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October 11, 2010

Scale Matters

Proponents of shutting down Vermont Yankee blithely assure us that "sun and wind and conservation" will replace the more than 5 million megawatt-hours of carbon-free electricity the nuclear plant produces per year. Even without quibbling about cost and subsidies, this replacement is highly unlikely anytime soon. Let's look at some numbers:

The world's biggest offshore wind farm, Thanet, opened two weeks ago off the coast of southeast England (where it's very windy). Although the plant is rated at 300 megawatts peak output as opposed to Yankee's 650 megawatts, the output of the plant will be less than 800,000 megawatt-hours annually or about 15% of what Yankee supplies since wind almost never blows at the ideal speed. [see below for the difference between a megawatt and a megawatt-hour.] Thanet covers 13.5 square miles. We'd need almost seven of them this size to replace Yankee. Do you think we have enough ridgeline available? How long do you think it would take to get permission to use it? to actually build? Moreover, we'd need backup for times that the wind isn't blowing much.

This is NOT an argument against solar and wind energy; it's an argument FOR the need to keep nuclear in the mix if we're going to reduce dependence on foreign oil and CO2 emissions anytime soon. Just to recap: the largest offshore wind farm so far operational has 15% the capacity of middle-size and middle-aged Vermont Yankee. BTW, the Cape Wind project, which looks like it may finally be built after years of delay, is planned to be slightly larger than Thanet although it is in not quite as windy a place. It will take about five Cape Winds to replace Yankee's output in the New England Grid.

Just last week the world's largest solar photovoltaic plant opened in Ontario, Canada: it has a rated capacity of 80 megawatts peak output and occupies 850 acres. Obviously a solar plant doesn't operate at its peak output for very many hours of the day. Located where it is, it's unlikely that it'll produce more than 100,000 megawatt-hours annually. We'd need about fifty copies of the world's biggest solar pv project to replace Yankee. BTW, the largest solar photovoltaic project in the US is in Florida. It's rated at 25 megawatts of peak capacity.

Solar thermal projects are expected to have better output than solar photovoltaic and to provide some possibility of using stored heat to generate electricity when the sun is down. The California Energy Commission has just approved a proposal by a company called BrightSource to build a solar thermal plant in the Mohave Desert. The plant will be rated at 392 megawatts peak and, according to BrightSource, is expected to produce 1,172,000 megawatt-hours annually – a little less than one quarter of what Yankee produces now. This plant is the biggest solar thermal project yet approved in the US – it doesn't have its environmental permits yet.

So we're not going to replace Yankee with sun and wind anytime soon – no matter how much we subsidize them. Conservation can save electricity in the applications we're using it for today; but we actually want to use more electricity, not less, so that electrically-delivered energy can replace the fossil fuel we use for heating and transportation. More on that in We Need to Use More Electricity.

It will be very important to Vermont's future and its green footprint whether the new Vermont legislature votes to allow the Public Service Board and Nuclear Regulatory Commission to decide whether Yankee is safe to operate and whether the new governor will sign such a bill if it passes. The last legislature voted down a bill which would put this decision out of the political arena. Frankly, the prospects aren't very good although they may get better as legislators – and their constituents – realize that a replacement for Yankeee's electricity isn't blowing in the wind and will be much more expensive.

It may seem strange that this post is appearing while there are news stories about tritium from Yankee showing up in a new well and even some of Yankee's supporters backing off. Nevertheless, the legislature is not the place to grapple with the engineering details of plant safety. IMHO, if Yankee isn't safe to operate and can't be fixed in the opinion of the Nuclear Regulatory Commission, we ought to be working on its nuclear replacement. That's equally true whether we're trying to avoid the security and economic cost of foreign oil or think that world is in imminent danger of overheating caused by burning fossil fuels.

The difference between megawatts and megawatt-hours: The peak capacity of a plant is expressed in megawatts ( a megawatt is a million watts), the maximum instantaneous flow of electricity the plant can produce. The output of a plant, the amount of electricity it produces that we can actually use, is measured in megawatt-hours. We consumers buy it in smaller chunks called kilowatt-hours; there are a thousand kilowatt-hours in a megawatt-hour. If a one megawatt plant ran at peak capacity for an hour, it would generate one megawatt-hour of electricity. If it ran full-out for a year, it would produce 8,760 megawatt-hours of electricity because there are 8,760 hours in a year. Using the peak output to compare plants is only meaningful with plants of the same technology. A nuclear plant runs at close to peak 90% of the time except when it's down for maintenance or refueling; a coal plant is similar. Wind turbines and solar plants obviously only operate when the wind is blowing or the sun is shining and, even then, the wind is not usually blowing at the perfect speed or the sun shining at the perfect angle and intensity for peak output. So a megawatt of solar peak capacity, for example, only results in 20% or less as many kilowatt-hours of usable electricity as a megawatt of nuclear capacity. Understanding this distinction (which most of the press doesn't), is crucial in understanding how much solar or wind would actually be needed to replace Yankee. We would need about 3,250 megawatts of solar peak capacity to replace the more than 5 million megawatt-hours of electricity generated annually by the 650 megawatt Yankee plant! The biggest solar project currently planned for Vermont has a peak capacity of two megawatts.


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