1) Propeller Length
The bigger the propellers the more power you can generate from the wind. So the number one rule of thumb is get the wind generator with the largest propeller. Ok why don’t all wind generator manufacturers put huge propellers on their turbines? The answer is that they can’t.
| Prop Diam Area sq M Area sq ft |
Air X 1.15 1.04 11.18 |
Whisper 100 2.1 3.47 37.29 |
Whisper 200 2.7 5.73 61.65 |
Whisper 500 4.5 15.91 171.24 |
RB900 |
For a given alternator or motor size, there is an optimum propeller length, to maximise the power that can be generated. Too short and not enough power is generated. Too long and the RPM starts to stall and slow down as well. The propellers must therefore be tuned to the motor and there is a optimal length after which the output declines.
We recommend that when choosing between wind generators we should look for the longest propeller ( read largest horsepower ) in the same price range.
2) Power Output
| Rated Power Kw per/mth |
Air X 400 @ 12.5 ms 38 @ 5.5 ms |
Whisper 100 900 @ 12.5 ms 100 @ 5.5 ms |
Whisper 200 1000 @11.6 ms 158 @ 5.5 ms |
Whisper 500 3200 @ 12 ms 538 @ 5.5 ms |
RB900 |
You buy a wind generator to produce power. Rated power or peak power is not the power produced. A wind generator with a advertised peak power output of 400 watts will not produce 10 kilowatt hours of energy a day. It may only be producing 38 kilowatts a month or just a little over 1 kilowatt a day.
Notice how the electricity produced a month bears no semblance to the rated power. So look for the average monthly output assuming that you can trust the figures claimed.
3) How do we tell if power output claims are credible ?
| Mtr/sec BetzLimit |
2.2 0.42 |
2.7 0.74 |
3.1 1.21 |
3.6 1.83 |
4.0 2.6 |
4.5 3.5 |
4.9 4.46 |
5.4 5.38 |
5.8 6.23 |
6.3 6.94 |
Betz limit says that capturing 59.6% of the energy in the wind is the best compromise between stopping the air and letting it pass through the turbine unaffected. Generally it is unlikely that a wind turbines can achive 35% effeciency. In other words “too good to be true”.
Betz limit above is the output per square foot. So first convert the swept area (diameter) to square foot and multiply by the the Betz limit figures for average wind speed quoted above, usually 5.4 meters per second. So for the whisper 100 swept area of 37.29 sq feet multiply 5.38 ( @ 5.4 meters ) equals 216.28 kilowatt hours per month. The claim of 100 kilowatt hours is within the betz limit (216.28 ). It is 27.78% efficient (100/216*60) which is below 35% so the manufacturer’s claim is credible.
4) Other impact of the propellers ?
The angle at which the propellers are mounted determines whether te wind generator is optimised for low or high wind speeds. Higher angle 15 or more optimised it for low wind speeds and low angle such as 5 degrees optimise it for high wind speeds. It can’t have 0 degrees as that represents a flat surface. So a manufacturer will optimise the angle for best performance in wind speeds between 4 to 8 meters a second.
This angle can only be determined through wind tunnel testing. The design of wind generators is not just a case of fitting a set of blades to an alternator or generator. The manufacturers must conduct wind tunnel test to design and tune their blades.
5) Generator
The more powerful the generator the higher the peak output and the also the average output that can be generated. Also of importance is the weight. Since these motors sits on a pole about 10 meters above ground heavier motors require stronger guy wires and larger mast. The analogy to power to weight ratio in motor vehicles applies. The higher the power to weight ratio determines how fast the motor vehicle can travel for a given engine capacity.
Brushless motors should always be selected or maintenance cost will be high. It is best not to choose machines that depend on software to slow the machine down in high winds as this adds to the complexity and reliability of the machine.
6) Construction
Stainless steel and marine grade aluminium ensures that the elements will not rust or corrode the equipment easily. Some manufacturers offer marine grade aluminium as an option. This is to save cost so it is logical to go for marine grade aluminium if offered.
Propeller blades are often made of reinforced plastic or carbon fibre. This allow for cheaper mass production techniques. Pure aluminium propellers are expensive to manufacture.
The shape and design of the nose cone or spinner is important as it diverts wind to the ‘muscle’ part of the propellers. The spinner can increase the RPM by as much as 100 for a given wind speed
7) Furling
What happens when the wind speed gets too high? If the electrical connection is accidentally cut the voltage produced spirals and the wind generator can spin out of control to self destruction. Most manufacturers quote a survival wind speed and design braking systems or furling to turn the propellers away from the wind.
