Pioneer tackles next wave of wind challenges

DENMARK: Since retiring as Siemens' chief technology officer in 2014, Henrik Stiesdal has devoted much of his time to working on floating offshore wind and energy storage. He talks about his new ideas and the challenges of scaling turbines towards 12-15MW and beyond by 2023-24.

“I decided against concentrating on a specific concept, like most floater developers do. Instead, I asked myself some basic questions: how should we manufacture floaters, and what is already available for us to learn from and build on?” says Henrik S

For Henrik Stiesdal, "retiring" meant taking a few weeks off to settle down into his new life after working for turbine manufacturers — Vestas, Bonus Energy and then Siemens — for more than 30 years. But he had work still to finish.

"I had already decided not to re-engage in wind-turbine design, mainly because I did not want to start competing with my former colleagues and friends still with the company," Stiesdal says.

"Besides, the onshore and offshore turbines of Siemens and other OEMs generally perform very well, which was another reason for me to focus on something else."

He decided to focus on two wind-industry needs still looking for the best solutions. The first falls within the offshore wind sector: the development of lightweight, low-cost floating foundations suited for flexible deployment and rapid large-scale industrialisation of production.

The second is low-cost energy storage, building on the innovative rock-heating technology he pioneered while at Siemens, and brought further in dialogue with the merged Siemens Gamesa Renewable Energy (SGRE).

The main aim is to come up with a low-cost storage technology that can be series-produced by existing companies, but there is no prototype yet.

Energy storage research is also one of the areas Stiesdal focuses on as part of his part-time professorship at the Danish Technical University (DTU).

A second part-time professorship, at the University of Maine in the US, is dedicated to research in further developing floating wind-turbine structures.

"Offshore wind has huge potential in many regions of the world, but, quite often there are situations where water depth is too great or the seabed conditions prevent using fixed-bottom foundations, or both" he says.

"For the floater design, I decided against concentrating on a specific basic concept, either semi-submersible, spar or TLP, like most other floater developers do. Instead, I asked myself some basic questions: how should we manufacture floaters? What is already available for us to learn from and build on?

"Other questions emerged after taking a closer look at all the issues, such as: how could we best benefit by procuring components through existing wind-industry supply chains? It turned out there are many similarities, especially with tubular steel-tower manufacture and nacelle cast components like the rotor hub."

Stiesdal observed that in-situ production of large components for wind turbines, such as on-site welding and slip-formed concrete towers, has proven to work technically if not commercially, and he felt this would also apply also for (heavy) floater concepts built at the port it would be shipped from.

This reinforced his idea that large-scale industrialised production using (semi-) standard serial components from existing supply chains was the way to go.

Additional questions to look at were: why would we need a floater in sections, and could a floater be transported in one piece?

All research pointed in the same direction: to produce floating foundations like a tubular steel tower and make optimal use of existing infrastructure, volume effects and fast ramp-up by adding moderate volumes.

Also, to keep mobilisation and assembly costs low, no part of the floater should be bigger than the largest section of the wind-turbine tower, and the number of parts should be comparable with that of wind-turbine main pieces.

"TetraSpar is a lightweight modular-design floater weighing about 1,000-1,500 tonnes for 6-8MW turbines. It mainly requires ‘standard’ piping in structurally stiff triangles, joined with bolts or pins," he explains.

"This rather simple tetrahedral structure has a retractable keel with a 1,500-cubic-metre displacement attached to the three buoyancy element assemblies via cables or chains. When pulled up in the harbour and during sea towing it only requires 6-8-metre depth."


The floating turbine offers semi-sub stability during towing and, once hooked on the keel on site, is brought to bottom position and ballasted with water, he adds. This pulls the floater below the water surface and the structure acts as a spar with far greater stability.

Initially, Stiesdal pursued the development of his floater as an open-source platform, and certification body DNV GL provided valuable assistance.

"This open-source idea did not work out, unfortunately, because end users, including developers, did not like it based upon their ‘if everybody owns it, nobody owns it’ reasoning," Stiesdal explains.

"We then decided to go for a classic licence-based commercial model and founded a separate company, Stiesdal Offshore Technology. We’re talking to big players for a prototype demonstrator with a 3.6MW direct-drive SWT-3.6-130 turbine, with the aim to install it late this year or early next year."

Stiesdal continues to closely follow wind-power developments. Since leaving Siemens, the 6MW direct-drive offshore platform he pioneered has been developed further to 7MW, and most recently into the second 8-9MW successor with an enlarged rotor.

On next-generation 12-15MW offshore turbines like GE’s 12MW Haliade-X with a 220-metre rotor, he says: "Let me start by repeating that I have always been very bad at making predictions on future turbine sizes.

"Having said that, the first big question for me is whether bigger turbines can also be competitive? An inherent bottleneck here is the square-cube law, which dictates that when turbine size grows, power generation potential increases with rotor diameter squared, but mass increment relates to diameter cubed.

"In other words, bigger turbines become inherently heavier in tonne/MW and more expensive in €/MW, even though impacts have been hidden by the huge technology and design-related progress experienced from the onset."


To decisively lower the levelised cost of energy (LCOE) for offshore wind, modularisation and industrialisation of production are essential, especially large-scale volume manufacture of standardised products, Stiesdal stresses.

Assuming a given annual factory output in megawatts and an increase in turbine size from 6MW to 12MW, then the number of units produced is halved.

This, in turn, has a negative impact on the learning curve, the benefits of which are expressed in the progress rate (PR), whereby a PR of 0.85 means that unit costs drop by 15% each time output numbers double.

The third point Stiesdal makes relates to non-turbine factors: "The foundation for a 12MW turbine does not cost twice as much as a 6MW foundation. The same logic applies to other elements, such as fewer cables and reduced total service visits required for the larger turbines, for a project of the same size in megawatts," he says.

"These non-turbine factors have so far been dominant in the offshore wind sector — in fact since the building of Vindeby in 1991. Over and over I have been surprised how well the next-generation bigger offshore turbines performed.

"This generally good performance of any next generation might also apply again to the next generation 12-15MW-plus turbines."

Stiesdal further notes that GE’s 12MW giant clearly has a focus on a big rotor, and its 316W/m2 specific power rating is very close to the 318W/m2 of the "classic" Siemens SWT-3.6-120 introduced in 2009.

"Such large rotor and modest rating configurations offer a high capacity factor. This is highly relevant to another question: when do you want to produce electricity? We increasingly experience that with high wind-penetration rates and surplus wind-power conditions, the market price can drop to almost zero.

"In opposite circumstances, when wind power contributes little, electricity prices have become sky-high. A low specific power rating could therefore become a main turbine marketing factor in future," he says.

Stiesdal adds that if — despite the high development costs and smaller numbers — GE succeeds in entering the market with a reliable, service-friendly and well-priced turbine, the market will go for it immediately.

"Competitors will have no other choice then but to follow, perhaps faster than they planned less than a year ago. On the other hand, commercially entering the market in 2023-24 with a fully validated, certified and bankable 12-15MW-plus product represents a big challenge for everybody, with a tight schedule," he concludes.

Henrik Stiesdal will head the judging panel for the Windpower Monthly Awards, entries for which close this month