Wind farms built from the ground up
Credit: By: RYAN D. WILSON, News Editor, Clay Center Dispatch, www.zwire.com 20 January 2011 ~~
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Wind turbines just don’t pop up overnight.
On Tuesday Bruce Graham, from Cloud County Community College’s Wind Energy program, took Lion’s Club members through the whole process of what it takes to build and maintain a wind turbine.
In constructing a wind turbine, workers dig out a hole to build the base and lay down what’s called a “mud mat” – a thin layer of concrete a few inches thick that’s put on the ground so crews aren’t working in the mud.
He showed what the foundation for the turbine looked like before they started pouring any concrete.
“Right out in the center you can see four poles sticking up,” he said, “They’re going to use those here in a little bit to hold the pedestal that’s a form for the very top of the foundation. Right in the center here, it’s hard to see, but there’s an orange circle – that’s where the bolt cage is going to go. That’s the bolts that hold the wind turbine.”
Next Graham showed pictures of workers staging the rebar.
“Each piece of rebar has to be propped up off the wood mat so when you pour the concrete, the concrete can come underneath that rebar and hold all that rebar together,” Graham explained. “You can see back in the back a crane lifting all that. Those pieces of rebar are about two inches (thick) in size. One person can’t even pick up one piece of rebar, it’s that heavy.”
Each piece of rebar is precut, pre-formed and they’re put together “kind of like an assembly line or erector set,” Graham said.
“They’re all ready to go, they just have to put them in the right spot,” he said.
One wind turbine foundation has 41 tons of rebar in it, “and if you take that times 67 turbines, that’s a lot of rebar out there” in a wind farm, Graham said.
As the concrete for the foundation is poured, PVC pipe is put around the bolts in the bolt cage to keep the concrete from grabbing a hold of the bolt.
“You don’t want the concrete to hold the bolt, you want the bolt to hold clear down to the bottom ring which is called the embedment ring,” Graham said. “There’s a nut under that and you want to be able to stretch those bolts just like a rubber band all way up, put the nut on them and they will hold the turbine. The turbine can actually move back and forth and stretch the bolts, but the bolts don’t break – they hold them all in place.”
Forms to pour around the bottom are put up as the same time the central pedestal is set up, Graham explained, showing pictures of workers setting up forms.
“Remember those four poles that started out?” he said. “That’s where that pedestal will rest on.”
The pedestal is the only part that will stick out above the ground, the rest of the foundation is buried, Graham said.
When the concrete is poured for the foundations, it is “very, very thick,” Graham said.
“It has to be thick because they’re going to dome it up,” he said. “It can’t be soupy or sloppy or it will run out of the forms and you wouldn’t have anything.”
The foundation also has a lot of concrete in it – 525 cubic yards per foundation, which is 50 semi-loads.
When they pour, they dump three trucks at once and use a conveyor belt to hold the concrete and direct it where it needs to be.
“They can’t pour these things one truck at a time because there’s 50 of them and it will take all day to get done,” he said. “The wind farm near Concordia was built in the middle of the summer, so it gets very, very hot, so that real stiff concrete sets up real quick. They’ve only got a few hours to be able to do it.”
The concrete is poured as “a monolithic pour,” meaning it is poured starting at the bottom, never stopping as they go all the way to top. Stopping at all during the process would create a cold lap.
At the Concordia wind farm they started at 5 a.m. when it was cooler. They also used chillers on the ladder to keep concrete cooler as they mixed it in to keep it from setting up so quickly, Gram said. They also painted trucks white to reflect some of the heat.
“Here you can see workers finishing off the top,” Graham said, then he pointed to a chute where the concrete comes down. “This worker is moving that (chute) around. He’s just follows around in a circular pattern, filling that foundation pour and the workers then smooth it up as they go.”
Workers also use vibrators “so it all settles out and there aren’t any voids in the concrete.”
To finish the concrete, worker use a jitterbug and remote-controlled truck dumping concrete. A curing agent is put on as the last step in pouring the concrete to keep the surface from curing too quickly and chipping off the top.
All the dirt that was excavated will be put back on top of the foundation.
“They’re going to put every bit of dirt that was in that hole back in that hole,” Graham said. “And you’re thinking, there’s a great big rock in the middle of that. What they’re going to do is compact that dirt so tight that all of that dirt will still fit in there. So that compaction actually becomes part of foundation. It’s pushing down with extra weight on this tower so it won’t tip over. So the dirt itself becomes part of the engineering of the foundation.”
Graham showed pictures of the working putting in underground power lines from wind turbine to wind turbine. They did with three large rolls of wire spooled from one trench to another. These lines eventually connect to a substation that’s connected to the power grid.
PVC pipe also laid underground with the power lines, which fiber optic communication wire will be blown through after the tower constructed.
After dirt has been backfilled, the only part of the foundation sticking up is a 16-foot pedestal with 144 foundation bolts, where the tower will be placed on top in four sections.
Once it’s assembled the tower will be about 300 feet tall.
“It takes a large crane to be able to put these pieces together,” Graham said, he showed pictures of one very large crane with a much smaller “tail off” crane opposite it. “They use it (the smaller crane) so that when the big crane picks up the tower it doesn’t slide across the ground.”
The little crane is actually quite big, it just looks very small in comparison to the large crane, which can lift 700 tons 300 feet in the air. The tracks on the crane are 45 feet wide.
When they begin to stack the tower, they only do it two sections at a time and stack several towers at a time.
“They can’t stack all four sections at a time,” Graham said. “Just the hollow tube on top of that will cause a natural frequency vibration and it will collapse the whole tower just because the wind’s blowing over the top of it.”
The last two sections of the tower are stacked when the nacelle can be placed on top of it, which dampens that vibration so that they don’t collapse.
Before the bottom section is put up, workers pour grouting around the bottom to level it “so this tower sits 100 percent flat on the foundation,” Graham said. “They don’t want any voids, any air pockets.” Vents are also installed in the bottom circulate air flow and prevent moisture from building up inside the tower.
On the inside of the tower, a very strong magnet attaches the ladder, landings and doors on the inside of the tower.
“They don’t want any drilling, welding or anything damaging the outside of that tower because it will weaken it, so everything is attached with magnets on the inside – even the front doors,” Graham said.
When lifting the nacelle, which is 70 tons, workers will use the smaller crane as a counter weight to the nacelle, Graham said, showing pictures of how it was done. Something this big requires tag lines tied to it to keep the wind from blowing it around.
The nacelle “is where all the component parts are,” Graham explained. “It’s about the same size as a Greyhound bus. It has the generator, the gear box, the transformer, all the component parts.”
The part sticking out in the front with three holes in it is the hub and is where the blades will be attached. The blades are 148 feet long, about the as long as one of the higher buildings in town. Graham showed pictures of the blades laying on the ground from various angles to show that they aren’t flat like they appear to be from a distance.
“They’re not flat, not straight but curved and made to catch as much wind as they possibly can,” he said.
The process of attaching the first blade is called “stabbing the blade” because that’s exactly how it looks.
“They’ll pick the blade up, they put it in there and there’s two people on the inside of the nacelle who actually put the nuts on the bolts and attach that blade to the rotor. Then they’ll rotate it around and attach the next one.”
Power is generated through the rotor spun by wind catching the blades at the top. The power is fed through feet of copper wires from the top to the bottom of the tower.
Graham showed an aerial view of another wind farm in construction to show that workers required a fairly large footprint around the turbine when constructing it.
“A lot of people say they destroy the earth … and they do – they come in and make a big pog here,” he said. “But when they get done, right out in the middle you see that little bolt ring, that’s the only part that won’t be put back just exactly the way it was.”
The only footprint that turbines take up when they are complete are the bolt rings and access roads from turbine to turbine. Farmers can farm right up to the base of towers, Graham said. He said he’s often asked many acres will be taken away from farm with wind towers on it.
Typically it’s about 2 to 3 percent, or at most three acres in a 100-acre field, and that includes the turbine itself and access roads.
“And they’re going to pay you well to have the wind turbine there,” Graham said. “The bigger the turbine, the more the farmer or landowner gets.” The national average is about $10,000 per turbine per year, plus payment of royalties on all energy produced from the turbine and payment for every foot of access road and transmission lines, he said.
“The farmer makes out really well if they put turbines on his land,” Graham said.
In maintaining a turbine, four areas a wind technician has to be skilled in are hydraulics, mechanical, electrical and computer networking. A technician can’t just do well in one area, because they are required to keep a wind turbine running, Graham said.
Maintenance includes changing oil filters, greasing, tightening bolts and “a lot of the stuff an auto mechanic does, only at a lot higher altitudes,” Graham said. They also do a lot of testing of batteries, diagnostics and trouble shooting.
Technicians don’t use conventional wrenches. Because everything’s big they use hydraulic tools, including hydraulic wrenches that fit in the palm of one’s hand and costs $3,000.
Every bolt needs a different wrench, so there are a lot of expensive tools needed to maintain a tower.
Bolts are tensioned when they are tightened, which is different than torquing with a conventional wrench and more precise. Tensing bolts tighten by stretching the bolt and hand tightening bolts.
Graham also showed pictures of the inside of a nacelle, which is something very few people see because they’re not typically allowed.
All the towers are monitored through a computer system with 300 sensors that monitors everything from wind speed and electrical output to how hot the gear box is and oil levels.
Graham, from Clay Center, started Cloud County Community College’s wind program as a night class with four students. Now there are 110 full-time students in the program, with five full-time faculty and five adjunct faculty. The program now has its own wind towers.
One of the hardest things they teach students is how to put up a 50-meter met tower (a tower that monitors the wind) but it has to be put up all at once, and “is like putting up a 150 string of spaghetti that’s been cooked.” Every semester students put up and take down that tower.
The other experience students really like is high and low rescue.
“You climb up a perfectly good turbine, get off the top of that, use a rope and down to the ground,” Graham said. “If you were ever working up there in the nacelle and there was fire down in the base of the turbine, you can’t go back down the way you came up, so you have to learn how to come off the top and rescue yourself. Students really like that part – some of them do, some of them leave there fingerprints in the ladder as they go up.”
Graham also briefly discussed assessing properties as potential sites for wind turbines, what wind is ideal for wind turbines, and what’s required to acquire properties for wind turbines.
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