Hopes pinned on multi-rotor wind turbines

In the interview, Sven Störtenbecker, research associate at HAW Hamburg, explains the potential of multi-rotor wind turbines.

EEHH: Please explain to us the technical concept and operating principle of multi-rotor wind turbines. What sets them apart from conventional three-bladed wind turbines?

Sven Störtenbecker: “Multi-rotor wind turbines (MR-WT) offer a viable way to counter the challenges posed by the relentless growth in the size of wind turbines, such as the increasing proportion of self-weight in blade loads, as well as the production and transport of large components. Instead of a large, single rotor, the desired swept area, and thus performance, is achieved in an MR-WT by means of several, smaller rotors. Due to the so-called square-cube law, which describes the relationship between the surface area and volume of a body, as originally formulated by Galileo Galilei, the combined mass of the small rotors is less than that of the equivalent single, large rotor. Simply put, the swept rotor area – and accordingly the power output and yield of a rotor – scales with the square of the rotor diameter, while the volume – and thus the mass as well as the cost – scales with the cube.

Every turbine on the multi-rotor has its own generator and control unit, enabling reaction to localised wind gusts and turbulence. The individual turbines are connected to each other via a steel support frame. Depending on the site and requirements, this support frame can be mounted on a floating platform (offshore) or on a tower (onshore or offshore) by means of a fixed-floating bearing arrangement. Yaw tracking is conducted over the entire multi-rotor structure as one. By reducing blade mass, the design saves substantial amounts of fibre-reinforced composites. The steel of the support frame, which conventional single wind turbines do not feature, is easily recyclable.

Using 0.5 to 1 MW turbines designed according to current engineering expertise, developers can configure target capacities as needed. Since the components, especially the rotor blades, are smaller, they are much easier to transport and manufacture. Economies of scale, akin to those in the automotive industry, would help to drive down component costs and defect rates through series production – a benefit largely unattainable for ever-larger single rotors. Moreover, studies based on high-resolution simulations and measurements show that the wakes behind an MR-WT recover much faster. Consequently, MR-WTs could be placed closer together within a wind farm, making more efficient use of the limited installation space.”

EEHH: What sparked the research and development of multi-rotor systems at HAW Hamburg? What was the impetus behind it?

Sven Störtenbecker: “The multi-rotor initiative at HAW Hamburg and the CC4E (Competence Center for Energy Transition) was initiated by Prof. Peter Dalhoff. During his tenure at certifier Germanischer Lloyd, now DNV, he was inspired by the work on multi-rotors by researcher Peter Jamieson. In the “X-Multirotor” research project, 20 MW multi-rotor concepts were investigated at HAW Hamburg in collaboration with wind turbine manufacturer Siemens Gamesa, among other things with regard to rotor counts and topology as well as their impact on support frame loads. In a follow-up project called “X-Multirotor DfM (Design for Maintenance)” in collaboration with energy supply company EnBW, the focus was on the operational and maintenance lifecycle of offshore multi-rotor systems. Both projects demonstrated positive aspects of both CAPEX and OPEX for MR-WTs.”

EEHH: A small community has formed around this topic, meeting annually as part of a conference. What is the focus of these events?

Sven Störtenbecker: “In 2023, HAW Hamburg hosted a two-day multi-rotor seminar in Hamburg in cooperation with the University of Strathclyde, bringing together international speakers and guests from the fields of research, industry and certification. The seminar included presentations on the latest multi-rotor research, practical testing and operational insights, panel discussions and networking. Following further seminars in Strathclyde in 2024 and in Hamburg in 2025, the next will be held again in Strathclyde at the beginning of 2027. Presentation slides from the 2025 Multi-rotor Seminar can be accessed via the HAW institutional repository.”

EEHH: Do you see a commercial future for multi-rotors? Are there any countries or markets where they are already gaining commercial traction?

Sven Störtenbecker: “Chinese wind turbine manufacturer Mingyang commissioned a 2 x 8.3 MW floating dual-rotor (OceanX) in 2024 and announced plans for a 2 x 25 MW multi-rotor turbine based on the same design at the end of 2025. The underlying concept was developed by northern German engineering firm aerodyn under the name nezzy² and initially trialled on a smaller scale in the Baltic Sea.

Norwegian company Wind Catching Systems is pursuing a different multi-rotor approach. They hope to develop a floating offshore multi-rotor system with 40 x 1 MW turbines in the next few years backed by government funding; it is expected to be built off the Norwegian coast. Other examples from industry, each with its own approaches to the development and operation of multi-rotor systems, are EnerOcean and Myriad.

Multi-rotor wind turbines therefore not only demonstrate potential to drive down the levelized cost of electricity from a researcher's point of view; they are also actively being developed and tested by industry.”

Sources for further information

https://www.tgs4c.com/news/mingyang92s-oceanx-the-largest-single-capacity-floating-wind-platform-launches-nid30167.html

https://www.offshorewind.biz/2025/10/21/chinas-mingyang-unveils-plans-for-massive-50-mw-floating-wind-turbine/

https://www.offshorewind.biz/2025/01/28/norway-grants-eur-100-million-to-deploy-multi-turbine-windcatcher-tech/