Sustainable Energy Solutions

1
Module Renewable Energy Engineering Lecture
topic Tidal Current Energy Resource
Assessment, Modelling and Exploitation Delivere
d by Dr. Scott J. Couch
2

• Relevant topics covered by Prof. Bryden
• Tide generation by the Earth-Moon-Sun system
• Tidal interaction with land and resulting
  dynamics
• Energy available in tides
• Technologies proposed for tidal energy
  extraction
• 1-d modelling of energy extraction

3

• Generic tidal regime classification
• Offshore deep ocean
• Unbounded nearshore coastal ocean
• Tidal streaming
• Hydraulic current
• Resonant systems
• M2 tidal component (amplitude (m) phase
  (degrees))

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• Requirements for renewable energy production
• Energetic and persistent tidal resource
• Currently proposed technologies limited to 50
  metres maximum depth
• Access to the local or national grid
• Regimes 3, 4 and 5 are therefore most relevant
  for exploitation

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• Tidal streaming example (amplification of tidal
  velocities through a flow constriction).
• Similar principles to the so-called venturi
  effect, but important physical differences

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Hydraulic current example (Naruto Strait, Japan)
Source http//133.31.110.195/D/inetpub/wwwroot/ww
w/text-English/page-07/page-07-01naruto.htm
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Images http//en.wikipedia.org
Resonant channel example

• Bay of Fundy (Minas Basin) recorded tidal range
  of 17 m
• Gradual tapering and shallowing constricts the
  tidal flow
• Standing wave established A standing wave
  arises when the incoming tidal wave and an
  earlier, reflected tidal wave constructively
  interfere. The interaction can create very large
  tidal amplitudes and associated tidal currents.

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• Numerical Modelling Background
• Large numerical modelling community focussed on
  tidal flow modelling has spun up since the early
  two-dimensional models of the 1950s and 1960s.
• Typical application of these models is very
  diverse
• Governing equations are applied on a grid
  structure using a finite-difference,
  finite-element or finite-volume approach. This
  produces a regular array of flow velocity and
  surface elevation data throughout the tidal cycle
  (output at selected intervals).

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Numerical Modelling Formulation
GOVERNING (SHALLOW WATER) EQUATIONS
Conservative form of the continuity equation
Conservative form of the x-directed momentum
equation (momentum flux)
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• Application of tidal numerical models in
  Renewables
• Numerical models can be used to simulate a site
  of interest for energy harvesting with a high
  degree of accuracy . The bugbears of this
  approach are
• Generating a solution requires reliable data to
  generate the model driving conditions at an
  upstream location.
• Simulating a real site with any degree of
  accuracy requires a significant investment of
  time.
• The availability of an experienced modeller and
  analyst. Beware, as with all complex problems,
  numerical modelling or otherwise, rubbish in
  rubbish out!
• Models and physical understanding need further
  development to accurately simulate the impact of
  energy harvesting on the system

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• Examples of research application of numerical
  models in the marine renewables context
• Generic properties of all idealised cases
  considered
• Uniform depth, d 35 m friction coefficient,
  n 0.025 sm-1/3 Coriolis parameter, f 0 wind
  stress, Wx Wy 0 ms-1.
• Simulations spun up from a cold start over an
  equivalent quarter tide (amplitude 3.5 metres),
  then boundary conditions maintained at steady
  state.
• Cell size, ?x ?y 80 metres.
• Energy extraction per cell assuming swept area
  of 20 metres, lateral device spacing of 2
  diameters, peak delivery of 750 kW per device and
  overall conversion efficiency of 25 6MW power
  extracted from the flow per cell.

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• Pseudo 1-d tests
• 9.6 km long channel, 80 m wide (1 cell)
  applying 2-d code.
• Two test-cases
• 1st with no energy extraction
• At steady state frictional losses balanced by a
  pressure gradient to produce typical friction
  slope effect
• 2nd with 6MW power extracted from the flow at
  the mid-point of the domain.
• At steady state energy extraction balanced by
  a pressure gradient
• Friction slope effect from case 1 still
  apparent but dwarfed by impact of energy
  extraction on the system.
• Depth-averaged flow velocities increase
  downstream of extraction site through the
  continuity equation to balance head loss.

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• 2-d blockage idealised tests
• 9.6 km x 4.8 km channel.
• Idealised island located
• in centre of the channel.
• Three test cases
• 1st with no energy extraction.
• 2nd with 6MW power extracted from the flow per
  cell along the y-axis centreline in both
  channels.
• Results broadly similar to the pseudo 1-d test
  cases.
• 3rd with 6MW power extracted from the flow per
  cell along the y-axis centreline of the upper
  channel only.
• Overall flow velocity reduced in the upper
  channel and increased in the lower channel
• Significant upstream and downstream flow
  asymmetry (vortex shedding downstream).

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A B C D

T
T
Energy extraction site
S
S
A B C D
Depth-averaged flow velocity vector map for
2nd 2-d test case (both channels exploited).

Discharge per metre width at four
representative cross-sections.
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Energy extraction site
Depth-averaged flow velocity vector map for the
3rd 2-d test case (only northern channel
exploited).
Discharge per metre width at four
representative cross-sections.
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S S T T
Along channel depth variation for representative
sections S (1st 3 lines) and T (last line)
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3-dimensional analysis
Comparison of 3-d stream-wise flow development
with varying levels of kinetic energy extraction.
Mid layer
(a) elevation profiles, (b) layer integrated
velocity profiles through the water column for
s-layer 5 (of 10).
Bed layer
Surface layer
Layer integrated velocity profiles through the
water column, (c) s-layer 1 (bottom), and (d)
s-layer 10 (top)
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Representative vertical velocity profiles across
the domain length indicating influence of energy
extraction (set at 50)

• Minimal upstream effect (overlapping profiles).
• At extraction site, (i) increased flow velocity
  near the sea-bed and surface, (ii) decrease in
  flow velocity in layers where energy is
  extracted, and (iii) enhanced shear between s
  0.65 and s 0.75, and reduced shear between s
  0.15 and s 0.25.
• Downstream flow velocity is slowly re-distributed
  towards upstream values.

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Modelling the Energy Extraction Process
The impact of energy extraction on the tidal
system (e.g. resource, environment (physical,
biological and chemical)) is not well understood.
A large focus of Professor Brydens research
group is in addressing these issues. In previous
lectures, the formula for deriving the kinetic
energy in a moving flow of water was
presented This formulation is correct from
first principles, and is correctly applied when
describing the incident energy available to a
device for extraction. Initial attempts by our
research group to use this formulation to
describe the impact of extraction on the tidal
resource surprised the research community .
? is the fluid density, A the swept area and U
the fluid velocity
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Two outcomes from modelling analysis made us
question the use of the kinetic energy flux when
considering the response of the tidal system as
opposed to the device performance
friction slope h1 h2
u1 u2 Section 1 NO energy
extraction Section 2

• Assuming a uniform width channel, u1h1 u2h2,
  therefore u2 gt u1, and hence the kinetic energy
  is increased downstream.
• Sensitivity analysis demonstrates that more than
  100 of the raw available kinetic energy can be
  extracted from the system!
• .

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These findings appear counterintuitive. Dynamic
and non-linear effects associated with tidal
flows ensure that life is not quite as simple as
we would like, or that previous research in the
field had suggested! Research we are currently
pursuing is having better success at accurately
simulating the impact of energy extraction on the
system, and is therefore leading to enhanced
understanding of the underlying physics of the
effect of energy extraction on the tidal system,
and on flow device interactions.
The physical response of tidal flow to energy
extraction indicates that the kinetic energy in
the system is not necessarily the only
consideration when quantifying the available
resource, even although it is only the kinetic
energy in the system that the device will
directly interact with.
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• Context
• Although currently an unexploited energy source,
  a number of competitive advantages are offered by
  the harvesting of clean, sustainable energy from
  tidal currents.
• Continued research and development effort is
  required to
• Improve device efficiency, reduce costs and
  understand maintenance and reliability issues.
• Enhance understanding of the short and long term
  physical, biological, chemical and social impact
  of harvesting tidal current energy.
• Provide regulatory guidance and interface with
  the relevant stakeholders, be they device
  developers, energy carrier and supply companies
  and ultimate end-user.

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• In the specialized area of resource assessment
  and modelling, the scientific goal is to develop
  existing systems which are currently able to
  accurately predict the pre-development tidal flow
  development (see accompanying image), to be able
  to produce realistic simulations of the flow
  development, energy potential and impacts on the
  various related systems when energy harvesting
  facilities are installed

Image characterising the tidal current and
therefore kinetic energy potential of Yell Sound
in the Shetland Islands (produced by colleagues
based at The Robert Gordon University).