Tidal Hydrodynamics

UK resource mapExtracting energy from a tidal stream has the potential to alter the tidal dynamics over a large area. As well as being important for the performance of neighbouring tidal energy schemes, these areas can also be geographically separated due to changes that occur to the propagation of the tides around, for example, the UK.

The dynamic relationship between the tidal stream resource and the resistance presented to the flow by a large number tidal devices results in a complex, multi-dimensional problem for the design and operation of arrays of tidals turbines. Understanding the changes that devices cause to the tidal dynamics is important so that devices may be optimally located, and so the power output and environmental consequences can be accurately determined.

Several research themes are being investigated into tidal basin hydrodynamics, including the analysis of idealised tidal basins, analysis of idealised arrays of tidal devices, the study of tidal stream energy extraction from proposed sites for energy extraction around the UK, and the modelling of the tidal resource.

Idealised Tidal Basins

Idealised basins

A basis set of idealised geometries have been identified to broadly characterise the types of locations where energy extraction by tidal stream turbines has been proposed. These idealised geometries provide an insight to the problem and important flow phenomenon that are present at different sites.

The basis set consists of four coastal basins: (a) a channel between two disconnected basins; (b) a coastal inlet, estuary or bay; (c) a headland; and, (d) an island located near a much larger land mass. The research has been conducted using both analytic (through, for example, analogy to electrical circuits) and numerical models, and have been used to inform analyses of specific sites of interest for tidal stream development.

Idealised Tidal Turbine Array Models

Analytic and numerical models if tidal hydrodynamics have been developed to model the local effects of tidal power devices based on Linear Momentum Actuator Disk Theory (LMADT).

LMADT

LMADT, based upon conservation of mass, momentum, and energy arguments, allows the performance of idealised turbines to be analysed analytically and provides a useful benchmark for more detailed models of tidal turbine performance. A key outcome of LMADT for tidal turbines is the importance of the blockage ratio (the ratio of device frontal area to the cross-sectional area of the surrounding flow passage) in determining the device thrust and power. The effect of free surface deformation may also be investigated analytically, and has been shown also to allow further increases in device thrust and power to be achieved.

Multiscale LMADTThe Tidal Energy Research Group has pioneered the concept of "scale separation", wherein two scales of flow phenomenon can be identified for a cross-stream array composed of a large number of tidal turbines in a wide channel. This allows the local flow phenomenon around a single device to be analysed separately to that around the array of turbines, and demonstrates that both the "local blockage ratio", the ratio of device frontal area to the cross-sectional area of the surrounding flow passage, and the "array blockage ratio", the ratio of array frontal area to the cross-sectional area of the tidal channel, are important in determining device and array performance. Although the thrust and power achieved by the devices is less than that of a cross-stream array spanning the entire channel cross-section (as would be predicted based solely on the local blockage ratio), constructive interference effects between the devices mean that the thrust and power are greater than that predicted solely on the basis of the array blockage ratio. Consequently, tidal arrays not occupying a significant fraction of the tidal basin cross-section can still benefit from significant performance improvement by deploying turbines to maximise the constructive interference effects. The predictions of the analytic work has been supported by both CFD and experimental models.

Tidal Resource Analysis

Real sites, such as the Pentland Firth and the Severn Estuary have been investigated to determine the available tidal stream resource and the environmental impacts of energy extraction using the continuous Galerkin Finite Element Method (CG-FEM) solver Telemac2D and the discontinuous Galerkin Finite Element Method (DG-FEM) solver ADCIRC. Complex interactions exist between the natural tidal resource and energy extraction, as energy extraction can move the tidal regime closer to, or away from, resonance with dominant tidal constituents.

Although tidal energy is largely considered to be a predictable renewable resource, the tidal stream resource, which can vary over much smaller spatial scales than the tidal range rosource, is challenging to predict due to the multitude of interactions between tidal constituents and the bathymetry of a tidal site. Machine learning is being utilised to develop better tools for understanding and forecasting the tidal stream resource, allowing more accurate long term assessments of the resource to be made.