A fluid mechanic view on urban wind energy

Abstract

The world energy demand is rising with an increasing need for renewable and sustainable energy sources. Wind energy plays a key role in this transition. Urban areas are largely unexploited for wind energy extraction. Urban wind energy offers great potential as a decentralized renewable energy source. However, one of the main issues to date is a limited understanding of the urban wind resources. The flow in an urban environment is naturally sheared, as part of the atmospheric boundary layer, and in addition, highly turbulent, due to the many obstructions in the flow. This thesis aims to contribute to an improved understanding of the urban wind resources. Specifically, the performance and flow field of a roof-mounted vertical axis wind turbine is examined, as well as the interaction of freestream turbulence and a turbulent boundary layer. The thesis presents experimental work in which these problems are investigated on a lab-scale. Experiments are conducted in a wind tunnel and a water channel, enabling controlled flow conditions. Specific parameters in the flow, such as turbulence intensity and velocity shear, are deliberately varied using active grids. The flow is primarily evaluated with Particle Image Velocimetry and Laser Doppler Velocimetry, complemented by Constant Temperature Anemometry and surface pressure measurements. Two model buildings are placed in the flow, represented by surface-mounted cubes. A drag-driven vertical axis wind turbine of the Savonius type is positioned on the roof of the model buildings and its power output is measured. Both the streamwise position and the height of the turbine above the roof are varied, and the impact on the flow field and power output is assessed. In addition, the influence of varying wind directions and vertically sheared in flow on the turbine performance are examined. The influence of turbulence intensity is investigated fundamentally on the evolution of a turbulent boundary layer and on the flow around the model buildings. Again, the impact on the turbine's power output at various positions is evaluated. It is demonstrated that a turbulent boundary layer is not permanently matured ahead of its natural evolution by the presence of freestream turbulence and that the relative state of evolution of both the boundary layer and the freestream turbulence has to be considered when assessing the turbulent boundary layer. A key finding is the substantial impact of a roof-mounted wind turbine on the flow field and, thus, on the available power. This suggests that in contrast to common practice, including an actual turbine in the analysis is significant. A central high turbine position is found to maximize the power output for a uniform wind rose. However, for individual wind directions, the ideal turbine position and height vary. Turbulence intensity has a significant impact on the flow around the model buildings and consequently also on the power output of the turbine. High levels of turbulence intensity reduce flow separation on top of the building, resulting in higher velocities where the turbine is placed and, thus, higher power output. Conversely, velocity shear has only limited influence on the flow field and power output of the roof-mounted turbine. Overall, a methodology to assess a roof-mounted vertical axis wind turbine in a controlled environment was developed, examining the influence of various parameters, such as in flow conditions and turbine position.