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Low efficiency and high noise levels are two of the main problems associated with wind turbine rotors, claims Johann Poschl of Kapfenburg in Austria, the inventor of a new type of small wind energy converter. The "Whisperwing" uses a spiral form of blade that rotates about a horizontal axis and looks rather like a Darrieus turbine laid on its side -- but with a spiral blade. The turbine, it is claimed, has low tip losses and the chosen blade form optimises chord profile with the airflow, enhancing the efficiency, says the company.

When generating, the device begins to produce power at lower wind speeds than conventional wind turbines, according to its maker. A further claimed advantage is the compact size of the unit with its lower visual impact.

Initially the rotors are being offered in three sizes up to one metre in diameter, and rated at up to 750 W. The designer claims the device can be produced in diameters up to 16 metres. The rotor is apparently thought to be much less sensitive to wind direction than those on traditional wind turbines, but the Whisperwing is in any case self aligning, it is claimed. The rotor can be also be reversed and used as a fan.