Transcript Group 4 – Marine Energy

Marine Current Modelling
For Energy Production
Group 4 – Marine Energy
James Glynn
Kirsten Hamilton
Tom McCombes
Malcolm MacDonald
To Recap …
Project Definition
• Investigate the characteristics of the tidal
resources in Scotland and demonstrate
how to match those resources with the
appropriate Marine current technology
Plan
Stage 1 - Investigation
• Resources
• Technology
Stage 2 - Analysis
• Methodology to match technology with resource
• Case study
Stage 1 - Investigation
A. Resource Investigation
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Identify current weaknesses in available resource data
Obtain ocean topography charts
Select sites in the west of Scotland based on current research,
specific criteria, and surface velocity
Calculate shear profiles according to bathymetry of selected
sites
Correlate shear data with surface velocity to derive velocity
distributions
B. Technology assessment
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Generic Modelling of three tidal technologies: horizontal axis,
vertical axis and oscillating hydrofoil devices
Oceanographic & Hydrodynamic Suitability of each type
Potential packing densities
Define suitable conditions for each technology in terms of
depth, velocity, surface roughness etc
Stage 2 - Analysis
A. Methodology
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Develop a frame work to assess each technology
suitability for each resource characteristic
Define rules to match technology with particular
resource characteristic
B. Case study
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Demonstrate the implementation of this
methodology for at least one Scottish West Coast
site, most likely Hebridean Sounds
Prove robustness of work and further qualify
methodology
Literature Review
• There are a number of relevant research projects that have been
undertaken in marine energy
• We are going to briefly discuss some sources of information,
highlight the conclusions, relevant information and differentiation
between past research and our proposal
• Information Sources discussed in following section:
 Past projects
 Tidal Resource Stream
 DTi Work
 Carbon Trust
 Baraj et al
Past Projects
03/04 “Base-load Supply Strategy”
• Area of concern:
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To investigate the potential of marine current energy to meet a
proportion of Scotland's base-load energy supply.
Review current research, existing technology & Engineering
Challenges.
Environmental impact, Planning requirements, grid connection, marine
current resource availability, and economics and policy issues.
• Conclusion:
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Primary advantage of tidal stream technology is its predictability,
renewable status and abundance.
Marine current technology has proven base-load potential, dependant
on large scale energy storage
• (e.g. Cruachan is limited operating at 400MW for 22 hours).
• Important points:
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Resolution of Tidal Calculations Grid>1km2
• Assumed non varying bulk tidal velocities within grid elements
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Idealised general tidal calculation & validation using sinusoidal
assumptions, port tidal charts & ignoring bathymetrical effects
Past Projects
04/05 “Developing and Growing a Scottish Marine
Current Turbine Industry "
• Area of concern:
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Mainly an overview of MCT’s and their commercialisation
potential.
Main thrust of technical investigation aimed at blockage effects in
a channel.
• Conclusion:
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After uncovering blockage effects, group stated this limit would
not be reached anyway due to economic and technical
limitations
• Important points:
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Discussion of potential for further work such as 3d modelling,
and including bathymetry in calculations, which we aim to do.
Takes steps towards the much-requested, site specific resource
quantification.
Early Tidal Stream Studies
• Most other studies refer back to these reports:
1)‘UK Tidal Stream Energy Review. 1992-1993’
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DTI, Binnie & Partners, Sir Robert McAlpine & Sons Ltd. & IT Power
First official attempt to evaluate the UK’s tidal resources
Identified a 25 sites in UK waters, combined annual output of 58 TWh/yr
Sites can be collected into 7 main locations; 3 of them in Scotland
Report excludes sites where the depth < 20m, speed <2m/s, area <2km2
Report concluded that although UK resource was large, the unit cost of
energy would be relatively high
2)‘The Exploitation Of Tidal /Marine Currents,1994-1996’
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EC Joule Programme
Report identified a total of 42 sites, combined annual output 31 TWh/yr.
Included a number of such sites at depths of <20m, speeds >1m/s At most
locations the output is generally between 40% and 70% of that from the
1993 report.
The West Coast of Scotland output is significantly increased by the
inclusion of shallow water sites
DTI Commercial Tidal Research
3)“Development, Installation And Testing Of A Large-scale Tidal Current
Turbine”, Oct 2005
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IT Power Marine Current Turbines, Seacore, Bendalls Engineering, Corus
Aim: Seaflow was a project to develop and test a commercially-sized marine current
turbine. Objectives were to test the feasibility of constructing and operating such a
machine, and to evaluate the likely longer-term economics of using such tidal turbines
to generate electricity.
Conclusion: The Seaflow turbine was successfully installed and operated. It proved
the basic physics of power extraction from tidal flows, and showed that useful
electrical power can be generated from horizontal-axis turbines.
Relevance: Useful data acquired for site selection and for technology review
4)“The Commercial Prospects for Tidal Stream Power”, 2000-1
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Binnie, Black & Veatch, IT Power Ltd
Aim: Study based on a conceptual design of 1MW and examined the cost and energy
outputs for schemes with different numbers of units located in various water depths,
peak flow velocities and tidal ranges
Conclusion: the proposed scheme was practical, robust and capable after
development of delivering unit costs in the range of 4-6p/kWh
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Assumption in 1993 report led to considerable lower energy output estimates and higher
energy costs than found for this report
The problems associated with constructing installing and operating the system are suggested
as the next appropriate step
Relevance: The size of and cost of extracting the UK resource
Bahaj et al
5)‘Fundamentals applicable to the utilisation of marine current turbines
for energy production, 2003’
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Bahaj A.S.; Myers L.E.,
Aim: This paper reviews the fundamental issues that are likely to play a major role in
implementation of MCT systems. The paper reports issues such as the harsh marine environment,
the phenomenon of cavitation, and the high stresses encountered by such structures are likely to
play a major role on the work currently being undertaken in this field.
Conclusion: The need to exploit marine energy is increasingly recognised and the engineering
capability to do so is now here following experience with offshore structures and new
developments in offshore piling. However, there is limited technical knowledge of how to optimise
the design of kinetic energy turbine rotors for use in water; there are also various important factors
unique to the marine current resource that need to be investigated through a process of model
testing, and technical analysis.
Relevance: Fundamental principles relating to turbine devices
6)‘Initial evaluation of tidal stream energy resources at Portland Bill, UK,
February, 2006’
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Blunden, L.S. Bahaj, A.S.,
Aim: To better estimate available energy resources at the Portland Bill, a two-dimensional tidally
driven hydrodynamic numerical model of Portland Bill was developed using the TÉLÉMAC
system, with validation using tidal elevation measurements and tidal stream diamonds from
Admiralty charts.
Conclusion: The results of the model were used to produce a time series of the tidal stream
velocity over the simulation period and may be used in future work to optimise the location of
turbine arrays at the site.
Relevance: The methodology used in this research can be adapted
Recent Tidal Stream Research
7) ‘Atlas of UK Marine Renewable Energy Resources, Dec 2004’
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Produced By: Garrad Hassan
Area of concern: aim to present an accessible overview for the potential
renewable energy resource in the UK.
Covers wind, wave and tidal predominantly
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Important points:
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Fairly encompassing, though the tidal resource model resolution is sufficient only
for initial identification purposes, and would not help calculating potential energy
yield to a high degree.
Carbon trust state: “The tidal stream resource is highly site specific”, thus
intimating the potential for discrepancies in an overall resource characterization.
There is scope therefore for the development of site specific methodologies and
models.
The GH report for the Scottish executive also has a number of assumptions,
which we feel facilitates something of an over-optimistic quantification of
resource availability.
B&V and RGU feel a flux method will aid in a greater understanding of
environmental and economic factors, relative to resource flow, capture and
packing arrangements
8) Carbon Trust
• The Carbon Trust is presently responsible for an initiative known as the
Marine Energy Challenge.
• Programme has the aim to ‘assess the potential for marine energy devices
to achieve a competitive cost of electricity generation against other
renewables and fossil fuelled power generation’ (Carbon Trust, 2004).
• Area of concern: Resource and technology assessment on a grand scale.
Big consultancy collaboration.
• Conclusion: Previous estimates poor.. Probably represents best approach
so far. 12-13TWh available. Economic assessment of predominant
technology types.
• Important points:
 Mean spring peak velocity taken at 5m depth..
 Previous data sources as primary input, +/-30 resource estimate
uncertainty. Also use a depth averaged flow: rule of 7ths
 Advocate further modelling to clarify resource for specific sites
 The Carbon Trust in conjunction with B&V are attempting to evaluate
benefits of different approaches and in particular understand the linkages
between resource potential and device development.
• CT would like flux assessments for individual UK sites
Other Sources
• Marine Energy Group & Meeting the 2020 Target reports
for S.E
• Various articles on tidal flows/sediment transport/fluid
mechanics and calculating wind resource for possible
cross over data etc
• World Energy Organization- various
• House of Commons: Energy Report white paper.
• Bowditch, N The American Practical Navigator Chapter
9: Tides and Tidal Currents, and Nautical Charts.
• Scottish Enterprise (2005): Marine Renewable (Wave
and Tidal) Opportunity Review
• Scott J. Couch & Ian G. Bryden (of RGU) The Impact of
Energy Extraction on Tidal Flow Development. [Flux
method referred to by B&V in Carbon Trust consultation]
Stage 1: Part A. Resource Investigation
Work undertaken
Surface Tidal Data
• Based on available data and previous work by others in the are, we
have ascertained several potential sites
• Some have been previously overlooked, such as the Sound of
Harris. An updated interlink/infrastructure will render this option
feasible, if the wind farm proposals on Lewis go ahead.
• Narrow focusing channels such as that between the islands of Islay
and Jura hold potential as high tidal velocity sites, but due to their
relative size & depths may have been previously over looked.
• Several Headlands with gentle curvature and protrusion into the tidal
flow also hold potential for sites
• Tidal Profiles are typically calculated by interpolating measured data
from ports around the site. The accuracy of this method depends on
the proximity of the site to the ports.
• Further sinusoidal theoretical tidal models can be implemented to
ascertain surface flow velocities. But are based on assumptions of
tidal frequency and sinusoidal nature.
Tides
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The rotation of the earth upon its axis gives rise to a transit period of the moon
of 12h 50m. Maximum declination about 23.5 degrees.
The moons orbit around the earth takes 27.3 days. 384403km average
distance from earth
The equatorial declination of the moon varies between north and south, and
the orbit paths repeat every 18.6 years. Thus tides are highly predictable,
though pressure, surge resonant effects and thermal currents all play a part.
Primarily, Newton’s law of gravitation allows the gravitational pull of the sun
and moon on the earth’s waters to be resolved into vectors, at any point, and
represents the main tide governing effect, in conjunction with the second law
of motion:
Fdm= GMmRe ; Fds = GMsRe (tractive forces)=>sun ‘pull’ is 46% of moon
dm3
ds3
Sublunar point and antipode in combination with centrifugal forces of the
earth’s rotation allow ‘bulges’ in the seas at 180degrees. Since the earth
rotates, around the equator, we will generally experience two high tides and
two low tides per day.
In channels, this gives rise to a head difference out of phase by the channel
characteristics, allowing flow to be calculated.
Sea lochs are not affected by blockage as much as a positive head is
generated.
Mapping
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Existing Marine & Bathymetric Atlases
 1 Minute or Coarser Measurement Grids
 General Bathymetric Charts of the Oceans (GEBCO)
 United Kingdom Digital Marine Atlas (UKDMAP)
• Navigational Admiralty Charts
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United Kingdom Hydrographic Office (UKHO)
Accurate Low Scales – 1:10,000
• High resolution information
Detailed Coastal & Channel Bathymetry
Tidal Flow Diamonds
Hazardous areas, shipping routes & military zones
Costly – Licence Required
Mapping
General Site Selection Principles (DTI 2005)
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Higher currents are only found around certain features, such as:
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Channels or constrictions between islands - fast and rectilinear flow
Headlands in the path of moderate flows - best when the headlands are large and do not
protrude too sharply into the flow, otherwise the flows are fast but turbulent, and the high
currents may be in different places on ebb and flood
Estuaries or other resonant water volumes - good sites with rectilinear flow
Narrow entrances to enclosed tidal lakes - can have very high currents, but only over a small
area.
‘Using these observations, large-scale maps can be used to predict possible sites,
but in many places there is insufficient published data to verify whether an actual site
is suitable.
As marine current exploitation develops, there will be a need for a detailed inventory
of potential sites.
On small-scale maps, areas that do have high currents appear very small, though in
reality each one may be several kilometres long in the direction of flow, and have
space for many turbines, potentially generating tens or even hundreds of megawatts.
Many suitable areas are several kilometres from the shore, and would be suitable for
development as tidal farms, though they would be prohibitively expensive for a single,
isolated turbine.’
We used the above principles to select a number sites for analysis in addition to sites
already proposed by the DTI
Mapping
• Ordinance Survey Maps
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Site location & surrounding
topography
Reliable Scaled & Detailed Site
Geometry
• Admiralty Charts
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Identify possible sites
Vectorise admiralty charts
Channel & sea floor bathymetry
Combined site 3-D Mapping
Accurate cross section of
resource flow boundary geometry
Enables higher resolution flow
modelling & characterisation
Investigating flow characteristics
Shear Distribution Methodology
Rationale
• Requirements exist for
some means of
identifying key flow
characteristics in sections
of interest
• If shear profile is known
for a channel section,
velocity, pressure, flow
rates and fluxes may be
determined
• Ideally a model for
arbitrary channel sections
will be developed
Rationale
• Requirements exist for
some means of
identifying key flow
characteristics in sections
of interest
• If shear profile is known
for a channel section,
velocity, pressure, flow
rates and fluxes may be
determined
• Ideally a model for
arbitrary channel sections
will be developed
Existing methods include:
Chezy & Manning Equations
Semi-empirical equations
C & n are resistance coefficients
Although derived from wall shear
stress considerations, these do
not directly take them into
consideration
Both are concerned with bulk flow
characteristics, e.g. mean velocity
Rationale
• Bulk characteristics
assumes uniform flow
• Valid only as first order
analysis tool
• However can provide
useful insight
• Will form a basis of our
method alongside:
Algorithm to resolve
arbitrary geometry,
roughness and velocity
for free surface flow
Rationale
• Bulk characteristics
assumes uniform flow
• Valid only as first order
analysis tool
• However can provide
useful insight
• Will form a basis of our
method alongside:
Algorithm to resolve
arbitrary geometry,
roughness and velocity
for free surface flow
Define Geometry
Discretise Geometry
Bulk Characteristics
Perimeter Roughness
Define Governing
Equations
Boundary Conditions
Solve on grid
Methodology
• Channel section from bathy
data
• Discratise into profile made up
of combination of 4
constituents, ensuring channel
area and wetted perimeter are
equal
• Assume roughness coefficients
for sections
• Ignoring “imaginary” parts in
calc for Pi :
Defining Geometry
• But…it was found:
More efficient to discretise in sections with finite elemental
size dx and find dy, hence gradient & normal
Can now resolve components of tangential and normal
shear on grid
Boundary Conditions
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Vertical and horizontal distribution of shear:
No-slip condition at perimeter of domain
Turbulent mixing region at free surface – surface flow may retarded or accelerated
Profile likely to follow law of wall, log law and law of wake, depending on Re, Fr and τ,
modelled using roughness coefficients and may be modelled by similitude with
boundary layer
Arbitrary profile shown above right used for examples: square channel, Sound of
Islay
Results: Rectangular Channel
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Results: Rectangular Channel
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Results: Sound of Islay
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• Preliminary results: wall shear is only resolved in
vertical direction due to programming constraints
not yet overcome
• Final 1D result will show more realisable
distribution at surface and wrt horizontal shear
Approximations & Assumptions
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Approximations and assumptions:
1D: velocities found will only show
only in-plane components implying
laminar flow
Irrotational flow: eddies modelled
by BL equations
Incompressible
Pressure effects neglected in
vertical sense (although BL profile
is consistent with a favourable
pressure gradient in the in plane
direction)
Also this method only solves for
vertical component – yet
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Program will be modified for:
Conservation of momentum in 2D:
Wall shear will be represented by:
From Blasius & Goldstein
And using Thwaits approximation
for BL thickness:
Giving
Which includes the effects of
pressure gradient
Turbulence Modelling
Split profile into 3 layers:
1. Inner layer - law of wall
and Prandtle inner law.
Viscous shear dominates
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Outer layer – von
Karman outer (log) law.
Turbulent shear
dominates
Overlap law – must
satisfy 1 & 3
Laminar sub-layer
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Outer Layer
Log Law
region,
Fully
turbulent
overlap
layer
Blending region
Wake region
Outcome
• Bathymetric profiles (channel cross-sections) from
mapping data will be inserted into TOMS software,
‘Topological Oceanographic Modelling for Shear’.
• This will provide shear profiles of flow having inserted
values for surface flow and roughness coupled with
Manning’s equation.
• The shear profiles will enable the velocity profiles for
sites of varying cross-section to be calculated
• A simple tool to quickly asses the effect the topography
of a sites affects the velocity of the flow
• Results can generate Reynolds numbers, Froude
numbers, etc., and inlet velocities for use with BEM
calculations for turbines, and also allow velocity
distribution to be reckoned for, say, out of plane load
distributions on turbine areas
Next Stage:
Part A. Finish TOMS code & verify
the results
Part B. Technology Investigation…
Marine Current Modelling
For Energy Production
Group 4 – Marine Energy
James Glynn
Kirsten Hamilton
Tom McCombes
Malcolm MacDonald