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Q: We never know when, how hard or how long wind will blow. How can we count on it as a reliable energy source? A: The short answer is we can't, but the long answer is way more interesting and important!
"Reliability" is obviously an important concern for our electricity and other energy systems. We certainly want the energy we need to be available for use when we need it. In the Great Lakes Region, people are used to have electricity available 24 hours a day, 7 days a week, except perhaps when severe weather strikes and outages result from damage to utility wires.
For a power plant, reliability is only one measure of performance. One aspect is "availability." That's expressed as a percentage of the year that the power plant is ready to make energy, should it be called upon to make energy. For a wind generator, this measure answers the question, "If the wind blows hard enough right now, will the wind generator be ready to make energy, or might it be out of service for some reason?" Modern wind generators have exceedingly high availability, generally in the 95% range or even a bit higher.
Modern utility scale wind generators are also monitored in real time by systems that sense vibration and measure several parameters of wind generator operation and wind speed and direction. Thus, wind farm operators are notified right away if anything does start to go wrong, and they are usually able to send a maintenance or repair crew to a particular wind generator, prior to a problem that causes an emergency shut-down.
Another aspect of reliability is called loadability or load factor. For wind generators, this is often expressed as capacity factor. This factor is also expressed as a percentage. The question it answers is, "Compared to the amount of energy that would be produced if this generator operated at full power capacity for all 8760 hours in a year, how much energy does it actually produce?"
This is a measure of performance where wind generators score much lower. The best, most windy locations in the Great Lakes Region might have winds blowing strong enough to make as much as: (1) some energy every single day of the year; (2) some energy in 90% of the hours of the year; or (3) at a capacity factor of nearly 50%.
Many of the better sites in the Great Lakes Region have wind capacity factors on an annual basis of 40 to 45%. That means a 1 megawatt (MW) wind machine would produce about 3,500 to 4,000 megawatt-hours (MWh) per year. That's 1 MW multiplied by 8760 hours per year and multiplied by 40% (which equals 3,504 MWh) or 45% (which equals 3,942 MWh). In order to be commercially viable right now, given the installation costs and typical performance of a utility scale wind generator, wind farms in the Great Lakes need to be installed where the average capacity factor will be 30% or higher.
Also, keep in mind that the more accurate wind forecasting gets, and the longer in advance winds can be forecast, and the more land area that gets covered by wind farms, the more accurate wind energy production can be anticipated each hour. As forecasters get better data and experience and utility operators do, too, it turns out to be not too difficult to blend significant percentages of variable-output wind energy into the overall utility mix.
To Summarize: Wind generators are very likely to produce energy whenever the wind blows strong enough to make the blades turn. Utility operators have the full time job of continuously balancing electric power production and electricity demand. Adding a bunch of wind energy to the mix does cost a little bit more, in terms of that utility operational function. On the other hand, wind energy -- when the wind is blowing -- helps to reduce the need to use the most expensive fuel and operating costs in that particular hour. Recent studies in the U.S. have shown, savings that result because wind energy represents a long-term, fixed price addition to a utility operating portfolio are generally equal to or greater than the extra costs involved with managing the electric grid to accept the variable output of wind generators.
