Wind Power Siting

by Larry Elliott

For many people the idea of producing household electrical power from a wind turbine is a romantic notion, a dream that rarely becomes a reality. Still for others, especially those living far from an electrical line or experiencing outrageous utility bills, it becomes a necessity. There are thousands of homes across the country now being powered by a wind turbine or combination of wind and other alternative electrical power inputs. Each installation's success or failure depends heavily on planning and correct installation. It is the critical planning and siting stage of an installation that will be discussed in this article.

Wind As Fuel

Cars, boats, planes, power plants or garden tractors, these all have something in common, they are machines that produce useable work or power by consuming a fuel. The amount of work they do or power they produce is directly related to their size and how much fuel is available for their consumption. In the case of a wind turbine, its fuel is the wind. The power available from any turbine is dependent on how much wind is available to drive the turbine. The quantity of wind is expressed in terms of wind speed or velocity. The higher the wind speed, the greater the potential output power we may expect from a wind turbine.

Betz's Equation

In order to illustrate just how important this relationship between wind speed and power output can be, a little math and physics is in order. A formula that describes power to wind speed relationship in a wind turbine was developed in 1927 by a German scientist George Betz. This formula states that the power available from a turbine is proportional to the cube of the wind's speed. In this equation P is the power produced in watts, E is the efficiency of the wind turbine in

percent, Rho (r) is the density of air, A is area of the areo turbine in silhouette in square feet, and S is the wind speed in miles per hour. The power which can be expected from a wind turbine is equal to the efficiency of the turbine multiplied by the energy delivered per unit time by the wind to the turbine. The energy delivered per unit time is equal to:

where m(t) is the mass of the wind impinging on the turbine blades per unit time and S is the wind's speed. The quantity m(t) is equal to rAS.

A combination of these two equations yields m(t) S 2

2 Betz's equation. In an average form this equation can be reduced to:

by assuming standard air density and normalized turbine efficiency.

Power by the Cube!

Basically all this math boils down to: the power available from the wind is proportional to the cube of its speed. As an example of this, let's assume we have a turbine that produces 100 watts in a 8 mph wind. At 16 mph you may expect this turbine to double its output to 200 watts, but instead it will produce over 800 watts. Thus it can be seen that a doubling of wind speed increases power available by a factor of eight times. A very small change in wind speed translates to a rather large increase in available power. A more dramatic look at this change would be the following. Assume that you have a wind turbine located at a marginally windy site that produces 100 watts in an 8 mph wind. If you had an increase in wind speed of only 1 mph your output would be 133 watts or an increase of 33%. Even small changes in annual average wind speed can determine whether or not your site is a cost-effective candidate for wind power.

How To Determine Wind Speed

Average wind speed is the critical factor that determines the economic effectiveness of wind machines. Let's look at some methods of determining wind speed. For those individuals who have lived for several years at a particular site, you probably have some idea of how

Wind

1000 Feet"

500 Feet

Height over Smooth Terrain Vs. Percentage of Maximum Wind Speed often you have windy days. For instance, how many days per week do you experience winds that raise dust, extend flags and streamers, or blow paper and cardboard about the yard. These winds are usually in the area of 8-12 mph. Another good indicator of your average wind speed would be trees and shrubs permanently deformed in the direction of the prevailing winds. Normally an average wind speed of at least 10 mph is needed to cause permanent deformation. If your site exhibits these characteristics, then perhaps further investigation is warranted. For those of you who have a site that really couldn't be described as windy, based on these observations, an alternative to wind power should be considered.

Use A Recording Anemometer!

If you feel your site is windy, and you are serious about installing a wind turbine, there is no more accurate method of site assessment than to install a recording anemometer. In an area of the country such as the great plains states or along a sea coast, a check with the local weather station might be sufficient to determine average wind speeds. But in most cases, the anemometer is truly your only source of accurate information on average wind speed. Don't consider wind power without a thorough measurement of the wind speed at your specific location. A recording anemometer should not be confused with an anemometer which measures only instantaneous wind speed. Rather than measuring a wind speed at any given moment in time, a recording anemometer measures cumulative wind speed. It constantly records wind speeds as a numerical count and then you simply need to divide this numerical count by the period of time over which you have been recording. This gives you an average wind speed over an extended period of time. In most cases, four months should be the minimum recording interval and one year is preferred. If you are going to spend a lot of hard earned money on a wind system, this extra eight months could mean the difference between a good investment and a bad one.

Proper Tower Placement

Although a recording anemometer is a very accurate instrument, its output will only give you wind speeds at a specific location. In areas of rolling hills or tree cover, the wind speeds can vary 30% or more between sites only 100 feet apart. The location of an anemometer on a specific site, as well as height above the ground and any obstruction, is critical to recording the highest winds available. For those of you who may be living in a very flat and wide open area this may not be as critical, but in rough terrain turbine location is everything. Referring to Figures 1 and 2, you can see how terrain can have an effect on wind speeds at certain elevations. Figure 1 shows a percentage of maximum wind speed to be expected over smooth terrain. At less than 50 feet above the ground, over 70% of maximum winds can be expected.

In Figure 2 we see that less than 10% can be expected at the same elevation when installed over rough terrain. On level land with no nearby obstacles, a 40 foot tower should be the minimum height for your anemometer or turbine. It is essential to measure windspeed at the actual height you plan on installing your turbine. Figure 3 illustrates a rule of thumb for tower height above obstacles and should not be ignored if maximum power is to be achieved. Remember, an increase of only 1 mph in wind speed gives a 33% increase in power. Obstacles or short towers are only robbing you of power. If you are considering placing your turbine on a hill to gain wind speed, you must be careful exactly where you place the turbine. Place the turbine high enough on the hill to enter the smooth undisturbed windstream.

Height over Rough Terrain Vs. Percentage of Maximum Wind Speed

As you can see the siting of a wind turbine is not a matter of simply erecting a tower and putting a generator on top. Only through accurate wind speed measurements on your particular site can you hope to install a wind system that is capable of supplying the power you need. In future articles we will look at methods of sizing your system and selecting a proper turbine output voltage. May your days be windy.

Larry Elliott is CEO of Cascade Wind Electric and is an expert in Jacobs windmachines and windmachine siting.

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