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Excellent Read on Wind Energy  RSS feed

Dale Bunger
Posts: 45
Location: WI, USA (Zone 5) Continental ~33" avg. rainfall
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I am interested in installing a Wind System on my property and have spent the last few weeks gathering information and looking at ideas/concepts.
In my search for information, I came across a book at our library that I suggest should be required reading for anyone interested in the topic.
The book is called "Wind Power, Renewable Energy for Home, Farm, and Business" by Paul Gipe
It is over 400 pages of theory, description, examples, and history.
He discusses types of Wind Turbines, towers, site considerations, permitting, safety, and tons of other great stuff.
The information is around 10 years old and seems to be a little dated when it comes to inverters and control, but the basics regarding rotors, generators, and blades types is timeless.
There are also formulas for nearly every thing you would ever want to calculate and realistic numbers regarding what to expect from a system.
It is also amazing at how many "new" and "game changing" ideas we see on the Internet that have already been tried and proven to less than effective. Bummer.... so many of my "outside of the box" thoughts have already been tried

The following is my attempt to summarize some of the more interesting things that I've picked up. Some are obvious, some took me by surprise, and some are probably still debatable. Enjoy...

"Power is proportional to the area intercepted by the wind turbine. Double the area intercepting the wind and you double the power available."

"When the wind strikes an object, it exerts a force while attempting to move it out of the way. Some of the winds kinetic energy is given up or transferred, causing the object to move."

Seems obvious, but it really is a simple as Mass(of the air) x Speed(of the air) = Energy
They call this the Power Density and it varies based on elevation, temperature, and a few other factors.

So when thinking about the size of a wind turbine, it is all about the swept area of the rotor. (Spinning Part) The energy from the wind is what we are trying to intercept and regardless of horizontal vs. vertical, drag vs. lift, etc., etc., etc... you simply cannot get anymore energy than the area of wind you are capturing.

"Power is a cubic function of wind speed. Double the speed, and power increases eight times"

"Wind Speed - No other factor is more important to the amount of wind power available to a wind turbine than the speed of the wind. Because the power in the wind is a cubic function of wind speed, changes in speed produce a profound effect on power."
"Consider the power available at one site with a wind speed of 10 (units are not important), and another site with a wind speed of 12. Although there's only a 20% difference between the wind speeds, there's 73% more power available at the windier location."

So the starting point in any calculation is the area that is being intercepted by the rotor. On a horizontal turbine, this is the area defined by the blade diameter. On a Vertical Axis turbine, this is the height of the turbine x its width. On some of the ducted designs, this would be the area of the ducted inlet.

This is the "MAXIMUM" amount of energy that is available to be captured. EVER. However, even this number is not "really" available.

"The maximum that we can capture at the rotor, the theoretical limit, was derived by Albert Betz. The Betz limit is 59.3% of the power in the wind available to the rotor." This is where many of the "Ground Breaking" designs claim to have broken free from the constraints of conventional wind turbines, and as with any theory, it is meant to be tested, but I would be extremely skeptical of any of these designs until they are proven by an unbiased group...

My searches have led me to believe that rotor design and efficiency appears to be the area with the most innovation, opportunity, and unfortunately, deception. Most of the incredibly cool looking designs that claim to have revolutionized the wind industry are short lived and almost none of these ideas have had any commercial success. Not that this is the only measuring stick, but it is whats proven at this point.

There are 2 main types of rotor design, ones that use "drag", like a cup and others that utilize "lift", like an airplane wing.

It is generally accepted that most drag style devices, including Savonius Vertical designs or old farm style wind mills, are limited to around 20% efficiency. However, they are capable of capturing more "low wind" energy and are typically more efficient and useful than lift style systems at these low wind levels. The downside is that there is very little energy to be capture at this point.

The lift style devices including most horizontal designs, and the Darrieus style of vertical designs are capable of efficiency approaching 40%. This is considerably more efficient than the drag style units, but they need a stronger wind to be effective.

So, the swept area of the rotor determines how much wind you can intercept, wind speed affects how much energy is available in this area, rotor design affects how much of this energy is actually able to captured, and now we are left with the efficiency of converting the mechanical power to electrical energy. Most modern designs are pretty efficient with the direct drive models being slightly more efficient than units that require a gearbox to speed up the generator portion of the unit. (Note, almost all large generators must use a gearbox due to the very slow main shaft speed, but many smaller designs use dedicated generators that are driven directly from the blades.)

So when you see a 6kW or a 10kW unit, this is really only an indication of how much energy the generator could "possibly" produce under near perfect conditions. Start by calculating the swept area of the rotor, the wind speed in your location, the rotor efficiency and the overall efficiency of the generator and you are getting close to what you could actually expect.

It seems like a really rough rule of thumb is that with a reasonable design, in a reasonably well placed site, the most you should expect is around 25% of the rated output on a extended basis. Anything too much beyond these numbers is either an actual breakthrough or more likely a scam or manipulation of numbers.

So if my homestead uses approximately 800kWh per month or approximately 26.6kWh per day or approximately 1.1kWh per hour, then I would need between a 5kW - 6kW wind turbine, operating at 20% efficiency, to offset my electrical use.

And most important, regardless of the design, the swept area would need to intercept at least 30 square meters or 300 square feet.

Sorry that it took so many words to get to the end of this, but the book was "really, really, good"

Matu Collins
Posts: 1976
Location: Southern New England, seaside, avg yearly rainfall 41.91 in, zone 6b
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Thanks for the summary, you were very clear and I understand this better now.
Barry Fitzgerald
Posts: 43
Location: Welland, Ontario, Canada
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Great post with lots of info, I would not have minded more detail. I guess I should just read the book.
r john
Posts: 134
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Need to be careful using the Betz limit because it is based on very specific assumptions.
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