Advancements in offshore vertical axis wind turbines

Abstract

In the past few years, the journal Energies received various original research manuscripts on offshore vertical axis wind turbines (VAWTs). These articles highlight the heightened interest (see also Hand and Cashman) in expanding the penetration of VAWTs beyond the small to medium-scale units found, for instance, in the built environment. This editorial aims to highlight various articles received by Energies that may be relevant to floating offshore VAWTs. The general landscape of the state-of-the-art progress in this area is also provided, but this article is not meant to be a literature review. This editorial is also intended to entice the growth of this field of wind energy science. In a recent review, Arredondo-Galeana and Brennan highlight major advantages of floating offshore VAWTs that contribute towards improved stability due to their generally low center of gravity (CoG), as well as the lower over-turning moment (OTM). This corroborates with other literature, such as that by Borg et al. and Borg and Collu. The severity of generator or gearbox failures is also reduced due to easier accessibility, while wind-farm power density is improved given the possibility to reduce turbine spacings as opposed to the HAWT alternative. Other risks predominant in HAWTs, such as yawing system failures, are eliminated in the case of VAWTs. On the other hand, it is well known that the power coefficient of VAWTs can be in the order of 20% less than HAWTs, a fact that has led to HAWTs leading the way in both industry and research. Nevertheless, this ball-park figure refers to the ideal condition when the HAWT rotor is perpendicular to the wind direction. In practice, floating offshore turbines can exhibit 6-degrees of freedom (DOF) motions due to the wave motion that can degrade performance substantially unless extensive control engineering is implemented. Figure 1 (source: Arredondo-Galeana and Brennan) shows that current VAWT offshore design initiatives have mostly focused on buoyancy and ballast foundation types, highlighting the lack of penetration of other technologies at the pilot scale level.