November 23, 2004

Cape Wind output

Data from the Cape Wind measuring station in Nantucket Sound is available on line (click the title of this post), with the wind speed updated every 10 minutes (you have to "reload" the page yourself to update it). Right below the table of data is an estimate of what the complete 420-MW complex would have generated over the previous hour, and at the bottom of the page is a table of data from the previous 12 hours. As I write this, the generation figure is 16 MW-hours, or 3.8% of the plant's capacity, equivalent to 5% of the average electricity use on Cape Cod and the Islands. This falls rather short of the developers' claim that it will provide three-quarters of the Cape and Islands' electricity.

Note about wind speed and electricity generation: The wind turbine produces its rated capacity of electricity only within a specific range of wind speed. The 3.6-MW GE turbines that Cape Wind will use produce their rated capacity at 14 meters/second (m/s), or just over 31 miles/hour (mph). Below that, the output falls off exponentially, so that at about 9 m/s (20 mph) they will produce half, at about 7.5 m/s (17 mph) aquarter, and at about 6 m/s (13.5 mph) an eighth of their rating. Below 3.5 m/s (8 mph), the turbines produce nothing (yet continue to use power themselves). If the wind gusts above 27 m/s (60 mph), they shut down to avoid damage. (Data are from the GE brochure.)

(The Cape Wind data are in knots. 1 knot = 1.15 mph = 0.514 m/s. 1 m/s = 2.24 mph.)

Since I've started writing this, the previous hour's output would have dropped to 10 MW-hours, about 4.3% of the Cape and Islands' average need; the currect wind speed of 6 knots (6.9 mph, 3.1 m/s) is below the speed at which the turbines would start producing even a trickle of electricity. That is, the whole massive installation would be producing nothing right now. Check in often, and ask yourself whether the depredations of such a project are worth it.

Note about Cape Wind's claims: The developers say the complex will provide three-quarters of the electricity used on Cape Cod, Martha's Vineyard, and Nantucket, which represents an average power need of 230 MW, 3/4 of which is 172.5 MW, which is 41% of the capacity planned for Cape Wind. This claim is not based on the actual measurements made in Nantucket sound nor on the experience of existing offshore wind facilities. It is based solely on the inflated generic assumptions of the American Wind Energy Association, which says that onshore wind turbines produce 30% of their capacity and offshore 40% (this figure is called their load factor). In fact, onshore wind turbines typically produce from well less than 20% up to 25% and offshore between 20% and 30%. It appears that Cape Wind's actual output would be at the low end, half of what the developer claims. And that's an average. Two-thirds of the time, it will be producing less than that. Often (like today) it will be producing close to nothing, not even enough to make up for its own electricity needs. So it will certainly not be replacing any more reliable power source on the grid.