Multi Rotor Wind Turbine Designs

Single VS Multi Rotor
Jan Bartl, HVL



The growing demand for large-scale offshore power generation led to a considerable increase in wind turbines’ rotor size over the last decades. Although these gigantic turbines produce considerably more energy, also the turbines’ production and operation costs increased significantly. While a rotors aerodynamic power output is well-known to scale with the square of its radius, a rotor blade’s mass scales cubically with the radius. Consequently, not only the costs for the rotor blades’ material and manufacturing increase, but also transportation, installation, operation, and maintenance become more expensive.

From an economical point of view these basic relations set a limit to a wind turbines rotor size. Naturally, also the question must be raised if not multiple smaller rotors installed on only one floating offshore structure could significantly reduce the levelized cost of energy for a wind power plant. Initial studies on multi-rotor systems (MRS) have disclosed economic benefits that mostly stem from a cost reduction up to 15% of the multi-rotor’s support structure compared to a single rotor of the same power output [Jamieson and Branney, 2014].

Project description

When it comes to the design of a lightweight, reliable, and efficient floating offshore MRS many different design parameters have to be taken into account. For a given rated power output of an MRS, the number and size of the single rotors are the obvious first parameters that can be varied. Furthermore, the design of the support structure and its interaction with the rotors are considered to play a very central role for an MRS power output, loads and wake dynamics. The proposed project will investigate the wake flow behind a set of porous discs representing the single rotors of an MRS. Different numbers of discs and arrangements will be investigated, which the main objective to achieve a fast recovery of the wake flow.

Instead of traditional wind tunnel experiments with moving air, the current project will be conducted in water, while the MRS arrangement is moved relative to it. All three components of the wake flow are measured by an Acoustic Doppler Velocimeter (ADV) installed in different relative distances behind the rotor/disc while it is laterally traversed. The results will be compared to computational results provided in a  recent publication by Bastankhah and Abkar (2020), which showed a large potential for mitigating wake effects by the use of a MRS in a wind farm.

Prerequisites and qualifications

The prospective student should have:

  • High motivation for offshore wind energy
  • A professional attitude with good organisation
  • Computational skills are advantageous

Jan Bartl - jaba@hvl.no,
David Lande-Sudall dla@hvl.no