Several weeks ago on the way to Joshua Tree National Park from Los Angeles, I passed through the largest field of wind turbines I’d ever seen. And, as if on cue, strong gusty winds suddenly whipped through the San Gorgonio Pass, buffeting my lightweight rental car. I was inspired by the sight of wind turbines extending out to the horizon, and wondered what goes into decisions on where to build these icons of green energy.
|Wind turbines in Southern California's San Gorgonio Pass. (Bill Stein)|
Fortunately, I didn’t have to wait long. Just weeks later, Kate Mullin, resource modeling analyst from WindLogics, a St. Paul-based company that provides forecasting and optimization solutions for sustainable power, gave a most informative talk to the Twin Cities Meteorological Society. She talked about the process by which wind power developers estimate the future energy production of a wind farm. While the following is by no means a complete summary of Kate’s presentation, I thought I’d share some of the facts we learned that I found interesting.
There are highly detailed maps that depict wind across the United States, providing a basis for determining locations that may be best suited for the construction of wind turbines. These maps display average annual wind in meters per second. You can multiply meters per second by 2.2 to get an approximate miles-per-hour measurement.
|Minnesota is just northeast of the nation's strongest average winds.|
Although there are several small and isolated pockets of high winds in western parts of the country, the majority of larger-scale strong winds occur in the central part of the United States. Minnesota is among the windiest parts of the country with southwestern Minnesota – owing in part to the Buffalo Ridge formation – experiencing the strongest average winds.
|Areas of southwestern Minnesota average winds of nearly 20 mph.|
An analysis of potential sites for wind farms includes mesoscale weather forecast model data, such as the Weather Research & Forecasting Model (WRF), as well as terrain-based models that take a closer look at smaller geographic grids (1km and 50m resolution).
Meteorological towers (“met” towers) are used to capture hourly data such as wind speed, wind direction and temperature at heights of up to 100 meters. Wind flow models incorporate the data and produce forecasts of wind conditions for an extended period of time, such as 30 years. Ultimately, the data generated will determine if a project is financially viable during the pre-construction phase.
The actual design of a wind farm incorporates a variety of factors. First, it’s a function of where land can be obtained and if it’s subject to any land use restrictions. Additional considerations include the form of power transmission, whether the site is close to housing and development, and environmental restrictions.
(Kate noted that regulations for wind farms in Canada are particularly stringent given environmental and other concerns. In the United States, Iowa and Texas are among states considered the most “wind-friendly.”)
After the potential energy production from a wind farm is calculated, other factors that can cause production losses must be considered. One of these factors is wake, a concept we might think of when it comes to a cyclist drafting behind another cyclist in a velodrome or a car following closely behind a large truck on the highway. In wind circles, it refers to the effect that one turbine has on additional turbines that follow down the line. Kate shared a particularly interesting graphic depicting wake that most everyone in the wind industry has seen. It’s an image of the Horns Rev offshore wind farm in Europe’s North Sea.
|Clouds form in the wake of the front row of wind turbines at the Horns Rev wind farm in the North Sea. (Aeolus)|
Some of the other factors that can reduce the productivity of wind turbines include the availability of the proper turbine model (different models are built to maximize different wind environments) and curtailment, which occurs when wind conditions are favorable, but for one reason or another the wind energy is not sent to the power grid.
Weather can also cause wind turbines to become unproductive. For example, icing is a particular concern for wind turbines in our part of the country. And, interesting enough, there are some parts of the country where wind turbines must be turned off when bats are most active so they don’t get caught in them.
So the next time you see a wind turbine, whether in the California desert or on the snow-covered prairie in southwestern Minnesota, take pleasure in knowing that energy is being captured from resources that are continually replenished, but also know that a ton of work – including complex wind forecasts – go into the building of them.