Wind Turbine Tower Design
Site Assessment
Wind is caused by the sun unevenly heating the Earth’s surface. The difference in temperature between one region and another causes a pressure difference, which creates airflow from high pressure areas to low pressure. The fastest and strongest winds are located in the upper atmosphere, while the slowest and weakest are at ground level. Ideally, the higher a turbine is installed, the better it will perform. It would be a good idea to consult a wind map to determine the mean annual wind speed in your area at different heights to establish a proper cost benefit analysis for an installation. Generally, a 5+ meter per second mean annual wind speed will suffice to justify the cost. You can check out the global wind atlas at the following link to verify your local wind speeds: https://globalwindatlas.info
The next thing that needs to be done is to make sure the installation site is suitable. That means no large buildings or trees that will obstruct clean wind from hitting the turbine. There should be at least a 100 ft radius between the turbine tower and any large objects, and the tower should be high enough to put the turbine above the tallest obstacles. You should also beware of local zoning laws, and consider the safety of neighbors if you live in a suburban area. Checking in with your local municipality to make sure your installation isn’t violating any regulations before starting a project like this would likely save some headaches down the road.
Tower Types
Knowing the condition of the soil is important as well, as it will determine how you anchor the turbine tower to the ground. Some turbine towers are self supporting concrete or steel tubes, like the industrial turbines that most people are used to seeing. Others are mounted to lattice towers, which can also be self supporting but instead of the tower being a pipe or concrete tube, it's created with a system of trusses or latticework. These are common for commercial installations like at a farm or business. For cost efficiency, though, most residential micro turbines are mounted to a guyed tower that's either a thin lattice type or, typically, a large steel pipe that's hoisted into place with a winch and a gin pole, and then secured by stainless steel cables called guy wires that are attached to heavy concrete anchors in the ground.
All turbine towers sit directly on a reinforced concrete foundation, regardless of the type, but the foundations for guyed towers can be considerably smaller because the wind load is shared by the guy wires and their anchors in 4 directions, whereas self supporting towers require a foundation that's buried deep into the ground, is heavy, and heavily reinforced in order to protect against high wind loads.
Anchors
Other anchoring options for guy wires are available if you have the right soil conditions, such as screw augers that screw into the ground, duckbill anchors that are driven into the ground, rock anchors that attach directly to large boulders or bedrock, or even just a deadman anchor, which is similar in concept to a concrete anchor but is just any large, irregularly shaped heavy object that's buried deep into the ground with a guy attachment, like a large horizontal pipe. If in doubt, you should consult an inspector with your local municipality and have them test the soil at your chosen location and recommend the right approach.
Guy Wire Connections
The guy wires should be stainless steel and sized according to the load. For example, a small 4-6ft diameter turbine on a 50ft tower would require at least 3/16" diameter guys, 8-10ft turbines would need 1/4"+, 12ft turbines should be 5/16"+, etc. Most residential micro turbines don't get much bigger than that, though, increasing tower height will call for increasing wire size as well.
Large turnbuckles are used to make the connections between the anchors and the guy wires, which allows for adjustment and tightening of the wires after the tower is raised in place so there's no wobbling. Any slack in the wires will result in movement that allows the tower to build momentum in high wind scenarios as it wobbles back and forth and increases the load on the guy wires. Not only will movement affect how well the turbine blades track the wind and extract power, but it increases the risk of snapping the guys and crashing the turbine into the ground. The guys are attached to thick reinforcing brackets on the tower with stainless steel quick links.
Large turnbuckles are used to make the connections between the anchors and the guy wires, which allows for adjustment and tightening of the wires after the tower is raised in place so there's no wobbling. Any slack in the wires will result in movement that allows the tower to build momentum in high wind scenarios as it wobbles back and forth and increases the load on the guy wires. Not only will movement affect how well the turbine blades track the wind and extract power, but it increases the risk of snapping the guys and crashing the turbine into the ground. The guys are attached to thick reinforcing brackets on the tower with stainless steel quick links.
Note that the wires are looped through the quick links and have a thin sleeve inside of the loops, called thimbles. These protect the wires from chaffing and kinking over the links. Another important thing to keep in mind when making and connecting guy wires is to make sure the saddle of the wire clamp is on the 'live' side of the wire, which is the longest side (see image below). If the saddle is on the dead side of the wire, then the wire will eventually if not immediately come apart under a load. It's commonly called "saddling a dead horse". It might seem just as strong, but it's a tried and tested recipe for disaster.
The brackets for connecting the guys to the tower should be welded directly over any welded joints in the tower pipe to provide the best support. That also means the joints in the tower should be planned accordingly, so you'll need to investigate your installation site and decide on anchor placement and guy length before building the tower itself. The angle of the guy wires after the tower is raised shouldn't exceed 45°, so if your tower is 50ft long then your ground anchors will be 50ft away from the base of the tower. Using Assuming the ground is relatively flat and level, then you can use Pythagorean theorem to approximate the length of the guys as the square root of the result of a^2 + b^2 = c^2
Example:
C = √(a^2 + b^2)
= √(50^2 + 50^2)
= √[(50 * 50) + (50 * 50)]
= √(2500 + 2500)
= √5000
= 70.71 ft
Example:
C = √(a^2 + b^2)
= √(50^2 + 50^2)
= √[(50 * 50) + (50 * 50)]
= √(2500 + 2500)
= √5000
= 70.71 ft
Add another 12-16" to allow for some slack to make the looped ends for connecting and adjusting. You would be lucky to find a single pipe for a turbine tower that's 50ft long, so there's bound to be a joint to connect two shorter pieces together in this case, ideally at the halfway point. This location should also have guys attached for reinforcement, as shown below. Again, joints are the weakest points, so every joint should be reinforced by guys to prevent the turbine from buckling in high winds. The uppermost guys should be attached approximately 6" below the tips of the turbine blades to prevent a collision in high winds. Most turbine blades will flex in high wind, so there's a real risk of colliding with the guys if they're connected to the tower anywhere behind the blades. Always connect them below the blades, even if you think there's enough room to sneak them higher.
Tower & Foundation
The tower itself should be made from schedule 40 or schedule 80 steel pipe. All joints should be reinforced with sleeves on the inside of the pipe, and brackets mounted at right angles to the outside of the pipe that will also make the connections to the guys. The top of the tower will also need to be fitted with a sleeve that accommodates the yaw pipe on the turbine, and a slip ring if you choose to use one. A slip ring is a type of electrical connector that allows 360° rotation without tangling the transmission lines. Here's an example.
The diameter of the tower pipe will depend on the swept area of the turbine, because the swept area determines how much wind and thus how much force will be exerted on it. As a general rule, the tower pipe diameter should be at least 2" for 4ft turbines, 2.5" for 6-8ft turbines, 3" for 10ft turbines, and 3.5" for 12ft turbines.
The base for a guyed micro turbine tower should be made of reinforced concrete that's large enough to accommodate a steel hinging mechanism for raising and lower the tower, and it should be heavy enough to resist lifting when the tower is being raised or lowered, as well as the lateral forcing from the wind and tightening the guys. It doesn't need to be enormous like a self supporting tower, but don't cheap out, either. The base should be heavier than the tower and the turbine combined, X2 for good measure. Concrete weighs approximately 150lb per cubic foot. A 3" diameter schedule 80 pipe weighs around 10.5 lbs per foot. A 50ft tower made of it would weigh 525 lbs, plus at least another 100 lbs for a typical 1 kW wind turbine. So you would want at least a 600lb base, which would require 600 / 150 = 4 cu.ft of concrete, or a 2ft square slab that's 1ft deep. Adding just 6" to those dimensions would more than double the weight.
The diameter of the tower pipe will depend on the swept area of the turbine, because the swept area determines how much wind and thus how much force will be exerted on it. As a general rule, the tower pipe diameter should be at least 2" for 4ft turbines, 2.5" for 6-8ft turbines, 3" for 10ft turbines, and 3.5" for 12ft turbines.
The base for a guyed micro turbine tower should be made of reinforced concrete that's large enough to accommodate a steel hinging mechanism for raising and lower the tower, and it should be heavy enough to resist lifting when the tower is being raised or lowered, as well as the lateral forcing from the wind and tightening the guys. It doesn't need to be enormous like a self supporting tower, but don't cheap out, either. The base should be heavier than the tower and the turbine combined, X2 for good measure. Concrete weighs approximately 150lb per cubic foot. A 3" diameter schedule 80 pipe weighs around 10.5 lbs per foot. A 50ft tower made of it would weigh 525 lbs, plus at least another 100 lbs for a typical 1 kW wind turbine. So you would want at least a 600lb base, which would require 600 / 150 = 4 cu.ft of concrete, or a 2ft square slab that's 1ft deep. Adding just 6" to those dimensions would more than double the weight.
Stub at the top of a tower with a 6 wire slip ring and locking collar installed to support the turbine.