An Introduction to Tidal Power

1
An Introduction to Tidal Power

• Professor Ian G Bryden
• University of Edinburgh

2
The Tides

• Definition
• The rise and fall of the ocean surface under the
  influence of the gravitational and dynamic
  influence of the Earth/Moon/Sun system
• The first effective theory was produced by Newton

3
Newtons Theory
4

• The Earth Moon system rotates around a common
  centre of mass (CoMs) and the radius of this
  circulation is given by r.
• The separation of the centre of mass of the Earth
  (CoMe) from the centre of mass of the Moon (CoMm)
  is given by R.
• If the Earth were not itself rotating, each point
  on, or in, the Earth would rotate about its own
  centre of rotation, the radius of the rotation
  would also be given by r and the period of
  rotation would be equal to the rotational period
  of the Earth-Moon system.
• This results in acceleration towards the local
  centre of rotation.

5

• At the centre of the Earth, the centrifugal
  acceleration, resulting from the rotation,
  exactly matches the gravitational acceleration.
• At all other points, there is an imbalance
  between gravitational and centrifugal effects.
• At the point B the centrifugal effects exceed the
  lunar gravitational attraction.
• At the surface of the Earth, there will be a net
  flow of water from CD to AB.
• The equilibrium theory suggests, therefore, the
  establishment of tidal bulges in the fluid
  surrounding the Earth.

6

• The Earth of course rotates and the two tidal
  bulges, in order to maintain their position with
  respect to the Moon, have to travel round the
  Earth at the same rate as the Earths rotation.
• The Moon rotates around the CoMs every 27.3 days
  in the same direction that the Earth rotates
  every 24 hours.
• Because the rotations are the same direction, the
  net effect is that the period of the Earths
  rotation, with respect to the Earth Moon system,
  is 24 hours and 50 minutes.
• This explains why the tides are approximately an
  hour later each day.

7
Further Lunar Influences on the Tidal Period

• The Lunar orbit is not circular but is elliptical
  in form and the tide producing forces vary by
  approximately 40 over the month.
• Similarly, the Moon does not orbit around the
  Earths equator!
• Instead there is 280 between the equator and the
  plane of the lunar orbit. This also results in
  monthly variations.

8
Influence of the Sun on the Tides

• The Earth Sun system is also elliptical but with
  only a 4 difference between the maximum and
  minimum distance from the Earth to the Sun.
• The relative positions of the Earth, Moon and Sun
  produce the most noticeable variations in the
  size of the tides. In particular the Spring-Neap
  cycle

9
New Moon- Spring Tide
In this configuration, the influence of the Moon
and Sun reinforce each other to produce the large
tides known as Spring Tides, or Long Tides. A
similar superposition also exists at the time of
Full Moon.
solar tide
Earth
Sun
Moon
Lunar
tide
10
Half Moon- Neap Tides
When the Sun and Moon are at 90o to each other,
the effect is of cancellation as shown.
This configuration results in Neap Tides, which
are also know as Short Tides.
Moon
solar tide
Sun
Earth
Lunar
tide
11
The Presence of Land and the Resulting Tidal
Dynamics

• The oceans are not all of a constant depth and
  the presence of continents and islands severely
  influences the behaviour of the oceans under
  tidal influences.
• Coriolis Force which, in the Northern hemisphere,
  diverts moving objects to the right and, in the
  Southern Hemisphere, diverts moving objects to
  the left, has a substantial influence on the
  tides.

12
Semi-enclosed Basin in the Northern Hemisphere

• On the way into the channel the water is diverted
  to the right towards the lower boundary. When the
  tidal forcing is reversed, the water is diverted
  towards the upper boundary. This results in a
  substantially higher tidal range at the basin
  boundaries than at the centre.

diversion of
outflowing water
Open
boundary

diversion of
inflowing
water
13

• The net result of this effect is to generate a
  tidal wave which processes anti-clockwise
  around a point in the centre of the basin.

14
Tidal Structure in the North Sea
15
Energy Available in the Tides

• It has been estimated that the total energy from
  the tides, which is currently dissipated through
  friction and drag, is equivalent to 3000GW of
  thermal energy worldwide.
• Much of this power is in inaccessible places but
  up to 1000 GW is available in relatively shallow
  coastal regions.
• Estimates of the achievable worldwide electrical
  power capability range from about 120 GW of rated
  capacity to approaching 400 GW.

16
Extracting Tidal Energy1Tide Mills

• The extraction of energy from the tides is not a
  new idea. Mills, which used tidal flows in bays
  and estuaries to drive machinery to grind cereal,
  were used in medieval times.
• Despite the global nature of tidal energy, there
  is little evidence of tide mill development
  outside southern England and, even there, the
  distribution is mainly localised to Hampshire,
  West Sussex and the Fal and Tamar estuaries in
  Devon and Cornwall.

17

• Tide mills were generally used in areas with only
  small streams where good sites for conventional
  watermills are uncommon.
• Tide mills frequently suffered from damage
  resulting from tidal surges.
• This, and changing labour markets following the
  First World War, resulted in traditional tide
  mills becoming rare and of historical interest
  only.
• More recently, however, the tides have been
  seriously re-examined as a potential source of
  energy for industry and commerce.

18
Eling Tide Mill
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21
Tidal Barrage Systems

• Essentially modern electrical generation
  developments of the traditional tidemill
• In the nineteenth and twentieth centuries, there
  were numerous proposals to exploit the tidal
  energy potential of the Severn Estuary. None have
  yet been developed.
• The world's first serious scheme to exploit tidal
  energy was constructed in France, at La Rance in
  Brittany, between 1961 and 1967 and consists of a
  barrage across a tidal estuary to utilise the
  rise and fall in sea level induced by the tides.

22
Tidal Barrage Systems

• Designed to harness the rise and fall of the sea
  by enclosing tidal estuaries eg
• LaRance, Severn, Solway

23
LaRance

• The worlds first serious scheme to exploit tidal
  energy was constructed in France, at La Rance in
  Brittany, between 1961 and 1967.
• It consists of a barrage across a tidal estuary
  to utilise the rise and fall in sea level induced
  by the tides.
• This scheme has proven itself to be highly
  successful despite some early teething problems.

24
La Rance Tidal Barrage
Now 36 years old! Currently undergoing a 10 year
maintenance programme
25
Possible Sites World Wide
26
Ebb Generation

• This is the most likely approach to be used
  commercially
• Sluices are opened during the flood tide allowing
  the basin to fill up.
• Sluices are closed at high tide and during the
  ebb tide a head is initially allowed to develop
• Once a sufficient head has been developed between
  the basin and the outer waters, gates are opened
  and water allowed to flow out of the basin
  through turbines.

27
Flood Tide- Sea water flows through sluices into
basin
Open sea
Within barrage
flow of
water
through
sluices
28
High Tide- Sluices closed to retain water in basin
Open sea
Within barrage
flow of
waer
through
sluices
29
Ebb
Tide(a)- water retained in the basin to allow a
useful head to develop
Open sea
Within barrage
30
Ebb Tide(b)- sea water flowing through generators
Open sea
Within barrage
flow of
water
through
turbines
31
Ebb Generation
32
Flood Generation Mode

• In this alternative to ebb generation, the
  sluices are are closed at low water and a head
  develops during the flood tide.
• Gates are opened once the head is sufficient to
  drive the turbines.

33
Flood Generation
34
Two Basin Systems

• Double basin system have been proposed to allow
  an element of storage and to give time control
  over power output levels.
• Typically, he main basin would behave,
  essentially like an ebb generation single basin
  system.
• A proportion of the electricity generated during
  the ebb phase would be used to pump water to and
  from the second basin to ensure that there would
  always by a generation capability.

35

• Multiple basin systems are unlikely to become
  popular, as the efficiency of low-head turbines
  is likely to be too low to enable effective
  economic storage of energy.
• The overall efficiency of such low head storage,
  in terms of energy out and energy in, is unlikely
  to exceed 30.
• It is more likely that conventional pump-storage
  systems will be utilised.
• The overall efficiencies of these systems can
  exceed 70 which is, especially considering that
  this is a proven technology, likely to prove more
  financially attractive.

36
Two Basin Systems
37
Combined Generation and Storage
38
The Financial Implications of Tidal Barrage
Development

• Severn Estuary could provide in excess of 8 of
  the UKs requirement for electrical energy .
• La Rance took 6 years to complete. No electricity
  could be generated before the total project was
  completed. This is a major disincentive for
  commercial investment.

39
Environmental Opposition to Tidal Barrages

• Environmental groups, although generally in
  favour of the exploitation of alternative energy
  sources, are suspicious of the likely
  environmental changes large estuary based schemes
  would produce.
• One politician in the UK likened the proposed
  creation of a barrage across the Severn Estuary
  to the formation of a large stinking lake.
• Similar opposition has been voiced against any
  development of the tidal resource in the Solway
  Firth between Scotland and England. It is
  anticipated that public and political opposition
  will limit the development of tidal barrage
  schemes in the short term.

40

• An ebb generation system will reduce the time
  tidal sands are uncovered. This would have
  considerable influences on the lives of wading
  birds and other creatures.
• The presence of a barrage will also influence
  maritime traffic and it will always be necessary
  to include locks to allow vessels to pass through
  the barrage.
• This problem will be less problematic for an ebb
  system, where the basin is potentially kept at a
  higher level, than it would be with a flood
  generation system, in which the basin would be
  kept at a lower than natural level.

41
Tidal Currents

• Typically small in the open ocean.
• Local geographical effects can enhance flow
  speeds.

In the Pentland Firth there is evidence of tidal
currents exceeding 7m/s. Other sites, in Europe
alone, with large currents include, the Channel
Islands and The Straits of Messina.
42

• In the open ocean tidal currents are typically
  very small and are measured in cm/s at most.
• Local geographical effects can result in quite
  massive local current speeds. In the Pentland
  Firth to the North of the Scottish mainland, for
  example, these is evidence of tidal currents
  exceeding 7m/s. The kinetic energy in such a flow
  is considerable.

43

• It has been estimated in a recent report for the
  European Commission Directorate General for
  Energy (Cenex 1995) that the European Resource
  could represent a potential for 48 TWhr annual
  energy production
• If even a small fraction of this potential were
  exploited it could represent a major contribution
  to the European energy market.
• More recent studies studies, including one
  commissioned by the Scottish Executive, suggest
  that the UK resource alone could exceed 40TWhrs
  per annum!

44
Tidal Current Resource
World-wide - 400 TWh/year achievable with
technology currently on drawing board
UK Resource - 36 TWhr/year 40-50TWhrs/year
ETSU 1999 Bryden 2002
45
Tidal Current Devices

• Must convert energy in moving water into
  mechanical movement
• Horizontal axis devices
• Vertical axis devices
• Linear lift devices
• Venturi devices
• Must be held in place against fluid loading
• Fixed to sea bed
• Anchored floating

CREE
46
Tidal Conversion Concepts
CREE
Horizontal axis turbine
Vertical axis turbine
Venturi based device
Linear lift based device
47
Vertical Axis Turbines

• The rotational axis of the system is
  perpendicular to the direction of water flow.

48

• A horizontal axis turbine has the traditional
  form of fan type system familiar in the form of
  windmills and wind energy systems.

49
Device Location

• The energy flux is so high in many locations that
  the real engineering challenge is not energy
  conversion but in securing the conversion systems
  against the flow.
• Should a system be
• suspended from a floating structure
• mounted on the sea bed
• How should either the system itself or, in the
  case of a moored system, anchors be secured?

50
Moored Systems
This concept has advantages of mobility and
accessibility. There are, however, possible
problems concerning the stability of the surface
pontoon and the generator/turbine. How is the
anchor attached?
51
Loch Linnhe Turbine
Small floating demonstration device in the early
1990s Study conducted by IT Power Ltd and funded
by Scottish Nuclear
52
Fixed Systems
Provides a stable platform but the construction
and installation costs could be very much larger.
53
Technology options holding a turbine in place
Shallow water options
Deeper water options
54
Prototype Systems
CREE
ENERMAR Tested in 2000 in the Strait of Messina
(between Sicily and the Italian mainland) A large
vertical axis floating generator
55
Prototype Devices
CREE

• SeaFlow (Marine Current Turbines Ltd)
• Rated power output of 300kW,
• mounted on a vertical pillar fixed into the sea
  bed.
• In Bristol Channel off Lynmouth

56
Prototype Devices
CREE

• Stingray (The Engineering Business Ltd)
• Tested in Yell Sound, Shetland during 2002 to
  2003
• Uses a unique linear foil system
• Novel barge based installation system

Stingray awaiting installation in Yell Sound
Artists impression of Stingray
57
Prototype Devices

• Hammerfest Strom
• Grid connected, sea bed mounted horizontal axis
  system which was installed in Norway in 2003.

Installation process
Artists impression
CREE
58
Systems under development
Hydroventuri Ltd Energy extraction system based
upon utilisation of the pressure differential
created in a venturi
Lunar Technology Ltd Uses a horizontal axis
turbine in a protective/flow enhancing cowl
60kW device being installed
1.5MW device concept
CREE
59
SeaGEN awaiting installation in Strangford Lough
60
Systems under development
CREE

• TiDel (SMD Hdrovision)
• Tethered twin horizontal axis system

61
The Sea Snail (my device)

• Support system for tidal energy extraction
  systems
• minimal sea bed preparation
• System is prefabricated requiring minimal on-site
  construction
• Installation requires the use of a tug
• Easily removed for maintenance, etc.

CREE
62
Kinetic Energy in Moving Water
where

• is the water density (kg/m3)
• A is the cross sectional area of the channel (m2)
  and
• U is the component of the fluid flow velocity
  (m/s)

63
Influence of Flow Speed on Energy Flux in a
Simple Channel
Mean consumption Glasgow
Mean consumption Edinburgh
64
But Influence of Flow Statistics
Obviously vital that the full tidal statistics
are considered and not just the spring peak!
65
Tidal Current Energy Flux Density
CREE
66
What Makes a Good Site(Hydrodynamics)

• Sufficient Current Speeds over a full monthly
  cycle!
• (dont rely only on peak spring currents)
• Flow stability
• Sufficient Water Depth to allow devices to
  operate away from the sea bed and sea surface
• Bidirectional flow
• It will be very difficult to operate effectively
  if the current is heavily asymetric
• Sheltered from wave influence through either
  coastal geography or water depth

67
What Makes a Good Site(environmental and social)

• Proximity to economic grid connection points
• Some design concepts cannot coexist with shipping
  and fishing activity- is an exclusion zone
  acceptable?
• Proximity to service capabilities

68
Energy Extraction

• Mechanisms reflect those in wind power
• eg formulation of speed power curves
• Case 1 Fixed Rotational Speed

69
Case 2 Variable Speed

• In energy conversion term, it would be
  advantageous if a turbine could be maintained
  with a tip speed ratio at the optimal value to
  ensure that the power coefficient Cp is kept
  close to the maximum possible. As tidal current
  speeds vary more sedately than wind speeds, this
  might be more practical for a tidal turbine than
  for a wind turbine.

In this case, the power output simply follows
the cube power law
70
Regulated Power Curves

• In principle, the output will be regulated so
  that it rises up to the Rated Power, then
  flattens off.

71
Depth Speed Profile

• The horizontal speed of water in a tidal flow (U)
  varies with depth below the surface. This
  variation may be complex in form. It has,
  however, become common to represent the variation
  parametrically as following in power law of the
  form

? is the vertical distance above the sea bed
(m) H is the water depth (m) n is the power law
coefficient
72

• As the power density is proportional to the speed
  cubed, the ideal descriptor of current speed is
  given by the cube root of the mean speed cube
  over the swept area
• If the turbine is of a horizontal axis type, this
  is given by

r is the turbine radius z0 is the height of the
hub above the sea bed. u(?) is the flow speed a
distance ? above the sea bed.
73
Influence of Current Speed Statistics

• As with wind power, the mean power can be
  determined by using the speed/power curve and the
  speed probability density curve, which is given
  by ?(u)

So that the probability an instantaneous
measurement of the velocity component ux would
fall between U1 and U2 would be
And the mean power output is given by
74
Parametric Speed Spectra

• It may prove convenient to use a parametric form
  of the tidal current variation. One of the
  simplest being of the form

A F are related to residial current speeds, B,
C, D and E are amplitude terms, T0 is the period
of the semidiurnal variation, T1 is the period of
the Spring-Neap cycle, Ux(t) represents the E-W
current speed and Uy(t) represents the N-S
current speed.
75
Examples of Parametrically Defined Tidal Forms
Spring mean 3m/s Neap Mean 1.5m/s
Spring Mean 3m/s Neap Mean 2m/s
76
Optimal Rotational Speed-fixed speed turbine
(unregulated)

• The optimal rotational speed of a turbine is a
  function of the form of the CP-l curve and the
  flow statistics eg

Using the parametric distributions A and B
defined earlier and with a 14m diameter turbine
(Optimal is defined as maximising the mean power
output)
77
Influence of Tidal Statistics on Energy
Conversion Potential

• If a fixed speed device is utilised, the optimal
  rotational speed, which delivers the highest mean
  power output is highly dependent upon the nature
  of the flow statistics.
• If is assumed that it is possible to identify
  this optimal rotation, then it becomes possible
  to establish a maximum achievable effective
  energy conversion coefficient Ceff.

Ceff is, in effect, the mean effective value of
the power coefficient Cp.
78
Optimal Unregulated Turbines
79
Influence of Residual Current on Ceff Values

• Assuming Neap component is 50 of spring
  component!

Optimal Unregulated turbine
80
Optimisation Rated Power and Rotational Speed in
a regulated turbine

• The situation is more complicated in the case of
  a regulated turbine.
• Consider distribution B the optimal rotational
  speed and the rated speed is a function of the
  rated power output!

81

• Influence of Rated Power on the form of the
  optimal power curve in a fixed speed turbine

82
Influence of Rated Power/Speed for an optimal
variable speed turbine
The value of Cp remains at the peak value of the
Cp-l curve until the rated power is achieved and
then falls off rapidly to ensure a constant power
output by reducing the efficiency of energy
conversion
83
Influence of Rated Power on Average Power Output
84
Observations of Conversion Effectiveness in an
Optimised Turbine

• The mean Ceff is closely related to the value in
  the peak of the Cp-l curve
• A well matched unregulated turbine should achieve
  a Ceff of more than 75 of the peak value in the
  Cp-l curve
• The size of the rated power only influences the
  Ceff if the rated power is much less than 75 of
  the maximum unregulated power output at which
  there should be less than a 10 reduction with
  respect to the unregulated case.
• These observations aid in the assessment of
  likely power outputs, even in the absence of
  detailed technical descriptions of the
  technology!

85
Assessment of Energy Flux at a Site Level

• Necessary to consider temporal variation over the
  semi-diurnal and spring/neap cycles
• Also necessary to consider the variation in
  current flow spatially
• In some sites, Energy Hot Spots may move
  between flood and ebb tides
• Need to identify regions of spatial stability for
  device installation

86
Identifying Limits to Extraction
The extraction of energy from a tidal flow will
alter the underlying hydraulic nature of a tidal
environment. This will set limits to how much
energy can be extracted without causing
unacceptable changes What those limits are will
depend upon the site
Based on a simple 1 dimensional channel model
87
Influence of Energy Extraction

• Hypothesis
• The extraction of energy from a tidal flow will
  alter the underlying hydraulic nature of the flow
• This may, depending upon the nature of the tidal
  environment, reduce the underlying flux
• It may have environmental consequences
• It may have design consequences
• It may also have financial consequences

88
The Simple Static Channel

• Horizontal channel bed
• Linking 2 infinite oceans
• Flow driven by a known head dh
• Ignore, for now, dynamic effects

Q is the discharge rate(m3/s) g is the
acceleration due to gravity(m/s2) Per is the
wetted perimeter (m) b2h ?0 is the bed sheer
stress(kg/m/s2), C is the Chezy friction
coefficient
89
Natural Boundary Stress Calculation

• The boundary stress can be determined in terms of
  the Chezy coefficient. But in the UK it is common
  to use the Manning Friction coefficient

n is the Manning roughness factor (sm-1/3) R is
the hydraulic radius (m)
The natural boundary stress equation can be
written, therefore as
90
Energy Extraction Hypothesis

• In the presence of the artificial extraction of
  energy, flow in a channel will experience
  retarding forces resulting from the natural
  boundary friction and from the artificial
  extraction processes themselves.
• The forces resulting from extraction can be
  considered, in cases where vertical flow
  structure can be neglected, as resulting from an
  additional component of the boundary stress, so
  that the net effective shear would be

91
Calculating the additional stress
Consider a flow with longitudinal velocity
component U passing through a cross sectional
area A. There will be a retarding force,
resulting from the extraction of P (Watts), which
is equal to
This can be modelled as an equivalent boundary
stress, tadd, given by
?x is the length over which the energy is being
extracted and Per is the wetted perimeter
Perb2h
b
h
92
Boundary Conditions

• Upstream
• There is an initial drop in the elevation head as
  a result of flow acceleration
• This drop in elevation can be related to the
  speed of flow just downstream from the entrance
  to the channel

93
Boundary Conditions

• Downstream
• Assume that the jet output from the channel
  does not rapidly mix with the ambient waters
• A condition of velocity continuity is assumed.
• Mixing will, of course, occur eventually but this
  three dimensional effect will manifest itself
  outside of the channel constraints and will not
  be considered here.

94
Solving the Equations

• By integrating the flow equation from the known
  depth at the downstream boundary, establish the
  upstream depth as a function of the discharge
  rate, Q.
• Establish an iteration to determine the value of
  discharge, Q, compatible with chosen upstream and
  downstream water depth
• This allows a the determination of depth and
  speed between the upsteam and downstream
  boundary.

95
Zero Energy Extraction

• Abrupt drop in water depth at entrance to
  the channel
• Associated with a sharp increase in flow
  speed
• Decrease in depth along the channel
• Acceleration of flow along the channel

96
10 Kinetic Energy Flux Extraction

• Substantial head drop over the extraction
  vicinity
• Overall flow speed reduced by 2.6 in the
  extraction vicinity
• Speed increase downstream of energy extraction

97
Sensitivity to Extraction
98
Kinetic Energy in the Channel
This shows the consequences of extracting 25 of
the raw kinetic flux from a channel of length
4000m, width 200m, assuming a manning coefficient
of 0.035m-1/3s Note the head drop over the zone
of extraction and the INCREASE in kinetic
flux! If the only energy in the system is
kinetic, then this would be impossible!
99
Where does the energy come from?

• Compare the charts for 25 extraction and zero
  extraction

Notice that the kinetic flux is much higher in
the zero case than in the exploited case! The
extracted energy is being drawn from the whole
flow environment and not simple removed from the
kinetic flux! A full understanding requires
consideration of potential energy and frictional
losses, some researchers have even suggested the
concept of Total Flux, which includes potential
energy, frictional energy and pressure
100
Simplifying the 1D Analysis
In the case of a constant width channel
(bconst), this can be rewritten in the form
I have also written the equation in terms of U
(m/s), the longitudinal component of the flow
velocity rather than the discharge Q(m3/s)
The effective boundary stress, once again is the
sum of the natural stress
And an artificial term representing the energy
extraction
101
Simplifying the 1D Analysis

• If the flow speed and depth along the channel is
  assumed to be constant and the artificial energy
  extraction distributed along the entire length,
  L, then

?h is the head drop along the channel (m)
This can be further simplified if U2/hgltlt1
102
Simplifying the 1D Analysis

• The Total head drop is give, therefore, by

In the absence of artificial energy extraction,
this can be written as
Hence
Uo is the unexploited flow speed
103
Flow Speed in the Exploited Channel
The equation relating the channel speed, Uc, to
the total head drop, Dh
Can be written to include the extraction
If P is related to the kinetic flux
The total head drop in the exploited channel can
be written
104
Flow Speed in the Exploited Channel
By equating the head drop in the exploited and
unexploited channel, we can write
This can be rewritten as
ALSO
105
Suggest a new key parameter
Based upon a simplified form of the 1d model, but
is starting to look significant in the 3d results
106
Influence of Flow Change on System Design

• If a system is designed to operate in the
  unexploited flow, then large changes in the flow
  speed resulting from exploitation will result in
  reduced system performance
• The mechanical power output of a system should be
  expected to be dependent upon flow speed and
  device power coefficients
• Flow speed reduction will result in requirements
  for changes in the turbine control system to
  maintain optimal power characteristic, in effect
  to maintain a appropriate values of the turbine
  power coefficient i.e. how to keep the operation
  close to the peak of the Cp-? curve

That is the subject of another study! Here we
will assume the control is being appropriately
handled and look at the energy flux itself
107
Influence of Flow Change of Required System Size
Assuming a horizontal axis turbine design, the
power conversion is
Consider a flow speed reduction
Uex is the flow speed after exploitation Uraw is
the undisturbed flow speed Red is the
proportional flow speed reduction
Assuming that the turbine control strategy could
maintain a constant value of the power factor,
the diameter of the device would need to be
increased
Dactual is the diameter the turbines actually
need to be (m2) Dapparent is the diameter
suggested by considering the unexploited flow
speed only (m2)
108
Example The 100MW Farm

• 50 devices each designed to deliver 2MW at 3m/s
• This corresponds to a peak in the Cp-? curve of
  0.4
• Each turbine needs to have a diameter of 21.5m
• If the channel flow speed is reduced by 10, then
  the turbine diameter would need to be increased
  to 25m, with obvious economic consequences!

109
Beyond the simple channel

• The simple channel gives some insight into the
  complexity of extracting energy from free surface
  flow but real tidal flows are generally multiply
  connected and exhibit long wave form properties
• More sophisticated analysis requires solution of
  the shallow water momentum flux equations (in 2
  dimensions)

Associated with the continuity equation
110
Extensions of the Shallow water Equation

• Inclusion of Artificial Energy Extraction
• Inclusion of Depth effects

Retarding force over an area ?x?y in the U,V
direction
Introduction of a transformed vertical dimension
and then solution of the governing equations on a
layer by layer, defined by s, basis
111
The Simple Island ModelSimulation Domain

• Initially a 2 dimensional simulation but
  capable of extension to 3 dimensions
• A 3.5m M2 tidal wave, was run from a cold
  start up to ¼ of the tidal period,
• The inlet and outlet boundary conditions
  were then maintained in a steady state.
• The extraction planes were one cell width
  with an extraction figure of 6MW per cell.

112
Exploitation of the Northern Channel

• Note reduction in flow speed in the northern
  channel 67m2/s (1.75m/s at a water depth of
  38.3m)) to approximately 50m2/s (1.31m/s at a
  water depth of 38.2m). and corresponding increase
  in the southern channel

113
Influence on Energy Extraction in Three Dimensions
This shows the reduction in flow speed along the
central stream line of the extraction zone As
expected, the simulation predicts the presence of
a reduced flow speed wake
114
Influence on Energy Extraction in Three Dimensions
This shows the increased flow in the vicinity of
the sea bed The energy extraction zone is, not
unexpectedly, resulting in flow diversion under
the zone and (not shown here) around and above
115
Resource Assessment

• The most recent, and most reliable, assessment
  was conducted by Black and Veitch in 2004 and
  concluded that the UK potential was equivalent to
  22TWhr/annum (6 of UK consumption)
• Resource is small in comparison with wind
• But is concentrated in sites with very high
  energy densities, offering the prospect of
  compact high output developments

CREE
116
Specific Technical Issues- Tidal Current

• Installation
• High energy flux densities and minimal slack
  water periods
• Intervention and maintenance
• Maintain in-situ or return to base?
• Erosion and corrosion
• Increases the maintenance problem

CREE
117
Environmental Concerns

• Tidal Current
• Impact and entanglement with marine life
• Flow impedance modification
• Habitat disturbance, especially during
  installation

Interaction with other users of the sea (fishing,
leisure, transport)
CREE
118
Advantages of Tidal Current Power

• High energy density
• Small devices
• Low visibility
• Predictable resource
• Suitability for energy storage

119
Marine currents high energy intensity
A tidal current turbine gains over 4x as much
energy per m2 of rotor as a wind turbine
120
Visual Impact
wind farm
10 to 20 MW / km2
...and a low visual impact
marine current farm
50 to 100MW / km2 (I challenge these figures!)
121
Predictability
122
Tidal Farms

• It is likely that, if tidal currents are to be
  commercially exploited, the generators will have
  to be mounted in clusters (tide farms?).
• If this is done, then, as with wind turbines, the
  devices will have to be sufficiently spread to
  ensure that the turbulence from individual
  devices does not interfere with others in the
  cluster.

123
Tidal Farms
Commercial Development will require tidal energy
conversion systems to be grouped in clusters
(tide farms) Problems will include wake
interactions and the influence of energy
extraction on the local and regional environment
Top View
124
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