The numbers didn’t add up to David Baumann. How could the proposed 200-megawatt Sugarland Wind project in Florida be expected to produce as much energy as the 200-megawatt Top of the World wind farm in Wyoming?
You didn’t have to be a wind engineer to see that Converse County, outside Casper, Wyo., is a heck of a lot windier than anywhere in Florida. And yet the Sugarland developers were claiming that their Palm Beach County wind farm would meet the power needs of “up to 60,000 U.S. households” – same as what Duke Energy said about Top of the World when it began operating in 2010.
So Baumann penned a piece for the Berkshire (Mass.) Eagle with the headline, “Wind power numbers just don’t add up,” and in the piece he said wind companies “flippantly throw out boilerplate numbers that have no basis in scientific fact.”
He might be right – in some cases.
Baumann’s article led us to poke at the “number of household” claims that utilities and developers make for big projects, both solar and wind. What we found is that sometimes they’re solid, and sometimes they’re not.
First, though, we should explain what is and isn’t meant by a households figure. It’s not how many households a project powers at any given moment in time – wind power production waxes and wanes, as does household energy use, so a single number would be difficult if not impossible to calculate. Instead, the households figure tells you how many homes could be fully powered by a project in a year based on a project’s annual production.
So let’s start with Sugarland. It’s impossible to know exactly how the developers, a company called Wind Capital Group, came up with the 60,000 figure; they never returned the messages we left asking for more information.
But here’s what we can say: Based on the Energy Information Administration’s figure of 11,496 kilowatt-hours of energy use per U.S. household per year, 60,000 homes use 689,760,000 kWh in a year, which can also be expressed as 689,760 megawatt-hours/year. A 200 MW wind farm, going at full tilt 24 hours a day, 365 days a year, would produce 1,752,000 MWh annually – but, of course, that doesn’t happen. Wind farms sometimes work near 100 percent capacity, other times at 10 percent, other times at 60 percent — it varies throughout the day. The percentage of the total possible production that a project actually yields in a period of time is called the capacity factor. According to a recent federal study, wind power plants that went into operation in 2010 had an average capacity factor of 33.5 percent.
If the Florida plant were an average performer, then, it would produce 586,920 MWh in a year – enough to power 51,000 average U.S. homes. But it is highly unlikely a wind farm in Florida will produce anywhere near that much power. A 2008 study prepared for the state of Florida [PDF] noted that “no Class 3 regimes, which are generally the minimum for economically viable wind farms, have been identified,” in Florida.
The Sugarland number looks even worse when you base the calculation on Florida energy use. Floridians use a lot more power than average Americans – 14,328 kWh per year. So even if Sugarland turned out to be as productive as the average new U.S. wind farm – again, that seems very unlikely – it would still only meet the power needs of around 40,000 Florida homes. In order to hit the 60,000 number for Florida homes, Sugarland would have to operate at an impossible 49 percent capacity factor.
And yet there, in a June 9 Miami Herald story about Sugarland, was the 60,000 figure, with no scrutiny at all (which isn’t meant to come down too hard on the Herald; we’ve done the same thing in the past here at EarthTechling).
Baumann’s suspicion was that Wind Capital Group simply used a multiplier to come up with the 60,000 number. Searching the Internet, we found the frequent assertion that a megawatt of wind power would provide enough power for “225 to 300 homes.” Where these numbers originated is anyone guess, but if Wind Capital Group did use them, you’ll note they latched onto the high side of the estimate.
The lesson here is that you can’t easily convert use a simple formula to convert megawatts of nameplate capacity into households powered. The power plants are too variable in their capacity factors, and the areas served are too variable in their energy consumption.
Fortunately, not all of the numbers we looked at closely are as shaky as the Florida ones appear to be. California, with its massive population and its aggressive renewable portfolio standard, is the source of a steady stream of renewable energy project announcements. Just in the past couple of weeks, EDF Renewables and San Diego Gas & Electric put out word that a 140 MW project in Kern County had gone online, while Pacific Gas & Electric and SunPower unveiled plans to build a 100 MW solar PV plant in the Central Valley.
Reps from both companies explained that instead of using a one-size-fits-all multiplier – X number of megwatts equals Y number of households – they used actual estimates of how much power the specific projects would produce and the average household energy consumption in their service territories in making their estimates.
So for the proposed Henrietta solar plant, PG&E estimated around 241,000 MWh of power will be produced in a year. The average PG&E household uses 6,700 kWh/year. Thus: 36,000 homes powered. That’s a number that fits with the nature of the project, which plans to use SunPower tracking PV technology. In documents filed with the state of California regarding other projects [PDF], the company has said similar systems typically yield 30 percent capacity factors.
So if anything, PG&E might be undercounting this plant’s production potential, because its numbers assume a capacity factor of 27.5 percent.
For the Kern County wind farm, EDF Renewables said it relied on figures from SDG&E when it said the project would meet the energy needs of 56,000 homes. The utility told us it average household electricity consumption in its service area is 6,000 kWh/year. So the 56,000 homes figure assumes a capacity factor of 27.4 percent, well within reach.
One thing you might have noticed in these cases is that both projects were able to give bigger households-powered figures because Californians are electricity misers compared to the rest of the nation. That solar plant that will power 36,000 homes would power fewer than half that number in Florida, even if it produced an equivalent amount of electricity in the Sunshine State – because Florida households use twice as much power, on average.
The phenomenon is even more pronounced with European wind power projects. In the U.K., for instance, average annual household electricity use is 3,300 kWh/year, according to Renewable UK. That 140 MW EDF Renewables wind farm, if operating just as efficiently in the U.K. as it is expected to in California, would power around 102,000 U.K. homes.