richard-rowland-perkins.com

In my first post in this series, I talked about hydropower, specifically hydroelectricity. I mentioned that hydropower was one of the oldest forms of renewable energy, but wind power is arguably older. People have been using wind power to propel ships (yes this counts as wind power) for over 5000 years. There are historical references to wind driven irrigation systems dating back over 3000 years. And the first practical windmills were used in Afghanistan to power mills and pump water over 1300 years ago.

Both technologies were first used to generate electricity within the same decade: hydroelectricity in the early 1880′s, and wind generated electricity in the late 1880′s. (I suppose you could call that aeroelectricity, though no one does ) The image to the left shows the Charles Brush windmill in Ohio, completed in 1888. It is often cited as the first wind powered electric generator, though James Blyth of Scotland actually beat him to the punch by about six months. So how does wind power generate electricity exactly?

Wind power simplified

Wind power works in much the same way that hydropower does, in fact. Wind turns the turbine blades, which are connected through a gear box to a generator. The generator is, as mentioned in the previous post, a loop of conductive wire spinning in the presence of a magnetic field. The movement of the electrons getting pushed around by the magnetic field is AC electric current! Victory!

Now it’s time for a trickier question. How does the wind turn the turbine blade? To understand that, it helps to know a little bit about something called Bernoulli’s Principle.  Simply put, the idea is that as a fluid or gas increases its speed, it must decrease its pressure. One simple way to demonstrate this concept is to think about an inflated balloon. As long as the neck of the balloon is pinched closed, the air inside it is trapped (stationary) at high pressure. You know it’s at high pressure because the walls of the balloon are stretched out, right? But what happens when you open the neck of the balloon? The compressed air jets out through the opening at high speed. The air has expanded from the area of high pressure (the balloon) into the area of low pressure (the room), exchanging pressure for velocity.

Alternatively you can think about traffic on the freeway. Packing cars closer together is analogous to increasing the pressure of an air flow. The closer the cars pack together, the slower everyone must drive to avoid collisions. Conversely, the fewer cars there are on the road, the faster everyone can drive safely.

How does this apply to a wind turbine? The magic is in the shape of the individual turbine blades, which are a type of airfoil, like an airplane wing. A turbine blade splits the wind that passes by it into two streams. The wind traveling over one side of the blade has farther to travel than the wind passing over the other. So it has to speed up to make the trip, and the pressure on one side of the blade drops below the pressure on the other. The result is a force called lift. Mounting the blades in a circle around a central hub turns that lift into a rotational force or torque, which spins the turbine’s rotor. Isn’t science neat? (Sorry, my geek is showing…  )

The hub spins pretty slowly in modern wind turbines: 10-20 revolutions per minute is a typical number. Given that the electric current used on most utility grids oscillates at 50 or 60 cycles per second, a mechanical gear box is used to increase shaft speed before connecting to the generator. As with the hydropower post, the caveats about simplification apply, both to the generator and the turbine technology. And as with hydropower, wind power comes in a variety of shapes and sizes.

Vertical axis wind turbines (VAWT)

Though most people think of windmills when they think of wind power, VAWT systems have a longer history. And vertical axis turbines, like the one in the picture to the left by Mariah Power,  are enjoying a resurgence in the small scale wind market. Why? VAWT systems allow installers to keep the generator close to the ground, where it can be easily serviced. They also can be installed without too much heavy equipment, which is an advantage to do it yourself home installers as well as medium scale enterprise customers. Systems of this type commonly have capacities ranging from 1kW to 5kW. They work well in low wind speeds, and don’t have to be turned to face into the wind. These features make them well suited for close to ground applications, where wind speed and direction varies greatly. The trade-off is that they generally have lower efficiency than their horizontal axis counterparts: for the same wind speed you can generate less electricity with a VAWT than you could with a HAWT. For these reasons, you probably wouldn’t consider using a VAWT in a location where the wind speeds were strong enough to support a HAWT.

Horizontal axis wind turbines (HAWT)

There are small scale HAWT devices on the market. Small wind turbines like those manufactured by Skystream are usually rated at 1kW-3kW capacity, and are designed for home users or small enterprise installations. But big, three bladed, horizontal axis turbines with capacities from 1MW-5MW, like the ones at left from Vestas, are the real workhorses of the wind power boom. It’s fairly common to see large farms of these towers planted in rural locations with strong, steady winds. Large is the operative word here. The towers are 200-300 feet tall and each blade may be 100 feet or more long. Transporting components to remote construction sites can be challenging, expensive, and energy intensive (tons of CO2 intensive concrete and diesel fuel for large trucks and heavy equipment). But despite the embodied energy of wind farm construction, most well designed wind farms become greenhouse gas neutral within a year of operation. Offshore wind farms tend to have lower construction related greenhouse gas footprints because it takes less energy to transport large components by boat than by truck or plane. And some of the best wind resources are located offshore as well, both for wind speed and wind consistency.

For land based and offshore wind farms like these, each tower is its own independent generator. Each turbine delivers the power it generates to a central electrical substation, where it is combined and conditioned to match high voltage transmission standards before being pumped onto the electric utility grid. Why the conditioning? Unfortunately even for large scale wind installations like this, wind speed is still much more variable than water pressure at the bottom of a giant dam. So the output of an electricity generating wind farm varies a lot faster than the output of a typical hydroelectric storage project. This is one of the reasons electric utilities don’t always jump at the chance to build more wind farms. They have to account for the variability of all of the sources they connect to their grid.

And this is where energy storage techniques like the pumped storage hydro systems I mentioned in the last post come in handy. Though it’s clean, renewable, and greenhouse gas free, wind power is a variable or intermittent source. Sometimes utilities respond to an increase in the variability of their electricity sources by installing additional dispatchable power to smooth out the supply curve. Unfortunately, if the dispatchable power is a greenhouse gas emitter, like natural gas or coal (yikes!), then much of the greenhouse gas benefit of the renewable power is lost. But pumped hydro storage combined with wind turns the variable wind power into dispatchable power, without significantly increasing its greenhouse gas footprint.

That’s a real win for the environment. But as I’ve said before, you never get something for nothing. Pumped storage hydro is a net energy consumer. So combining it with wind results in a lower overall efficency of conversion from wind to usable electrons than direct connection to the grid. And with the increased capital costs of building pumped sotrage reservoirs, you get a double hit on the cost per kWh of electricity. That’s why you don’t see too many wind farm developers building their own on-site pumped storage facailites. But one day, that might change. The market for energy is evolving as people educate themselves about the effects of how we generate and consume power. Perhaps with a carbon tax or a cap and trade scheme that places an appropriate value on so-called externalities, an approach like this may become cost competitive. After all, we probably should place a higher value on keeping the planet hospitable for human life. It’s the only one we’ve got.

Filed under: Professional