Abstract
Tidal turbines are subject to large hydrodynamic loads from combinations of currents and waves, which contribute significantly to fatigue, extreme loading and power flow requirements. Physical model testing enables these loads and power fluctuations to be assessed and understood in a controlled and repeatable environment. In this work, a 1:15 scale tidal turbine model is utilised to further the fundamental understanding of the influence of waves on tidal turbines. A wide range of regular waves are generated in both following-current and opposing-current conditions. Wave frequencies range from 0.31 Hz to 0.55 Hz & wave heights from 0.025 m to 0.37 m in a fixed 0.81 m/s current velocity. Waves are selected and programmed specifically to facilitate frequency domain analysis, and techniques are employed to isolate the effect of non-linear waves on turbine power and thrust.Results demonstrate that wave action induces large variations in turbine power and thrust compared to current only conditions. For the range of conditions tested, peak values of thrust and power exceed current-only values by between 7%–65% and 13%–160% respectively. These wave-induced fluctuations are shown to increase with wave amplitude and decrease with wave frequency. Following wave conditions exhibit greater variations than opposing for waves with the same wave height and frequency due to the lower associated wavenumbers.A model is developed and presented to aid the understanding of the high-order harmonic response of the turbine to waves, which is further demonstrated using steady state coefficients under assumptions of pseudo-stationarity. This approach is proven to be effective at estimating wave-induced power and thrust fluctuations for the combinations of waves, currents and turbine state tested. The outcome of which shows promise as a rapid design tool that can evaluate the effect of site-specific wave–current conditions on turbine performance.
Highlights
Tidal stream power is a highly dense renewable energy source, which is both predictable and reliable
The International Electrotechnical Commission (IEC) recommends that waves are considered in resource characterisation process (IEC TS 62600-201:2015, 2015), and advises wave measurement programmes where wave orbital velocities are likely to be greater than 20% of the rated current speed (IEC TS 62600-200:2013, 2013)
The components of interest have been extracted from the measured force spectra, before taking the maximum value of an Inverse Fast Fourier Transform (IFFT) including only these components. This approach effectively isolates waveinduced maxima from those incorporating turbulence. As these maximum values are crucial for design, the results presented in Sections 4.2 and 4.3 include the response to the fundamental wave frequency, along with these maximum wave-induced values
Summary
Tidal stream power is a highly dense renewable energy source, which is both predictable and reliable. Recent studies suggest that the theoretical available tidal power, in the UK alone, is 95 TWh/year (The Crown Estate, 2012), yet at present the tidal resource remains largely unexploited. To take advantage of the resource potential, Tidal Stream Turbine (TST) devices have been actively developed, with the first arrays currently being installed and commissioned (Atlantis, 2017). Steynor et al / Journal of Fluids and Structures 84 (2019) 199–217
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