Resource characterization of sites in the vicinity of an island near a landmass

Highlights

Resource characterization of coastal sites defined as an island near a landmass.

Undisturbed kinetic or dissipated power do not approximate maximum power extracted.

Numerical results exceed those from an analytical model except for long islands.

Increased offshore depth and lower blockage both reduce the maximum power extracted.

Power extracted can be maximized with extraction in strait and offshore of island.

Abstract

Renewable energy technologies are undergoing rapid development, the global aim being to achieve energy security and lower carbon emissions. Of marine renewable energy sources, tidal power has inherent predictability and large theoretical potential, estimated to exceed 8000 (TW h)a−1 in coastal basins. Coastal sites in the vicinity of an island near a landmass are prime candidates for tidal stream power exploitation by arrays of turbines. This paper characterizes numerically the upper limit to power extraction of turbines installed at such sites. It is demonstrated that the maximum power extracted from the strait is generally not well approximated by either the power dissipated naturally at the seabed or the undisturbed kinetic power of flow in the strait. An analytical channel model [C. Garrett and P. Cummins, “The power potential of tidal currents in channels,” Proc. R. Soc. A Math. Phys. Eng. Sci., vol. 461, no. 2060, pp. 2563–2572, Aug. 2005] provides lower predictions than the present numerical model of available power in the strait due to the analytical model not accounting for changes to the driving head resulting from power extraction and flow diversion offshore of the island. For geometrically long islands extending parallel to the landmass, the numerically predicted extracted power is satisfactorily approximated by the power naturally dissipated at the seabed, and there is reasonable agreement with the estimate by the channel analytical model. It is found that the results are sensitive to choice of boundary conditions used for the coastlines, the eddy viscosity, and bed friction. Increased offshore depth and lower blockage both reduce the maximum power extracted from the strait. The results indicate that power extracted from the site can be maximum if extraction is implemented both in the strait and offshore of the island. Presence of the landmass and increasing island dimensions both enhance power extraction.

Keywords

  • Tidal energy;
  • Resource assessment;
  • Numerical modelling;
  • Strait;
  • Island;
  • Landmass

1. Introduction

Development of renewable energy technologies has undergone remarkable progress in the past decades motivated by the security of supply, finiteness and unstable price of fossil fuels [1] and [2] and the effects on the climate associated with carbon emissions [3]. Renewable energy sources such as wind and solar are stochastic and as such, backup generation is required during those time periods when generation is unable to meet demand. Tidal currents have the advantage of being completely deterministic, and therefore quite predictable, making power-grid integration more straightforward. The ebb and flow motions of tidal currents make tidal power production intermittent, and so backup would be required during slack water as the tide turns and possibly during neap tides. Tidal farms exploit the relatively high energy densities of tidal streams, thus limiting their footprint in comparison to wind and solar farms.

The first pre-commercial tidal arrays are under construction and in the next ten to twenty years it is expected that the first multi-megawatt commercial arrays will become operational. The success of such tidal projects depends on correct estimation of the tidal resource and assessment of the associated environmental impacts. Tidal energy comprises both potential and kinetic energy; hence resource assessment requires information on sea surface elevations and current velocities. Typically, data are measured at the site using acoustic Doppler current profilers (ADCP), and the tidal signal time history reproduced using harmonic analysis [4]. The data are very useful for validation of tide models. However, there are limits to ADCP deployment, owing to the cost of field measurement campaigns. Lack of spatial data coverage and measurement errors add to uncertainty in theoretical model calibration.

Power extraction alters the local flow hydrodynamics, and this must be accounted for in predictive models used for tidal resource assessment. Such models can be classified into three categories. Analytical one-dimensional (1D) models determine the maximum average power extracted from an idealised channel connecting two infinite ocean basins [5] or an infinite ocean basin with an enclosed bay [6] based on accessible parameters such as amplitude of tidal head difference driving the flow, peak flow through the channel, seabed friction, and channel dimensions. However, such analytical models assume idealised seabed conditions and channel geometry, and uniform power extraction. These limitations are largely overcome by using two-dimensional (2D) and three-dimensional (3D) models. 2D models solve the shallow water equations (SWE) to compute free surface elevations and depth-averaged velocities, and permit a localised representation of power extraction by tidal turbines. Although 2D models are computationally efficient, they neglect vertical flow behaviour. 3D models compute the flow velocity over the entire water column and model the power extraction profile over the water column, leading to a more realistic representation of power extraction. The resulting improvement in accuracy is at the expense of greatly increased computational load, limiting 3D models to small- and medium-scale domains, unlike 2D models which are routinely applied to medium- to large-scale domains [7].

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