For a number of years, the planners on the ERCOT (Texas) grid assumed that wind projects would provide less than 9% of their nameplate capacity towards meeting peak demand. That assumed number for “Effective Load-Carrying Capability” (ELCC) was based on the fact that wind production is not dependable and may be inversely correlated with demand, especially during a hot summer day when there is no breeze to find.
Groups like the Sierra Club pressured ERCOT planners to use a model that did not simply assume a low number, but used selected past history to make wind appear more reliable — even if that meant an optimistic assumption that could result in capacity shortfalls under certain circumstances.
Because Texas hit a record peak demand in the past day, I went looking to see how well wind was performing and how well the new model is working out to predict its contribution to meeting the demand.
I located a page that provides data tables about ERCOT generation sources. I’m sure the tables can provide useful graphs with some moderate scripting or programming effort. Maybe someday soon, I will invest the time required, but for now I will just wish that the data sources would produce a few more graphs rather than keep burying their data in zipped CSV tables. (I know. I’m whining.)
However, I did find an illuminating graph in the file downloaded from the “Daily Aggregated Wind Output” link that illustrates the hourly variability of wind production.
(Click to enlarge.)
How would you like to operate a grid with that kind of generation variability?
That graph, with its sharp peaks and valleys, tells me that ERCOT’s 8.75% assumption for wind capacity value wasn’t too far off the mark.
The below quote from the Texas Sierra Club article titled ERCOT FINALLY VALUES WIND PEAK CAPACITY USING ACTUAL DATA, however, illustrates the risk that people who depend on reliable electricity might be taking if ERCOT allows its assumptions to be modified by pressure groups that bear no responsibility when their suggested models provide a poor basis for making decisions.
If the latest CDR, released in early 2014, used the up to date 16.3% carrying capacity of wind, (in line with the numbers presented at the recent ERCOT meeting) an additional 1,452 MW of capacity would be estimated to be added to the Texas market by 2018, therefore moving the reserve margin in 2018 from 13.4% to 15.5%. It may not sound like much out of context, but this is huge! Major investment and incentive decisions in new power plants are heavily influenced by the margin of reserves the state estimates it has. ERCOT’s current reserve margin target is 13.75%. So we essentially cross over from inadequate to adequate reserves in 2018.
Thus, by more accurately representing wind’s capacity at peak electricity use, Texans and investors in Texas will be able to see more clearly what future resources might be needed. In essence, simply by using a more accurate figure for wind capacity, Texas just gained an improvement in its reserve margin by some 2%.
Way to go wind!
You might need to read the above very carefully to see the logical flaws. If you are an experienced grid operator, you might notice the problem a little more quickly.