Harvest the Wind
(Page 4 of 7)
June/July 2002
By Michael Hackleman and Claire Anderson
Wind plant rotors have either two or three blades. Wind plant expert Mick Sagrillo recommends three-bladed rotors. Sagrillo explains in his wind power video (see "Resources," Page 78) that although each blade spoils, or reduces, the wind for the blade behind it, a three-blade rotor is able to yaw—move—freely. A two-blade machine goes through cycles of yawing and stopping, resulting in vibration that fatigues the machine's fasteners and welds fairly quickly, Sagrillo says.
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MORE TOWER POWER
Three main types of towers are suited for residential wind plants: freestanding, guyed and tilt-up towers. Most of us have seen freestanding towers gracing old farm pastures. Because they have a small footprint, freestanding towers are ideal for homeowners who have limited space. However, freestanding towers are expensive due to the amount of metal used in their manufacture.
Guyed towers are the cheapest towers available. A guyed tower consists of a slender pole tied to the ground by at least three guy lines. Both guyed towers and freestanding towers can be installed piece by piece—a task that's time-consuming and requires a crew of people—or can be assembled on the ground and lifted into place by a crane. Once in place, maintenance must be performed on both guyed and freestanding towers by climbing up to the wind plant: a job not for the faint of heart.
Tilt-up towers are a variation of guyed towers, but are braced with at least four guy lines for safe and easy raising and lowering. Unlike freestanding and standard guyed towers, they appeal to many people for their ease of maintenance. With the use of a winch and a small farm tractor or other large vehicle, the tower can be safely lowered to the ground and wind plant checks can be performed on terra firma. The tower is then raised again.
Budget between $1,300 and $10,000 for a tower, based on the size of the wind plant.
Even if you are connected to the grid, you may still want to include a small battery bank in your system to provide backup power if the grid goes down. If you are off the grid, then you will need to decide how large you want your battery bank to be. (You'll probably want to evaluate the benefits of a part-wind, part solar power system.) The battery bank must be sized to absorb the influx of wind-generated electricity and bridge the gap of windless (and sunless) periods.
Battery banks for small systems are usually built around 6-volt, deep-cycle, lead acid batteries. Similar in size and weight to a car battery, they have a service life of four to five years. Larger systems use batteries with larger cases, or one large single case for the whole bank. These cost more but are higher quality and may last as long as 10 years or more. The trick to a longer service life is periodic maintenance and limiting the depth of the discharge to no more than 50 percent of the bank's capacity.
While lead-acid battery technology is less efficient than some newer types, it is nearly a 100-percent-recycled industry, and at this point it offers a better overall cost/benefit ratio than nickel-cadmium or nickel-metal-hydride (NiMH) batteries for the typical renewable energy system.
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