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An Introduction to Tidal Power

- Professor Ian G Bryden
- University of Edinburgh

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

Newtons Theory

- 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.

- 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.

- 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.

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.

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

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

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

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.

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

- 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.

Tidal Structure in the North Sea

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.

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.

- 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.

Eling Tide Mill

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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.

Tidal Barrage Systems

- Designed to harness the rise and fall of the sea

by enclosing tidal estuaries eg - LaRance, Severn, Solway

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.

La Rance Tidal Barrage

Now 36 years old! Currently undergoing a 10 year

maintenance programme

Possible Sites World Wide

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.

Flood Tide- Sea water flows through sluices into

basin

Open sea

Within barrage

flow of

water

through

sluices

High Tide- Sluices closed to retain water in basin

Open sea

Within barrage

flow of

waer

through

sluices

Ebb

Tide(a)- water retained in the basin to allow a

useful head to develop

Open sea

Within barrage

Ebb Tide(b)- sea water flowing through generators

Open sea

Within barrage

flow of

water

through

turbines

Ebb Generation

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.

Flood Generation

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.

- 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.

Two Basin Systems

Combined Generation and Storage

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.

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.

- 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.

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.

- 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.

- 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!

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

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

Tidal Conversion Concepts

CREE

Horizontal axis turbine

Vertical axis turbine

Venturi based device

Linear lift based device

Vertical Axis Turbines

- The rotational axis of the system is

perpendicular to the direction of water flow.

- A horizontal axis turbine has the traditional

form of fan type system familiar in the form of

windmills and wind energy systems.

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?

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?

Loch Linnhe Turbine

Small floating demonstration device in the early

1990s Study conducted by IT Power Ltd and funded

by Scottish Nuclear

Fixed Systems

Provides a stable platform but the construction

and installation costs could be very much larger.

Technology options holding a turbine in place

Shallow water options

Deeper water options

Prototype Systems

CREE

ENERMAR Tested in 2000 in the Strait of Messina

(between Sicily and the Italian mainland) A large

vertical axis floating generator

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

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

Prototype Devices

- Hammerfest Strom
- Grid connected, sea bed mounted horizontal axis

system which was installed in Norway in 2003.

Installation process

Artists impression

CREE

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

SeaGEN awaiting installation in Strangford Lough

Systems under development

CREE

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

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

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)

Influence of Flow Speed on Energy Flux in a

Simple Channel

Mean consumption Glasgow

Mean consumption Edinburgh

But Influence of Flow Statistics

Obviously vital that the full tidal statistics

are considered and not just the spring peak!

Tidal Current Energy Flux Density

CREE

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

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

Energy Extraction

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

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

Regulated Power Curves

- In principle, the output will be regulated so

that it rises up to the Rated Power, then

flattens off.

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

- 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.

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

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.

Examples of Parametrically Defined Tidal Forms

Spring mean 3m/s Neap Mean 1.5m/s

Spring Mean 3m/s Neap Mean 2m/s

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)

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.

Optimal Unregulated Turbines

Influence of Residual Current on Ceff Values

- Assuming Neap component is 50 of spring

component!

Optimal Unregulated turbine

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!

- Influence of Rated Power on the form of the

optimal power curve in a fixed speed turbine

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

Influence of Rated Power on Average Power Output

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!

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

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

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

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

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

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

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

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

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.

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.

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

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

Sensitivity to Extraction

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!

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

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

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

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

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

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

Suggest a new key parameter

Based upon a simplified form of the 1d model, but

is starting to look significant in the 3d results

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

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)

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!

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

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

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.

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

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

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

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

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

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

Advantages of Tidal Current Power

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

Marine currents high energy intensity

A tidal current turbine gains over 4x as much

energy per m2 of rotor as a wind turbine

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!)

Predictability

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.

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

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