Force Measurements on a VAWT Blade in Parked Conditions

Anders Goude,Morgan Rossander

doi:10.3390/en10121954

Abstract

The forces on a turbine at extreme wind conditions when the turbine is parked is one of the most important design cases for the survivability of a turbine. In this work, the forces on a blade and its support arms have been measured on a 12 kW straight-bladed vertical axis wind turbine at an open site. Two cases are tested: one during electrical braking of the turbine, which allows it to rotate slowly, and one with the turbine mechanically fixed with the leading edge of the blade facing the main wind direction. The force variations with respect to wind direction are investigated, and it is seen that significant variations in forces depend on the wind direction. The measurements show that for the fixed case, when subjected to the same wind speed, the forces are lower when the blade faces the wind direction. The results also show that due to the lower forces at this particular wind direction, the average forces for the fixed blade are notably lower. Hence, it is possible to reduce the forces on a turbine blade, simply by taking the dominating wind direction into account when the turbine is parked. The measurements also show that a positive torque is generated from the blade for most wind directions, which causes the turbine to rotate in the electrically-braked case. These rotations will cause increased fatigue loads on the turbine blade.

Highlights

One of the key aspects of the design of a wind turbine is survivability at high wind speeds.Typical turbines will operate at a limited rotational speed once the rated wind speed has been reached, and at some point when the forces become too large, the turbine will be stopped, which significantly reduces the forces
Since the turbine can move during electrical braking, one would expect that the forces are relatively independent of wind direction
While some of them could be coincidences, the most significant one is the region 210◦ to 250◦, which shows lower forces. Note that this is a common wind direction and should not be dismissed due to insufficient data. It appears that at least for this direction, there is an offset in the relation between the wind speed at the measurement position and at the turbine blade

Summary

Introduction

One of the key aspects of the design of a wind turbine is survivability at high wind speeds. Typical turbines will operate at a limited rotational speed once the rated wind speed has been reached, and at some point when the forces become too large, the turbine will be stopped (cut-out), which significantly reduces the forces. A second design condition is the extreme wind speed. According to the International Electrotechnical Commission (IEC) 61400-1 standard, the extreme 50-year gusts are defined as 70 m/s, 59.5 m/s and 52.5 m/s for Classes I to III. This study focus on the vertical axis wind turbine (VAWT). It is a technology that has received an increased interest recently, especially for offshore applications due to its low center of gravity [1,2,3]

Results

Discussion

Conclusion