This master’s thesis examines whether wind energy variability might be reduced – and thus capacity credit obtained – by balancing the characteristics of three separate sites in North Carolina and Tennessee. It was published in the Duke Environmental Law and Policy Forum, Spring 2005, pp. 341-381.
“As wind is added at the other two sites, portfolio variance decreases, but so does mean portfolio output.” (p. 372)
A “best” portfolio (to provide the least variability) for January was calculated – based on 1 years’ 10-minute wind samples – as (roughly) half the turbines at site A, a fifth at site B, and a fourth at site C. For August, the best distribution was a fifth at site A, a third at site B, and two-fifths at site C. These theoretical portfolios would have a capacity credit of 21% in January and only 7% in August (based on theoretical January outputs of 53%, 58%, and 39% at sites A, B, and C, respectively, and August outputs of 47%, 19%, and 13%, respectively).
If a grid manager wanted the least variability all year, then several turbines at each site would have to be turned off at different times to provide the best balance over geographically dispersed locations, thus lowering the overall capacity factors (and vastly increasing costs, both economic and environmental).
Note the explanation of the difference between traditional capacity credit (how much load is available 95% of the time) and “effective load-carrying capacity” (ELCC), which is now frequently used for variable sources such as wind. The author says it is a very complicated formula, but it appears to maximize, e.g., wind’s capacity credit by also considering the likelihood that other sources would not be able to provide necessary load at any time. That is, as long as the rest of the system is robust, wind is allowed a high ELCC.
But that still indicates that wind provides energy, not reliable capacity. Geographic dispersion provides some capacity credit, but the necessary redundancy only underscores the madness of trying to use wind to replace other sources.
Download original document: “Can wind be a “firm” resource?”
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