Multi Rotor Wind Turbine Designs

The world’s growing demand for large scale offshore power production has led to a considerable increase in wind turbines’ rotor size during the last years. 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 of large offshore wind farms become more expensive. From an economical point of view these basic relations set a limit to a wind turbines rotor size, which recently inspired some new innovative concepts for offshore wind farm installations.

Instead of installing only one rotor on single structure, multirotor concepts accommodate multiple smaller rotors on one bottom-fixed or floating offshore structure, which could potentially reduce the levelized cost of energy for a wind farm significantly.

Despite these promising economic advantages, there still are many design parameters for multirotor wind farm installations that need further investigation. Two of these challenges are concerning the drag force and wake losses multirotor systems produce, and how these are dependent on rotor number, size and spacing.

A first set of lab experiments on different designs of multirotor representations has been performed at the Western Norway University of Applied Sciences in spring 2021. While measurements of drag forces for different rotor spacings only showed minor differences, the turbulent wake flow was observed to recovery significantly faster for larger inter-rotor spacing. Inter-rotor jet streams were found to enhance the turbulent mixing in the wake, resulting in a significantly lower velocity deficit in the near wake. These first results indicate that multirotor wind farms could allow for smaller spacing between the single support structures in a farm setup, if properly designed.

Webinar speaker

Jan Bartl, Associate Professor, Institute for Mechanical and Marine Engineering (IMM), Western Norway University of Applied Sciences