Towards a wind energy climatology at advanced turbine hub heights

The size and generation capacity of large utility-scale wind turbines have increased fairly rapidly for the past few years and this trend will continue. Typical turbines in the 1990s were rated below 1 MW, with rotor diameters of around 30 m to 50 m and hub-heights 40 m to 60 m above ground. In contrast, recent technological advances in wind turbine design have increased the generation capacity above 1 MW and raised the hub-height of the machines used in new wind farm projects to around 80 m above ground level. Some turbines currently under development for deployment during the second half of this decade will be rated between 2 MW and 5 MW generation with rotor diameters near 100 m and hub-heights from 100 m to 120 m above ground. These advanced turbines will take advantage of the higher wind speeds aloft to generate more wind energy, but for optimum siting and design of future wind farms specific knowledge of the local and regional wind climate at these heights is necessary. Measurements of the wind characteristics over a wide range of heights up to and above 100 m are useful not only to characterize the local and regional wind climate but also to validate wind resource estimates derived from numerical models, and evaluate changes in the wind characteristics and the wind shear over the rotor swept area. The vast majority of existing wind energy anemometer measurements have been taken at heights of 50 m or lower. State programs, supported by the United States Department of Energy (DOE), that are placing anemometers and vanes on existing tall (90 m +) communication towers have started to fill this data gap. The National Renewable Energy Laboratory (DOE) has obtained much of these measurement data and has begun analyzing important wind climate parameters such as wind speed, power, and shear from the tall towers. The distribution of tall tower stations varies from state to state. This paper will concentrate on the results from the states of Kansas, Indiana, and Minnesota that have the greatest number of tall towers with measurement data. Analysis of local and regional wind climatology, plus horizontal and vertical gradients of wind potential from the tall-tower data in these states will start the process of developing a comprehensive climatology for wind energy development areas of the United States.