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wind towers: height and Types

The electric output of small wind turbines can be greatly increased by simply increasing the height at which the turbine is suspended in the tower.

Obviously, that's not the height in itself that explains any spectacular increase in their productivity. Variations of 20% in the wind speed can determine variations of more than 50% on the electricity output: that's the "cube rule": the output of a wind turbine system is proportional to the cube of wind speed.That happens because wind speed can increase sharply with higher heights; more height means less obstructions to the wind and more speed, and more speed represents more productivity of the wind system. That's a well known rule: the productivity of a wind system increases more than proportionally with the increase in the wind speed (see Box).

Obstacles & wind speed

Fences, trees, buildings and many other obstructions can hugely reduce wind velocity close to the ground, or increase the wind turbulence, severely reducing the productivity of wind turbine systems.

This is particularly apparent with urban wind-electric systems; they have very small towers - most of the time just a mere pipe - and that, and the many obstructions limiting the wind speed in our cities and towns, can make them very ineffective...

How tall should a turbine tower be?

The towers of small wind turbine systems are supplied together with the other elements; manufacturers demand their turbines to be mounted on their towers and within specific height limits.In other words: the goal of the tower is to place the turbine above wind turbulence and any obstruction limiting the wind speed. The goal is not to place it as high as possible...

A very general rule of thumb recommends that the height should take into account the height of the surrounding obstacles; or, more precisely, the height should allow the bottom of the turbine blades to be at least 30 feet/9 meters above the top of any obstacle within 300 feet/90 meters of the tower.

Height and capacity of a wind turbine
The height of small wind turbines doesn’t depend on the capacity of the turbine system. Size and tower height are rather un-related. A 5-kWh turbine can have a tower from, say, 30 to 140 feet in height… See also: Small Wind Systems Location

Local and state rules about Wind Turbine Systems height

Legislation and covenants may impose height restrictions to wind tower height: rules imposing maximums of 30-40 feet/9-12 meters are common, and they may affect very negatively the productivity of wind systems.

Types of small wind turbine towers

There are two main types of wind turbine towers: the self-supporting or free-standing type, and the guyed type.Tilt-down towers are a good choice for small wind turbine systems. They are more expensive and very common in hurricane-prone regions, for an obvious reason: they can be lowered down. But they also offer to their owners an easy way of performing routine maintenance….

Guyed towers are the least expensive, easy to install and the most common; they involve different designs and materials (avoid aluminum towers: they are prone to cracking), but aren't easy climbable - an important feature for inspections or repairs - and require more land than self-supporting towers.

Non-guyed tilt-up/cylindrical towers use pipe or tubing and a self-supporting design. They do not use guy wires and have a smaller footprint. These towers can include climbing pegs but are relatively expensive.

Mega turbine towers
Mega turbines towers are made of tubular steel and their height is chosen according to specific site conditions. In the Texan Snyder Wind project, in Scurry County, the 21 Vestas V90 3.0 megawatt turbines are mounted on 345 feet (104 m) towers, because the best wind conditions are at that height. That's a high height, compared to most other cases..

Lattice configuration

Lattice towers use welded steel profiles and are a cheap and tested solution. Most lattice towers aren't guyed, but there are also guyed configurations: three-legged lattice structures suspended on all three sides by guy wires. They are usually climbable.