In our last episode, we outlined some ways of estimating the wind resource in your area. We also
developed a method to determine the average wind speed at hub height for your proposed wind generator. The question before us now is: where on your property is the best place to put your wind generator tower?
The answer to this one is easy. Put the tower where it’s windiest! It is actually windier in some places on your property than on others. In this article, we will show you how to analyze your specific location so that you can maximize your wind resource.
Tools
The purpose of examining any potential renewable energy (RE) site is to optimize the RE resource, and therefore, the power output of the proposed RE generator. To accomplish this, you will need to gather some information about your site, such as:
• Location of vegetation and buildings
• Prevailing wind direction/directions
• System voltage (battery charging systems only)
• Surface roughness or topography.
From this information, we will develop a series of rules or guidelines that will help you qualify the resource at your site.
Vegetation and Buildings
Since any site analysis requires a specific location, we’ll use our homestead as an example. Figure 1 illustrates the layout of the buildings and most trees on our property. Also noted are the compass points, and a distance scale. The numbers indicate the approximate heights of the trees and buildings. The terrain in our area is somewhat rolling and relatively open, but spotted with buildings, overgrown fencerows, groves of trees, and the occasional woodlot (several hundred acres). While we live in farm country with some forested areas, this site need not be rural, strictly
speaking. It could very well be a small town, suburban development area, or the edge of a larger city.

Tower Height
Remember from “Tower Economics 101” that our arch enemy is turbulence. It robs us of our fuel, the wind, and puts unnecessary wear and tear on the wind generator. Therefore, rule #1 is: minimize turbulence as much as possible. We’ll return to turbulence later.
Another important consideration is that the wind generator must be at least 30 feet above anything
within 500 feet — a minimum requirement. Let’s call this rule #2. For anything larger than about 1 kiloWatt (kW), the “30 foot” above rule must include the blades as well.
As an example, let’s assume that we will install a large wind system, say a high voltage 10 kW wind generator with a 24 foot rotor diameter. If we add the radius of the rotor (12 feet) to the 45 foot silo and the “30 foot above rule”, we find that the minimum tower height for this location is 87 feet. Since an 87 foot tower is not readily available, we’ll opt for the next closest size, 90 feet.
Notice that we went up in height rather than skimp with an 80 foot tower. This is the minimum tower height for this location given a 10 kW wind generator.
We have the wind generator, system voltage, and tower height selected. Our next question is where do we put it to best utilize our wind resource.
Prevailing Winds
To answer this question, we need to know the seasonal wind patterns for this proposed site. This
should be fairly obvious for your location if you have lived there for a least a year. If you are new to the area, ask your neighbors.
At our place, the wind blows quite regularly during the fall, winter, and spring. Summer winds are restricted to frequent thunderstorms. While brief, these storms are usually accompanied by high winds for several hours. Winter is dominated by winds out of the north and northwest. Fall and spring bring winds from the south and southwest. While the winds from a summer storm
come from any direction, they arise most frequently out of the west. We can see a pattern developing here (Figure 2).

The best place for the tower which optimizes our prevailing wind directions is west of the barn. We now have rule #3: after you determine the direction of the prevailing winds, site the wind generator upwind of any buildings or trees.
But Mick, you say, the wind sometimes blows from the northeast, east, and southeast. At those times, the tower and generator are downwind of the trees and buildings and in the zone of turbulence. Won’t we have problems when the winds are from these directions?
Unless we install a movable tower (something no one has perfected yet), the answer is yes. Because the wind blows from any compass direction some of the time, we have to compromise. Since we are interested in where the wind blows most of the time, we have to accept that occasionally the tower is in a less than ideal location. This brings us to rule #4: in a site analysis,
tower placement minimizes those compromised locations, thereby maximizing our wind resource.
Wire Restrictions
Since we know the rotor size and building height, we can easily determine the minimum tower height at this location. The distance of the wind generator to its controller is of little concern because it is a high voltage system. We can locate the tower almost anywhere on
the property without worrying too much about wire size. In a wind system, your wire run includes the distance from the tower to the controller plus the tower height, and back again. Because wire runs in a wind system can add up fast, higher voltage systems are preferable.
Why is this, you ask. The power that a wind generatorproduces is a function of volts multiplied by amps.Thus, voltage and current are inversely related. If thevoltage doubles, the current carried by your wires is cutin half, for a given amount of power.
The power lost in a wire run is a function of the wire’s resistance (a function of the length and diameter)multiplied by the square of the amps that the wirecarries. For a given wire gauge, doubling the voltagemeans that the wind generator can be located fourtimes the distance and still have the same power loss! With wind generators, higher system voltages areeasier to work with and more cost effective than lower system voltages. Which brings us to rule #5: raising
system voltage gives you considerable flexibility in where you can put the tower.
Low Voltage Dilemma Now, let’s muddy the waters by lowering the voltage. Rule #3 would favor the location west of the barn. A tower near the house would be downwind of buildingsand trees. It would see a lot of turbulence and reduce our power output. “Tower Economics 103”
diagrammed the zone of turbulence behind such obstructions. But let’s suppose that we already have a low voltage system in the house, and upgrading to a higher voltageis not an option. The distance from the proposed towersite west of the barn to the house is 400 feet. Add the 90 foot tower height, and we have a low voltage dilemma. We cannot run 12 Volts for nearly 500 feet.
Now what do we do, Mick? We can opt to move the tower closer to the house. If we place the tower within 30 or so feet of the house, we reduce the wire run to under 150 feet. This will
require some gonzo wiring, but is acceptable cost-wise. But now the only place for the tower is downwind of thebuildings, which puts us into a zone of turbulence (see
Figure 2).Installing a wind generator tower is not like plopping PVs on the ground somewhere. It is a considerable project, involving concrete. We want to get it right the first time (concrete is not very forgiving). Therefore we must determine the extent of the turbulent zone at our proposed tower site.
Mr Wizard!
Now comes the fun! You will need a “giant weather balloon”, the kind available from scientific mail order stores. (Admit it; you always wanted one of these as a kid. Now, you have an excuse to buy one!) Take it down to your local FTD florist, the folks who sell helium filled anniversary and birthday balloons. Have them fill it with helium. You might want to do this outside. Few florists have enough room (especially florists specializing in cacti) or doors wide enough to
accommodate six foot weather balloons.
Once you get the damn thing home in one piece, it’s time to conduct our experiment. Whatever you do, don’t let the balloon go or it’s back to the florist. Get someone to help (Mr. Wizard always has an assistant) rather than attempt this alone. Tie the balloon to a nylon cord about 100–120 feet long. This is your tether line. The other end of this cord should be securely fastened to a stake driven into the ground. Place the stake where you want the tower to be. Tie a second
nylon cord, about 200 feet long, to the balloon. This is your tag line. As your helper lets out the tether line with the balloon, tie streamers made of four feet long pieces of yarn or crepe paper every ten feet along this line. Keep the tag line from getting tangled up in the tether line and
streamers. Tie the tag line off in the upwind direction once the balloon is in the air. Get the balloon to fly directly over your proposed wind site, as in Figure 3. Why, you ask, are we doing all of this? Elementary, my friend. We are trying to determine the boundary layer where turbulence ends and smooth flowing laminar air begins. The streamers in the turbulent zone will ruffle in the wind, whipping around in all directions. Once you cross the boundary layer and reach the undisturbed air, the streamers straighten out and only blow downwind of the tag line. Count off the streamers to determine the height of the smooth flowing air. This is the minimum height for your wind generator tower.

Trees, Trees, Trees
Let’s assume that we have not just one tree, but an entire forest of trees. We have done the above
exercises, and we know that the forest canopy is about 60 feet tall. Is a wind generator still practical? Sure, but we need to get the generator at least 30 feet above the top of the tree line. In this case, we need to know the approximate age of the trees, their species, and the height that these particular types of trees will reach at this location. Why? Because of rule #8: trees grow, towers don’t!If the forest is made up of maples and ash, and the canopy is 60 feet high, you can be fairly confident that these trees are mature and will not grow much taller. You would be safe with a 90–100 foot tower. If the 60 footers are Douglas fir trees and you install a 90 foot tower, you can be assured that in not too many years the wind generator will be engulfed by branches. In
most areas, Doug fir will be more than 90 feet tall. Your time and investment will have been wasted.

And More Trees!
Let’s complicate the situation a little. Assume that you own ten acres of cleared land in a national forest made up of oaks or aspen trees. Either species will mature at around 40 feet in your area. Your house is only 18 feet tall and situated smack dab in the middle of the ten acres. That puts you at least 500 feet away from the trees. Since you are 500 feet away from the nearest obstacle except the house, you can get by with a 48 foot tower. Right?
Not in this case. While it may seem that you are an adequate distance away from the trees, you are
essentially in a sheltered hole! The tree canopy is the effective ground level as far as the wind is concerned. A 48 foot tower at this location would be analogous to an eight foot tower on open ground. The minimum tower height in this instance would be 70 feet (40 foot trees plus the “30 foot rule”). If you wanted to install a 1 kW wind generator with a ten foot rotor, a safer bet would be to include the radius of the rotor (or length of a blade) at five feet, to this height. This would
put the tower height at 75 feet. Since there are no other obstacles above the forest canopy, prevailing wind direction is of little concern to us. One final note about trees: avoid putting towers amidst a dense group of trees. It complicates the installation and jeopardizes the safety of the installer or service people climbing the tower. You don’t want tree branches rubbing on the tower. It’s not safe for the tower or the trees.

To Tree or Not to Tree
Folks who live around tall trees may feel discriminated against when it comes to tower height. They’re right. But that’s the price one pays for living in a tall neighborhood. Figure 6 graphically represents what I am talking about.
If we assume a given location with a given wind speed at any point in time, Figure 6 depicts how tall your tower must be at a minimum to compensate for the surrounding vegetation. Does this mean that you should cut down all of your trees? Hardly! Just ask anybody in Kansas about the value of shade. Does this mean that a wind system in an area with tall trees is impractical? Not at all. No matter where you live, you still have to install the wind generator on a tower. So what we are discussing is the incremental cost increase of an additional 30 feet of tower compared to
the cost of the entire system. (“Tower Economics 102”)

Low Voltage Options
Since many Home Power readers already have low voltage systems, and a goodly number also probably live near tall trees, let’s revisit this issue. Going back to rule #5, I recommend increasing system voltage first. Resistance to change aside, this will always be the cheapest option. In addition, systems need not be restricted in total power output. A 12 Volt system, for example, is limited to a power output from a few hundred watts to about 1.5 kW. Due to the amount
of current that must be generated, building 12 Volt wind equipment larger than this capacity is just not practical. Because of this, 12 Volt systems larger than 1.5 kW are not even commercially available. There are, however, a few tricks that are available to low voltage system users facing extremely long wire runs. Many of the newer wind generators utilize permanent magnet alternators. These alternator’s produce something called three phase wild ac, which is rectified to DC with diodes in the system’s controller before travelling to the battery bank. A three phase wind system utilizes three wires to transmit current down the tower to the controller. A DC system only
uses two wires, positive and negative. The advantage of three phase ac is that the current
produced by the alternator is split up and transported equally by its three wires. By contrast, all the current produced by a DC generator is transported down a single wire. Three phase alternators can use lighter gauge wire for a given wind generator capacity at a given distance. Therefore, three phase permanent magnet wind generators offer greater flexibility in siting — they allow longer distances from the battery.

High Voltage Transmission
One final trick that can be used for low voltage systems is to actually install a high voltage wind generator, but then step the voltage down at the batteries. With a three phase ac alternator, the solution is simple: use a three phase step-down transformer before the rectifiers. These transformers are relatively inexpensive when compared to cost of enormous wires. Transformers are only about 85% efficient, so some power is lost in the process. The efficiency can be boosted somewhat by adding capacitors to the system. While a 15% loss of efficiency may be considered
extreme by some, it is a small price to pay when the option is a much-compromised tower location tethered to welding cable-sized wires. There is no such thing as a DC transformer for systems
utilizing a DC wind generator. However, a linear current booster (LCB) performs essentially the same function as an ac step-down transformer. An LCB is a high speed switching device that will allow the user to transmit high voltage DC, reducing the voltage at the battery bank. LCBs are also about 85% efficient. They are, however, more expensive than transformers. While neither the transformer nor the LCB is a cheap option, they do allow the user to cut down on the size wire needed. In many cases, these devices more than pay for themselves in money and aggravation when compared to the cost of heavy-duty wires. But their big advantage is that they allow the user to site their wind system at the most ideal location rather than compromise with less than ideal tower placement. We have customers using these devices that have sited their wind generators as far as 2000 feet from their battery bank.
Sounds of Silence Broken
You may be thinking, why not virtually eliminate the wire run by just mounting the tower right on top of their house. Don’t even consider this idea for several reasons. First of all, few roofs have the structural beefiness to support the loads presented by a wind generator and tower, let alone when the wind is trying to blow them over.Second, even if your roof was strong enough or the tower was short enough or the generator was light enough, there is another serious drawback to this idea.
Any rotating electrical generating device produce a harmonic that we can perceive by touch as a vibration. That harmonic vibration is transmitted down the tower. Touch one sometime and you will feel what I mean. If the tower were attached anywhere to a building, the building itself would begin to resonate, similar to a guitar’s sounding box. This resonance would vibrate the building, doing who knows what kind of structural damage over time. In addition, the resonance would
develop into sound inside the building. Unless you are deaf and don’t mind things walking off horizontal surfaces or jumping off walls whenever the wind is blowing, this is a pretty bad idea.
Recap
Proper siting of a wind generator tower is actually easier than initially meets the eye, if you remember thefollowing guidelines:
1. Minimize turbulence.
2. Install the tower at least 30 feet above anything
within 500 feet.
3. Note your prevailing winds and stay upwind of
any obstacles.
4. Minimize compromises in location, voltage, and
tower height.
5. Consider higher voltages.
6. Rougher surfaces produce gustier winds.
7. If downwind of obstacles, compensate with a
taller tower.
8. Trees grow, tower don’t.
9. Never attach the tower to your house.
You now have the tools to properly site your wind generator. Take the time to ask questions and do it right. You will be rewarded with a system that takes the best advantage of one of Nature’s greatest and most powerful gifts: the wind.

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