Air flow, penetrating wind turbine, creates a pressure force on blades to lead the turbine to rotation, thereby converting the kinetic energy of air flow into mechanical energy. Wind turbine can have a horizontal axis of rotation (propeller) or a vertical rotation axis (Savonius, Darrieus, and their hybrids). The main performance of any wind turbine is the conversion coefficient of wind energy into mechanical energy, which characterizes wind turbine efficiency.
According to the classical theory (the law) of Albert Betz ideal propeller is able to convert the kinetic energy of wind into mechanical energy with the maximum conversion coefficient 59.3%. The ideal propeller – it disc with an infinite number of narrow blades, rotating at an infinitely high frequency, with the plane of the disk should be strictly perpendicular to the direction of the wind.
Real propeller has a finite number of blades and a finite speed of rotation. In the real central part of the propeller (the center of the disk) cannot participate in the conversion of wind kinetic energy as in this place there is no blade – a place occupied by the propeller hub and nacelle of generator. In the real propeller is overflowing air flow through the ends (tips) of the blades, which does not allow the ends of the blades to participate in the conversion of wind energy. Plane (disk) of the real propeller in real wind conditions do not positioned strictly perpendicular to the oncoming flow.
At present, the best real propeller in real conditions, when the propeller disc does not have time to located perpendicular to the direction of the wind, when the airflow is turbulence, when the density and temperature inside of airflow are constantly changing, cannot convert more than 35% of the kinetic energy of wind. A propeller turbines have practically reached the limit of aerodynamic and technological excellence, and increase their productivity is no longer possible.
Why is Wind Turbine Efficiency Important? Turbines of similar size would take similar amount of land, use roughly the same amount of labor to build, and require similar materials and infrastructure. All these factors lead to the difference between turbines of the same size not being the cost, but productivity. Since only one turbine can occupy a given spot, it is more profitable in the long term to use turbines which will produce the most energy from the available wind..
Wind turbines with vertical axis of rotation which use the principle of Darrieus are not limited by the Betz law as propeller turbines are. Because propeller turbine uses aerodynamics of rotation, while vertical axis turbine uses aerodynamics of linear movement, and the cylindrical geometry of the VAWT is vastly different from propeller (which has a flat disk geometry). This allows blades of a vertical wind turbine to generate energy from the same body of airflow more than once.
Dr Ken Brown (Melbourne University Department of Mechanical and Manufacturing Engineering) was the first to achieve Darrieus wind turbine efficiency of more than 60%. (published in magazine UniNews Jun 1993). And in his 2005 article "Experimental studies of Darrieus rotor” Dr Gorelov D.N. reported that his vertical turbine during tests in the wind tunnel had wind power efficiency of more than 60% (magazine “Thermophysics and Aeromechanics” Vol 12, №2, publisher – Siberian Branch of the Russian Academy of Sciences).
Our own extensive research and testing of scale prototypes in the wind tunnel and full-sized turbines in real wind conditions resulted in creation of ANew-type of turbines, what we believe to be the most efficient wind turbine in the world today, in some conditions capable of extracting up to 70% of kinetic energy of the wind (after all mechanical and electrical losses).