The technical advances that could make wind power viable everywhere

Wind power is often described as relying on “mature technology” and, in many contexts, that’s correct. Today, well-sited wind farms in the US and EU generate electricity at a lower cost than coal.

But numerous difficulties remain with the way we build wind turbines, and these difficulties raise the price of the turbines, which in turn determines where they can profitably go. High turbine costs mean that, unless the wind at a site is quite strong, there are typically better ways to spend money.

Fortunately, while wind is mature technology, it hasn’t reached an evolutionary dead end. Plenty of incremental changes can make wind even more affordable—and in many cases, the necessary technology is already being tested.

Capacity vs. reality

Jose Zayas, the director of the Department of Energy’s Wind and Water Power Technologies Office, compared the wind power situation with that of the car. Both are extremely well understood technologies where effective products are already on the market, but neither technology is at a dead end. Innovations in cars—from cameras and sensors to hybrid and electric drive systems to self-driving experiments—show that there are still plenty of new directions for automobiles.

These sorts of evolutionary changes have already been at work in the wind industry. One major example to this point has simply been scale; average rotor sizes have doubled since the 1980s. That has helped bring the cost of wind power down dramatically. Over the same period, the levelized cost of wind power dropped by an average of seven percent each year, driving total costs down by over 90 percent. As a result, enough wind power has been installed in the US to avoid 115 million tonnes of carbon emissions in 2013 alone.

Despite that progress, wind currently accounts for less than five percent of the total electricity generated in the US. That stands in stark contrast with the total potential for wind power, which is more than 10 times our current electrical consumption.

We aren’t taking full advantage of this for several reasons. It’s partly a matter of manufacturing capacity; wind has only been booming for about a decade, and it takes time for companies to respond to that demand. But two interrelated factors have also slowed wind’s adoption. Many of the best areas of the US for wind power are in the most sparsely populated states, far from the high-capacity transmission grids that support more populous regions. And many of the most populous regions have wind resources that we simply can’t harvest economically at the moment.

Making energy from wind

Figuring out how to change this situation is the focus of a recent report prepared by the US Department of Energy. It details some of the technological limitations holding back wind power, as well as the solutions that might make wind more economical in general, allowing the industry to expand to sites where the wind simply doesn’t blow as hard.

To start, the DOE report defines the energy you can get from wind as the product of a simple equation:

Credit: DOE

Here, Cp is the coefficient of power of the hardware, which we can assume is roughly a constant. (The report says “most turbines extract around 48–50 percent of the available wind power after accounting for aerodynamic and mechanical losses and other considerations.”) ρ is simply the air density; higher, which means lower altitudes, is generally better. Unfortunately, that factor isn’t something we can control once a site is chosen.

That leaves just two factors to be influenced by the turbine hardware. One is A, the area occupied by a rotating turbine. Since turbine blades describe a circle, the area is πr², where r is simply the length of the blades. Given that this term is squared, changing the length of the blade has a big impact. So while moving from a 75m blade to an 80m one is just a 6.7 percent change in length, it provides a 14 percent boost in turbine power.

But the remaining factor, U, is even more significant since it’s raised to the power of three. U represents wind speed, and going from a typical wind speed of seven meters per second to eight nets you a whopping 50 percent boost in turbine power.

Of course, things are more complicated than that. Wind speed is typically distributed along a complicated curve centered on a typical value. Meanwhile, rotors have a “cut-in speed”—if the wind speed falls below that value, they simply don’t rotate. Rotors also have a “cut-out speed,” at which point they have to be stopped for safety reasons. In between, the power generated ramps up until it reaches the maximum capacity of the generating hardware. This results in the complicated chart below.

The expected yield from a wind turbine (green) is dictated by its capacity (orange) and the typical wind speed at the site (blue).

Credit: DOE

Still, the complications don’t change the basics: if you lengthen the rotor blades, you can access a bit more power. And if you manage to get those blades in front of some higher wind speeds, you can access a lot more power.

Fortunately, a remarkably simple solution can put more wind in front of the turbines, and it works at almost every site—build a taller tower. Wind speed tends to increase with altitude, so raising the rotor will shift the system into a higher wind regime. “Simple analysis suggests that gains of 20-45 percent are possible by increasing the height of the towers from 80m to 140m,” says the DOE.

With bigger blades and taller towers, we can essentially harvest wind anywhere. So why don’t we?

Why we can’t have nice things

Part of the argument against these tweaks will sound familiar to anyone who has ever been told “no”—money. Bigger and taller means more materials and higher costs. Of course, these costs may be more than offset by increased productivity, especially over the lifespan of hardware that could be producing for decades.

But even when the economics make sense, a wind turbine might not. Many hold-ups are purely physical. When it comes to turbine blades, for example, long blades mean that the tips travel at very high speeds and experience very high stresses. The blades are made of fiberglass held together with epoxy resins, and a lot of work goes into ensuring that they are capable of withstanding these strains.

At GE’s research center in India, they have a room—and some expensive equipment—dedicated to breaking these materials to find out just how much strain they can handle. On a recent visit there, I saw an impressive amount of shattered fiberglass scattered around the room, along with a set of pieces that hadn’t yet been tested.

GE’s Anil Rajanna said that his materials scientists do modeling of various fiberglass structures, and they can even get a nearby production facility (one normally used to make the actual blades) to spit out a test version of the scientists’ new ideas. Then, the team proceeds to break the new material and determines if it behaves in the way their model suggested.

Here, a GE researcher prepares a sample in a machine that will proceed to torture it.

Credit: Greg Russ

Yet even as materials evolve that can provide longer blades, we face an additional problem: getting them from where they’re made to where they’re used. While light for their size, blades are completely enormous. Sizes today extend beyond 80m (pushing football field length). The biggest blades are generally used on off-shore turbines, which means they can be shipped by sea, but that’s not an option for most sites. And while turbines as long as 75 meters have been sent on trucks, the route can’t contain things like sharp turns or low bridges.

It’s also not enough to slap bigger blades on smaller hardware. The added weight of the blades creates mechanical stresses on all of the hardware they’re attached to.

“When you go to the low wind speed terrains, you need a longer length of the blade,” said Anil Rajanna of GE India. “And when you put this longer blade on the top of the hub, which could vary from 80m to 90m to 100m to above, you’re putting a mass up there. So anyone who looks at this [asks], ‘How can you make these blades lighter when you put it up there?’ So that the rest of the components see less of the load from these blades.”

Part of the solution is to work on lighter blade materials. But the nacelle and drivetrain have to be redesigned and reinforced, as well. “It’s more a complex optimization of the system,” said Rajanna’s fellow researcher Kannan Tinnium.

“The challenge of just growing the rotor naturally,” the DOE’s Zayas told Ars, “is of course that the loads—both the aerodynamic load, but also the gravity load because of weight—go up. Innovation has been needed to counteract those two.”

Otherwise, materials costs simply go up in tandem.

Building towers

Surprisingly, it’s not the blades that cause the most problems; it’s the towers. Currently, towers are manufactured as cylindrical segments. Because these cylinders have to fit under overpasses, they are generally limited to 4.5 meters in diameter, a size the DOE calls “structurally sub-optimal.” (The optimal diameter appears to be nearly twice that size.) To compensate, current towers have to use thicker steel, which adds significantly to the cost. It also boosts the mass, which further complicates transport.

Even with those parameters, it’s still possible to build towers up to 160m tall with the 4.5m segments. It just makes less financial sense.

There’s another, more practical limit that comes when building towers taller: putting the requisite hardware on top of them.

“The mass of a 3MW nacelle is approximately 80 metric tons without the gearbox and generator installed,” the DOE report notes. “The availability, scheduling, and logistics of the larger crane classes required to lift progressively larger wind turbine nacelles onto taller towers is increasingly challenging.”

3MW, it should be noted, is becoming a mid-range capacity, as the largest turbines are now above 6MW. The demand for heavy lifts is such that one crane manufacturer recently introduced a new model that will be effective up to 140m.

Developing tech

Fortunately, some of these problems have already been solved. Germany, where the solar market receives most of the attention, has also pushed to tap wind in areas of the country with lower wind speeds. As a result, nearly half the turbines installed there in 2013 were at heights of 120m or greater (the vast majority of installs in the US went on top of towers that were under 100m). And plenty of technologies in use in Germany already haven’t been used in the US at all.

However, the DOE is also looking at (and in some cases funding research on) ways to change the entire process of tower building. Most of these would involve doing more of the construction on site. So, rather than shipping a fully formed steel cylinder, it might be possible to send the steel partially unrolled and then weld it together on-site. Another alternative is to construct the towers out of concrete. Yet a third approach would use corrugated steel segments, which can be formed on site (and could also cut the metal requirement by up to 30 percent).

Back in 2002, the DOE funded a company to develop space frame towers where an internal metal lattice is covered with a robust fabric. The technology was eventually bought by GE, which has since introduced it as a product. This allows the construction of towers in places where metal cylinders can’t be shipped at all.

A similar approach is being tested for turbine blades, where a fabric-covered internal frame could be built where it’s needed, rather than shipped. These would also save weight relative to existing blades. A European company is also selling a blade that is shipped in segments and then bolted together at the site of use.

Hoisting lots of heavy hardware into place adds to the cost of wind power.

Credit: NREL

On top of all this, efforts are under way to cut down on the mass that needs to be lifted just to hoist the nacelle to the top of the tower. Some solutions are prosaic. Large towers in Europe are often topped off by using two cranes to hoist the nacelle and its contents, rather than one; this simply hasn’t been done in the US. A number of companies have introduced direct drive systems where there’s no gearing between the rotor and the generator. That eliminates one component that would otherwise have to be hoisted to the top of the tower. A spokesman for one of the companies that offers gearless turbines, Goldwind, told Ars that it has the potential to dramatically reduce maintenance costs as well.

Zayas suggested that while gearboxes have improved so that they’re no longer a major maintenance drain, the lower tower-top mass can be decisive. All of the large turbines meant for offshore use are direct drive systems.

Trending positive

Even without an aggressive effort to change technologies, the inexorable economics of wind power have dramatically changed the production landscape in the US. The DOE estimated back in 2008 that wind technology could allow a 30 percent net capacity on about 1.6 million square kilometers of the US, mostly in the central areas of the country. By 2013, that figure had jumped to over 2.7 million square kilometers, largely in the same geographic regions.

If we can raise the height of the rotors to 140m and drop the energy required to make them economical, things change radically. Over 4.5 million square kilometers of the US are put in play (more than half the total land area), and wind’s geographic range expands considerably, including large areas in the north and southeast.

“It opens new territories in the US which have historically not seen themselves as having a good resource potential,” Zayas said.

It’s not simply the US that could see benefits. In India, Anil Rajanna made many of the same points that would appear in the DOE report that was released a few days after our conversation.

“If you look at an Indian market context per se, most of the market right now is towards the class 3, which is the very low wind speed market,” Rajanna told Ars.  “One of the things where technology plays a role is, how do you build technologies where customers, in building these wind farms, make their returns? And, at the same time, where do you build technology with respect to the cost?  How do you build technologies where these then can be transported to the place where the customers want to put their wind farm?”

If the economics can work out, Zayas argued that there are some good reasons for many areas to want wind. Some of them are economic, as new jobs and lease payments for the sites pump jobs into the local economy. As coal is displaced, residents will also suffer from far fewer pollutants. Not having to provide sufficient water for steam-using fossil power plants will also put less of a strain on local resources. And there’s also the prospect of generating energy closer to where it’s needed.