A typical wind energy system consists of the turbine itself mounted on top of a tower and a controller consisting of power electronics which controls the power output of the turbine under various wind and grid conditions. Typically a controller will come as part of a package with the turbine making turbine and tower the major decisions in choosing your system. There are many different turbine manufacturers with many models of turbines to chose from as well as several tower configurations that vary in difficulty for erecting.
Choosing a small wind turbine model
Wind turbines come in many different makes and models. Spending a little extra time while choosing a model will improve economics of your project as well as make it much more successful in the long run.
Before choosing a turbine you should figure out if the electricity produced will be consumed completely on site, if you want to sell excess energy back to the utility company or if you prefer to be off the grid. The turbine model you choose will be greatly affected by where the electricity will ultimately be used and what compensation you will receive from selling electricity back to the utility. If your facility will be net metered or off the grid a load analysis for your home should be done to determine your monthly electricity usage, for net-metering, or your peaking load and daily usage for off grid applications.
Turbine manufacturers have published power curves that estimate the monthly output for their turbines based on average wind speed at the site and the Rayleigh distribution of wind speeds around that average.
Home Power Magazine is a good resource for comparison of small wind turbines. See the following articles on their website: "Apples & Oranges: Choosing a Home-Sized Wind Generator" and "Wind Generator Buyer's Guide."
Vertical Axis Wind Turbines (VAWTs)
Vertical axis wind turbines (they look like egg beaters) have been around longer than horizontal axis wind turbines (the most common type of wind turbine). Some vertical axis turbines have been very effective at producing energy. The concern for consumers is that many manufacturers of vertical axis wind turbines make inflated or fraudulent claims about the technology.
No matter what anyone says, vertical axis wind turbines still need to be placed on tall towers for ground clearance. Wind turbines are placed on tall towers because the wind speeds are higher and there is less turbulence higher above the ground, producing greater energy yield.
Some claim that vertical axis wind turbines are more efficient and operate in lower wind speeds than horizontal axis wind turbines. Modern horizontal axis wind turbines are actually better at capturing the wind than vertical axis machines given the same amount of swept area (which is the size of the circle traced by the outside tip of the blades). This is because vertical axis machines always have at least one blade traveling into the wind creating additional drag on the machine as it spins. Additionally, neither horizontal nor vertical axis of machines will produce much energy in low wind speeds because there is little energy to capture from low-speed wind.
Vertical axis wind turbines should not be mounted on rooftops, even if that makes them easy to install. Buildings and other obstructions will create turbulence in the wind as well as decrease its speed, significantly reducing the wind turbine’s ability to produce energy, even for a vertical axis turbine.
For further information about Vertical Axis Wind Turbines:
- Robert Preus – Thoughts on VAWTs: Vertical Axis Wind Generator Perspectives from issue 104 of Home Power magazine
- Mick Sagrillo – The Myths & Mysticism of Vertical Axis Wind Turbines
Buyers should beware of used, refurbished, or remanufactured machines. Even though these machines may initially be less expensive that comparable new machines, the initial savings can be negated very quickly by costly repairs from a machine that is nearing the end of its life time. Before purchasing a used machine read through this article by Mick Sagrillo from the November 2002 American Wind Energy Association newsletter about buying used wind equipment.
Choosing a Tower
Small wind system manufacturers often give you several different tower options to best match the economic, maintenance and space needs of your project.
There are four basic types of tower options for home and farm sized turbines: guyed monopole, guyed lattice, (self-supporting) lattice and (self-supporting) monopole.
Guyed Monopole: this type of tower is generally the least expensive. The tower is a tilt-up style tower and is generally installed using a gin-pole and winch. Guyed monopoles generally have 4 guy wires for support. Maintenance on the turbine is performed by tilting the tower down. The height of this type of tower is limited by the weight of the machine and generally 10 kW machines are the largest that can be placed on such towers. Tilt up towers require much space for guy wires and for tipping up and down of the tower. Check with manufacturers to determine what type of tower they recommend with particular turbine models.
Guyed lattice towers are generally more expensive than monopole designs but can support larger turbines at higher heights. Towers are generally installed using gin-pole and winch. Guyed lattice towers also come with the advantage of either being able to tip the tower down or relatively easy climb-ability for regular maintenance (climbing, if done with appropriate safety precautions, gear and training, is much safer than tilting a tower up and down every time maintenance is required). Guyed lattice towers require much space for tipping up and down of tower and guy wires. Again, check with turbine manufacturers to see what tower options they recommend with various turbine models.
Lattice towers, in most cases, are required for machines larger than 20 kW in size. The installation cost of these towers is generally more than that of a guyed lattice tower because a crane is needed to lift the tower and turbine into place. Maintenance is performed by climbing the tower. If major work is required on a machine a crane may be required to remove the turbine from the tower.
Monopole towers are generally regarded as the most aesthetically pleasing, but they also tend to be the most expensive and so tend to be used only with machines rated 10kW or more. All utility scale turbines are put on monopole towers. Check with your turbine manufacturer to determine what tower options they recommend with your turbine model.