Flow diagnostics and optimal design of vertical axis wind turbines for urban environments

The gap between the supply and the demand of electricity is rapidly increasing throughout the world, due to the decommissioning of old coal-fired power plants, and the stringent regulations on carbon emissions. Power suppliers need renewable energy generating technologies which are both environment friendly and sustainable in a long term. The requirement of renewable technologies is resulting in an increased financing of the research and development in the solar and wind energy sectors. This increase in the research and development in these renewable sectors can be observed all across the developed world and in particular, the European Union, as a result of its commitment to its Renewable Energy Directive. In the United Kingdom, the wind energy sector has predominantly grown in the recent years, with the construction of multiple wind farms, both on-shore and off-shore. This is resulting in significant advancement in the designs of large wind turbines. However, the abundantly available wind energy in the urban areas is still an energy source to be explored. The current research study is aimed at exploring the design and analysis methodologies of small-to-medium sized wind turbines for urban applications, as the published research in this area is severely limited. A drag based Vertical Axis Wind Turbine (VAWT) has been considered in the present study as these types of VAWTs are more suitable for urban environments. Such VAWTs operate at lower wind speeds, have multidirectional structure, simpler construction, lower manufacturing and maintenance costs, robust design, long lasting operational life, smaller size, low level of noise and vibrations, and possess the ability to self-start and be packed closer together. Improvements to the standard drag based VAWT designs are required to make the best use of their inherent design benefits, which coupled with a more refined and effective design can lead to a wide spread use of drag based VAWTs in urban centres, increasing renewable energy production. Detailed investigates on the effects of various innovative geometrical features on the performance of the VAWT have been carried out. This has been achieved through the use of advanced Computational Fluid Dynamics (CFD) based techniques. The first aspect of the study is to carry out a detailed flow diagnostics of a standard drag based VAWT (the baseline model). Various innovative geometrical features have then been integrated with the baseline model to analyse their effects on the performance of the model. For this purpose, three dimensional models of the VAWT have been numerically analysed for the flow of dry and clean air in urban environments. Furthermore, sliding mesh technique has been employed to rotate the rotor of the VAWT. An in-depth qualitative and quantitative analysis of the global and local flow related parameters has been carried out, while instantaneous torque variations have been monitored throughout the rotation of the VAWT. The second aspect of the study is to critically analyse the start-up process (accelerating rotor) in order to determine the entire operating range of a helical VAWT design. Advanced CFD based technique, known as dynamic mesh, has been employed for this purpose. The advantage of dynamic mesh over sliding mesh is that sliding mesh assumes a constant operating point of the VAWT, while dynamic mesh generates the complete performance map of the VAWT; from the start-up to the constant operating point. Hence, the use of dynamic mesh technique is essential in understanding the transient accelerating behaviour of the VAWT. The third aspect of the current study is to develop an innovative in-house built CFD package for aerodynamic design analysis of VAWTs. The developed package is based on existing open source CFD solvers. Predictions from the developed package have been validated against a well-known and widely used commercial CFD software (ANSYS). The developed package has been shown to predict the complex flow structures in the vicinity of the VAWT, and its performance parameters, with reasonable accuracy, and hence can be used as an inexpensive tool for CFD based design analysis of VAWTs for small-to-medium sized enterprises (SMEs).