EGU2020-19231
https://doi.org/10.5194/egusphere-egu2020-19231
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Bio-optimisation of a tidal channel

Rory O'Hara Murray1 and Matthew Lewis2
Rory O'Hara Murray and Matthew Lewis
  • 1Marine Scotland Science, The Scottish Government, Aberdeen, UK
  • 2School of Ocean Sciences, Bangor University, UK

Scotland has ambitious decarbonisation and climate change objectives, such as generating 100% of gross annual electricity consumption from renewable sources by 2020. Tidal stream energy is a renewable and predictable source of energy that converts the kinetic energy within tidal currents, into electricity, using a hydrokinetic device such as a horizontal axis turbine. However, economically viable tidal stream development is currently confined to areas of exceptionally high current speeds, and this can severely limit the choice of area. If the speed threshold required for an economically viable tidal site can be lowered then the number of potential sites could increase dramatically.

It is well known that macro-algae (e.g. kelp) grow in perspective tidal energy sites, as they requiring similar water depths and current speeds. Furthermore, kelp is known to grow in dense patches, reaching from the sea-floor to the ocean surface, and can modify tidal current speeds. Indeed, observations have shown that “kelp forests” can locally reduce current speeds by a third (Jackson and Winant, 1983). This local reduction in current speed will cause an increase in speed elsewhere, in order to conserve mass. Therefore, we hypothesise that by adding a kelp forest in the vicinity of a tidal channel, the current speed and tidal stream resource could be increased sufficiently for the site to become economical.

A three dimensional finite volume hydrodynamic model has been used to model an idealised tidal channel. The drag imposed by kelp was theoretically calculated and represented in the model as a sub grid scale momentum sink. The changes to the current speed resulting from this bio-optimisation of the tidal channel were investigated and show that the current speed in the centre of the channel can be increased. Kelp were then added to a previously developed hydrodynamic model of the Pentland Firth and Orkney Waters to investigate how such bio-optimisation could influence an area currently being considered for substantial tidal stream development. The changes on both the areas of suitable tidal stream development and the power yield are investigated.

Acknowledgements

Matthew Lewis wishes to thank Aaron Owen and Ade Fewings at SuperComputingWales, and Fearghal O'Donncha at IBM-research Ireland for fruitful discussions, and the METRIC grant, EP/R034664/1.

References

Jackson, G. A. and Winant, C. D. (1983). Effect of a kelp forest on coastal currents. Continental Shelf Research, 2(1), pp.75-80.

How to cite: O'Hara Murray, R. and Lewis, M.: Bio-optimisation of a tidal channel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19231, https://doi.org/10.5194/egusphere-egu2020-19231, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 02 May 2020
  • CC1: Comment on EGU2020-19231, Iain Fairley, 04 May 2020

    Hi Rory,

    Nice presentation and interesting idea. What are Marine Scotland's thoughts on the planting of kelp? How would the presence of additional kelp affect things like seal use of tidal channels and interaction with turbines?

    All the best,

    Iain

    • AC1: Reply to CC1, Rory O'Hara Murray, 04 May 2020

      Hi Iain,
      I'm not sure what official stance MS has on kelp planting or harvesting. I know thee have been some tentative proposals to licensing and operations on harvesting of existing wild kelp. These are unlikely to be consented anytime soon due to environmental concerns/uncertainties. In terms of planting large farms in tidal channels. I think it is fair to say that substantial environmental impact assessments would have to be conducted first.  Still, I think it's interesting to theoretically look into possibilities of sharing marine space between different economic sectors.
      Chees, Rory

      • CC2: Reply to AC1, Iain Fairley, 04 May 2020

        Thanks Rory. I agree - especially as multi-use of space is recommended by the EU for marine spatial planning. 'Talk' to you tomorrow!

        Iain

  • CC3: Comment on EGU2020-19231, Sam Fredriksson, 05 May 2020

    Hi

    Thanks for the interesting presentation. Two questions, How large is the model domain? Is it shown in slide 10 fig a)

    In the conclusions you ask if present Kelp should be represented in out models. What is your own thoughts about that?

    //Sam 

     

    • AC2: Reply to CC3, Rory O'Hara Murray, 05 May 2020

      Hi Sam,

      Yes, slide 10 (a) is the Orkney model domain. 

      OK, in terms of representing Kelp in models, maybe we should be... This work was really just a bit of fun, as we're making big changes to how energy is dissipated in the model, but the changes are quite noticeable and it would be interesting to see if adding kelp in this way to models where we know kelp grow improves the model calibration.  You could argue that a calibrated model would take naturally occurring kelp into account, but adding it would be a kind of spatially varying friction in the model.

      Rory

      • CC4: Reply to AC2, Sam Fredriksson, 05 May 2020

        Thanks.

        Ok

        Guess it depends on how you calibrate your model. Do you do it by spatially varying bottom friction? Anyway it is often better to have as much information in the model as reasonable before you start the calibration.