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When More is More: Multi-Rotor Turbines

 

The offshore wind industry is one of scale. For decades, the direction for turbine development was to build them bigger. The sweep of blades have more than quintupled in size since the turn of the millennium. Every year a manufacturer announces a new bid for creating the world’s longest blade. Danish developer Vestas holds the title with its 115.5-metre bladed V236-15.0 MW. A year before that, it was Siemens Gamesa’s SG 14-222 DD at 108-metres.

 

It’s an approach that has worked, to great effect. The larger capacity and output afforded by ever more powerful machines has driven down the once prohibitive cost of wind energy, making it a feasible competitor and alternative against fossil fuels. Costs have dropped as much as 49 percent over the last decade. Experts believe it’ll be half as cheap as predictions forecasted in 2015. Developments are coming together globally, from the United States to Vietnam.

Growing Pains

With offshore wind firmly established as the fastest growing source of renewable energy, new questions–and problems–are arising from the field. Transport is becoming increasingly tricky, as the size of blades outpace the vessels that carry them onto the site. “Vessels built early this decade are already becoming outdated as turbines grow, making owners reluctant to commit to expensive newbuilds that could be obsolete before they are profitable,” notes analysts from Rystad.
 

There’s also the limits of physics to consider. A single blade from GE’s Haliade-X weighs 35 metric tons. Components scale accordingly to handle the weight of larger blades, with nacelles weighing upwards of 500 tons. Altogether, turbines can weigh thousands of tons. Reconciling this sheer heft with the durability needed to survive extreme storms and climates will be a challenge. For the sky to truly be the limit, materials will need to be stronger and lightweight at the same time.
 

Communities living within sightline of the farms will also have to welcome these behemoths into their proverbial backyard. A challenge that, according to RWE Renewables CEO of onshore wind and pholtovoltaics Katja Wünschel, may be even more difficult to hurdle than technical limitations: “There will be opponents who say ‘I don’t want to have this gigantic turbine next to me’”.

When And Why Is More Better?

In April 2016, a rather strange looking turbine spun to life near the Danish city of Roskilde. The machine differed from its neighbours in one very noticeable way: instead of a single rotor, it had four.
 

The prototype, which was launched under Vesta’s project “Babylon”, was a partnership experiment between the Danish turbine manufacturer and the Technical University of Denmark (DTU). One of the goals was to find out how multi-rotor turbines perform in real weather conditions. The findings: an annual power gain of 2 percent, which researchers attributed to the rotors interacting with each other.
 

Theoretically, multirotor turbines like Vestas’ prototype increase the sweep radius and output of a single structure without the stress bigger blades impose on the tower. “With multi-rotors, a second turbine downstream will produce more energy and will be subjected to less load and stress, because the turbulence is correspondingly smaller,” says Mahdi Akbar, assistant professor at Aarhus University’s Department of Engineering.
 

Equally important given the complexity of moving large components, parts can be moved around without requiring the development of new ships that can handle the load. Smaller turbines will be easier to transport and install in areas that can be challenging to reach without special vessels. This flexibility can prove to be an advantage in the coming years against a resurgent oil and gas industry that’s snapping up more vessels for work.

Bringing Multi-Rotor Tech To The Fore–Again

While the interest around multirotor turbine development appears to be a riposte against increasingly larger blades, conceptually manufacturers and scientists have been playing around the idea for nearly a decade. 
 

In the 1930s, German engineer Hermann Honeff put forth plans for a triple rotor turbine that would generate 20MW. Fifty years later, Lagerway Wind would launch several multi-rotor systems. Due to issues with vibration, these were ultimately taken down. Lagerway claims that although the six-rotor system performed well, customers went with traditional turbines.
 

But that was the 80s and 90s, when single-rotor turbines and wind energy itself was only in its infancy. Now that the market is considered mature and conventional turbines are encountering limitations, the need for innovation is fuelling research into alternative designs. And when a four-rotor system can cut manufacturing costs by around 15 percent, the industry is snapping to attention again.

Out Of The Lab, Into Our Oceans

Much of the development around multi-rotor systems has been on the conceptual level. But over the past decade, investments into the tech have grown, driven by the imperative to further increase output while decreasing the levelised cost of energy (LCOE). Analysts predict the multi-rotor market to reach a Compound Annual Growth Rate (CAGR) of 6.7 percent by 2031.
 

Several developers are already piecing together the future of multi-rotor systems. Norway’s Wind Catching Systems sees not four or five rotors atop one structure, but a hundred. Called “Wind Catchers”, estimates claim five of these thousand foot structures can generate the same output as twenty-five traditional turbines, while occupying a fraction of the space.
 

Offshore Renewable Energy (ORE) Catapult’s predictions are more conservative, but no less impressive. According to Research and Innovation Director Dr. Stephen Wyatt, the UK may see 40-rotor structures rising in its oceans between 2030 and 2040.
 

In Bremerhaven, a 1:10 scale prototype of a two-rotor turbine called Nezzy2 floats in a flooded pit. Should tests show promise, German energy company EnBW plans to move the experiment out onto the deeper waters of the Baltic Sea, a step closer to multi-rotor’s dreams of commercial viability.
 

Conventional turbines remain unparalleled in output and capacity, powered by blades that are only increasing in size. Multi-rotor systems won’t be edging these out anytime soon, if at all. And they don’t have to. These innovative machines can help harness the full potential of offshore wind by squeezing in where larger installations are no longer feasible in terms of acreage and logistical limitations.
 

The larger turbine blades grow, the harder it becomes to balance logistics, cost, and physics. Do we have anything to gain by adapting more yet smaller blades?

 
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Date published
Date modified
08/06/2022

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