# ASK THE EXPERTS: Tall Towers vs. Efficiency

Intermediate
A vertical-axis wind turbine mounted on a short tower and located within the trees is a recipe for wind energy system disappointment.

I’ve heard over and over that tall towers are very important for wind energy generation. But, how important? Is there no wind to be captured on short towers? Is there a dramatic difference between a tower just above the treetops and a tower way above the treetops? How do I decide tower height, balancing the cost of a taller tower with the increased production?

Also, sometimes I wonder if some wind experts get fixated on tall towers and don’t pay enough attention to other important factors. How do I decide whether to spend my money on a more efficient machine or a taller tower? Thanks for any light you can shed on these issues.

Marvin Chessly • Memphis, Tennessee

Wind energy is generated by sustained wind power over a period of time. Wind power depends on the machine’s efficiency, but variations in efficiency can be 20% or more. Wind power depends on the machine’s rotor radius, squared—doubling the blade length will quadruple the power capture. But most importantly, wind power depends on the cube of the wind speed—doubling the wind speed will give eight times the power.

High above everything, the wind blows strong and straight. Closer to the treetops, it is slowed by friction and diverted into turbulent eddies. Below the level of treetops and roofs, there’s a zone of high turbulence, with very little potential for energy capture. Gusts may come, zooming around and knocking over the garbage cans, and brief peaks of power might be experienced and captured, but only by a wind turbine that could respond quickly enough, and none of these exist. These types of winds yield no sustained power, and generate no real energy at this lower level.

Wind turbine experts often recite the 30/500 rule: “Put your turbine at least 30 feet above any obstacle within 500 feet.” How much difference does it make? When you apply the cube law, the result is that there’s twice as much energy 30 feet above the treetops than there is 10 feet above the treetops.

In most situations, small-scale wind energy is not a particularly viable technology, although wind power is fun for many, and it can also work fairly well at certain exceptional sites, or on very tall towers, where the wind speed is suitable. But all too often we see completely unrealistic installations on rooftops or among tall trees. Anyone who studies the energy production of such installations will conclude that wind turbines are a complete waste of time. And this is a shame because it tarnishes the reputation of renewable energy in general.

Hugh Piggott • Scoraig, Scotland