When it comes to wind power, you might think bigger is better. Taller structures, larger blades, and more powerful generators will produce more electricity, so of course it’s best to build all of these as big as we possibly can. Except, according to the data and industry experts, that’s not what’s happening. Instead, wind turbine manufacturers are deliberately putting the brakes on the maximum generation capacity of their turbines—holding back the peak possible power. And the result is that turbines produce more electricity, not less.

The trend of decreasing "specific power" among turbines in the U.S. was highlighted in the 2017 Wind Technologies Market Report, which tracks a number of trends in wind power and was produced by a group of scientists at Lawrence Berkeley National Laboratory for the DOE.

“Specific power is the relationship between the maximum nameplate capacity—or the amount of total generation plausibly generated by a wind turbine—relative to the swept rotor area of the blades,” report author Ryan Wiser told Popular Mechanics in an interview. “It's the ratio of those two factors.” The larger a turbine’s blades get, and the smaller its generator, the smaller its specific power is going to be. And in the case of the U.S., where rotor sizes are increasing but generator size increases aren't quite keeping pace, the ratio is dropping, very much on purpose.

At first glance, it might not make sense that wind turbine designers would be aiming for a lower specific power. After all, if you’ve got a giant turbine with very large blades, you'd imagine that it would be wise to couple that with a powerful generator to wring every last drop of electricity from those giant blades. That might be true if your goal was to maximize the sheer amount of electricity generated, but what American utilities care most about is actually how stable their generators are.

what American utilities care most about is actually how stable their generators are

One of the biggest downsides to renewable sources like wind and solar is that these sources can fluctuate wildly, producing zero energy one day and more energy than necessary the next. One solution to the problem is giant battery farms, but what utilities really want from a wind turbine is the ability to produce a certain amount of electricity continuously over a long period of time without fluctuations.

In the energy business, this characteristic is called "capacity factor," and it tells operators how close a generator is to producing its maximum amount of energy 100 percent of the time. If a particular turbine can produce a maximum of, say, five megawatts—the turbine’s nameplate capacity—and it does that 24/7, then it has a capacity factor of 100 percent.

But no real turbine could possibly go at full speed all the time. Sometimes the winds just don’t blow, or aren’t very strong, or maybe the turbine has to be shut down for maintenance. A few years ago, wind turbines in the U.S. averaged a capacity factor around 33 percent, says Wiser, meaning they were only generating about a third as much power as their hypothetical maximum over a given period of time.

More stable power means cheaper power

But with a lower specific power, reaching maximum capacity is suddenly much easier. Even with lower wind speeds, larger blades can max out smaller generators pretty easily, meaning those turbines operate at maximum power more often. This results in a more stable power supply and a higher capacity factor.

“That boost in capacity factor from about 33 percent just a few years ago to the low 40 percentages or even mid 40 percentages today is primarily a result of this decline in specific power,” says Wiser.

More stable power means cheaper power, and a lower nameplate capacity means turbines can be built in places that don’t get as much wind. That opens the door for building renewable energy infrastructure in places that wouldn’t have been possible even a few years ago.

“As we begin to exhaust some of the very vigorous wind speeds of the nation and we are moving towards somewhat less attractive wind project sites in terms of annual average wind speeds,” says Wiser, “these low specific power turbines will help enable wind projects to thrive in a wider diversity of resource sites.”

“These turbines are what helps make wind economic in the northeast, in the Great Lakes region, and perhaps even into the southeast in the future,” he says.

These effects aren’t all going to be felt immediately, says Wiser, because the U.S. still has more land for wind farms than it knows what to do with. But in a few years—and right now in a handful of parts of the country—low specific power turbines are making their impact known. This just shows that bigger isn’t always better.

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