On-site fabrication process makes taller wind turbines more feasible

On-site fabrication process makes taller wind turbines more feasible
Model of a turbine constructed with Keystone Tower System's spiral tapered welding process. Credit: Keystone Tower Systems

Wind turbines across the globe are being made taller to capture more energy from the stronger winds that blow at greater heights.

But it's not easy, or sometimes even economically feasible, to build taller towers, with shipping constraints on tower diameters and the expense involved in construction.

Now Keystone Tower Systems—co-founded by Eric Smith '01, SM '07, Rosalind Takata '00, SM '06, and Alexander Slocum, the Pappalardo Professor of Mechanical Engineering at MIT—is developing a novel system that adapts a traditional pipe-making technology to churn out wind turbines on location, at wind farms, making taller towers more economically feasible.

Keystone's system is a modification of spiral welding, a process that's been used for decades to make large pipes. In that process, steel sheets are fed into one side of a machine, where they're continuously rolled into a spiral, while their edges are welded together to create a pipe—sort of like a massive paper-towel tube.

Developed by Smith, Takata, and Slocum—along with a team of engineers, including Daniel Bridgers SM '12 and Dan Ainge '12—Keystone's system allows the steel rolls to be tapered and made of varying thickness, to create a conical tower. The system is highly automated—using about one-tenth the labor of traditional construction—and uses steel to make the whole tower, instead of concrete. "This makes it much more cost-effective to build much taller towers," says Smith, Keystone's CEO.

With Keystone's onsite fabrication, Smith says, manufactures can make towers that reach more than 400 feet. Wind that high can be up to 50 percent stronger and, moreover, isn't blocked by trees, Smith says. A 460-foot tower, for instance, could increase energy capture by 10 to 50 percent, compared with today's more common 260-foot towers.

"That's site-dependent," Smith adds. "If you go somewhere in the Midwest where there's open plains, but no trees, you're going to see a benefit, but it might not be a large benefit. But if you go somewhere with tree cover, like in Maine—because the trees slow down the wind near the ground—you can see a 50 percent increase in energy capture for the same wind turbine."

Solving transport problems

The Keystone system's value lies in skirting wind-turbine transportation constraints that have plagued the industry for years. Towers are made in segments to be shipped to wind farms for assembly. But they're restricted to diameters of about 14 feet, so trucks can safely haul them on highways and under bridges.

This means that in the United States, most towers for 2- or 3-megawatt turbines are limited to about 260 feet. In Europe, taller towers (up to about 460 feet) are becoming common, but these require significant structural or manufacturing compromises: They're built using very thick steel walls at the base (requiring more than 100 tons of excess steel), or with the lower half of the tower needing more than 1,000 tons of concrete blocks, or pieced together with many steel elements using thousands of bolts.

"If you were to design a 500-foot tower to get strong winds, based on the force exerted on a turbine, you'd want something at least 20 feet in diameter at the base," Smith explains. "But there's no way to weld together a tower in a factory that's 20 feet in diameter and ship it to the wind farm."

Instead, Keystone delivers its mobile, industrial-sized machine and the trapezoid-shaped sheets of steel needed to feed into the system. Essentially, the sheets are trapezoids of increasing sizes—with the shorter size fed into the machine first, and the longest piece fed in last. (If you laid all the sheets flat, edge-to-edge, they'd form an involute spiral.) Welding their edges assembles the sheets into a conical shape. The machine can make about one tower per day.

Any diameter is possible, Smith says. For 450-foot, 3-megawatt towers, a base 20 feet in diameter will suffice. (Increasing diameters by even a few feet, he says, can make towers almost twice as strong to handle stress.)

Smith compares the process to today's at-home installation of rain gutters: For that process, professionals drive to a house and feed aluminum coils into one end of a specialized machine that shapes the metal into a seamless gutter. "It's a better alternative to buying individual sections and bringing them home to assemble," he says. "Keystone's system is that, but on a far, far grander scale."

Behind Keystone

Smith, who studied and electrical engineering and computer science at MIT, conceived of a tapered spiral-welding process while conducting an independent study on wind-energy issues with Slocum.

Running a consulting company for machine design after graduating from MIT, Smith was vetting startups and technologies in wind energy, and other industries, for investors. As wind energy picked up steam about five years ago, venture capitalists soon funded Smith, Slocum, and other wind-energy experts to study opportunities for cost savings in large, onshore .

The team looked, for instance, at developing advanced drivetrain controls and rotor designs. "But out of that study we spotted tower transport as one of the biggest bottlenecks holding back the industry," Smith says.

With Slocum's help, Smith worked out how to manipulate spiral-welding machines to make tapered tubes and, soon thereafter, along with Slocum, designed a small-scale, patented machine funded by a $1 million Department of Energy grant. In 2010, Smith and Slocum launched Keystone with Rosalind Takata '01, SM '06 to further develop the system in Somerville, Mass. The company has since relocated its headquarters to Denver.

In launching Keystone, Smith gives some credit to MIT's Venture Mentoring Service (VMS), which advised the startup's co-founders on everything from early company formation to scaling up the business. Smith still keeps in touch with VMS for advice on overcoming common commercialization roadblocks, such as obtaining and maintaining customers.

"It's been extremely valuable," he says of VMS. "There are many different topics that come up when you're founding an early-stage company, and it's good to have advisors who've seen it all before."

Opening up the country

Keystone is now conducting structural validation of towers created by its system in collaboration with structural engineers at Northeastern University and Johns Hopkins University. For the past year, the startup's been working toward deploying a small-scale prototype (about six stories high) at the MIT-owned Bates Linear Accelerator Center in Middleton, Mass., by early 2015.

But last month, Keystone received another $1 million DOE grant to design the full mobile operation. Now, the company is working with the Danish wind-turbine manufacturer Vestas Wind Systems, and other turbine makers, to plan out full-scale production, and is raising investments to construct the first commercial scale machine.

Although their first stops may be Germany and Sweden—where taller wind towers are built more frequently, but using more expensive traditional methods—Smith says he hopes to sell the system in the United States, where shorter towers (around 260 feet) are still the norm.

The earliest adopters in the United States, he says, would probably be areas where there is strong wind, but also dense tree cover. In Maine, for example, there's only a small percentage of the state where wind power is economically feasible today, because trees block wind from the state's shorter turbines. In the Midwest, wind energy has already reached grid-parity, undercutting even today's low-cost natural gas—but in areas like New England and the Southeast, taller towers are needed to reach the strong winds that make economically feasible.

"Once you're at the heights we're looking at," Smith says, "it really opens up the whole country for turbines to capture large amounts of energy."

Explore further

Engineers design, test taller, high-strength concrete towers for wind turbines

More information: Tapered Spiral Welded Structure, www.google.com/patents/US20110179623

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Citation: On-site fabrication process makes taller wind turbines more feasible (2014, November 6) retrieved 19 May 2019 from https://phys.org/news/2014-11-on-site-fabrication-taller-turbines-feasible.html
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User comments

Nov 06, 2014
Pretty cool idea. Eliminating concrete eliminates a lot of the unavoidable CO2 emissions caused by traditional manufacture of wind energy.

How are the towers anchored to the ground though? Do you just dig a hole and stick it in, and then fill the tube with gravel?

Nov 06, 2014
How are the towers anchored to the ground though?

A concrete base can be laid down on site without having any prefabricated sections.
But the "stick it in a hole" approach might just be enough if the hole is deep enough. Since there is no limit to how high up (and low down) the tower can be built it's probably just a matter of figuring out what is cheaper.

The transport of the blades is still an issue, though. Longer blades are better, but the longest blades are also having a hard time being moved on roads. Potentially there is a similar approach for making the blades on site with a mobile "mini-carbon fiber fab"

Nov 06, 2014
Vestas has an 8 MW turbine in operation. I thought our PG&E Boeing Mod 2.0 machine was big with its 300-foot blade, slowly turning. It was 2.5 MW.

Nov 06, 2014
Will we be printing major components of generation systems on-site?

It is something I would never have predicted, only a few years ago.

Nov 06, 2014
A concrete base can be laid down on site

It can, but that still poses the problem of producing the cement without expending fossil fuels.

In some senses, concrete itself is a non-renewable material, because there's no method to grind it back to powder for re-use. You have to quarry limestone to make more of it.

Nov 06, 2014
It can, but that still poses the problem of producing the cement without expending fossil fuels.

Old type powerplants also use concrete (and LOTS more of it per MW capacity). So while not optimal it's a saving either way from the way things are being done now.
In the end any building is just a one-time "CO2 expenditure" while the mode of generation will continually reap the benefits in that regard (or add to the problem depending on whether it's a renewable or a fossil power plant) over decades.

Let's keep things in perspective.

Nov 06, 2014
"In some senses, concrete itself is a non-renewable material, because there's no method to grind it back to powder for re-use."

Look up concrete recycling. Got a calciner?

Nov 06, 2014
"In some senses, concrete itself is a non-renewable material, because there's no method to grind it back to powder for re-use."

Look up concrete recycling. Got a calciner?

Concrete is recycled into gravel or road filling. One can make new concrete using old concrete, but there's no practical way to separate the calcium carbonate to recycle the cement binder.

Old type powerplants also use concrete

They do, but that doesn't excuse anything. It's a problem that has to be solved.

(and LOTS more of it per MW capacity).

That sounds like it wouldn't actually be true, considering that conventional powerplants are a couple orders of magnitude more powerful than wind turbines, per unit.

In the end any building is just a one-time "CO2 expenditure"

You forget that wind turbines need to be continuously rebuilt. Even the foundations erode because the rebar rusts and cracks develop over time.

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