Free-standing towers are those that require no additional support: they are self-supporting. Because they have a relatively small footprint, they are ideal for applications where space is limited. For example, if you only have an acre of land.
The most common free-standing tower is the lattice or truss tower, like the one shown in Figure 9-9, Lattice towers
resemble the latticework of arbors* The Eiffel Tower in Paris is an example of a lattice structure* Lattice towers are secured to a massive foundation and are engineered to withstand powerful winds* Because of the large amount of metal that goes into them and because of the need for a sturdy anchoring foundation, they are one of the most expensive tower options*
Free-standing lattice towers are typically installed in 20-foot sections* The sections are usually preassembled and
welded or bolted together prior to delivery* When the sections arrive, they are bolted together on the ground, then lifted onto the foundation via a crane, and bolted in place* Some installers assemble lattice towers in the upright position piece by piece using a device known as a vertical gin pole* This is risky and time-consuming and generally avoided by experienced installers* Yet another method is to install a lattice tower with a hinge at the base. The tower is assembled on the ground, the wind turbine is attached, and then the tower is tilted up using a crane, thanks to the hinged base*
Another option for a free-standing tower is a monopole* This tower consists of rigid metal steel pipe* It is assembled on the ground, 20 feet at a time, then lifted and bolted onto a hefty concrete foundation using a crane* Monopole towers require a lot of steel and a very large foundation; therefore, they are the most expensive type of tower*
Whatever choice you make, be sure that the tower is strong enough to withstand both the winds in your area and the weight of the wind turbine — both of these can be quite substantial. Several of the largest home-sized wind turbines weigh around 1,000 to 1,500 pounds. The Jacobs 31-20, admittedly the heavyweight of the residential – or business-sized units, weighs 2,500 pounds!
Guyed towers typically consist of steel pipe or lattice uprights supported by cables called guy wires, a. k.a. guy cables, that run from the tower to anchors in the ground (Figure 9-Ю). (Please note that it is guy wires or cables, not guide wires.) Guyed towers are the cheapest towers and are therefore very widely used in the small wind industry. Steel pipe can be used for the masts of all household-sized wind turbines. The steel pipes are assembled
section by section, secured by bolts or slipped together, then erected using a crane.
For larger small wind turbines, the lattice tower is often the tower of choice. They’re strong, mass-produced for the telecommunications industry and therefore widely available, and they are cheaper than other options. They’re also available in different strengths. Like free-standing lattice towers, they can be assembled in the horizontal position on the ground then
tilted into place, or they can be assembled upright, one section at a time or in their entirety, using a crane.
In all guyed towers, strong steel cable is attached to the tower and anchored in the ground using an assortment of anchoring mechanisms, depending on the soil type. Stranded wire cable ranging from one quarter to one half inch is typically used.
Installers use three guy wires for each section of the tower, located 120 degrees apart. Manufacturers typically specify guy
radius — that is, how far out the anchors for the guy wires need to be for optimal strength.