Put a wind turbine on high, open ground with nothing upwind, and raise the rotor at least 10 meters above any obstacle within 150 meters. Wind speed rises with height as v2 = v1 × (h2 ÷ h1)^α, so with the farmland exponent α = 0.20 a hub at 24 m sees 1.19 times the speed measured at 10 m, which is 1.19³ = 1.69 times the power.
Part of Advanced Wind Turbine
Siting a turbine means choosing the ground and the rotor height so that the rotor sees fast, smooth air; it is the cheapest way to gain power.
High, Open Ground Beats A Better Blade
The carrier’s rules are three. Prefer elevation, since hilltops, ridgelines and coastal bluffs are ideal. Keep clear exposure, with no buildings, trees or hills within 150 meters upwind, because obstacles create turbulence that damages turbines. And put the turbine at least 10 meters above any obstacle within 150 meters; taller towers always produce more power.
Two other pages in this corpus state this rule differently. Tower Height says 30 feet (9.1 m) above anything within 300 feet (91 m), and Wind Power says 10 meters above any obstruction within 100 meters. The carrier’s rule is the strictest of the three on both height and distance, and this page uses it. Tower Height gives the reason for the distance: an obstacle’s turbulent wake extends roughly 10 times the obstacle’s height downwind, so a 15 m tree line disturbs the air for about 150 m behind it (Terrain Effects & Turbulence).
Wind Speed Rises With Height By A Power Law
The Tower Height page gives v2 = v1 × (h2 ÷ h1)^α with an exponent α that depends on how rough the ground is:
| Ground | α |
|---|---|
| Open water, ice | 0.10 |
| Flat open grassland | 0.14 |
| Farmland with hedgerows | 0.20 |
| Small town, scattered trees | 0.25 |
| Forest, suburbs | 0.30 |
| City center | 0.40 |
Because power goes as the cube of speed, the power gain from a taller tower is (h2 ÷ h1)^(3α). Relative to a 10 m height, from that formula:
| Hub height | α = 0.14 | α = 0.20 | α = 0.30 |
|---|---|---|---|
| 15 m | 1.19 | 1.28 | 1.44 |
| 20 m | 1.34 | 1.52 | 1.87 |
| 30 m | 1.59 | 1.93 | 2.69 |
| 40 m | 1.79 | 2.30 | 3.48 |
Rough ground gains the most from height. The Tower Height page prints a table of its own with larger values than its formula gives: for 30 ft against 20 ft at α = 0.14 it shows 1.29, while (30 ÷ 20)^0.42 = 1.19. The numbers above are recomputed from the formula, and the printed table should not be used.
Worked Example: From A 10 m Reading To A 24 m Hub
An anemometer on a 10 m pole in farmland (α = 0.20) records a mean of 4.0 m/s.
- Speed at 24 m: 4.0 × (24 ÷ 10)^0.20 = 4.0 × 1.191 = 4.77 m/s.
- Power at the same distribution of gusts: 1.191³ = 1.69 times.
- For the 3 m rotor of Average Wind Speed & Power Curve: 178 W average at 4.0 m/s becomes 297 W at 4.77 m/s, a gain of 1.66 times once the cut-in and rating are included.
A hub at 24 m is within the carrier’s ideal band of 15 to 25 m and under its 30 m ceiling for a guyed pipe tower without engineering analysis. The step from 4.0 to 4.77 m/s moves the site from “moderate” toward the upper part of the carrier’s 4-5 m/s band (Wind Resource Assessment).
Method: Choose The Spot
Step 1: Mark the prevailing wind direction from the Beaufort observations and the lean of trees.
Step 2: Walk 150 m upwind of each candidate in that direction and note the tallest obstacle. The hub must clear it by 10 m.
Step 3: Prefer the higher ground, then re-check the obstacles from there.
Step 4: Check the cable run. The turbine’s battery bank should be at or near the tower base. A long DC run at 12 V loses power in the wire just as hydro does (Battery Bank Sizing).
Step 5: Keep a fall zone clear. If the tower falls it sweeps a circle of its own height around its base. Keep houses, paths and livestock out of it, and plan the guy anchors inside it (Foundation & Guy Wire Anchors).
Common Mistakes
- Siting for convenience — next to the house, behind the barn.
- Measuring wind at 3 m and building at 24 m without scaling it up.
- Using the printed gain table in the Tower Height page.
Safety
A falling tower covers its own height
A guyed pipe tower and turbine can fall if a guy or anchor fails. Keep the base of the tower away from houses, roads and power lines by at least its height, and never raise or lower it in strong wind.
What’s Next
- Learn next: Terrain Effects & Turbulence for the wake behind obstacles
- Combine with: Guyed Tower (Steel Pipe) to build the height you have chosen