How Accurate is Accurate Enough?

Anemometer accuracy varies greatly. For most Wind Turbines, useful wind speeds range from 9 mph to 36 mph (4 m/s to 16 m/s) Within this wind speed range anemometer accuracy and measurement repeatability are of utmost importance. As stated in the paragraph above, any measurement error is amplified to the 3rd power (V3). Below is a table illustrating anemometer measurement accuracy errors and their effects on your results. All anemometers used for wind assessment studies are calibrated within this wind speed range to MEASNET specification.

Where is the wind?

Plan you wind turbine placement carefully, as wind can significantly vary by micro-location. The exception being flat land without trees, rolling hills or other obstructions for hundreds of yards. Local topography has a huge effect on speeding, slowing and channeling of wind near the ground and up to 100 yard/meter heights. Verify your observations with an anemometer. Log wind data for at least a month before making conclusions, and if you are unsure, consider getting professional advice. Be careful who you choose, a wind turbine sales rep may have conflicting interests. In the end, the most cost effective solution is to test more locations with multiple anemometers which you can later sell on eBay or classified ads.

What about seasons?

To get a direct comparison between two locations, you need to measure the wind simultaneously since weather patterns differ month to month and year to year. This is the reason why all large scale projects require a minimum of 1 year of detailed wind data before before installing a wind turbine. For a small-scale wind turbine for your home or office, you should not underestimate these seasonal effects and use good judgement when shortening your wind survey time.


Which wind turbine is best for me?

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Choosing and comparing different wind turbines

The hardest part for most people is comparing between different wind turbine systems because every manufacturer quotes power output at a different wind speed. A calculator can help put in perspective two different wind turbines. Calculator only requires that you know a manufacturers quoted power output at a particular wind speed and the average wind speed at your location. It does the math and shows you how two wind turbines will compare at your location.

Will I save money?

As you may already know, most wind turbines are expensive and if not situated correctly, they are a money loosing proposition. Maintenance does not help the long-term financial picture, so choose quality over price. The last thing you need, is to have to climb a pole or a roof in the middle of winter to replace worn bearings. Watch out for the cheap propeller type wind turbines from China; you will get what you pay for.

How do I tell which turbine is best for me?

If you are unsure, feel free to contact us, and we will help you make the right choice free of charge.


Anemometer Error Charts

Anemometer Accuracy Error Chart @ 5m/s (11mph)

Wind Speed Error Wind Speed Error Wind Speed Error V3 Wind Power Error
± 0.05 m/s ±0.11 mph ±1.0 % ±3.0 %
±0.10 m/s ±0.22 mph ±2.0 % ±6.1 %
±0.15 m/s ±0.34 mph ±3.0 % ±9.3 %
±0.20 m/s ±0.45 mph ±4.0 % ±12.5 %
±0.25 m/s ±0.56 mph ±5.0 % ±15.8 %
±0.30 m/s ±0.67 mph ±6.0 % ±19.1 %
±0.40 m/s ±0.89 mph ±8.0 % ±26.0 %
±0.50 m/s ±1.12 mph ±10.0 % ±33.1 %
±1.00 m/s ±2.24 mph ±20.0 % ±72.8 %

Anemometer Accuracy Error Chart @ 25m/s (56mph)

Wind Speed Error Wind Speed Error Wind Speed Error V3 Wind Power Error
±0.05 m/s ±0.11 mph ±0.2 % ±0.6 %
±0.10 m/s ±0.22 mph ±0.4 % ±1.2 %
±0.15 m/s ±0.34 mph ±0.6 % ±1.8 %
±0.20 m/s ±0.45 mph ±0.8 % ±2.4 %
±0.25 m/s ±0.56 mph ±1.0 % ±3.0 %
±0.30 m/s ±0.67 mph ±1.2 % ±3.6 %
±0.40 m/s ±0.89 mph ±1.6 % ±4.9 %
±0.50 m/s ±1.12 mph ±2.0 % ±6.1 %
±1.00 m/s ±2.24 mph ±4.0 % ±12.5 %

So you live on a hill? (Placement)

If you live on a slope, you may be able to take advantage of your terrain to gain extra power output. If prevailing wind direction is mostly up hill, try place a wind turbine as far up the slope as possible. You will find the highest wind speeds there…even up to 10 % faster than the prevailing wind speed. This can give you up to 30 % gain in power output! (Test it with an anemometer to see the difference.)

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Guidelines for Anemometer or Wind Turbine Placement

For sparsely placed non-solid objects like trees:

  • Stay at least 2x the tree height upwind.
  • Stay at least 5x the tree height downwind, ideally 10x.

For sparsely placed solid objects like houses:

  • Stay at least 3x the house height upwind.
  • Stay at least 10x the house height downwind, ideally 20x.

For thin objects like poles:

  • Stay at least 20 pole diameters away.

Summary

If wind prevails from a single direction, the top of your house (or a hill) may be the best place. Up to 30 % gains in wind speed are shown in the above image where red contours represent highest wind speeds close to 7 m/s when the prevailing wind speed is 4 m/s. Wind accelerates as it flows past most solid objects’ tallest/widest points (the top of a hill, edge of a cliff, roof line) as shown in the above computational fluid dynamics (CFD) simulation picture. Object’s shape, orientation and wind direction are critical to maximizing wind speed gains. On the flip side, if a wind turbine is placed in the wake (wind shadow) of an object, losses, turbine noise and vibration can result and will shorten your turbines lifetime and power output significantly.


Watch Out!!!

Watch out for wind turbines which quote power output at high wind speeds (above 15 mph or 7 m/s) for the following reasons:

Reynolds Number (Re) …say that again!

In simple terms, Reynolds Number denotes a relation ship between air speed and an objects size. Basically, what Reynolds number tell us is that if air is moving past 2 objects are at the same Reynolds Number, then it will behave identically. To illustrate, lets compare a baseball and a softball, which is about twice big. If we throw a curve ball with both at 60 mph, they will curve differently. Now, since the baseball is half the size, if we throw it at 120 mph, it will behave the same as a softball at 60mph, as far as the air is concerned. Thus, an object half the size needs to go twice as fast to maintain identical aerodynamic behavior.

What do baseballs have to do with wind turbines?

A wind turbine with small blade width (narrow blades & small diameter) needs to spin very fast to achieve good performance...twice as fast as a turbine with double the blade width. So what? Well, as we all probably noticed from riding in cars, noise tends to greatly increase with increasing speeds.

And it gets even Worse.

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The darkest side of small blade width is inefficiency. Just look at the Reynolds Number Effects plot. The Vertical Scale denotes L/D…which is an engineers way of denoting aerodynamic efficiency and the horizontal scale is Reynolds Number. What the plot tells us that at low Reynolds Numbers, we don’t have any hope of achieving reasonable efficiency…this translates directly to power output.

Real numbers

For a 10 ft (3 m) diameter wind turbine spinning in 10 mph (4.5 m/s) wind, average Reynolds number will be 110,000 (1.1E5) based on a 6 inch wide blade (measured at 2/3 radius).  From the Reynolds number chart above, we can see how low the efficiency will be compared with larger wind turbines. There is no way to get around these basic physical concepts, no matter what a sales rep may say, so if you have a choice, always go bigger for a better return on investment.

So how do we fix it?

We can’t…but we can sure as heck avoid it! Since it is much harder to design an good efficient airfoil for low Reynolds numbers and because low wind speeds are prevalent over populated areas, here are some things to look for when choosing a wind turbine.

What to look for in a Wind Turbine:

  • A good design will quote power output at reasonable wind speeds…below 9 m/s or 20 mph. Idealy below 7 m/s (15 mph).
  • When comparing two designs, one with a larger blade width will stand a better chance of good power output at low wind speeds.
  • Always ask for power output charts plotted against wind speed and compare them to the average wind speed you measured around your house or wind site with an accurate anemometer.
  • Choose quality over price. Maintenance of wind turbines is complicated expensive and dangerous. Spinning blades can cause serious injuries and even death.

Noise!

Blade tip speed is the governing factor in noise, and it is proportional to diameter and rotation speeds (RPM). In simple terms: Power = Force * Speed. Thus, for identically sized wind turbines, one with less efficient airfoils will rotate faster to get to the same power output.

Generator Efficiency

Generator efficiency can vary from 70-98 % and proper generator sizing for a wind turbine is crucial for higher efficiencies)