You’ve decided you want to make electricity with the wind.

You have your eye on a high-quality wind generator, and

you’ve chosen the balance of systems (BOS) components.

What’s left is the biggest and most important job—choosing

and installing the tower.

The mounting structure for a photovoltaic (PV) array

puts the solar energy collectors up in the fuel—sunshine.

Towers for wind generators do the same thing. Wind is the

fuel for a wind generator, and to collect it, you have to get

your machine above obstructions. Buildings, trees, and hills

block the wind, slowing it down and causing turbulence.

The standard guideline is to site a wind generator at least

30 feet (9 m) above anything within 500 feet (150 m). The

entire rotor needs to be well above obstructions, so start

your measurement from the tip of the lowest blade. Doing

less is shortchanging your investment in wind energy—it’s

like putting solar-electric panels in the shade.

Your tower needs to support the weight of your wind

turbine and handle the thrust loads put on it by the wind.

It’s easy to underestimate the severity of the environment

that wind generators work in. If you ever see a catastrophic

failure of a wind-electric system, you won’t forget it. And if

you make the tower too short, you won’t get much energy.

Purchase and install a tall, sturdy, permanent tower, so

your wind energy experience will be long lasting and as

productive as your wind site allows.

Tower Perspectives

It’s easy to get focused on the wind generator as the

primary component in a wind-electric system. After all,

it’s the collector—the machine that converts the energy

in the wind to electricity. It moves, which is exciting and

attracts attention. But it is quite often not the most expensive

component in the system. The BOS components can easily

cost more than the turbine, and the tower can cost two to

ten times as much as the turbine, depending on the site

and situation. Take a realistic view of your plans to tap

wind energy by looking at the total system cost, not just

the turbine cost. Costs for a typical off-grid installation are

shown in the table on page 66.

 

A similar situation occurs when

it comes to installation. Students

attending wind system installation

workshops often expect that they will

spend a lot of time dealing with the

wind generator. In fact, most of the

installation time of a six-day wind

workshop is spent with the tower.

Assembling the wind generator and

attaching it to the tower takes only

a few hours, while assembling and

installing the tower can take two to

four days.

Tower Types

Three basic tower types are used for

almost all home-scale wind generator

installations. Tilt-up towers make

maintenance easy, with no climbing.

Fixed, guyed towers are very common,

climbable towers. Freestanding towers,

with no guy wires, are costly, but

attractive, and also climbable.

Tilt-up towers. My advice: If you have space for a tilt up

tower, use one! You will never have to climb your tower

(in fact, you won’t be able to). All maintenance will be done

with your feet on terra firma. If there’s any trouble with the

machine, you can have it down in less than an hour, and

back up in the same time once you’ve done the repair.

Tilt-up towers come in heights up to around 130 feet (40

m) for small-scale machines, with various sizes for different

machine weights and thrusts. The most common tilt-ups

are tubular steel, with sections of pipe coupled together,

and guy wires attached at each joint. All the guy wires on

one side of the pole (from each of the pipe joints) make up

a set of guy wires. For tilt-up towers, four sets of guy wires

are required, with three sets attached to one of the concrete

anchors placed at four separate points in a radius around a

concrete base at the center. The fourth set is attached to the

gin pole, which in turn gets attached to the fourth concrete

anchor when the tower is raised.

The major drawback of tilt-ups is the footprint needed.

You need a clear, open area for the tower, a diamond shaped

space (see diagram) that is as long as the tower

height plus the guy wire radius, and as wide as the guy

radiuses extending from the sides of the tower base. For a

100-foot (30 m) tilt-up tower, the guy radius will be about 50

feet (15 m); so a diamond-shaped area 150 by 100 feet will be

required. This area needs to be clear of trees and structures

so the guy wires can lie down cleanly. You’ll also need a

clear lane to drive a lifting vehicle, if you use one. Other

drawbacks: for minor repairs or service by people who are

comfortable climbing, a tilt-up can be less convenient than

a climbable tower. And you won’t enjoy the views from the

top of your tower!

 

Tilt-up towers consist of the tower pole and a “gin pole�

that is attached to it at 90 degrees. When the tower is down,

the gin pole sticks straight up in the air. When the tower is

up, the gin pole rests horizontally near the ground. The gin

pole is a big lever that allows you to easily lift the tower,

which pivots at its concrete base.

You can raise and lower the tower with a truck, tractor,

winch, come-along, or grip-hoist. The latter options allow

you to install towers in remote locations not accessible to

vehicles. The gin pole is generally 75 to 100 percent of the

guy radius in length. I prefer tower systems that use the full

guy radius for the gin pole length and permanently attach

the rear guys directly to the end of the gin pole.

Like all towers, tilt-ups have their hazards. Things can go

wrong. They can get dropped. Tow vehicles can slip. There

are real dangers if the anchors are not correctly positioned

and the guys get too tight while lowering or raising the

tower. You should do your homework before attempting to

install one, and always put the tower up without the turbine

on it the first time.

Fixed, guyed towers. Another type of guyed tower,

a fixed tower is lifted up once, and does not tilt down.

Guy wires hold it up, and any maintenance on the tower

or turbine is done by climbing the tower. These towers

come in various configurations, the most common being

triangular lattice sections, 10 or 20 feet (3 or 6 m) long, that

bolt together. You’ve probably seen this type of tower used

for commercial radio antennas and the like.

These towers must have a minimum of three sets of

guy wires, with an underground concrete anchor for each

set, and a concrete base under the tower itself. It’s possible

to install them one section at a time, using a different type

of gin pole, a vertical temporary crane that mounts on the

tower. The gin pole is moved up the tower one section at a

time, and is used to lift each succeeding section. This is a

slow, laborious process, and many people opt instead to lift

these towers with a crane.

While fixed, guyed towers don’t require the open area

that a tilt-up tower needs, you still must have open lanes

for the guy wires. The guy radius will be 50 to 80 percent of

the tower height, and the guy wires will be visible. Costs for

fixed, guyed towers are in the same general range as tilt-ups,

but these towers can be installed on many sites that will not

accommodate a tilt-up tower, mostly because fixed towers

don’t need as much cleared space, or as level ground.

Freestanding towers. If your budget isn’t tight, a

freestanding tower might be your first choice. No guy wires,

no tilting, and it only needs a modest clear space for the

tower base. The drawback, of course, is cost. Freestanding

towers rely on steel and concrete to hold them up instead

of guy wires—lots of steel and concrete. This means higher

cost for these materials, as well as for excavation, concrete

forms, rebar, and labor.

Freestanding towers take two basic forms. Most common

is the three-legged Eiffel Tower style, with tubular legs

connected by angle iron braces. The other option is a

monopole tower—a large, single tube, similar to what is

used for utility-scale wind turbines. These are often quite

expensive, and out of the financial reach of most small scale

renewable energy (RE) users. Both types are usually

assembled on the ground and lifted with a crane.

A freestanding tower will cost at least a third to half

more than a tilt-up or fixed, guyed tower. But the end result

may be worth it. Aesthetically speaking, most people prefer

not to look at guy wires. Less land clearing is necessary, and

the tower is less vulnerable to damage than a guyed tower.

Homebrew towers. Many RE enthusiasts like to do

things for themselves. While I have a great deal of respect

for home brewers, I urge you to be careful when it comes

to towers. This is no place for lightweight construction or

engineering guesswork. If you’re going to try to build your

own tower, do careful research. Look at engineered towers

and get a sense of the designs, as well as the size and quality

of hardware used.

When in doubt, overbuild. Better yet, stick with

engineered towers that are professionally designed for the

job. To obtain permits, you may need an engineer’s stamp

on your plans, anyway. Most tower manufacturers have

engineers on staff who can provide you with specifications

and calculations that will make your local engineer’s job

easier, and less expensive for you.

Choosing Your Tower

So how do you choose your tower? First of all, look at the

function. Each turbine manufacturer will tell you what

tower size (pipe diameter or lattice tower size) is necessary

to hold your wind generator. Using the 30-foot/500-foot

rule, determine how tall your tower needs to be. Consider

mature tree height, and remember that trees grow, while

towers don’t. Then look at what tower

options you have.

Look at your site. Is there space

for a tilt-up tower? Do you have the

available footprint for guy wires? Then

ask yourself whether you or someone

you hire is going to be willing to climb

the tower to do the regular, twice-a year

maintenance. And ask yourself,

your family, and neighbors about the

aesthetics. Take the time to go and look

at installed wind-electric systems to

get a sense of what you’re getting into.

Look at your budget. Many people

would love to have a freestanding

tower, but the cost is prohibitive.

Whatever your tower choice, avoid

the most common mistake in wind

system design—don’t make your

tower too short! Taller towers will always give you more

energy for your investment, and you will not regret going

higher. Take the time to research your tower choices, and

make the best investment for the long-term. If you don’t

have experience installing wind generators and towers, seek

qualified help. Tower installation is not something to be

taken lightly, but if you do it right, you’ll have a solid base

for making some or all of your electricity with the wind!

 

 

 

Ian Woofenden

©2005 Ian Woofenden