Title Development of a tidal flow model for optimisation of tidal turbine arrays Author(s)

347 Pages·2017·10.55 MB·English

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Development of a tidal flow model for optimisation of tidal
Title
turbine arrays
Author(s) Phoenix, Anna
Publication
2018-02-19
Date
Item record http://hdl.handle.net/10379/7145
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DEVELOPMENT OF A TIDAL FLOW MODEL FOR
OPTIMISATION OF TIDAL TURBINE ARRAYS

BY
ANNA PHOENIX
Dissertation submitted to NUI, Galway in partial fulfilment
of the requirements for the degree of PhD

Discipline of Civil Engineering
College of Engineering and Informatics
National University of Ireland, Galway
September 2017

Head of Discipline Supervisor
Dr Bryan McCabe Dr Stephen Nash
Declaration

I declare that this dissertation, in whole or in part, has not been
submitted to any University as an exercise for a degree. I further
declare that, except where reference is given, the work is entirely my
own.

Signed:

Anna Phoenix September 2017

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Dedication
I dedicate this thesis to my Mum and Dad. Without them I would
never have gotten this far. And to Donnacha, for always being there
for me.

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Abstract
Tidal current turbines have the potential to provide a proportion of
global energy requirements. Installations to date have been singular
test devices but commercial application will involve multiple devices
deployed in farms. The viability of large-scale tidal turbine arrays will
depend on the expected energy yield of the farm and the associated
hydro-environmental impacts. The effects are, as yet, still relatively
unknown although numerical models have been used to show that
they can be significant.

In this study a 2D hydrodynamic tidal flow model has been developed
to facilitate the modelling of tidal turbine arrays so that one can
determine an optimum array layout with regard to both power capture
and hydrodynamic impacts. This involved a number of model
development stages to facilitate (1) calculation of the available
resource, (2) simulation of energy extraction, (3) power quantification
and (4) implementation of the array optimisation algorithm.

The first model development stage was to establish and implement a
methodology which accurately quantifies available resource. This
research is based on horizontal axis turbines, whose fixed orientation
results in maximum energy extraction occurring when flow is
travelling perpendicular to the turbine swept area. The methodology
therefore utilises harmonic and ellipse analyses to determine the
primary direction of current flow at a site, which is then used to
quantify available power. This results in a higher degree of
sophistication compared to traditional approaches which are based
on the total velocity vector.

The tidal flow model was next developed to enable simulation of tidal
energy extraction via the momentum sink approach. This requires
parameterisation of the turbine thrust coefficient which is theoretically
defined based on the undisturbed velocity upstream of a turbine but

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is usually calculated using the local velocity at the turbine. A
sensitivity analysis concluded that if calculation of the thrust is based
on localised current velocities it is necessary to specify a localised
thrust coefficient. Based on this finding, a thrust coefficient chart has
been developed from which suitable localised thrust coefficients can
be determined based on the turbine grid cell blockage ratio for a
particular model grid cell. The third stage of development involved
incorporating a power quantification calculation into the energy
extraction model. The power extracted by an individual turbine is
based on the model-calculated turbine thrust.

The final stage of model development involved developing an
optimisation algorithm which determines an optimal array
configuration for maximum energy capture whilst employing spatial
and environmental impact constraints. This algorithm was
incorporated into the numerical model and application of the fully
developed model to test cases demonstrated that optimal arrays
should be staggered, thereby producing higher efficiencies than
symmetrical inline arrays.

This is the first optimisation model to relate hydro-environmental
impacts to the level of energy extraction. The fully developed model
could be extremely useful for determining the economic viability of
proposed arrays, enabling determination of environmentally-safe
levels of tidal energy extraction, and completion of a realistic and
accurate cost-benefit analysis for early stage tidal energy projects. It
is therefore, potentially, a very valuable tool for tidal energy
researchers and turbine developers.

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Acknowledgements
The author wishes to thank Prof Padraic O’Donoghue for facilities
granted and for supporting this research.

The author would also like to thank the Irish Research Council for
funding this research.

Thank you to my supervisor, Dr Stephen Nash, for his technical
advice, encouragement and guidance throughout the last four years.
I would also like to thank my research group director, Prof Michael
Hartnett, for his support during the course of this research. Many
thanks to all the engineering staff for helping me through my studies.

Big thanks to all my friends for the much needed chats and support.
Especially Raydo, for all the brain-storming and gossip sessions,
Moroney, for being a great housemate and of course Kima and
Cosmo. Special thanks to Donnacha for all his love and support,
even through the meltdowns. And to mum, thanks for everything.

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Table of Contents
Declaration ....................................................................................... ii
Dedication........................................................................................ iii
Abstract ........................................................................................... iv
Acknowledgements ........................................................................ vi
Table of Contents ........................................................................... vii
List of Figures ............................................................................... xiv
Mathematical Notations .............................................................. xxvi
1 Introduction ................................................................................... 1
1.1 Tidal Energy ............................................................................ 1
1.2 Numerical Modelling ................................................................ 3
1.3 Aims and Objectives ............................................................... 5
1.4 Thesis Layout and Content ..................................................... 6
1.5 Publications ............................................................................. 9
2 Literature Review ........................................................................ 10
2.1 Introduction ........................................................................... 10
2.2 Tides and Tidal Currents ....................................................... 11
2.3 Tidal Stream Energy Devices ................................................ 14
2.4 Tidal Stream Resource Assessment ..................................... 16
2.4.1 Introduction……………………………………………………….16
2.4.2 Kinetic Energy Flux………………………………………………17
2.4.3 Resource Assessment Methodologies………………………...18
2.4.4 Factors Affecting Accuracy of Resource Assessments……...20
2.4.4.1 Current Velocity Data used in Resource Assessments…….20
2.4.4.2 Formulation of available power equation…………………...22
2.4.4.3 Turbine Impacts…………………………………………………24

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2.5 Approaches to Numerical Modelling of Tidal Turbines .......... 24
2.5.1 Turbine Representation…………………………………………25
2.5.2 Momentum Sink Approach……………………………………...27
2.5.3 Array Optimisation……………………………………………….29
2.6 Hydro-environmental Impacts of Tidal Turbines .................... 30
2.6.1 Summary of Environment Impact Studies…………………….31
2.6.2 Pilot Studies………………………………………………………32
2.6.3 Laboratory and Numerical Model Studies…………………….34
2.7 Hydrodynamic Impacts of Tidal Turbines .............................. 40
2.7.1 Hydrodynamic Effects: Single Turbine………………………...40
2.7.1.1 Reduced Wake Velocities……………………………………...40
2.7.1.2 Accelerated Bypass Velocities………………………………...43
2.7.1.3 Turbulence………………………………………………………43
2.7.1.4 Water Levels…………………………………………………….44
2.7.2  Hydrodynamic Effects: Arrays………………………………….44 
2.8  Environmental Impacts of Tidal Turbines .............................. 46 
2.9  Summary and Conclusions ................................................... 48 
3 Numerical Modelling Theory ...................................................... 52 
3.1  Introduction ........................................................................... 52 
3.1.1  Model Description ................................................................. 53 
3.1.2  Justification of Model Choice ................................................. 55 
3.2  Governing Equations ............................................................. 55 
3.2.1  Finite Difference Formulations .............................................. 58 
3.3  Model Accuracy, Stability and Resolution ............................. 58 
3.4  Model Architecture ................................................................ 60 
3.5  The Nested Model ................................................................. 62 

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3.5.1  Nested Model Grids .............................................................. 62 
3.6  MSN One-way Nested Model ................................................ 64 
3.7  The Open Boundary Problem ................................................ 66 
3.7.1  Specification of the Open Boundary ...................................... 68 
3.7.2  Model Architecture ................................................................ 69 
3.8  Tidal Flushing ........................................................................ 71 
3.9  Tidal Harmonics and Tidal Ellipse Analysis ........................... 71 
3.9.1  Tidal Harmonic Analysis ........................................................ 72 
3.9.2  Tidal Harmonic Analysis Software ......................................... 78 
3.9.3  Tidal Ellipse Theory ............................................................... 83 
4 Resource Assessment ............................................................... 86 
4.1  Introduction ........................................................................... 86 
4.2  Tidal Resource Assessment .................................................. 87 
4.2.1  Traditional Methodologies for Available Resource
Assessment ........................................................................... 87 
4.2.2  New Methodology for Available Resource Assessment ........ 87 
4.3  The Shannon Estuary Model ................................................. 88 
4.3.1  General Description of the Shannon Estuary ........................ 88 
4.3.2  Model Description ................................................................. 90 
4.3.3  Model validation .................................................................... 92 
4.4  Development of Model to Implement New resource
Assessment ........................................................................... 95 
4.4.1  Importance of Considering Flow Direction ............................. 95 
4.4.2  Development of Resource Assessment Model ...................... 96 
4.5  Shannon Estuary Case Study Results .................................. 98 
4.5.1  Testing of the Ellipse Code ................................................... 99 
4.5.2  Available Resource Assessments ....................................... 102 

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