page: 1, 2 Anemometer Accuracy

Geography and Wind

So you live on a hill?

Since you live on a slope, to gain extra power output, you may be able to take advantage of your terrain. 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.)

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.

For sparsely placed solid objects like houses:

Stay at least 3x the house height upwind.
Stay at least 10x the house height downwind.

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% gain in wind speed!...see image above)
Wind accelerates as it flows past most solid objects' tallest points (the top of a hill, edge of a cliff, roof line) (see above CFD picture)
Object's shape and wind direction are critical to maximizing wind speed gains, and if improperly placed losses and noise can result. (If you are unsure, contact us and we will help you out, most of the time free of charge)

Watch Out!!!

Watch out for wind turbines which quote power output at high wind speeds (above 15mph or 7m/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 60mph, they will curve differently. Now, since the baseball is half the size, if we throw it at 120mph, it will behave identically 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?

What this means for Wind Turbines is that a wind turbine with small blade width, is that it needs to spin very fast to achieve good performance. 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.

The darkest side of small blade width is inefficiency. Just look at the Reynolds Number Effects plot below. 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 a hope of achieving reasonable efficiency...that translates directly to Power Output.

10ft diameter Wind Turbine spinning in 10mph wind

Average Reynolds Number = 110,000 (1.1E5) based on a 6 inch wide blade (@ 2/3 radius).

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 9m/s or 20mph.
Idealy below 7m/s (15mph).
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 every site with an accurate Anemometer.
Choose quality over price. Maintenance of wind turbines is complicated expensive and dangerous.

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)