Monday, August 04, 2008

Constraints to wind power

Jesse Ausubel has a pretty good essay which describes what he thinks the future of power production will look like. It's a somewhat rosy picture, and although I'm also optimistic (but not for the next few years), I disagree on a couple of points.

He, like I, thinks that we've got to get away from coal. I don't follow his reasoning for why he thinks we have to get away from coal. It seems he has identified a long term trend towards fuels with less carbon and more hydrogen, and he thinks we should make choices to perpetuate that trend. As near as I can tell, he's skipped the part about why the trend is a good thing. Perhaps he thinks consumers like lower carbon fuels because they tend to burn with fewer combustion byproducts, but he doesn't back this claim up with any market analysis.

I think we as a country need to stop burning coal because
  • we import a lot of oil to burn coal (we spend almost as much on oil to move the coal as on the coal itself), so that the price of coal-fired power is quite sensitive to the cost of oil,
  • it is politically possible to install lots more windpower, but coal is seeing opposition, and it is vital to our economic health to get a lot more electric supply,
  • wind power is inelastic supply, whereas coal power is elastic. That is, a coal plant will shut down if the price of electricity falls below it's operating costs, but a wind turbine costs almost nothing to run and will keep generating through a larger swing in electricity prices, which will make our electricity supply more predictable,
  • and finally and perhaps most importantly, because climate change matters.
My biggest point of disagreement is with Jesse's assertion that windpower is impractical due to land use constraints. Other, perhaps clearer-thinking people have made this same point. Jesse makes a very sobering calculation: he figures a wind farm produces 1.2 watts per square meter, average. To produce all of the U.S. grid's 450 gigawatts (average), you'd need a lot of land. Jesse calculates 780,000 square kilometers. The area of the U.S., for reference, is 9.16 million square kilometers, with 1.75 million square kilometers of cultivated cropland. I don't get quite as large a number as Jesse, but we'll take his 780k km^2 for now.

He figures that's just too much land. But this argument is trite. I'll skip over the point that farmland with wind turbines is still farmed land, and instead focus on a more basic question: How much is too much? I think too much is when the next wind turbine to be installed is projected to make no money. That could be all the farmland plus a lot of offshore turbines, or it could be just a few places in North Dakota. It won't be decided by people getting scared of erecting some more infrastructure on 44% of our existing cropland. Farms in the Netherlands in the 1800s were dotted with windmills, because that's what drove the pumps to keep the water out. Farms in the U.S. in the late 1800s were dotted with windmills, with parts shipped at enormous expense across the continent, because that's what pumped the irrigation water wells. Modern farms aren't currently dotted with wind turbines because they've been using oil instead.

Jesse's argument is also trite because it ignores the huge variation in windiness around the U.S. In North Dakota, the entire state is class 4 or above. That means the power available at 50 meters above the ground is 400-500 watts/meter^2. Even during the summer doldrums, the average power available is 300-400 watts/meter^2.

Jesse's 1.2 watts/meter^2 number comes from a wind farm in Lamar, Colorado. That wind farm has 108 1.5 MW turbines spread over a 11840 acre area. Multiply by a 30% capacity factor, and you get 1.01 watts/meter^2. (I'm not sure how he got the extra 20%.) Why is this number so low?

It's economics. The company that owns the wind turbines pays the company that owns the land on which the turbine is sited approximately $3000 to $6000 per year per turbine. The net present value of that payment stream is $60,000 to $120,000. The turbine costs $1,500,000, which is a lot more. Spacing the turbines farther apart slightly increases the power from each turbine, at small increases in royalty payments and road and cable construction costs. If land scarcity ever becomes an issue for wind farmers, I would expect $ per watt and watts per km^2 to go up. Note that $/watt may go up slightly, while watts per km^2 may go up a lot.

Consider that the first big wind farm, on the Altamont Pass, has a power density of 0.86 watts/m^2, which is lower than Lamar's density. If you follow that link, you'll note that wind farms vary from 0.24 watts/m^2 (Pierce County, N.D.) to 5.3 watts/m^2 (Braes of Doune, Scotland). I think land prices, more than turbine capability, is driving the energy density of these farms.

Note that the wind power map above quotes wind at 10 and 50 meters above the ground. Back when the Department of Energy began collecting data for these maps, those were considered the likely bounds of practically sized wind turbines. However, the Lamar turbine towers are 70 m tall. It turns out that the tower costs are mostly just steel, and the higher up you go, the faster the wind blows. After the industry got experience with the costs of siting, permitting, building, bird strikes, aesthetics, and so forth, it turned out worthwhile to spend more on steel in the tower and concrete in the foundation. As a result, watts per km^2 has gone up.

Is there a limit? Placing turbines closer together can collect more wind energy, but fundamentally most wind power is still being dissipated as turbulence and then heat higher up in the atmosphere. Bigger wind turbines reach farther up to capture more energy. It is hard for me to imagine that ground-based wind turbines are going to get substantially taller than they are now, and so I do not expect the average power yield to increase much beyond, say, 2 or 3 watts/m^2 average. 2 watts/m^2 across all of North and South Dakota would yield 750 gigawatts, which is why you hear wind advocates claiming that the Dakotas can power the rest of the U.S. They could, if you could transport the electricity to market.

Finally, I doubt very much that, even if windpower is wildly successful, it will ever account for anything like 100% of the U.S. grid's production. If many coal plants are forced out of production by lower cost wind plants, I would expect that some very efficient mine-mouth plants will remain. I will be astonished (and pleased) if wind ever produces half the U.S. capacity. If that ever happens, wind turbines will be a familiar sight, but not an overwhelming use of land.

Jesse also complains that wind turbines take significantly more steel and concrete than nuclear powerplants. Obviously the steel and concrete are factored into the current prices of turbines, so it's already part of the price comparisons being made. There are two future risks to large use of concrete and steel, however:
  • Wind turbine prices in the future could be more closely tied to raw material prices (which in turn depend on the cost of energy) than on the price of labor (which depends on the state of the economy). This question resolves to whether future wind turbine prices are more sensitive to the cost of imported energy than electricity from coal is. Coal fired electricity is fairly sensitive to oil prices, so I doubt this is a problem.
  • A large bump in wind turbine construction could use so much concrete and steel that it would distort the markets and cause large price increases.
The second issue got me to pull out the calculator again. Here are Jesse's numbers, actually Per Peterson's numbers, in context of the production necessary to build a 250 GWe average windpower grid (about half U.S. electric consumption):
  • Steel: 460 metric tons per MWe. The U.S. produces about 90 million metric tons of steel every year. Over the 30 years it would take to build a new US grid, wind turbines would require 1.3 years' worth of production.
  • Concrete: 870 cubic meters of concrete. The U.S. ready-mix industry produces about 350 million cubic meters a year, so we'd need 0.6 years' worth of concrete production.
These constitute a nice bump to domestic production, but are significantly less that ordinary year-to-year variation.

The bottom line: if the price is right (or even close), let's have all the wind turbines we can build, because it really could help with our foreign trade deficit, economic sensitivity to energy prices, and global warming.