Simple upgrade improves efficiency of Savonius wind turbines Monday, 23 May 2016

Finnish researchers have found a way to further improve the efficiency of the Savonius wind turbine, a vertical-axis wind turbine which consists of two identical semi-circular straight blades which are assembled symmetrically around the centre of rotation of the rotor. It converts the force of wind into torque, increasing its applicability for use in cities.

Originally invented by engineer Sigurd Johannes Savonius in 1922, the Savonius turbine's scoops are designed to experience less drag when moving against the wind than when moving with the wind.

Savonius turbines are cheap and reliable, with a simplified design that requires little or no maintenance. Unlike horizontal axis wind turbines, no pointing mechanism is required to allow for shifting wind direction, and the turbine is self-starting.

Other advantages of the vertical axis Savonius wind turbine are low noise level, and the ability to operate with low wind speed.

However, they are also less efficient — for their size, they are able to extract much less of the wind's power than other types of turbines.

It is this efficiency that Svetlana Marmutova from the University of Vaasa targeted. Marmutova studied the performance of the Savonius wind turbine under steady and unsteady wind conditions, looking for ways to improve its aerodynamic efficiency using computational fluid dynamics analysis.

According to Marmutova's doctoral thesis, the power coefficient (the ratio of the power extracted by the rotor to the available kinetic power in the air flow) depends on the level of gustiness — the more gusty the conditions, the more stable is the power coefficient.

This makes the Savonius turbines ideal for use in cities, where the nature of the wind differs greatly from wind conditions in typical onshore and offshore locations of wind farms. The relatively dense presence of obstacles like trees and buildings in urban areas generates significant turbulence within the wind currents, resulting in reduced average wind speeds, but an increased level of gustiness.

“[The] Savonius wind turbine can provide a good solution for wind power generation in cities due to its size and less sensitivity to the changes in wind speed and direction”, says Marmutova.

In order to improve the aerodynamic efficiency of a Savonius wind turbine, Marmuitova proposed adding a plane to in the centre of the rotor, in order to decrease the pressure behind the advancing blade, thus increasing the lift force. This relatively simple mechanical modification, Marmutova showed, could increase the power coefficient of the rotor by around 8 percent.