February 4, 2006

Tons of conrete, blasting of bedrock for small wind turbine

From "Tons of concrete, massive bolts to secure windmill to earth" (East Bay (R.I.) Newspapers, Feb. 2), here is a description of the platform for a relatively small 660-KW Vestas V47 wind turbine. Its total height will be 241 feet (tower 164', blades 77'), 100-180 feet shorter than models currently being pushed for utility grids (and not singly, as in this case, but in groups of dozens, sometimes hundreds.

This is going into the grounds of the Portsmouth Abbey School, who think they are going to get half of their electricity from the turbine. They are looking at the projected average output, however, ignoring the fact that a wind turbine generates at or above its average rate only a third of the time. And much of that time is likely to be when there is low demand.

Of course, they will still be connected to the grid, and any mismatch of supply and demand will be handled there. The school may gain some savings from net metering, at the expense of other customers on the system.
... The ingredients for that base rolled into the school aboard caravans of trucks. Twenty mixer trucks full of cement and eighty 27-foot long by one-inch diameter steel rods all sunk through bedrock in a 30-foot deep hole should keep the turbine firmly tethered to earth.

... Halfway down they struck rock, "solid rock all the rest of the way down."

The school located a licensed blasting company which agreed to take on the job. ...

Next, a 15-foot diameter corrugated steel pipe, of the sort used in drainage systems, was lowered into the hole and an outer two-foot ring of cement (120 cubic yards worth) was poured between the pipe and the bedrock to form an outer shell.

A team of laborers, among them Brother Joseph, Paul Jestings, the school's director of operations, and Henry duPont, ("Our wind turbine expert from Block Island") climbed down into the hole and threaded the 80 heavy threaded rods into their templates. It is to these rods that the turbine tower will be bolted.

Another corrugated pipe, this one narrower at 13-feet, was lowered into the hole and filled to the top with dirt. Then the two-foot space between the two pipes was filled with 80 yards of concrete, effectively sandwiching the bolts in solid concrete. The whole thing was capped with reinforced concrete and, once cured, will provide an immovable foundation for the turbine to come. ...
It seems rather a lot to put up with for such an intermittent and variable source of power.

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