Out here in the desert, we have a lot of those tall, metal, windmills. They have a windmill way up at the top to catch the wind. What I would like to know is if you put another windmill down lower on the same "post" would that increase the output of electricity it makes?

The best height for the turbine depends on where the wind is, apparently:

'NREL's advanced wind turbine program assumes that hub heights increase from 90 feet (27 meters), considered the base case, to 180 feet (55 meters) with only a 10% cost penalty. The tall tower is justified if there's a high wind shear, such as NREL believes exists on the American Great Plains. With wind shear approximating the 1/7 power law, doubling tower height will increase the power available in the wind 45%. NREL's research, says Sue Hock, has found wind shear greater than the 1/7 power law.

Minnesota, currently a hotbed of wind activity, will be the first region on the Great Plains to test the merits of tall towers in North America. Minnesota's Department of Public Service is seeking funding for a series of tall anemometry towers to measure the wind shear up to 60 meters above ground level at several sites in the state specifically to evaluate the production benefits of taller towers.

There are less benefits from tall towers in Britain, suggests Andrew Garrad, where there's stronger winds at 40 meters than on the American Great Plains. In some cases, such as National Windpower's Bryn Titli site in Wales, there's planning pressure for shorter towers to reduce the obtrusiveness of the turbines...' www.wind-works.org

(The article goes on to discuss other factors influencing height - FAA regulations, unsightliness, the type of tower and so on...)

I guess putting two turbines on one tower would not be very efficient - the tower would have to be made a lot stronger (and you'd have to synchronise the blades), which could be more expensive and problematic than just having another tower. If you wanted turbines on two levels in one area, it might be cheaper to mix short (single turbine) towers in with tall (single turbine) towers.

Thanks newnickname. I wondered about that. It just seemed like a awful lot of tower to waste. We have at least six tall towers here in Idaho along the highway between Twin Falls Idaho and Boise Idaho.

One consideration is where the maximum flow of air is at a height. The Altamont pass has a lot of windmills and they are set at a distance above the ground to capture the maximum amount of wind with the least amount of turbulence. Also Sheer.

Turbulence can be easily caused by wind moving along the ground, which could lead to whipping, gusts and multi directional winds. Wind Gennys work best if the point in one direction and are not subject to turbulence.

Another consideration is that the fans themselves cause a bit of turbulence as the air passed over them. Shaped like a wing, windmill fans generate ares of high pressure and low pressure. The wind forward of a windmill may be in straight lines, at the windmill it breaks up an "eddies" behind the windmill for a short distance the wind is in swirls, eddies and turbulence.

These large Gennys that we see are commercial grade, they are designed big to maximize the surface area of each fan (wing/blade) of a windmill to extract as much movement from the least amount of wind as possible. Size matters as well as in gear ratios. The larger the diameter of the fans, the faster the inner spindel to the Genny turns. More power can be had the longer the blades are. Thus there is somewhat of a need to make them as big as possible to maximize the different ratios of fan to Genny.

Interesting fact is that the windmill part (blades and tower) usually costs less than the generator part, the part that is spun to make electricity. The materials in a Genny, copper wiring, steel for magnets, plus all of the electronics to either convert from DC to AC or to convert 10 hertz (from slow spin) to 60 hertz (standard home current) cost more per Genny.

The size of a genny does not affect the price of manufacture as much as how many you build. Building two small gennys could be 3 or 4 times more than building a genny twice the size producing twice the amount of power.

In order for wind power to be an economical alternative to coal, natural gas, wood it has to be designed maximizing the materials while harvesting as much wind energy as possible.