Wave Modelling for Potential Wave Energy Sites around the Outer Hebrides

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This presentation looks at the methodology for assessing the temporal and spatial wave energy distribution for the north west coast of the Isle of Lewis. where the proses of creating a mesh, boundary conditions from wave buoy data and model calibration are reviewed. This results in a model that successfully predicts the near shore wave height within 13% for the year long simulation.

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  • The Isle of Lewis is situated 60 km off the west coast of the Scottish mainland 3 main devlopersPelamisAquamarineWavegen at peak time there was a proposed devlopment of 80MW This study focuses on wavegen and aquamarine north
  • This study uses Mike SW model to quantify the nearshore wave resourceMesh was created use bathymetry data from several sources varying in resolutionsThe mesh uses an unstructured grid with 4 “nested” increases in resolutions with a mesh resoultuion at the area of interest of 100m between mesh elements This domain is driven by wave data collected from a datawell MKIII wave buoy located on the outer north-west boundary (see previous slide)
  • Boundary conditions are taken from a DatawellMkIII wave buoy Uses the .SPT file and the highlighted data groupsThis can be used simply to create a frequency spectralHowever this leaves out important directional information and for the purose of wave energy modelling this should be included
  • When calculating the directional spectra additional data is used - This method uses the direction and spread for each frequency to calculate a spectrum the spectrum used in this study is based Gaussian function dependant on the direction and spreadThis function is used as it is simple to apply and has a low computation time requiment when compared to other techniques. for a single frequency the this method can be demonstrated- When the results from all frequencies are compiled the results can be written to a Mike 21 format. (dfs2 or dfsu)
  • The initial resource from the wave buoy and AWAC showsDirectional distribution Where a large proportion of the waves come from 290 degrees a second peak can be observed of 30 degrees indicating likely bi-modal sea states Wave height time seriesVery large resource in the winterLarge variation in wave height in the short term and seasonallyAWAC Tidal height variationComparison plot between measured AWAC data and DHI global tidal modelled dataAs AWAC data only covers a fraction of the simulation period modelled data is usedModel results shows good accuracy in predicting the phase and amplitude for the AWAC locationThese results suggest An angled driving boundary that allows the propagation of both peak swells And at least a years simulation to account for some seasonal variation
  • The model was calibrated using data from the first 2 weeks of March 2012- Using Bottom friction, White capping and Wave breakingSignificant wave height- This uses mainly bottom friction Kn values of 0.04 shows a reasonable agreementKn values of 0.32 shows a good agreement89% Agreement
  • Mean wave period- This uses the white capping delta dis parameterDelta value of 0.5 shows a reasonable agreement Delta value of 0.001 shows a good agreement 84% Agreement
  • When the entire AWAC data series is taken into account a good correlation can be seen between the simulated and measured outputsWhen this is quantified numerically Hs = 0.13 87%Tp =0.07 93%Tz =0.14 86%
  • When the annual spatial distribution is considered The offshore regions show higher level of wave powerLarger resource is shown at headland and lower power are shown in bays and inletsSite 1 and 2 represent 2 proposed development sites-Site 1 shows a lesser resource then site 2 - Where the line of + indicates the 10m depth contour from Brager to the Butt of Lewis
  • If the distribution of wave power along the 10m contour is reviewedIt can be seen there is a high level of variance along the coast.3 peak locations are Brager head land Siadar and the Butt of Lewis Site 1 has an average resource of 25kw/m2 and Site 2 has an average of 29kw/m2While site 2 offers a more energetic resource the general wave power at both site is considered to be high.. other location benefits may be taken into account such as existing infrastructure and sea bed type.
  • This research shows several new techniques and results that may benefit industry and academia.-The high level of interest in the north west coast of Lewis has provided the driving force to design a regional scale wave model-This study shows how the simulation was set up using measured wave buoy data-The calibration process was then shown in detail- Where the model was validated to simulated the nearshore wave climate to Hs = 87% and Tz 86%The spatial results show the distribution of wave power around the north west coast and in more detail around the proposed sites of interest The methods and results shown in study are of particular use to the development of wave models in industry and academia and additionally highlighting areas of high resource.
  • Wave Modelling for Potential Wave Energy Sites around the Outer Hebrides

    1. 1. Wave Modelling For Potential WaveEnergy Sites Around The Outer HebridesCharles Greenwood1, V. Venugopal2, D. Christie1, J. Morrison1 and A. Vogler11Lews Castle College, University of the Highlands and Islands2University of Edinburgh, Scotland
    2. 2. Outline• Proposed Developments• Methodology• Calibration Process• Validation and Results• Benefits to Industry and Academia
    3. 3. Proposed Development for LewisPelamis 10MWAquamarine 40MWWavegen 30MW Total of 80MW by 2020
    4. 4. Methodology - MeshMike 21 Spectral Wave (SW) Model
    5. 5. Boundary ConditionsDatawell waverider MkIIIwww.datawell.nlFrequencyDirectionSpreadPower Spectral Density
    6. 6. Boundary ConditionsSPT data used to create directionalfrequency spectrum
    7. 7. Offshore Resource CharacteristicsLarge seasonal variation in HsGood comparison to measureddataDirectional energydistribution
    8. 8. Model Calibration - HsCalibration Parameters•Bottom friction•White capping•Wave breakingScatter Index = 0.22Scatter Index = 0.1189% Agreement
    9. 9. Scatter Index = 0.218Scatter Index = 0.1784% Agreement!Model Calibration - Tz
    10. 10. Model ValidationScatter Index AgreementHs = 0.13 87%Tp =0.07 93%Tz =0.14 86%CalibrationPeriod
    11. 11. Spatial Distribution of Wave Power
    12. 12. Shoreline Distribution of PowerBarvas Siadar GalsonkW/m2Site 1Site 2Site 1 = 25kW/m2 Site 2 = 29kW/m2
    13. 13. Summary• Created a full directional frequency basedmodel for the Isle of Lewis• Demonstrated the Calibration process• Predict Hs = 87% and Tz 86%• Show spatial wave power distribution

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