Abstract
Production of electricity using hydrokinetic tidal turbines has many challenges that must be overcome to ensure reliable, economic and practical solutions. Kinetic energy from flowing water is converted to electricity by a system comprising diverse mechanical and electrical components from the rotor blades up to the electricity grid. To date these have often been modelled using simulations of independent systems, lacking bi-directional, real-time, coupling. This approach leads to critical effects being missed. Turbulence in the flow, results in large velocity fluctuations around the blades, causing rapid variation in the shaft torque and generator speed, and consequently in the voltage seen by the power electronics and so compromising the export power quality. Conversely, grid frequency and voltage changes can also cause the generator speed to change, resulting in changes to the shaft speed and torque and consequently changes to the hydrodynamics acting on the blades. Clearly, fully integrated, bi-directional, models are needed. Here we present two fully coupled models which use different approaches to model the hydrodynamics of rotor blades. The first model uses the Blade Element Momentum Theory (BEMT), resulting in an efficient tool for turbine designers. The second model also uses BEMT, combines this with an actuator line model of the blades coupled to an unsteady computational fluid dynamics simulation by OpenFOAM (CFD/BEMT). Each model is coupled to an OpenModelica model of the electro-mechanical system by an energy balance to compute the shaft speed. Each coupled system simulates the performance of a 1.2 m diameter, three-bladed horizontal axis tidal turbine tested in the University of Edinburgh FloWave Ocean Energy Research Facility. The turbulent flow around the blades and the mechanical-electrical variables during the stable period of operation are analysed. Time series and tabulated average values of thrust, torque, power, and rotational speed, as well as, electrical variables of generator power, electromagnetic torque, voltage and current are presented for the coupled system simulation. The relationship between the mechanical and electrical variables and the results from both tidal turbine approaches are discussed. Our comparison shows that while the BEMT model provides an effective design tool (leading to slightly more conservative designs), the CFD/BEMT simulations show the turbulence influence in the mechanical and electrical variables which can be especially important in assessing an additional source of stresses in the whole electro-mechanical system (though at an increased computational cost).
Highlights
With increasing awareness of the rise in global temperatures due to carbon emissions, there is a conscious need to meet electricity demand using renewable energy and reduce the consumption of fossil fuels
Once the turbine, which works connected to the electrical system, reaches an energy balance between the rotor torque and the electromagnetic torque, produced by the generator, a more stable period is presented
In the CFD/Blade Element Momentum Theory (BEMT) model, the interaction between the surrounding sea and the turbine rotor blades is managed by an energy balance, between the imposed hydrodynamic forces and the reacted body forces, and OpenFOAM resolves the Navier-Stoke equations
Summary
With increasing awareness of the rise in global temperatures due to carbon emissions, there is a conscious need to meet electricity demand using renewable energy and reduce the consumption of fossil fuels. Numerical simulation of the surrounding sea is a challenging task as it is characterized by its unstable behaviour and its intrinsic physical properties like turbulence. Ideal models based on momentum balance can be used for the simulation of the surrounding sea They have low computational cost but the intrinsic flow characteristics are lost. Models based on a domain discretization and solution of the fluid conversation equations for each control volume can be used. They involve the use of higher computational resources but simulate the fluid dynamic behaviour and keep the fluid physical properties. The performance of these type of models is discussed in this work
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.