Demand for electricity, a cornerstone of modern society, accounts for about 39 % of all energy use in the United States, even though electricity accounts for 30% of the energy used for heating. Electricity demand doubled from 1970-2000 and is on pace to increase another 20% by 2009. We expect electricity to be highly reliable, affordable, and secure, made more difficult because it must be used immediately at industrial levels; unlike the water supply, it can’t be stored. The key to success turns on providing power to supply demand precisely when consumers desire it, second by second. The goal is to forecast demand as accurately as possible, then assemble the most dependable, controllable supply in order to achieve confidant reliability, or capacity.
It is crucial to know how energy experts understand the difference among the terms rated capacity, capacity factor, capacity credit, and capacity value, in order to ensure reliability at the lowest cost. 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 expected to be available at any peak demand time (for planning purposes—by ordering supply well ahead of the time it is used, grid operators can lessen the cost by purchasing bulk supplies); 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.
Conventional generators--coal, natural gas, nuclear, and hydro—typically produce their rated capacities when asked to do so—or, in the case of large coal and nuclear plants, within a well understood time period, and maintain a steady energy level throughout their capacity factor time; wind generation produces only a percentage of its rated capacity, rarely, if ever, at its rated capacity (an expert analysis of over 7000 wind turbines in Germany revealed that none produced more than 80% of their rated capacity over a year).
Wind's variability means that its output varies continuously between zero and (rarely) 90% of its rated capacity, always skittering. Collectively, this fluctuating output is averaged over the course of a year to produce a capacity factor of between 20% and 33%. However, more than 50% of the time over the course of a year, a wind plant is unlikely to produce more than 15% of its rated capacity By contrast, conventional generators produce nearly 100% of their rated capacity at a steady level throughout the time they are chosen to be deployed.
In most wind rich areas of the world, the wind blows hardest at times of least demand while, conversely, it typically blows least, if at all, during times of highest demand; by contrast most conventional generators work independently of external factors (the exception perhaps being for hydro, in times of drought). This has grave implications for wind's capacity value, that is, for reliable expectations for its availability at any future time in a peak demand cycle. This is largely why NYSERDA's capacity credit for wind of 20% is so fictitious. Statistically, one might be able to show a history of wind availability at any critical point in time. 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' technology’s capacity value will approach zero for any wind facility.
Wind's skittering flux also has thermal implications, since the grid must balance supply and demand precisely on a less than second-to-second basis. In a system complicated by economic dispatch considerations (where the choice of power units typically is a function of selecting power supply with the lowest fixed price), it's not at all clear what fuel(s) wind power might erratically displace and what fuels would be deployed to follow and balance the wind flux. Tom Tanton, an energy expert in California, has suggested a protocol that could test this, but it would have to be applied to specific grids; it would not be a one size fits all formula. Wind flux behaves much like demand flux (though it is much less predictable). However, wind flux is in addition to demand flux. Consequently, it creates additional instability that must be compensated for.
It is likely the same conventional units used to balance demand flux will also be used to balance wind flux—but this will be on top of—in additional to—the operations necessary to balance demand flux, at increased costs in dollars and CO2 emissions. (Consumers must pay twice—once for the wind energy and once again for the compensatory generation.) System-wide, wind energy, on most grids, cannot contribute much as a method for abating significant levels of CO2, since it is unclear what volume of coal-fired CO2 wind might displace over time (it is more than likely to displace hydro or natural gas, with either no carbon abatement or very little) and to what extent the increased thermal activity flowing from fossil-fired compensatory generation would offset any CO2 savings wrought by the wind energy.
To illustrate how these concepts apply to wind technology, 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 PJM grid (which produces up to 140,000MW during the year). More than half the time, however, it generates 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—of 3.5MW from this plant. 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 wind plants.
Consider 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 corresponds to the way the grid must provide special means to integrate wind’s unreliability.
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. It can, by itself, power no homes.
Jon Boone
Oakland, MD
March 10, 2008
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