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.

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.