April 25, 2006

Why a 52-MW wind "farm" is worth only 4.3 MW

I have been asked to clarify the numbers from the "data" paragraph of the previous post, "Why is VPIRG hiding?".

Capacity factor: Searsburg started at about 24% and has now been around 21% for a few years. The national average as reported by facilities themselves to the Energy Information Agency of the DOE is 27%, but they apparently do not count out-of-commission turbines, so I think 25% is a fair estimate. Note that in the U.K., with the "best" wind resource in Europe, the capacity factor also is only 24% or so.

That's the easy one.

For most of the power curve between the cut-in wind speed of 9 mph and the rated wind speed of 30 mph, the power generation increases cubically in relation to the wind speed. That is, as the wind speed doubles, the power output increases eightfold.

Say the wind speeds are evenly distributed within that range, that is, it blows at 12 mph as often as at 24 mph and so on across the range. Because the turbine produces power at much lower rates at slow than at high wind speeds (one-eighth the power, e.g., at 12 mph than at 24 mph), such an even distribution of wind speeds would mean that the turbine is producing at lower rates much more often.

All this is to explain actual observations as shown in this graphic from German grid manager Eon Netz. The curve follows the total output (or infeed to the grid) of Eon Netz's wind plant and the number of hours that level was reached or surpassed. For example, the infeed was at least 2,000 MW during approximately 5,000 quarter-hours of the year and 1,000 MW during about 12,500 quarter-hours. Higher levels of infeed were seen during much fewer quarter-hours.

The heavy horizontal line shows the average infeed (which, as a percentage of the total installed capacity, represents the capacity factor, in this case 16.4%). That level was seen during about 12,500 quarter-hours, which is 35% of the year. The graph shows, therefore, that the average rate of production is seen only a third of the time. That is, only one third of the time the turbines produce at or above their average rate.

A higher capacity factor would simply raise straighten the curve somewhat; the average infeed, although higher, would still be seen only a third of the time.

This graph, from a Views of Scotland paper, shows the same thing even more clearly. Each bar shows how many hours of the year the infeed was in the specified range. The total heights of the first three columns represent two-thirds of the year but only one-fourth of the installed capacity. The capacity factor in Denmark is actually around 20% or less, so a rate of production at or above average was reached less than a third of the time.

That's the time the wind plant is reasonably productive. But that is not really how I got down to 4.3% effective capacity for UPC's proposed 52-MW facility in Sheffield and Sutton.

The "effective" capacity is a measure of how much other sources could be replaced by wind power for the supply system to remain reliable. It is also called "capacity credit." It is a much more speculative number, but study after study of the grid integration of intermittent, nondispatchable, and imprecisely predictable wind energy put it at about a third of the capacity factor.

The scenario is that even as, say, a 52-MW wind plant produces at an average of 13 MW, the grid cannot decommission or plan not to install a corresponding 13 MW but perhaps only a third of that. As more wind plant is added and therefore more often uses more of the rest of the system to balance it, that "credit" approaches zero (which the Irish Grid study I alluded to clearly states).

I'm a bit out of my depth here, but I would assume the capacity credit would be reflected in costs to utilities, which would be similarly unable to contract for less energy to an amount that is anywhere near the amount of wind power they may purchase. Thus, if Washington Electric buys UPC's production from 52 MW of turbines averaging 13 MW, it would be able to buy only 4.3 MW less from other sources.

wind power, wind energy, wind farms, Vermont, environment, environmentalism