Are you planning on constructing a wind generator? If that's the case, you'll want to know just how much power you'll be able to generate with your finished product. In this article, I am going to show you just how much available power is in the wind. Then I am going to explain how much of that power can actually be harnessed, and, finally, I will use real DIY wind generator examples to describe how much electricity can be created with typical do-it-yourself wind power systems.
The Amount of Energy in the Wind
The energy in the wind is calculated in watts. The formula is:
Watts = (.5) (rho) (A) (V^3)
Now, exactly what does all of this mean? Well, it's less complicated than it seems. "Rho" stands for air density, and, at sea level, one cubic meter of air weighs in at 1.23 kilograms: accordingly, we insert 1.23 for "rho."
"A" in the equation represents what's known as swept area. Swept area is a representation of the amount of area one's wind turbine blades cover-longer blades = more swept area. To determine your specific wind turbine's swept area, use this formula, A = pi x r^2. Or, to keep matters uncomplicated, A = 3.14 (length of one turbine blade squared).
"V" in the equation represents the velocity of the wind. In this space, you will insert the average wind velocity where you want to put your wind generator.
So, simply, the power of the wind can be calculated like this:
watts = .5 X air density X swept area X wind velocity cubed
How Much Energy You Can Actually Use
The above formula demonstrates how much power is in the wind. It does not explain how much energy you're able to extract from it. It is only possible to get 59.26% of the energy available in the wind. I won't go into the specifics at this point, but if you wish to find out more, you can research the Betz theory.
So, does this mean that we're able to use a wind generator to extract 59.26% of the total energy of the wind, as explained using the information above? No, it doesn't. Why? Mainly because there are inefficiencies in wind turbine rotor blades and in the gears and motors used to produce electrical power.
Alright, what's the bottom line? The bottom line is that using a small, well built DIY wind turbine you ought to be able to extract about 30% to 35% of the power in the wind.
Your Homemade Wind Turbine
Okay, let's dive into what's really important. How much electricity is your own personal DIY wind generator going to generate? Let's say that you're going to build a 6-foot wind generator, meaning each of your blades is 3 feet long. The wind in your neighborhood blows at 12 MPH. Before we do anything at all, we need to convert our figures into metric figures. Our rotor blades are 3 feet long, which equals .914 m. 12 MPH has to be changed into meters per second, which is 5.36 meters/second.
Our formula is.
Watts = (.5) (rho) (A) (V^3)
Let's fill in the simple numbers to start with.
Watts = (.5) (1.23) (A) (5.36^3)
Now we have to get our "swept area," (A). To do this we use the very simple equation A = 3.14 x length of one blade^2. Our blades are .914m long, so to get our swept area we use the equation.
A = 3.14 (.914 ^2)
A = 3.14 (.835)
A = 2.62
At this point we go back into our original formula and put in 2.62 for the swept area (A).
Watts = (.5) (1.23) (2.62) (5.36^3)
Watts =(.5) (1.23) (2.62) (153.99)
Watts = (.615) (2.62) (153.99)
Watts = (1.61) (153.99)
Watts = 247.92
As you're able to see, there are 247.92 watts available for our wind generator. Keep in mind, however, that your wind generator is only going to be able to draw out approximately 30% to 35% of the energy available. Which means that, actually, for your DO-IT-YOURSELF wind turbine.
Watts = 247.92 (35%)
Watts = 86.77
So, what you've got is a wind generator rated at 87 watts. What does that mean exactly? An 87-watt wind turbine makes 87 watts per hour of operation. So, if the wind at your location blew 24 hours a day at 12 MPH, you'd create 2088 watts each day, or about 2 kw (kilowatts).
Just what would this power?
The standard notebook computer uses approximately 35 watts per hour. A CFL light bulb uses approximately 25 watts each hour. A 42-inch ceiling fan on low uses approximately 24 watts.
So, if you sat in a room using a ceiling fan, a lamp and a notebook computer, you'd consume 84 of the 87 watts you were creating with your do-it-yourself wind turbine.
Of course, most people don't actually live like this. Our wind generator is not going to be rotating at 12 MPH for 24 hours a day, nor are we going to be sitting in our residence with a lamp on and a laptop computer operating for 24 hours a day. And, most importantly, there are great things known as batteries that we are able to use to store the energy our wind turbine produces.
Because of this while we are sleeping, or at work, or playing outdoors, we can use our wind generator to charge up our battery bank. Then, at night, for example, we are able to use that stored up energy to watch television or browse the web. Just how much power does one need to generate to go off the grid? Well, that varies largely between households.
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