Are vertical-axis wind turbines a scientific mirage or the future of offshore wind energy? A critical perspective a century after Darrieus’ patent

After one century from the original patent of J.M. Darrieus on a lift-driven “turbine having its rotating shaft transverse to the flow of the current”, vertical-axis wind turbines (VAWTs) are stuck in a dichotomy between being the most fertile field of research for aerodynamicists and a technology that still fails in turning into an industrial reality, with many companies going in and out of the market. After the research on VAWTs restarted in the early 2000s, significant progress was made in understanding their complex aerodynamics and modeling them with simulations of different fidelity, leading in turn to the definition of design guidelines able to reduce the performance gap with respect to horizontal-axis wind turbines (HAWTs) to less than ten percentage points. This did not persuade the consolidated HAWT industry to leave the most efficient and reliable horizontal-axis archetype, especially after the rush towards diffused onshore energy production by VAWT started in the 2010s failed. However, recent discoveries are putting again Darrieus VAWTs in the spotlight as a possible game changer in offshore wind energy, where their faster wake recovery could bring to farms with unprecedented energy density in a market where competition for marine space is strong. Different to other reviews made to date, the present study not only presents the evolution of VAWTs over the last decades, but also critically analyses the lessons learnt made along the way and highlights the recent discoveries that are opening after a century new prospects for VAWT technology. • Critical analysis of the evolution phases of Darrieus VAWTs. • Annotated overview of recent discoveries and new design paradigms. • A new perspective on unsteady aerodynamics in VAWTs. • Analysis of the prospects of flow entrainment for offshore applications.