So herein lies the dilemma...

I my previous post I explained how the power in the wind has a cubic increase as velocity increases, but the power from the generator in linear. To illustrate how
these inter-relate look at the graph below.

The three linear lines represent the power output in different wind speeds for three different stator designs. The purple line is the cubic wind speed which climbs sharply as wind speed increases.
When the stator lines are above the wind power (purple) line it means that there is not enough power in the wind to turn the turbine.
If we look at stator 1. It starts producing power at 200 RPM, but at speeds of over 400 RPM is is not very efficient. The inefficiency is represented by the gap between the wind power line and the stator line (i.e. how much power is in the wind vs how much power is being produced).
Now looking at stator 3. It only starts producing power at around 450 RPM, but at 600 RPM it is producing 1kW while stator 1 was only producing around 200 watts.
What this shows is that unless you can adjust the makeup of the generator as the wind changes, you cannot make a stator that is efficient for all wind conditions.

How do generators compare

As mentioned in a previous post the make up of the generator is important. The generator is made up of a set of magnets that are rotated by the propeller blades. These magnets are rotates so that they pass close to coils of wire. These coils are mounted on a stationary plate and are called the stator. Basically, the current is induce based on the number of turns of the coil passing through the flux of the magnetic force. The stronger the flux and the more turns in the coil the more current is induced. Remember, also that the magnetic flux is greatest closest to the magnet.
Different grade magnets and the larger the magnet the grater the flux.

The more current that is produced the harder it is to turn the rotors. So for light wind conditions you want the rotor spinning as soon as possible and it is better to design your generator to produce lower watts per revolution. For higher wind conditions you should opt for higher watt producing generators.

In the above graph 'stator 1' is designed to run at higher speeds that 'stator 3', but although it produces less power per revolution, it will be able to harness the winds power in much lower wind conditions. In the next post we will see how the wind power and the stator graphs inter-relate. This can be sued as a guide to help in designing your stators.

Lets talk generators

Up do now I have explained how to determine the power in the wind and the different ways to capture this power. Once the power has been capture it needs to be converted into electricity. This is done using a generator.

Before I get into how a generator works, you need to understand that all generators are not equal. Some are made to produce high power at low speeds and others can only produce high power at high speeds. And then you get all the combinations in between.

These differences have to do with the relationship between the magnets and the coil windings that induce the current. I will get into more detail about that in a later blog.

What is important is that, if we eliminate minor inefficiencies, a specific generator will produce twice the power if you double the speed. It is linear. As the rev's per minute increase so does the power output. This is very important, because if you remember the power in the wind is a cubic so in the next blog I will explain how these to equations fit together.

Different types of wind turbines

The are basically two sets of types vertical axis wind turbines (VAWT) and horizontal axis wind turbines (HAWT). As the names suggest the VAWT rotate on a vertical axis and the HAWT rotate on a horizontal axis.

There is a good page on the comparison of these types of turbines at http://www.otherpower.com.


Courtesy of http://www.windturbine-analysis.com/ (Note: Site is no longer working)

The bottom line is that if you are planning to build your own wind turbine it is easiest to go for the traditional horizontal design with the blades facing the wind like a propeller. The vertical design turbine needs to be precision engineered to generate decent amounts of power.

Graph of watts generated by 5m diameter blade with 35% efficiency

How much power can I get from the wind?

So how much power can I actually get from the wind? I turns out that the answer is not that simple, because there are a lot of factors at play. For instance wind speed, type and size of wind generator, efficiency of the generator etc.

So let us start by looking at the wind itself. The power in the wind is influenced by three factors, the wind speed, the area the wind is hitting and the density of the air.

So this sound logical to me. So air at sea level has a density of about 1.23 as you go higher the density decreases.

The formula to calculate the amount of watts of power in the wind is as follows:
Power in the wind(Watts) = 1/2 * rho * A * V^3
where:
rho = air density,
A = area in square meters
V = velocity in meters/second

So at sea level for a horizontally mounted propeller with a diameter of 2 meters and a windspeed of 5 m/s we get the following:
Area of the a circle = Pi * radius squared so for the propeller we get
Pi * 1^2 = 3.141
Power = 1/2 * 1.23 * 3.141 * 5^3
Power = 242 watts

Wow! that seems like a lot of power and as a result a smallish turbine in a moderate wind can produce a lot of power. Unfortunately, along came Albert Betz. He calculated that although there is all this pwer in the air you can't harvest it all, because it flows around things. So he mathematically proved that at best you can harvest just over 59% of the power.

So that gives us 242 * 0.59 = 142.78 watts

This is still not to bad, but then we need to take into account that the our generator is not 100% efficient. From what I can read a realistic percentage for a decent DIY wind turbine is around 35%.

242 * 0.35 = 84.7 watts

Luckily, if you look at the formula you will notice that wind velocity is cubed, so what this means is that as the wind increases we get a cubic increase in power.

At 35% efficiency at double the wind speed we get
1/2 * 1.23 * 3.141 * 10^3 * 0.35 = 676.1 watts

Now that looks much better.

How does it all work

So how does all this work. I simple terms, the wind turns a generator that creates an electrical current. This current is then used to charge the some batteries and the batteries are then used to power electrical appliances, lights etc.

But nothing is always that simple and easy. There are a whole lot of terms like charge controllers, regulators, inverters, induced currents magnets that you will encounter when building a wind generator and using it to power you home. So I will tackle each of the parts of the system in more detail, starting with the wind generator.