Comparison of the efficiency of various wind turbines – horizontal/vertical axis

This article introduces the efficiency comparison of various wind turbines, including common vertical axis wind turbines(Savonius and Darrieus) and typical three-blade horizontal axis wind turbines. Among them, the Savonius turbine is a drag type wind turbines, and the Darrieus wind turbine and the three-blade horizontal axis wind turbine are both lift-type. The efficiency comparison chart of various wind turbines and related videos are shown below. If you want to understand what the Savonius and the Darrieus wind turbines are, please refer to => Basic principles and types of Vertical Axis Wind Turbines.

In the wind turbine efficiency comparison chart, the ordinate is the efficiency, and the abscissa is the “Tip Speed Ratio”, which is the ratio of the blade tip speed to the wind speed. When the value is 1, the blade speed is as fast as the wind speed, A value of 2 means that the blade speed is twice the wind speed, and so on. The theoretical maximum value of wind turbine efficiency is called Betz Limit, and the value is about 0.59.

The maximum efficiency of a typical Savonius drag wind turbine is only 0.15, which occurs at a Tip Speed Ratio of 0.9. Since the efficiency increases very fast near zero speed, it means that it has a large torque at low speed and can be self-starting. However, due to the slow revolution speed, it is usually unable to exceed the wind speed and is less suitable for generating electricity, because the generator needs sufficient revolution speed to induce electrical energy.

The maximum efficiency of a typical Darrieus lift wind turbine is about 0.4, which occurs when the tip speed ratio is about 4. The efficiency is much higher than that of the drag wind turbine. However, when the Tip Speed Ratio is in the range of 0 to 2, its efficiency is almost 0. It means that there is no torque output in this range [Note 1] , so it is difficult for the Darrieus wind turbine to start by itself. But when the tip speed ratio exceeds a threshold value, the efficiency of the Darrieus wind turbine increases rapidly. Due to its relatively high revolution speed, it is suitable for generating electricity.

The maximum efficiency of a typical three-blade horizontal axis wind turbine is about 0.45, which occurs at the tip speed ratio of about 8. It can be seen that the horizontal axis wind turbine has a high efficiency (it has become the mainstream of power generation at present), but the higher tip speed ratio also leads to more serious noise.

Although the efficiency of the vertical axis wind turbine is lower than that of the horizontal axis, it is not much worse in theory, and the vertical axis wind turbine can accept wind in any direction, so no yaw mechanism is required, and the generator can be installed under the main shaft, the center of gravity is low, which also makes the structure simple, so the vertical axis wind turbine still has the potential for development

參考:Wind Energy and Wind Turbine, Calculation of wind energy and wind turbine efficiency.


 [Note 1] The wind turbine efficiency is equal to the wind turbine power divided by the total power of wind energy. And the wind turbine power is equal to the angular speed multiplied by the torque. When the efficiency of a wind turbine is low in the tip speed ratio range of 0~2, it means that even if it has speed, its torque output is very small.


 


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