The enduring lure of vertical-axis wind turbines
BY JASON DEIGN
Visit any wind farm in the world and you will find three-bladed turbines with upwind rotors and horizontal nacelles perched on top of tall towers. This horizontal-axis wind turbine design, sometimes called the Danish concept because of the country where it was first commercialised, has become an industry standard worldwide. But not everyone is convinced it is the way forward.
Back in 2010, UK architecture firm Grimshaw and engineers Arup unveiled what they thought would be the wind turbine design of the future. The biggest wind turbines then on the market had a capacity of seven MW per machine, and there were doubts over how much larger they could get. Grimshaw and Arup envisaged an offshore wind turbine that was more than twice as powerful, delivering 15 MW per unit, and looked very different to the ones on the market thus far.
Instead of three rotors on top of a tower, the Aerogenerator X – designed for a now-defunct company called Wind Power Limited – had two blades arranged more like those of a blender, spinning on a vertical axis at the base of the machine. The design was the fruit of an 18-month feasibility study involving the defence contractor QinetiQ plus the universities of Cranfield, Strathclyde and Sheffield, Wind Power Limited said at the time.
“The Aerogenerator X is considered one of the only real alternative solutions available to help deliver the UK’s offshore wind strategy in a reliable and cost-effective manner,” the company said. “It does not have the same weight constraints as a normal wind turbine and the blades do not suffer weight-induced fatigue. This new design is half the height of an equivalent horizontal-axis turbine and its weight is concentrated at the base of the structure.”
In the event, the constraints listed by Wind Power Limited have not stopped the Danish concept from scaling up. While the Aerogenerator X concept has languished on the drawing board, the GE Haliade-X has been certified up to 14.7 MW and the Chinese firms Goldwind and China Three Gorges have announced a 16 MW machine.
These real-life turbines, along with similar-sized offerings from other manufacturers, share the standard three-bladed horizontal axis design. Does this mean the brains behind the Aerogenerator X got it wrong? There are some who think not. After all, vertical-axis designs are already used routinely for small-scale onshore turbines, so the technology is established and relatively well understood. Companies such as Whirlwind Wind Turbines of Spain offer small vertical-axis turbines for domestic and industrial customers.
Vertical axes in floating offshore wind
Whirlwind Wind Turbines claims its products are well suited to gusty conditions and are easy to install and to maintain. Other vertical-axis wind turbines can be purchased on Amazon for the price of a top-of-the-range flat-screen TV. At the other end of the scale, some experts question the wisdom of using bigger and bigger horizontal-axis wind turbines offshore, where there is a growing trend towards putting the machines on floating platforms.
“Just looking at it from an engineering standpoint, I see the benefits of vertical-axis turbines,” says William Walker, senior associate at Stress Engineering Services, which in 2017 worked with Sandia National Laboratories of the USA on a study of the technology. “If you have the gearbox and all the important, expensive, heavy stuff down low, it makes it a lot easier to access when you have to do maintenance. And for a floater, when you put all the mass lower down, it can likely lead to reductions in loads on your mooring lines.”
Maintenance would be further simplified by the fact that vertical-axis wind turbines should not need complex controls for pitch, which is the angle of the blades, and yaw, which is the direction the nacelle is pointing in. As well as lessening the chance of faults, not needing these controls would cut the cost of the turbine, advocates believe. Further cost reductions would come from not needing a tower.
Beyond having easier maintenance and greater stability on floating platforms, vertical-axis wind turbine proponents believe the technology could be less prone to wake effects. These are what happens to the wind stream after it passes through the swept area of a turbine. With horizontal-axis turbines, the first machine facing the wind causes turbulence and reduces the amount of energy that can be produced by the machines behind it.
Wake effects are less pronounced with vertical-axis wind turbines, allowing the machines to be placed closer together and thus theoretically increasing the amount of energy that can be generated for a given area. Such benefits have prompted several modern start-ups to follow in the footsteps of the Aerogenerator X with vertical-axis offshore wind turbine designs.
Companies with a vertical approach
World Wide Wind of Norway, for example, has not one but four vertical-axis models, including a giant floating design with two sets of counter-rotating turbines that the company says could deliver up to 40 MW of power per unit. Hydro Wind Energy, meanwhile, is seeking backers for a sail-bladed floating design that incorporates an energy storage system under the turbine. And Swedish technology developer SeaTwirl has a product with three vertical blades supported by arms attached to a central tower.
“The company’s unique system ensures lower manufacturing costs, lower life-cycle costs and thus a lower overall cost,” says SeaTwirl on its website. “This is especially important for offshore structures.”
Other technology developers see a growing role for vertical-axis wind turbines onshore. One eye-catching concept, from Odin Energy of South Korea, is a tower of up to 12 floors, each containing a central turbine. Soo-Yun Song, Odin Energy’s vice president and chief technology officer, says the company is looking to install two 500 kW towers in Las Vegas, USA. The company also has plans for demonstration projects in South Korea and a 300-metre-high tower “at a famous tourist attraction in China”.
The towers are designed to create a pressure difference that increases the wind speed across the turbines, he says. “Odin towers can be installed in weak wind areas where existing wind power cannot be installed,” he says.
Despite these innovative concepts, the fact remains that the vertical-axis turbine market was valued at less than 13 billion dollars in 2022, little over a tenth of the 101-billion-dollar wind market overall. And for all those that believe in the technology’s potential, there are others that question its value. For example, says Kenneth Bhalia, chief technology officer at Stress Engineering Services: “If you’ve got a horizontal-axis wind turbine and it is a hundred metres up, you’ve got more stable wind speeds versus something that’s ten metres. Someone’s got to do an economic and technical analysis.”
Limited market appeal
Another common criticism of vertical-axis wind turbines is that they are not as efficient as horizontal-axis machines because the return half of each rotation of a blade happens against the wind. What is clear is that while vertical-axis wind turbines are useful for some niche applications, such as generating energy in places where there is a constraint on height or area, they struggle to beat traditional three-bladed horizontal-axis machines for large-scale, low-cost power production.
This in turn means that most of the wind energy supply chain is geared up to build and service three-bladed horizontal-axis machines, making it hard for vertical-axis models to compete. Even on floating platforms, where the traditional wind turbine approach requires massive ballasts and strong moorings for stability, it is hard to see vertical-axis machines displacing horizontal-axis models in the short term.
In 2022, the USA Interior Department’s Bureau of Ocean Energy Management auctioned five leases for the establishment of floating wind farms off the coast of California. The state has no supply chain to speak of for offshore wind, potentially throwing the market open for innovators to try new approaches.
However, most of the companies that won the leases were European developers such as Equinor of Norway and RWE of Germany, which all have a long history of working with three-bladed horizontal-axis wind turbines. The chances of them changing tack and developing projects with untested vertical-axis machines would seem slim.
“The Danish concept of three-bladed wind turbines has proven to be the most efficient so far,” says Christoph Zipf, press and communications officer at the technology-neutral wind industry body WindEurope. “Europe’s five wind turbine manufacturers have consistently improved this concept. Scale effects and technological innovations have led to impressive cost reductions for these turbines. WindEurope is tracking wind turbine orders across Europe. The new orders clearly show that the three-bladed turbine concept will continue to dominate new installations.”
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