Lift-Induced Wake Re-Energization for a VAWT-Based Multi-Rotor System

Abstract

In response to the urgent need for renewable energy amidst the escalating impacts of global warming, this study delves into the forefront of wind energy research, particularly focusing on the innovative Multi-Rotor System (MRS). The MRS concept offers a promising departure from the prevailing trend of scaling up traditional turbines, instead proposing a configuration comprising multiple smaller rotors mounted on a single support frame. This approach, identified for its inherent upscaling advantages, presents an opportunity to reduce costs and weight relative to conventional Horizontal Axis Wind Turbines (HAWTs). While the concept is not entirely novel, its application with Vertical Axis Wind Turbines (VAWTs) is. Recent studies have demonstrated the significant potential of the VAWT-based MRS, showcasing comparable power performance to large HAWTs while enhancing Operations and Maintenance (O&M) aspects. Furthermore, the adaptable support frames of the MRS facilitate the integration of wake control devices, such as external lift-generating wings, which introduce cross-flow loading to rapidly re-energize the wake. However, despite these advancements, understanding the wake dynamics within MRS systems, particularly concerning their implications for wind farm applications, remains limited.

The current work aims to investigate the intricate near-wake dynamics of the VAWT-based MRS and study the effects of external lift-generating wings on the deflection and recovery of the wake. To do so, a scaled wind tunnel model of such a VAWT-based system has been designed together with a set of removable high-lift wings. These, in turn, have been tested in the Open Jet Facility of the Delft University of Technology to gather insight into the behaviour of the near-wake. Tomographic Particle Image Velocimetry using Helium Filled Soap Bubbles has been deployed in combination with load measurements to gather data.

Load measurements revealed that the thrust coefficient behaviour of the MRS closely resembles that of single-rotor VAWTs, with an additional thrust induced when the external lift-generating devices are present, attributed to accelerated flow on the suction side of the wings. Furthermore, the PTV measurements have provided a detailed visualization of the near-wake, showcasing symmetric wake structures and lateral deflection induced by the presence of individual rotors. The introduction of external lift-generating wings significantly altered wake behaviour, inducing lateral contraction and promoting streamwise momentum recovery through enhanced vertical advection. Furthermore, analysis of velocity deficit recovery highlighted substantial improvements in power recovery behind the MRS with external wings.

The findings presented in this work underscore the potential of the VAWT-based MRS, particularly when such a system is equipped with lift-generating devices. The presence of such devices effectively manipulates the near-wake of the turbine, enhancing wind farm efficiency, and thereby advancing innovative wind energy solutions.