TOWER SELECTION

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(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 rimary component in a wind-electric system. After all, t’s the collector—the machine that converts the energy n the wind to electricity. It moves, which is exciting and ttracts attention. But it is quite often not the most expensive omponent in the system. The BOS components can easily cost more than the turbine, and the tower can cost two toTen times as much as the turbine, depending on the site nd situation. Take a realistic view of your plans to tap ind energy by looking at the total system cost, not justthe  turbine cost.

Tower Comparison

Tower Type Advantages Disadvantages

Tilt-up No climbing Large footprint Maintenance on ground Four sets of guy wires Medium cost Need relatively level site ipe locally available annot climb for minor work Takes longer to assemble Fixed, guyed Modest footprint Three sets of guy wires Lowest cost ust climb Uneven sites OK Crane cost Freestanding Small footprint High cost No guy wires Must climb Uneven sites OK Cost of crane Safest installation (crane) installation  A freestanding tower can be the most visually pleasing and adaptable to varied terrain, but is the most expensive.

Ian Woofenden

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-ayear 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.

Check the Regulations

When considering potential sites for your windgenerator, make sure to check local land use laws, zoning laws, and with building officials, for any regulations that will affect installing a wind turbine. Some local codes may restrict tower height or require a “fall zone” around your tower. Other issues about noise and aesthetics may come up, as well. However, if you live in a rural location, and aren’t within a mile of an airport, the height of your wind turbine’s tower probably won’t be an issue. And fall zones should be a concern only when you are installing or raising and lowering towers. Common sense should suffice about how close an installation should be to a residence or other structures. Fall zones are not required for utility poles, trees, or buildings. Properly engineered and installed wind generator towers are safe to install within range of people and buildings. If you’re worried about a tower falling, perhaps you should buy a sturdier tower, or not install a wind generator. Nonetheless, local restrictions may apply, whether they’re practical or not. You may need to start your project by educating and lobbying local government.

Guyed Towers have guy wires—lots of steel and concrete. This means higher cost for these materials, as well as for excavation, concrete forms, rebar, and labor. Also a larger footstep.

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 smallscale 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.

HOME BREW TOWERS

Many RE enthusiasts like to dothings for themselves. While I have a great deal of respect for homebrewers, 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 researc h. 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.