Elsevier

Renewable Energy

Volume 34, Issue 12, December 2009, Pages 2803-2812
Renewable Energy

The impact of tidal stream turbines on large-scale sediment dynamics

https://doi.org/10.1016/j.renene.2009.06.015Get rights and content

Abstract

Tidal stream turbines are exploited in regions of high tidal currents. Such energy extraction will alter the hydrodynamics of a tidal region, analogous to increasing the bed friction in the region of extraction. In addition, this study demonstrates that energy extracted with respect to tidal asymmetries due to interactions between quarter (M4) and semi-diurnal (M2) currents will have important implications for large-scale sediment dynamics. Model simulations show that energy extracted from regions of strong tidal asymmetry will have a much more pronounced effect on sediment dynamics than energy extracted from regions of tidal symmetry. The results show that energy extracted from regions of strong tidal asymmetry led to a 20% increase in the magnitude of bed level change averaged over the length of a large estuarine system, compared with energy extracted from regions of tidal symmetry. However, regardless of the location of a tidal stream farm within a tidal system, energy extraction reduces the overall magnitude of bed level change in comparison with non-extraction cases. This has practical application to many areas surrounding the UK, including the Irish Sea and the Bristol Channel, that exhibit strong tidal currents suitable for exploitation of the tidal stream resource, but where large variations in tidal asymmetry occur.

Introduction

Tidal currents have been identified as a renewable resource that has a strong future potential for economic energy harvesting. Leading developers of Tidal Energy Converter (TEC) devices are at the stage of full-scale testing of pre-commercialisation examples of their respective technologies. The UK is home to many of the most advanced technologies, and has the largest proliferation of devices under development [1]. Key to the UK achieving this position is the extent of the tidal current resource in UK territorial waters. As for any renewable technology, the local availability of the resource is what drives private and public funding interest. Although resource analysis and quantification remain the subject of significant research interest (e.g. [2], [3], [4], [5]), around half of the identified ‘technically extractable’ European tidal current resource is located in UK territorial waters [6]. Harvesting of the tidal current energy resource has therefore been highlighted as a key contributor to the future UK energy mix. This is reflected in strong government support of R&D in tidal current energy in the wider context of marine renewable energy (e.g. [7], [8], [9]).

TEC devices operate by intercepting the kinetic energy in strong tidal currents (typically through a turbine unit). This intercepted energy is then converted to electrical energy through a power take-off system (e.g. an induction generator) and conditioned for dispatch to the electricity network. Theoretically, this is very similar to the operation of a typical wind energy device. What is significantly different from the wind energy analogy is the environment that the TEC device operates in [10], and the potential interaction between TEC devices and their environment [3]. Given the proliferation of at-sea demonstration devices, the environmental impacts of operation of TEC devices are a timely issue to consider. Developers such as Marine Current Turbines Ltd. (MCT) and Verdant Power have had to conduct multi-million dollar environmental monitoring programs in order to obtain environmental permission for installation of development devices. This is potentially slowing down the development of TEC technology. The major research questions in the tidal energy context have already been identified [11]. Progress to-date has, however, been limited in addressing these research issues. The impact of TEC operation on sediment dynamics has yet to be explored in the scientific literature and has motivated the present study.

The tidal stream resource is generally estimated using histograms of depth-averaged tidal current speed over time periods that include the spring/neap cycle [12]. In terms of environmental impact, no consideration has yet been given to the change in large-scale sediment dynamics and morphodynamics resulting from tidal stream energy extraction. This may have varying impacts depending upon tidal asymmetry at the point of extraction. In regions of strong tidal asymmetry, a residual transport of sediment occurs in either the flood or ebb direction. A modest amount of energy extracted from such a location could potentially have a large effect on the residual sediment transport compared with energy extracted from a region of tidal symmetry. In the latter case, since sediment transport is equal in magnitude during the flood and ebb phases of the tide, energy extracted at such a location will not significantly affect the residual sediment transport and hence the large-scale morphodynamics of the region. Tidal asymmetry is discussed in more detail in Section 2.

The importance of considering the phase relationship between the M2 and M4 constituents is demonstrated here, initially with theoretical application to energy extracted from idealised domains (Section 2). A one-dimensional (1D) morphological model is then developed (Section 3) consisting of hydrodynamic, sediment transport and bed level change components. This model is applied to the Bristol Channel (Section 4), a tidal system that has a large tidal stream resource, and exhibits a large range of phase relationships between the M2 and M4 constituents and, hence, a large range of tidal asymmetries. The model simulations also include a range of sensitivity tests in order to demonstrate the robustness of the theory.

Section snippets

Tidal asymmetry

Pingree and Griffiths [13] noted that the mean bed shear stress distributions associated with the M2 and M4 tides in the Irish Sea tend to be directed into bays and to diverge from M2 amphidromic points, reaching maximum values at M4 amphidromic points. They further suggested that the interaction between the M2 and M4 tides determines the direction of net sand transport around the British Isles. Regions of modelled bed-stress divergence compared favourably with sediment bed load partings, based

Morphological model

To test the hypothesis outlined in Section 2, a 1D morphological model was developed and applied to a case study (the Bristol Channel) where large spatial variations in tidal asymmetry occur along the length of the channel (Section 4). The morphological model consisted of three components: hydrodynamic, sediment transport and bed level change models.

Bristol Channel

The Bristol Channel (Fig. 6) separates South Wales from South West England and extends from the lower estuary of the River Severn to the Celtic Sea. For the purposes of this paper, however, the term “Bristol Channel” is taken to include the Severn Estuary. Depths in the outer region of the Bristol Channel are around 50 m, and an extensive system of narrow channels and tidal flats in the upper part of the estuary reduces the depth to less than 10 m upstream of Newport. With a mean spring range of

Discussion

Results of a model parameterised on the Bristol Channel demonstrate that tidal stream energy extraction can have a significant impact on the large-scale morphodynamics of a large-scale tidal system, the magnitude of which depends upon the tidal asymmetry at the point of extraction. This supports the hypothesis presented in Section 2: energy extracted from regions of strong tidal asymmetry will have a much greater influence on large-scale sediment dynamics than energy extracted from regions of

Conclusions

A one-dimensional numerical model has demonstrated that a small amount of energy extracted from a tidal system can lead to a significant impact on the sediment dynamics, depending on tidal asymmetry at the point of extraction. The resulting influence on the morphodynamics is not confined to the immediate vicinity of the tidal stream farm, as would occur in the case of localised scour, but affects the erosion/deposition pattern over a considerable distance from the point of energy extraction (of

Acknowledgements

Tidal data for the Bristol Channel was provided by the British Oceanographic Data Centre and the UK Tide Gauge Network. We are grateful to an anonymous reviewer for constructive comments on the manuscript. This research formed part of Emmer Litt's MSc studies at Bangor University, funded by the UK Natural Environment Research Council (NERC). The project was also supported by the Higher Education Funding Council for Wales (HEFCW) and the Welsh Assembly Government.

References (45)

  • S.P. Neill et al.

    A model of inter-annual variability in beach levels

    Cont Shelf Res

    (2008)
  • L. Myers et al.

    Simulated electrical power potential harnessed by marine current turbine arrays in the Alderney Race

    Renew Energy

    (2005)
  • P.T. Harris et al.

    Sand transport in the Bristol Channel: bedload parting zone or mutually evasive transport pathways?

    Mar Geol

    (1991)
  • S.P. Neill

    The role of Coriolis in sandbank formation due to a headland/island system

    Estuar Coast Shelf Sci

    (2008)
  • S. Wang et al.

    The impact of climate change on storm surges over Irish waters

    Ocean Model

    (2008)
  • IEA-OES. International energy agency implementing agreement on ocean energy systems, Annual report;...
  • L.S. Blunden et al.

    Tidal energy resource assessment for tidal stream generators

    Proc Inst Mech Eng Part A – J Power Energy

    (2007)
  • Couch SJ, Bryden IG. Large-scale physical response of the tidal system to energy extraction and its significance for...
  • Prandle D. Extracting ‘free-stream’ tidal current energy in estuaries. In: Physics of Estuaries and Coastal Seas,...
  • G. Sutherland et al.

    Tidal current energy assessment for Johnstone Strait, Vancouver Island

    Proc Inst Mech Eng Part A – J Power Energy

    (2007)
  • Black & Veatch. Tidal Stream – Phase II UK Tidal Stream Energy Resource Assessment. A report to the carbon trust's...
  • Cited by (0)

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