A naval architecture first approach to examine the design principles required for operation of floating tidal turbines

The design of tidal turbine systems involves the evaluation of performance and loads across a very large parameter space, necessitating the use of simplified engineering models. Floating turbine systems are particularly challenging to evaluate due to the feedback between platform motions, the wave and current environment, and the turbine performance.

Blade element momentum theory, first developed in the early 20th century, is an important tool that has underpinned a lot of turbine design and analysis, with additional sub-models and corrections proposed over time to account for tip and hub losses, high thrust regimes, and dynamic inflow conditions. However, development and validation of the model specifically for tidal turbines and the conditions that they experience has been much more limited than for wind, leading to significant uncertainties related to the additional complexity that arises through motion of floating platforms.

The aim of this research is to improve the understanding of floating tidal turbine system behaviour, allowing the development of improved sub-models and greater model accuracy, thereby leading to reduced time and cost in system evaluation.

The extent of the impact of the motion and resulting phenomenon that affect the performance of the turbine will be identified and evaluated. Examples of these phenomena may include the added mass effects, dynamic stall, and mooring response. The capabilities and limitations of existing methods will be assessed by comparing analytical model results to alternative simulation experimental results where these data are available.

Turbine and system design considerations will be extended to include the naval architecture of the floating platform itself, which has a great impact on the motion and therefore the performance of the turbine. Of particular interest is the pitch motion of the platform induced by both wave forcing, and the thrust of the turbine suspended beneath the platform on a lever arm, and how thrust interaction, coupled with the motion of the platform, can result in differences in the efficiencies and loading on the tidal turbines that can be utilised for a performance and cost-benefit analysis of the whole system.

The results from initial model tests could be used to inform realistic ranges in pitch and surge for model scale experiments. Understanding the ranges of key criteria associated with performance will allow a method for identifying limiting operating conditions to be developed.

This project is part of the Wind and Marine Energy Systems and Structures Centre for Doctoral Training (WAMSS CDT) with collaborators at the universities of Strathclyde, Edinburgh and Oxford. Industry partners can be found on the CDT website (https://www.wamss-cdt.co.uk/). This project falls within the EPSRC energy and engineering research areas.