. Yet, at less than 18 tonnes, the turbine is relatively lightweight.

The LM 61.5 P blade was developed by LM Glasfiber, a Danish company – the largest blade-manufacturer in North America – in close collaboration with Germany's REpower Systems. Its modest weight results not only from LM Glasfiber's FutureBlade fibreglass and carbon-fibre technology, but also from a design concept that allows the blade to be lengthened without increasing the diameter at the blade root at the centre of the rotor.

Thin optical fibres, incorporated into the laminate on the lower half of the blade and connected to specially developed software, continuously monitor the conditions on the blade, such as load, temperature, damage and lightning strikes. Data gathered by the LM BladeMonitoring System can be used to improve reliability and allow more efficient preventive maintenance, which, in turn, reduces costs.

The design of a blade developed by Sandia National Laboratories in collaboration with California's Knight & Carver enables wind-farm developers to use a longer blade without reducing blade life. Unlike conventional straight blades, the Sweep Twist Adaptive Rotor, or Star blade, has a gently curving tip. This allows it to twist more in wind gusts, reducing the fatigue load on the blade.

Made of fibreglass and epoxy resin, the Star blade can function cost-effectively at sites with average wind speeds of 13 miles an hour (mph) – lower wind speeds than are required for most wind turbines. That means more sites would be available for potential wind farms, because locations with average wind speeds of 12.5-13.4 mph are almost 20 times more abundant than high-wind-speed sites, according to the National Renewable Energy Laboratory (NREL).

Four types of Star blade manufactured at Knight & Carver's facility in Howard, South Dakota, are undergoing field testing; initial results are expected by April. Knight & Carver plans to begin production later this year.

A wind turbine blade designed by Canada's WhalePower also targets areas with lower average wind speeds and was inspired by the shape of a humpback whale's flipper. Teeth-like protrusions along the front edge of the blade that give it a serrated appearance also channel the airflow more efficiently, increasing its lift force, its developer says. The company expects to finish testing the blade this summer and begin manufacturing it early next year.

Before the end of this year, Nordic Windpower plans to begin producing a turbine that uses two blades instead of the traditional three. A 30-year, $75m investment by the Swedish government in research and development led to the design of four test turbines, one of which has operated for 11 years with only minimal maintenance and no significant component failures.

The benefits of the two-blade concept are that it reduces materials, manufacturing and transportation costs and is easier and quicker to install. The concept of a two-bladed turbine is not new, but previous versions were noisy and had problems with fatigue load. The Nordic Windpower model has been engineered to respond flexibly to wind gusts, which "dissipates the force of turbulent wind, preventing the progressive stress that can start with blade-rocking and end in gearbox failure – the Achilles Heel of rigid turbine technology," according to Steve Taber, the company's chief executive. It also rotates at lower speed than that of its predecessors, reducing gearbox wear and tear, and noise.

Departing from tradition

Some new rotor designs bear even less resemblance to their predecessors. FloDesign Wind Turbine is developing a Mixer/Ejector Wind Turbine (MEWT), which, says FloDesign, has the potential to generate over 50% more power at significantly less cost than a conventional, three-blade, propeller-type, or horizontal-axis, wind turbine (HAWT).

Based on advanced aerospace technology, the MEWT looks more like a jet engine than a traditional windmill. Instead of three large rotating blades, it has more, smaller, faster-spinning blades that are surrounded by ringed devices called cambered shrouds. As wind is drawn into the turbine by a mixer/ejector pump, the shrouds redirect it towards the blades. This increases the volume of wind that flows through the blades, which generates more power. According to FloDesign, the MEWT "could even be used at severe sites where wind gusts and variability make HAWT installations impractical".

Another design that departs from the HAWT comes from Terra Moya Aqua (TMA), which has developed a turbine that employs a vertically oriented rotor instead of a horizontal-axis propeller. According to TMA, this configuration has several advantages. It is "the only wind turbine that has the ability, because of its unique configuration, to increase the speed of the wind as it reaches the power-producing blades."

While conventional wind turbines typically must be shut down when winds speeds exceed 50 mph, TMA claims its version is engineered to withstand winds of up to 70 mph for 10 minutes at a time. That would certainly prove advantageous, because each doubling of wind speed results in an eightfold increase in available energy.

 

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