Documents
Selected Extracts:
BACKGROUND
The size and rated capacity of commercial wind turbines have grown steadily since the early 1990s. Typical turbines in the 1990s were rated below 1 MW, with rotor diameters of around 30–50 m and hub heights 40–60 m. 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. The trend of larger turbines will continue; some turbines currently under development for deployment during the second half of this decade are rated at 2–5 MW of energy generation with rotor diameters near 100 m and hub heights of 100–120 m. These advanced turbines will take advantage of the higher wind speeds aloft to generate more wind energy. Specific knowledge of important wind characteristics at turbine hub height is still needed to optimize turbine design and wind farm layout. Physical measurements of parameters such as wind speed, wind power density, and wind speed shear at heights of 80–120 m were virtually nonexistent a few years ago and are still rare today.
Most wind energy anemometer measurements are at heights of 50 m or lower. A common practice in the wind energy industry is to analyze data from the shorter towers and extrapolate these data to turbine hub heights for wind farm design and wind energy prediction. This technique is much less reliable for hub heights of 80 m and higher. The decreasing influence of surface roughness on wind shear and increasing influence of lower atmospheric features such as low-level jets and thermal circulations makes simple extrapolation prone to large errors. Recently updated state wind resource maps (Schwartz and Elliott 2004) are used for regional wind farm siting. However, the maps are only validated for 50 m above the ground and the resource patterns depicted on the maps may not accurately reflect the distribution of the resource for levels 80 m and higher.
The wind energy community has recognized the need to fill the data gap. Programs instituted at the state level and, in large part, supported by the U.S. Department of Energy (DOE) place anemometers and vanes at several levels on existing tall (80 m+) communication towers. The wind resource group at DOE’s National Renewable Energy Laboratory (NREL) has obtained many of these measurement data. We have begun to analyze important wind climate parameters such as wind speed, power, and shear from the tall towers. The distribution of the tall towers varies among the states that participate in the program, because the tall tower program is new and the available funding to establish tall towers is variable. Tall tower data from Kansas, Indiana, and Minnesota (which have the greatest number of tall towers with measurement data) will be the focus of this paper. Analyses of data from the tall towers will start the process of developing a comprehensive climatology for wind energy development areas in the United States.
NEED FOR TALL TOWER CLIMATOLOGY
Measurements of wind characteristics over a wide range of heights up to and above 100 m are useful to: (1) characterize the local and regional wind climate; (2) validate wind resource estimates derived from numerical models; and (3) evaluate changes in wind characteristics and wind shear over the area swept by the blades. Developing wind climatology at advanced turbine hub heights for the United States benefits wind energy development. Regions where a climatology is most important include the central United States between the Rocky Mountains and the Appalachians, the interior western states between the Pacific Ocean and the Rocky Mountains, and the northeastern and mid-Atlantic states. Specific circulations such as the nocturnal low-level jet and the land-sea breeze influence many locations in these regions. These circulations may have a greater influence on the wind resource at 80–100 m than at 50 m. The strong winter winds aloft that frequently affect the northern and central tiers of the United States may also affect the wind resource. These winds are often prevented from mixing down to the 50-m level by a strong surface-based stable layer, but whether or how often these winds descend to the 100-m level is not known.
A tall tower wind climatology will better define areas in the United States where wind energy projects could be feasible, and may include regions where current 50-m measurements indicate the wind resource may not be sufficient for a profitable project. DOE supported projects that establish wind measurements on communication towers through grants from its State Energy Program (SEP) in fiscal years 2002 and 2004. Figure 1 shows states that have established tall tower measurements from SEP funding and in-state programs. Three states in the midwestern United States—Kansas (6 towers), Indiana (5 towers), and Minnesota (9 towers)—had sufficient regional distribution of these towers to begin an analysis of the regional wind climatology at advanced hub heights. Unfortunately, only a subset of the towers in Indiana and Minnesota was used in the analysis because of short periods of record or questionable data. The wind climates in these states are interesting because the southerly nocturnal low-level jet strongly influences the climates in Kansas edge of its area of influence.
CONCLUSIONS
NREL has started to analyze the wind climatology at advanced turbine hub heights based on data measured on existing tall towers in Kansas, Indiana, and Minnesota. The highest measurement level at these towers was 90–110 m. There are two significant findings from the analysis: (1) the difference in wind resource at tall tower sites in the central United States seems to be controlled by the strength of the noctural and southerly winds; and (2) the average wind shear exponent of 50-100 m at tall towers in the central United States is influenced by strong southerly winds and is significantly higher than the 0.143 often used for conservative estimates of the wind resource at turbine hub height. A common range of shear exponents at well-exposed sites is apparently 0.2–0.23; the windiest sites have slightly lower shear exponents (0.18–0.20). This finding may prove beneficial to wind energy development in the Midwest. If the shear exponent values above 0.2 are widespread, the wind speed of 100 m for many locations could be up to 0.5 m/s higher than previously estimated, which would make more locations attractive for development.
Tall tower programs are being implemented in Ohio, Iowa, Missouri, South Dakota, Virginia, and North Carolina. Tall tower data are also being collected in California, Texas, North Dakota, and New York. In time, tall tower data supplemented by data from remote sensing networks may help us develop a clearer picture of wind characteristics at advanced turbine hub heights across the United States and allow for a more systematic deployment of wind farm power plants to meet our energy needs.
| < prev | next > |