Wind turbines for low wind speeds defy Betz limit efficiency

Many designers of utility scale wind turbines dismiss the idea of using turbines to capture energy from light breezes and in areas where there isn't a lot of wind. Their argument comes from Betz's law which calculates the maximum power available from the wind. The law uses principles of conservation of mass and momentum of the air stream flowing through an idealized cylinder that extracts energy from the wind stream. Importantly, it assumes the use of aerodynamic propeller blades in the turbine. Betz's law says no turbine can capture more than 59.3% of the kinetic energy in wind. Practical utility-scale wind turbines can usually get to 75% to 80% of the Betz limit. So at low wind speed, there just isn't much energy left to harvest.

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But one way to get around the problem at low speeds is to avoid using aerodynamic blades and instead use something that looks more like a water wheel than a traditional wind turbine. That is the approach used by a Dutch firm called Archimedes for its Liam F1 Urban Wind Turbine. The turbine blades are in the shape of an Archimedes screw, notable in that hydro systems often use Archimedes screw designs in pumping systems characterized by low head pressures. They also typically have an efficiency curve that is flat across a wide range of flow rates.

archimedes turbineMarinus Mieremet, cofounder of Archimedes, puts it this way: "Generally speaking, there is a difference in pressure in front and behind of the rotor blades of a windmill. However, this is not the case with the Liam F1. The difference in pressure is created by the spatial figure in the spiral blade. This results in a much better performance. Even when the wind is blowing at an angle of 60 degrees into the rotor, it will start to spin. We do not require expensive software: Because of its conical shape, the wind turbine yaws itself automatically into the optimal wind direction. Just like a wind vane. And because the wind turbine encounters minimal resistance, it is virtually silent. “ Mieremet also says the efficiency is about 80%.

The wind turbine cuts in at about 4.5 mph. Its maximum output is 1.5 kW which it reaches when winds hit a little over 11 mph. The firm says it is targeting home owners with the Liam. 

Also, Archimedes isn't the first wind tubine firm to go with a design principle based on that of water wheels. Wind Sail Receptor in Nevada has a pinwheel-like design that functions more like a water wheel than like an aerodynamic turbine. 

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