The aim of this paper is to investigate the implications of site-specific conditions on the prediction of loading and power performance of a tidal stream device.
The design tool GH Tidal Bladed is used to model a generic 2MW turbine operating at a location within EMEC's tidal test site. Data collected at the site is analysed to evaluate the tidal flow characteristics, including annual mean flow distribution, typical velocity profiles and turbulence intensities. The coherent structure of the turbulence is estimated by spectral analysis of ADCP data, although the assessment is limited, it provides some understanding of the turbulent length-scales and suggests that a von Karman model can be used to extrapolate measured data to generate a 3D time history of the flow field. The application of such a model is discussed. A review of nearshore/site wave data is used to estimate a wave scatter diagram at the site.
Using site characteristics two forms of analysis are conducted. The first evaluates the impact of turbulent flow and waves on structural loading and the second compares the variation in annual energy yield when incorporating measured site data.
An annual energy yield prediction is evaluated using time histories of the electrical power generation and the annual mean flow distribution. This value is compared to a simplified harmonic model, based on Admiralty data, and a steady and dynamic power curve predication.
The need to evaluate the effect of turbulent flow and waves on structural loading is apparent. If cost-effective design solutions are to be achieved then it is a prerequisite to have a detailed description of the environmental conditions at a potential tidal turbine site--coupled with sophisticated, validated models which can incorporate the complex interaction of the environment with the operational behavior of the turbine.
The potential importance of turbulent flow and waves on loading and performance underlines the requirement for detailed tidal flow measurement studies, followed by the development of validated spectral models of tidal current flow.