7. Is the Answer Blowin’ in the Wind?

Is the Answer Blowin’ in the Wind?

To some, industrial wind power is as puzzling as the Bob Dylan song proclaiming that “the answer is blowin’ in the wind,” an answer which basically inspires more questions - such as, does it even make sense?

Likewise, why do they build these giant towers hundreds of feet tall and then stick thin, sparse blades up there that seem to turn sluggishly?  Why are wind turbines so different from those little windmills on the miniature golf course that spin busily in the breeze?  How is it worth it to spread them out on scenic hilltops when you could put them in less noticeable places? Why don’t they at least put them closer together?  The answers, my friend…Well, here are a few of them anyway.

First, those blades. Why are there only three seemingly thin blades rotating so slowly? Actually the tips of the blades are moving quite fast.  They seem slow because they are so large.  Their typical rotation is about 20 revolutions per minute (rpm’s), so the tips have to do a full circle in 3 seconds. If the rotor diameter is 300 feet the circumference is a little over 900 feet. 900 ft in 3 seconds translates to 200 mph. If you stand directly below one of these you can really sense how fast they are moving as they pass over you. 

In optimal conditions, these blades are extracting over 40% of the energy of the wind passing through them. The reason the blades are so far apart is that at the high speed that the blades move they leave a wake in their path. If the blades are too close together the wake created by one blade affects the efficiency of the next. Careful theoretical and experimental work has shown that the familiar three blade turbines you see on hilltops in our area are the most efficient design.

Similarly, each individual turbine leaves a wake downwind. If the turbines are too close together their wakes will interfere with each other. Typically they need to be separated by 3 to 10 times the diameter of the rotors. The actual layout depends on the prevailing wind direction in a specific location.

So, the blades are going much faster than you might think, and there’s a good reason they are thin and sparse-looking.  Now how about height and location?

The two factors that most affect wind speed are location and height above the ground.  Wind speed actually increases fairly predictably with height off the ground due to decreasing friction with the ground.  Separately, wind speed is always higher on top of a hill because it is not blocked by nearby features.

It should be obvious that where the wind speed is greater you will get more power. But what is not so obvious is HOW MUCH more. The power available in the wind increases with the cube of the wind speed.  That means if you double the wind speed, the available power isn’t doubled - it increases by a factor of eight. (2x2x2).  

So, say you have a small, 30 foot tall turbine in your yard,  where it receives wind at 8 mph. If the tower were instead 350 ft. high, it might receive wind of 16 mph, and having doubled the speed you increase the power by a factor of eight, as above.

Meanwhile at the top of the mountain the wind speed might be 1.5 times faster (24 mph) than down below. Now the speed is 3 times the original 8 mph. 3x3x3 is 27 so the power there will be 27 times as much as the original location with a 30 foot tower.  The power in the wind is also dependent on the area swept out by the blades of the turbine. In sum, one 300 foot diameter wind turbine on the ridge in a 24 mph wind can produce the power of more than 60,000 six-foot diameter turbines in the 8 mph wind in your backyard.

Though our last column touted the footprint and payback time of photovoltaic panels (PVs), it turns out that wind power has an even better profile, according to the latest Life Cycle Analyses (LCAs), which you will remember take into account all of the energy inputs for the manufacture, installation, operation, and decommissioning of the turbine. The LCAs indicate that a wind turbine will compensate for all those energy costs in less than eight months. Given the expected lifetime of 20 years or more, a turbine will generate more than 30 times the energy it uses.

One interesting way to compare different renewable energy sources is by considering how much land area is required to produce a given amount of power. Let’s compare electricity from wind to that from biomass.  At best, biomass can grow at a rate of 6 tons per acre per year. If this acre of biomass is harvested and used to generate electricity, it could produce around 5000 kWh per year. The Kibby Mountain wind farm in Franklin County covers around 3000 acres and produces around 300 million kWh each year. That amounts to 100,000 kWh per acre, or twenty times as much power as biomass. 

We don’t know about “THE answer,” but turbines are part of the answer, and they’re definitely blowin’ in the wind. 

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side.  He can be reached at pauls@maine.edu.  Cynthia Stancioff, MA, Public Administration, is an amateur naturalist and wordsmith.