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Gregory Meyer reports at CNBC that electricity generated by US wind farms fell 6 per cent in the first half of the year even as the nation expanded wind generation capacity by 9 per cent. The reason was some of the softest air currents in 40 years, cutting power sales from wind farms to utilities and the situation is likely to intensify into the first quarter of 2016 as the El Niño weather phenomenon holds back wind speeds around much of the US. "We never anticipated a drop-off in the wind resource as we have witnessed over the past six months," says David Crane.
Wind generated 4.4 per cent of US electricity last year, up from 0.4 per cent a decade earlier. But this year US wind plants' "capacity factor" has averaged just a third of their total generating capacity, down from 38 per cent in 2014. The EIA (U.S. Energy Information Administration) notes that slightly slower wind speeds can reduce output by a disproportionately large amount. "Capacity factors for wind turbines are largely determined by wind resources," according to their report "Because the output from a turbine varies nonlinearly with wind speed, small decreases in wind speeds can result in much larger changes in output and, in turn, capacity factors." In January of 2015, wind speeds remained 20 to 45 percent below normal on areas of the west coast, but it was especially bad in California, Oregon, and Washington, where those levels dropped to 50 percent below normal during the month of January.
Some also speculate the the increase in the number of wind farms may be having an effect. Since wind turbines extract kinetic energy from the air around them, and since less energy makes for weaker winds, turbines make it less windy. Technically speaking, the climate zone right behind a turbine (or behind all the turbines on a wind farm) experiences what's called a "wind speed vacuum," or a "momentum deficit." In other words, the air slows down and upwind turbines in a densely packed farm may weaken the breeze before it reaches the downwind ones. A study in 2013 also found that large wind farms could be expected to influence local and regional atmospheric circulations. "If wind farms were constructed on a truly massive scale," adds Daniel Engbar, "their cumulative momentum deficit could conceivably alter wind speeds on a global scale."
(Score: 2, Informative) by stingraz on Friday September 04 2015, @01:53PM
Between Feb and March you can see a week long period when output drops by nearly 90%. Then it happens again at the end of March. Interspersed are 2-3 day periods of similar low production. At other times, there are days when production is easily 20x that low. There is no storage system on the planet that can deal with this kind of intermittent production and mind you, this is data averaged over all of Germany, not just one locale. Locally, it'd be much, much worse.
That can happen at the scale of a country like Germany, yes. However, The European power grid is fairly well integrated at a scale much larger than this; the UCTE continental synchronicity zone spans an area from Portugal to Turkey, and from Italy to Denmark. Significant power flows (at multi-gigawatt levels) are possible within that zone, and the EU is pushing for a further buildout of cross-border transmission capacities, which should happen in the next 10-15 years. This should increase transmission capacity into and out of Germany alone by around 100%. Connections to the Nordic zone (Scandinavia) and the British Isles are also available today in multi-gigawatt capacities, and further buildout to connect either to (mostly) Norway better is happening as we speak.
Wind power becomes much more evenly available once your grid spans more than 1500km (~1000mi) in all directions (research shows that wind power aggregates over all of Europe + North Africa would virtually guarantee that power output never drops below ~30% of rated power, and rarely exceeds 50% either). This is directly linked to the size of low-pressure systems, which is to say that solar output will basically correlate in an inverse way to wind power.
Viewing a system the size of Germany is too narrow-minded a perspective; the current scale of most existing continental power grids is already beyond that, and future developments make your point even more moot. The main obstacle seems to be the people that oppose transmission capacity (line) construction in the mistaken belief that this would not be necessary for "decentralized" renewable energy scenarios, when in fact it's quite the opposite.