In recent diaries on this site I've noticed a number of misleading statements about renewable electricity technology, and wind energy in particular, mostly from pro-nuclear advocates. Of course, this is nothing new, but I've seen particular incorrect statements about wind limitations often enough that it is worth a brief diary to clear up some misconceptions.
Remarks about wind intermittency are frequent here. Just in the past few days, I've seen comments like "even under the best conditions, wind requires a very high amount of backup capacity" and "there's a real limit to how much wind can used on the power grid, due to its intermittent nature (15%, I've read)," and several others.
To understand the effect of intermittency, we need to start with an important distinction: A single turbine has very different output characteristics from a farm of turbines, and the output characteristics from a network of many wind farms are different again.
An individual commercial wind turbine is undisputably intermittent, but this volatility in generation cannot be extrapolated to farms of many turbines. There are three key wind speeds that describe a turbine's output. The turbine starts producing electricity (albeit a small amount) at the cut-in speed, reaches maximum generation at the nominal speed, and shuts down for safety at the cut-out speed. Typical values are 4-5 meters per second (10 mph) cut-in, 14 m/s (30 mph) nominal, and 25-30 m/s (55-65 mph) cut-out. (The figure on the right shows a typical power curve - this one is for the Vestas V52 turbine, which can be adjusted to trade off a little power to reduce sound levels as needed.) At very good wind sites that yield a 45% load factor, a typical turbine might generate at full capacity on average 7 hours a day, less than full capacity 14 hours, and nothing for 3 hours a day.
But wind conditions vary across a farm of turbines - and they vary even more between wind farms. So it makes absolutely no sense to take the intermittency of a single turbine and apply it to a large number of wind farms. Yet this is precisely the fallacy repeated here so often.
To address the misconceptions about wind energy limitations, here are a collection of facts about wind. This is not meant to be a definitive answer, just a selection of things I've learned about wind energy that I believe are relevant and (if the above comments are any guide) not well known here.
Many older studies rely on wind observations too close to the ground, and may be less applicable to modern larger turbines that operate at heights where the winds are stronger and more consistent.
Wind energy and storage technology is evolving rapidly, so we should be wary of "wind needs this" or "wind cannot provide that" statements.
There is generally a very low correlation in wind conditions between distant wind farms.
There can be a high correlation in wind conditions between neighboring wind farms; in some situations this can be useful, as one farm may operate as a predictor for another.
As more wind farms are built, the increased diversity of wind conditions means their total combined contribution to the grid becomes smoother and more predictable.
No form of generation is immune to shutdowns, and wind farms spread this risk unlike some other sources (e.g. hydro, coal and nuclear plants) that concentrate generation capacity at one site. We saw an instance of this recently, when Japan's biggest nuclear site (with one sixth of its nuclear generation capacity) was shut down by an earthquake.
Affordable large-scale storage of excess generation may not be far away, and it will significantly improve the economic viability of weather-dependent sources such as wind.
Most of the research I've seen to back up the points about correlation between wind sites is specific to New Zealand where I live. However, I did a bit of searching and found some relevant American scientific literature. The most interesting paper I found was from the National Renewable Energy Laboratory (NREL) but there are many other institutions and companies conducting renewables research.
The paper I want to cite regarding wind intermittency is: Wind Power Plant Behaviors: Analyses of Long-Term Wind Power Data by Y.-H. Wan, NREL Technical Report, September 2004. Here is a PDF link (1.4 MB). It's a long report, 66 pages, so I will summarize just a few of the most relevant parts from section 3.7 ("Spatial Diversity of Wind Power") here.
Analysis of two wind farms in the Buffalo Ridge region, SW Minnesota and Storm Lake, proved a good example of sites with high yet useful predictive correlation. The two farms are about 120 miles apart.
Here is figure 21 showing their one-minute average power over one particular week.
Clearly, SW Minnesota catches the prevailing winds ahead of Storm Lake farm and is a good predictor of Storm Lake's output.
The paper also presented results for four Texas wind farms. The distance between the two closest farms was 25 miles; the most distant pair of farms were 300 miles apart. Figure 23 shows their one-minute average output over an example 24 hour period.
The red and green lines represent the two closest farms, while the orange line is the farm most distant from the others. We can see that while three of the four farms had hours of no generation, there were always at least two farms generating at a combined rate of at least 45 MW.
The paper has a lot more detail with many graphs and tables of correlation results, and pages of other analysis. But I believe the above two graphs are sufficient to show my points. As more wind farms are connected to the grid, their combined output becomes smoother and more predictable.
As I mentioned in my points above, breakthroughs in storage of excess electricity (that will substantially improve the economic viability of renewable technologies) may not be far away. A couple of recent papers also from the NREL show interesting research in this field. One is Preliminary Assessment of Plug-in Hybrid Electric Vehicles on Wind Energy Markets (PDF) by W. Short and P. Denholm, NREL Technical Report, April 2006, that looks at the synergistic potential between wind energy and Vehicle-to-grid (V2G) capability from future PHEVs. The other, Improving the technical, environmental and social performance of wind energy systems using biomass-based energy storage (PDF) by P. Denholm, from Renewable Energy, Vol. 31(9), 2006, looks at the potential of baseload energy from wind by storing excess generation with compressed air energy storage (CAES) using biomass instead of natural gas.
In summary, there's a lot of debate over fossil fuel alternatives here. That's great, because this is a Democratic blog and these energy topics are important Democratic issues. (I diaried just yesterday about how coal gets Republicans elected.)
Daily Kos is no scientific forum, but people who assert energy facts should be able to provide up-to-date credible sources. I often see negative "facts" about wind and other renewables that cannot be backed up.
Be wary of general statements about wind energy limitations. There is a world of difference between the intermittency of one turbine and the smoother, more predictable combined grid supply from many wind farms.