Becky Blankenship and Jon Boone discussion

[Cohocton Wind Watch - Interactive Community]

To understand how grid operators consider the generating units that provide electric power, one should know the difference among the terms rated capacity, capacity factors, capacity credit, and capacity value. The first describes the optimal performance of a machine; the second is the actual performance over time expressed as a percentage of the rated capacity; the third is a statistical average of the percentage of the rated capacity of the energy that might be expected to be available at any peak demand time (for planning purposes); and the fourth is the percentage of a machine's rated capacity that grid operators can be confidant will be available at any 15 minute time ahead interval.


Let's use the Meyersdale wind facility in Pennsylvania as an example and plug in the numbers: with 20-1.5MW turbines, the plant has a rated capacity of 35MW. It has a proven capacity factor, over three years, of about 27%--meaning that over a year's time it erratically produces about 9MW for the grid (which produces up to 140,000MW). More than half the time, however, it produces less than 15% of its rated capacity--about 2MW. At peak demand on the hottest summer days, it often produces nothing. It has a capacity credit of about 10% (3.5MW), which is an average; statistically, one might be able to show a history of wind availability at any critical point in time--the capacity credit. Operationally, however, because wind behavior is randomly unique for any future time (in much the same fashion as a baseball player's batting average can't foretell the outcome of his next at bat), statistical history will not be good enough to ensure firm reliability. Consequently, wind's capacity value approaches zero for the Meyersdale plant--and all other such plants.


One might illustrate all this by talking about the internal combustion automobile. It, too, like wind, has a capacity factor of about 25%-30%, limited by a combination of operator choice (people generally don't them 24 hours a day each day of the year) and by the need for ongoing maintenance and continual refueling. However, when it is asked to work, it will do so with a high rate of reliability, which is well beyond 99.9% of the time. This is its capacity value. Contrast this with the windmobile, where one can never be sure if it will start or not--or where most of the time it's speed lurches between extremes, often stopping in mid-traffic without warning and requiring a host of new traffic systems and patterns to enable it, not to mention the borrowed cars, buses, taxis, and late appointments involved in going hither and yon. This activity parallels what the grid must do to "integrate" wind energy.


A 1600MW coal farm produces a steady stream of 1600MW about 80% of the time--day and night throughout the year with high degrees of reliability. It is also contained within a relatively small area and can be equipped with scrubbers to virtually eliminate noxious emissions, such as SO2, NOx, and Mercury. Contrast this with a 1600MW wind plant, with its 2000 wind turbines stretched out for hundreds of miles and delivering its energy in skittering bursts--one minute 1400MW, the next 80MW, the next 1000MW, the next zero MW--all the time accompanied by conventional generation working overtime and more inefficiently to balance this volatility. Wind energy cannot stand alone; it necessarily is a minor ingredient in a larger fuel mix.