Proper Siting of a Wind Machine
A wind turbine must be mounted in a good wind site, well above ground clutter in the strongest, smoothest winds. Wind site assessors begin the process of siting a wind turbine by determining the prevailing wind direction at a site.
Although winds blow in different directions at different times of the year, or even within the same day, they arrive from one or two directions predominantly over the course of the year. For example, in many places in North America, winds come predominantly from the south- west. They often blow from the north- west in the winter.
To determine the predominant wind flow, ask the advice of farmers, who work outdoors and hence are familiar with wind patterns, or contact a local airport. They may be able to provide you with a wind rose, a graphical representation of wind direction (Figure 1).
In a wind rose, the length of the spokes around the circle is an indication of how frequently the wind blows from a particular direction. The longer the line, the greater the frequency. In the wind rose in Figure 1, the winds blow predominantly from the southwest. A wind rose also indicates the percentage of total wind energy from each direction, which is very helpful. You can also find data on wind direction at the NASA Surface Meteorology site.
In an open site, with little ground clutter, a wind turbine can be located almost anywhere — so long as the entire rotor is mounted 30 feet above the tallest obstacle within a 500 foot radius and you’ve taken into account future tree growth, if trees are the tallest objects. Unfortunately, very few of us live on ideal sites. There’s almost always some major obstacles.
To site a wind turbine, first determine the prevailing wind direction, then look for a location for the tower that’s upwind of major obstacles. Although winds will shift so that upwind temporarily becomes downwind, situating your wind turbine and tower this way will ensure that it can take advantage of the strongest prevailing winds.
When siting a wind machine, it is also a good idea to minimize wire runs from the turbine to the controller and inverter to reduce line loss. As a rule, the higher the wind turbine’s voltage, the farther it can be sited from the point of use. When installing a turbine, contact the manufacturer or an experienced installer for recommendations.
Tower Height Considerations
Once you’ve identified the best site, you must determine optimum tower height. To produce as much electrical energy as possible at a site, the rule of thumb is that the entire rotor should be at least 30 feet above the tallest obstacle within a radius of 500 feet.
When calculating minimum tower height, don’t forget to take tree growth into account. If the trees on your property will grow 20 feet in the next 20 to 30 years, the life expectancy of a wind system, add that to the tower height for the best long-term performance.
If your site is within a quarter of a mile from a forest or good-sized wooded lot, the top of the nearby tree line is the height you want to exceed. Mount the wind turbine using the tree line as the height you must exceed. Don’t forget to factor in tree growth.
If you are installing a wind turbine in an area with more than 50 percent deciduous tree cover, the effective ground level is two thirds of the tree height. If trees are 60-feet high, for instance, the effective ground level is 40 feet. A 100-foot equivalent tower would, therefore, need to be 140 feet high to take into account the trees.
Bear in mind that the height recommendation is the minimum acceptable tower height. Savvy wind energy installers exceed the rule and see increased performance because of it. It usually costs very little to increase tower height by another 20 to 40 feet and the return on this small investment is quite impressive. We don’t know anyone who has installed a wind turbine who says, “I wish I’d bought a shorter tower.” However, we know lots of people who wish they had purchased a taller one.
Tall Tower Economics
When you talk to professional wind system installers, you may hear statements to the effect that it doesn’t make sense to mount a smaller turbine, for example, one with a seven- or eight-foot diameter rotor, on a tall tower. This is flawed reasoning. Tower height should be determined by the height of obstructions in the area, not the size of the wind turbine or the towers a manufacturer or dealer sells.
A 50-foot tower slightly downwind from a 65-foot-high tree line isn’t going to produce much electricity. Moreover, the turbine will produce even less electricity as the trees grow over the 20- to 30-year life of the wind system. Remember: energy output and the economics of the wind system are both proportional to V3 (the cube of the wind speed).
Although it is sometimes hard to justify a tall tower for a small turbine, that doesn’t mean that the right decision is a short tower. The right decision is to invest enough in your tower to make the most of your turbine’s potential — or choose another renewable energy system.
If you are thinking about installing a smaller wind generator, but are nervous about the cost of a taller tower, we recommend that you calculate how much more the tower will cost and how much more electricity the turbine will produce on a taller tower.
In our experience, installing a taller tower always results in the production of substantially more electricity. Even though it will always cost more money, the important question to ask is whether the increased tower height is justified economically by the increase in electrical production. In most cases, it is.
Aircraft Safety and the FAA
Another factor to consider when installing a wind turbine is its impact on aviation. If the tower exceeds a certain height and is within a certain distance from an airport, you will need to file for a permit from the Federal Aviation Administration (FAA). As for height, an FAA permit is required if the tower is over 200 feet, which is extremely rare for a small wind turbine.
The second condition that requires an FAA review and permit is if the wind generator is approximately two to four miles from a “public use” or military airport.
Whether you need a permit depends on the length of the runway. In such instances, the FAA will determine the height of the tower you can install. They may also require top-of-tower warning lights. Note that permits are not required when siting a turbine near private landing strips with no public access, airfields not shown on FAA maps, or landing strips that are not in use. If you’re hiring a professional installer, he or she can advise you on this matter.
Protecting Against Lightning
Although lightning is not attracted to tall metal objects, such as a wind generator tower, as is commonly thought, it is important to install lightning protection. Grounding rods attached to the tower bleed off the static charge created as air masses move across the Earth’s surface. They’ll reduce the likelihood of a direct strike. Ground rods are eight-foot long copper-coated rods. They are driven into the ground at the base of the wind turbine tower.
Grounding a tower minimizes lightning strikes, but does not guarantee that lightning will not strike your tower. Backup is needed in the form of lightning arrestors.
Lightning arrestors “bleed off ” electricity in case of a direct or nearby strike, protecting sensitive equipment. A professional installer will provide recommendations.
Surge arrestors should also be installed on the electrical wire running down the tower and the utility wiring for grid-tied systems. They protect against surges of electricity induced in the wire by lightning strikes. The surge protectors on the util-ity side of the system protect against lightning strikes in utility lines, which are much more frequent than on properly grounded wind turbine towers.
While protecting against direct lightning strikes is important, nearby strikes pose the gravest danger to a wind system. When lightning strikes the ground near a home, it creates an electrical current in the atmosphere and/or the ground. This current creates a voltage wave that resembles ripples in a quiet pond after a pebble is dropped into it.
If one of these waves crosses a conductor like a wind generator tower or buried wires, an electrical current will be created (induced) in the conductor. This current can fry sensitive electronic components of a wind system. Surge protectors in a wind system will help protect against this phenomenon.
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