Comparison of Analytical and CFD Modelling of the Wake Interactions of Tidal Turbines

The status of marine current tidal energy technology is currently in the research and development phase, with a few deployments and tests of prototypes under-way in some countries. There is huge pressure for tidal farms to be of GW scale in order to have a real, economically viable impact on renewable energy utilization targets outlined for 2020. A route to achieving this is the large scale energy farm philosophy, similar to wind farms, based on very large numbers of unit current tidal stream devices. However, this in-mature technology development raises different research questions which lead to further problems in the practical implementation of tidal stream devices. Thus, the aim of the work described in this paper was (i) to formulate simplified parameterised analytic models of individual and clusters of tidal stream devices using the concept of linear momentum actuator disc theory, (ii) to perform a detailed calculations of the flow f ield of multiple turbines using the developed models, and (iii) to compare the analytic model results with the results calculated using a CFD based Immersed Body Force (IBF) model. This study has been mainly focused on a new device, Momentum-ReversalLift (MRL), which is a cross flow type of tidal turbine developed by Aquascientific Ltd. Several analytic models have been developed to describe the f low characteristics downstream of the turbine, i.e. the wake velocity profile and to estimate the total power extraction from a tidal stream farm containing ideally tenth and hundreds of devices. The developed models showed the capability to examine the wake characteristics and to estimate the performance of clusters of turbines taking in to consideration the influence of turbine to turbine interactions. A small longitudinal spacing between turbines inflicted a massive energy shadowing that affects the performance of downstream turbines. Based on the analysis of the influence of wake interaction, more than 91% of the performance of an isolated turbine can be achieved with turbines spaced 20D apart. In addition, comparison of the wake velocity profiles and the power extraction calculated using both the analytic and CFD models showed reasonable agreement.