UKERC EDC: Publications

This resource links to a document we hold in our system

Abstract:

There is continued interest in tidal energy generation as one of the components needed in the UK portfolio of renewable energy generation schemes. As the understanding and assessment of UK tidal energy resources progresses, it has become apparent that tidal regimes and site characteristics can vary considerably from one location to another. This, together with a survey of the state of the art in tidal energy technology, suggests that tidal energy converter schemes capable of economical operation over a wide range of site conditions would be inherently attractive. As a step towards this goal, Edinburgh Designs present in this report the result of an 11 month investigation into the technical and economical merits of a floating, variable pitch, Vertical Axis Tidal Turbine (VATT) scheme.

Tidal energy converter concepts based on conventional, fixed-pitch vertical axis tidal turbines have not shown great promise so far, owing to relatively poor efficiency, uneven loads and cavitation-limited operation. These issues can be resolved by introducing individual, active pitch control of each turbine foil. In addition, the floating vertical axis configuration offers significant advantages:

Power train components and control system located above water, allowing for easy access and reducing maintenance costs and infrastructure,

Rectangular energy capture cross-section, making the use of high rated powers permissible even in shallow water sites

Versatility: the configuration is suitable for moored, pile or barrier installation

The aim of this study, as set out in the proposal for a floating variable pitch VATT, is to investigate the following points:
Technical analysis:

Impact of variable pitch on machine performance and energy output

Sensitivity of expected gains in energy production to less-than-optimal design parameters

Optimum pitch control strategy and optimum foil profiles for the task

Structural loading and mooring configuration

Economical analysis:

Tidal resource investigation and definition of a baseline site for the proposed device

Investigation of annual energy capture

Build and operation costs for baseline site

Optimisation of unit cost

Overall, the objectives of this study have been met. In particular, we list the following conclusions:

Active pitch control on a vertical axis tidal turbine provides a new performance and economic benefit over the standard fixed pitch alternative. This is supported by parametric studies carried out with both a semi-analytical mode land a CFD simulation.
Steady-state structural loads are well understood and a simple, robust mooring configuration has been studied for the device. This area, however, would benefit from further work to determine how transient loads affect the dynamic stability of the floating structure.
For a single production-sized unit, electricity unit cost comparable to that forecasted for a state-of-the-art horizontal axis tidal turbine seem achievable with the variable pitch VATT concept (8-9/kWhr on the baseline site).
Based on the available data (2001), the results of the economic study suggest that for shallow sites (less than 25m depth, lower peak current velocities) the variable pitch VATT device may have an economic advantage over the MCT horizontal axis device.

This report contains an executive summary, and is divided into the following sections: