Garth Bryans - Impacts of Tidal Stream Devices on Electrical Power Systems

8/13/2019 Garth Bryans - Impacts of Tidal Stream Devices on Electrical Power Systems

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Impacts of Tidal Stream Devices on

Electrical Power Systems.

Andre Garth Bryans, BSc, MSc

A thesis presented on application for thedegree of Doctor of Philosophy

School of Electronics, Electrical Engineering and

Computer Sciences

Faculty of Engineering and Physical Sciences

The Queens University Belfast

September 2006

Supervisors: Dr. B. Fox, Prof. Peter Crossley, Prof. T.

Whittaker and Prof. M. OMalley


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AbstractThe increase in fuel price, concerns over energy security and global warming have

fuelled a global drive towards renewable power generation. The majority of

renewable generation investment in Ireland like many countries is in the form of

wind generation. However concerns about the feasible level of wind generation

are leading to development and investment in other forms of renewable

generation, such as tidal stream generation, a near market ready form of variable

but predictable generation.

A review of the tidal stream systems under development was undertaken to

determine the operational limitations of the systems most likely to reach the

market first. The tidal stream resource was modelled within a 2D ocean model and

viable areas identified based on the operational limitations of the tidal energy

device seen as being closest to market readiness. The viable areas were analyzed

producing the profile and magnitude of the currently viable resource.

The impact of tidal generation on system operations was studied with

consideration to its effect on the system ramp rate, demand profile, capacity /

availability factor, generation capacity credit, unit commitment, net system

emissions, net generation cost and cost based price received by tidal generation.

Tidal generation was found to be manageable on the system considering the

currently viable resource.

The benefits that tidal generations predictability may offer in comparison to wind

generation was quantified in terms of the effects on emissions, market aspects,

and system operations. This involved the development of a unit commitment

model with methods for providing reserve against uncertainty in wind forecasting.

Tidal generations predictability was found to offer benefits over wind in most

aspects considered.

The grid connection of the viable tidal resource was studied in terms of

transmission loss adjustment factors, short circuit ratings, capacity of the

transmission system and cost of 33/38 kV grid connection. As a result of this

analysis the most attractive tidal resource around Ireland was identified. The


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impact of a prototype scheme on voltage control for a lower voltage 11 kV system

was also considered, and voltage was found to be within tolerable limits.

Acknowledgments

This work has been financed by Northern Ireland Electricitys SMART program

(Sustainable Management of Assets and Renewable Technology), the UK

Department of Enterprise, Trade and Investment (DETI) and Sustainable Energy

Ireland. Thanks is owed to Dr. B. Fox, Prof. M. OMalley, Prof. P. Crossley &

Prof. T. Whittaker who provided supervision and technical guidance in carrying

out this research. The School of Electronics, Electrical Engineering and Computer

Sciences, Queen's University of Belfast and the Electricity Research Centre,

University College Dublin provided technical support, and an administrative hub

for the project. Thanks is also owed to Kirk McClure & Morton Ltd., Danish

Hydrographical Institute, British Oceanographic Data Center, Prof. Jenkins,

Marine Current Turbines and The Engineering Business Ltd. who provided

technical support, software or data.


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List of Acronyms UsedACCC Alternating Current Contingency Calculation

ADCP Acoustic Doppler Current Profiler

CCGT Combined cycle gas turbine

CFL Courant Friedrichs Lewy criterion

DETI Department of Enterprise, Trade and Investment

DFIG Doubly fed induction generator

DHI Danish Hydrographical Institute

EAMC Energy averaged marginal cost

EDR Electrical down rating

ESB Electricity Supply Board

EU European Union

GR Generation reduction

LMP Locational Marginal Pricing

LOLE Loss of load expectation

MCT Marine Current Turbines

MIC Marginal incremental cost

MIP Mixed integer programming

MSL Mean sea level

NAP National allocation plan

NECD National Emissions Ceiling Directive

NI Northern Ireland

NIE Northern Ireland Electricity

PPA Power purchase agreement

REFIT Renewable Energy Feed In TariffROCs Renewable obligation certificates

RoI Republic of Ireland

SMART Sustainable Management of Assets and Renewable Technology

SMC System marginal cost

SONI System Operator Northern Ireland

SOP Scheduled Outage Periods

TED Tidal Energy Device

TG Tidal generation

TLAF Transmission Loss Adjustment Factor

TMPC Total marginal plant costTSO Transmission System Operator

TUoS Transmission Use of System

UC Unit commitment

UK United Kingdom


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Table of Contents

1. Introduction1.1. The Need for Renewable Energy 1

1.2. Incentives for renewable generation 4

1.2.1. Northern Ireland 4

1.2.2. Republic of Ireland 4

1.3. Existing renewable generation 5

1.4. Incentive for tidal energy 6

1.5. Available tidal resource 6

1.6. Tidal generation and system operations 6

1.7. Tidal generation and grid connection 7

2. Energy extraction from tidal stream

2.1. The Origin and Nature of Tidal Energy 8

2.1.1. The Tidal Generating Forces 8

2.1.2. The concentration of tidal energy on shelf seas 18

2.2. Generation from tidal stream 20

2.2.1. TED systems currently in development 21

2.2.2. Marine Current Turbines 23

2.3. Impact of tidal energy extraction 30

2.3.1. Effect on tidal regime 302.3.2. Effect on the local eco-system 31

2.3.3. Effect on human actives 32

2.3.4. Global scale 33

3. Resource assessment

3.1. Introduction 34

3.2. Shelf sea models 35


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3.2.1. The definition of closed boundaries and land cells 38

3.2.2. Selecting the Grid Scale Size 40

3.2.3. The setting of grid cell depths 43

3.2.4. Selecting a Time Step 44

3.2.5. The forcing of open boundaries 44

3.2.6. The inclusion of a nested grid 46

3.2.7. Bed friction 46

3.2.8. Eddy viscosity 47

3.2.9. Model Calibration 47

3.3. Development of an oceanographic database 48

3.3.1. Seabed depth 49

3.3.2. Seabed slope 49

3.3.3. Maximum spring tidal current velocity 50

3.3.4. Maximum neep tidal current velocity 51

3.3.5. Wave height 51

3.3.6. Tidal Phase 54

3.3.7. Distance from Ireland 54

3.3.8. Database integration 56

3.4. Determination of Power Output 59

3.5. Accessible Resource 63

3.6. Energy in the Irish Sea 63

3.7. Conclusion 64

4. System operation with tidal generation


4.2. The potential methods of development and

operational control

65

4.2.1. Tidal Superposition 66

4.2.2. Electrical Down Rating (EDR) 67

4.3. The effect of tidal generation on demand profile 70

4.4. The effect of tidal generation on the system ramp 73


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rate

4.4.1. Increasing amounts of tidal generation 74

4.4.2. Ramping on the system during a spring neap cycle 76

4.4.3. Comparison between wind and tidal energy 784.4.4. Effect of EDR on the system ramp rates 78

4.5. Capacity & availability factors 79

4.6. Capacity Credit 81

4.6.1. Scheduled outages 82

4.6.2. Calculation of loss of load expected 83

4.6.3. Identification of the capacity credit of tidal generation 85

4.7. Frequency response of a TEDs to disturbances 87

4.7.1. Inertia constant 89

4.7.2. Generator type 89

4.7.3. Ability of generators to provide inertial response 94

4.7.4. Method of calculating turbine inertial constant 95

4.7.5. Model of frequency response 97

4.7.6. Frequency response provided by MCTs TED 99

4.8. Power quality 104

4.9. Conclusion 107

5. Impact of renewables on thermal plant


5.2. Irish case study characteristics 110

5.2.1. Electricity markets 110

5.3. Unit commitment 112

5.3.1. Operating costs 112

5.3.2. Technical constraints 114

5.3.3. Ancillary services 114

5.3.4. Optimisation and unit commitment 116

5.4. Method of adding wind to the dispatch 118

5.4.1. Fuel saver approach 118


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5.4.2. Forecast approach 118

5.5. PLEXOS Model set-up 124

5.5.1. Small test case 126

5.6. Effect on emissions 1285.6.1. Carbon dioxide 129

5.6.2. Sulphur dioxide 132

5.6.3. Nitrogen oxides 130

5.7. Effect on operation of generation units 134

5.8. Effect on the market 135

5.9. The effect of tidal generation on plant usage 140

5.9.1. Effect of tidal penetration on existing generation 140

5.9.2. Effect of tidal penetration on new generation 142

5.9.3. Effect of tidal energy on the use of storage systems 143

5.10. Effect on carbon emissions 144

5.11. Conclusion 145

6. Grid connection6.1. Introduction 146

6.2. Transmissions loss adjustment factors 147

6.2.1. Method of calculating the TLAFs for the North Coast

and Larne.

147

6.2.2. TLAFs at Coleraine and Larne 148

6.3. Short circuit levels 151

6.4. Connection capacity and cost 152

6.4.1. The northeast cost 153

6.4.2. Maiden Islands 164

6.4.3. Arcklow 166

6.4.4. Carnsore Point 168

6.4.5. Malin Head 170

6.4.6. Shannon 173

6.4.7. Strangford 174


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6.4.8. All sites 175

6.5. Strangford case study 176

6.6. Conclusion 182

7. Conclusion

7.1. Tidal stream energy devices 184

7.2. Tidal stream resource available to Ireland 184

7.3. Impact of tidal generation on system operations 185

7.4. Grid connection and site development of tidal

resource

186

7.5. Recommendations for future research 187


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1. Introduction

1.1 The Need for Renewable Energy

Societys demand for energy is increasing at a rapid pace (see Figure 1-1) with developing

countries such as China rapidly increasing their demand for energy. The majority of the

energy used to meet this demand is in the form of fossil fuels. These are fuels that

originated as organic material having captured their energy from sunlight and converted it

to chemical energy. The organic material initially formed a spongy peat, which underwent

sedimentation to form fossil fuels such as coal, oil and gas. The majority of the peat was

formed during the Carboniferous Period about 360 to 286 million years ago. Therefore

fossil fuels represent a finite resource that once used will not naturally replenish in a

feasible time frame.

Figure 1-1. Annual global energy demand record and prediction (Energy Information

Administration, 2006), note this uses the US definition of Quadrillion (1015).

The rapid increase in demand (see Figure 1-1) in conjunction with concern over the security

of supply from nations acting as net exporters of fossil fuels has resulted in a rapid increase

in price of fossil fuels such as oil (see Figure 1-2).

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Figure 1-2. Average annual crude oil price in $ per barrel of oil (1 BBl = 159 litres) in the

United States from 1920 to 2006, not corrected for inflation (Oilnergy, website).

The effect of burning such large amounts of fossil fuels to meet the energy demand is to

release green house gases which act to trap heat around the earth by enabling high energy

radiation to heat up the earth but impeding the escape of low energy thermal radiation,

resulting in global warming (see Figure 1-3).

Figure 1-3. Change in surface temperature in the northern hemisphere over the last 1000

years (Intergovernmental Panel on Climate Change, 2001).

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Studies have indicated that if green house gas emission rates are not abated society will

face dangerous climate change (Schellnhuber et al., 2006). Whilst it is difficult to perform

an exact cost benefit analysis for avoiding dangerous climate change due to the

uncertainties, it is believed that the costs of reducing emissions is much less than facing the

consequences of climate change (Schellnhuber et al., 2006).

The Kyoto Protocol was signed in 1997 by nations agreeing to the United Nations

Framework Convention on Climate Change (United Nations, 1997). This agreement sets

binding emissions reduction targets for developed countries (or groups of countries such as

EU). A burden sharing agreement between EU member states (EU, 2002) has required the

United Kingdom (UK) and the Republic of Ireland (RoI) to limit their CO2 emissions to

87.5% and 113% of their 1990 levels, respectively. Also, the European National Emissions

Ceiling Directive (NECD) (EU, 2001a) aimed at reducing acid rain has set out legally

binding national limits for emissions of nitrous oxides (NOx), sulphur dioxide (SO2),

volatile organic compounds and ammonia. Of these three, the power generation sector

contributes towards the production of NOxand SO2. Within the NECD the UK must limit

its SO2and NOxemissions to 585 kt and 1,167 kt respectively, and Ireland must not exceed

42 kt of SO2 and 65 kt of NOx emissions per annum from 2010. However, Ireland is

finding the NOxtarget particularly challenging, with NOxemissions of 135 kt in 2001 due

to large economic growth and enlargement of the transport sector over the past decade

(Department of Environment, Heritage and Local Government (Ireland), 2003). A large

part of the emissions reduction is expected to be made through the power generation sector

through methods such as the introduction of different fuel types (limited options left),

exhaust gas cleaning methods, control methods and the introduction of renewable

generation. The development and introduction of renewable generation is seen to have a

double benefit, because in developing renewable generation forms to a commercial stage,

developing countries such as China can also begin to install renewable generation on a

much larger scale, resulting in a much greater global saving in emissions. Therefore to

incentivise the development of renewables the European Union has set a target of 22.1%

renewable generation by 2010, with the RoI and the UK expected to provide 13.2% and

10% respectively of their generation by renewable means (EU, 2001b). Northern Ireland

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contributes towards the UK renewable generation target and has been allocated a local

target of 6.3% by 2012 (Department of Enterprise Trade and Investment, 2004).

1.2 Incentives for renewable generation

A number of mechanisms have been established to encourage investment in renewable

forms of generation both in NI and RoI.

1.2.1 Northern Ireland

In NI a Non-Fossil Fuel Obligation was established in 1993, which placed an obligation on

the distribution company to provide 16 MW by installed capacity of renewable generation.

By 2005 this was increased to 45 MW. Further investment was incentivised by introducing

renewable obligation certificates (ROCs) in April 2005. The scheme works by requiring

supply companies to obtain a given percentage of their energy from renewable sources. For

non-renewable generation this may be achieved by buying ROCs from another company. If

a company has insufficient ROCs it is charged a buy-out fee - 32.33 per MWh in the first

year (Ofgem, 2005). The buy-out fund is then distributed to the suppliers in proportion to

the ROCs provided. The ROCs scheme provides an incentive to produce up to 6.3% of

electricity by renewable sources by 2012 (Department of Enterprise Trade and Investment,

2004; Department of Enterprise Trade and Investment, website; Department ofCommunications Marine and Natural Resources, 2005). However, if this is exceeded, the

spare ROCs can be traded with mainland UK, which has a target of 12% by 2012.

1.2.2 Republic of Ireland

Renewable generation in RoI has been incentivised through the Alternative Energy

Requirement scheme since the 1990s. This involves a bidding process for 10 to 15 year

contracts to provide renewable generation at a fixed bid price under a power purchase

agreement (PPA). Each type of renewable generation has a cap price which the applicants

in the bidding process can submit up to. This scheme was replaced in 2006 with the PPA

REFIT (Renewable Energy Feed In Tariff) (Department of Communications, Marine and

Natural Resources, 2006), which replaces the process of bidding with a fixed reference

price for each type of renewable generation (shown in Table 1-1).

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1.4 Incentive for tidal energy

The main thrust of Irelands response to the EU target has been provided to date by wind

generation. However, there is concern about the feasible level of wind generation that can

be absorbed by the system (Garrad Hassan, 2003), and there is a desire to achieve greater

diversity of renewable energy supply. Tidal generation (TG) offers an energy source which,

unlike wave generation, is not linked to the velocity of the wind, and is largely predictable

within the generation scheduling time-scale. Also, tests on a medium-sized prototype have

confirmed that the technology can deliver renewable energy at little extra cost to consumers

(Whittaker et al., 2003).

1.5 Available tidal resource

To understand the impact of tidal stream devices on the system, it is necessary to first

establish the magnitude and profile of the viable resource and the characteristics of the

generator. Therefore a review of the technology was conducted and the devices which are

near market ready are identified (see Chapter 2). To determine the tidal resource, an ocean

model is developed using the Mike-21 software, indicating the tidal flows around Ireland

(see Chapter 3). This work was conducted with the guidance of Kirk McClure & Morton

Ltd. An oceanographic database is developed using the Mike-21 data and data provided by

the British Oceanographic Data Centre which is in turn questioned to give the viable

resource and generation profile (see Chapter 3).

1.6 Tidal generation and system operations

The impact of tidal generation on system operations is studied with consideration of its

effect on: system ramp rate; demand profile; capacity / availability factor, generation

capacity credit; unit commitment; net system emissions; net generation cost; and cost-based

price received by tidal generation (see Chapter 4). Methods of controlling the effect of tidal

generation are identified as including installation at different locations with opposing times

of peak generation and reducing the tidal generation. The effectiveness of these methods is

considered with each of the operational issues studied.

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Tidal generation is used in conjunction with wind generation to clearly demonstrate the

operational difference between variable (wind) and variable but predictable (tidal) forms of

generation (see Chapter 5). To do this it is necessary to include wind and tidal generation

in the unit commitment model. This involves the development of methods of providing

reserve for the uncertainty in wind forecasting.

1.7 Tidal generation and grid connection

Given the location and magnitude of the resource it is necessary to quantify the issues for

grid connection. These include: transmission loss adjustment factors, short-circuit ratings at

the point of transmission system connection; capacity of the transmission system at the

point of connection; and cost of grid connection, including the necessary upgrades to the

33/38 kV system - see Chapter 6. Whilst most of these are available for the Republic ofIreland in the Forecast Statement, it was necessary to model each to obtain compatible

results for the system in Northern Ireland. The impact of the prototype scheme in

Strangford Lough on the 11 kV system was also considered in relation to voltage control.

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2. Energy extraction from tidal stream

2.1 The Origin and Nature of Tidal Energy

The development of renewable sources of energy, which are variable in nature, has

increased the importance for electrical engineers to develop an understanding of the

energy source and the forces driving the turbine. The following explanation will

demonstrate that tidal energy is a variable but, unlike wind, is accurately predictable.

This is a summary of a more extensive report produced as part of a project for

Sustainable Energy Ireland (Bryans, 2004).

2.1.1 The Tidal Generating Forces

The earth and moon orbit each other about a common centre of gravity, which is much

closer to the earth because the earth has a much greater mass than the moon. Therefore

all points on the earths surface follow the same circular path each lunar month

(ignoring the earths daily rotation), and experience the same centrifugal force (see

Figure 2-1). The gravitational force exerted by the moon on the earth is proportional to

the distance from the moon, so the gravitational force on the lunar side of the earth is

greater than on the far side of the earth. The sum of the gravitational force and the

centrifugal forces produces forces acting away from the earth at the equator whilst the

two forces tend to cancel each other out at the poles (assuming the moon to orbit above

the equator) (Pugh, 1987).

d

RR

Centre of gravity

Centrifugal force

Gravitational force

Figure 2-1.The gravitational and centrifugal forces in the earth moon system.

The tidal generating forces at any point on the surface of the earth can be given in both

the vertical and horizontal directions (see Figure 2-2).


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Earth

MoonMoon

v

Figure 2-2.The tidal generating forces in the earth moon system.Figure 2-2.The tidal generating forces in the earth moon system.

Gravitational forceGravitational force2d

mmGF emg = Newton, 1687 (2-1)

Centrifugal force (2-2)2RmF ec =

Where:

= the mass of the moonmm

= the mass of the earthem

= the angular velocity of the moon and earth about

the common centre of gravity.

G = gravitation constantem

ga2

=

In the centre of the earth the centrifugal force equals the gravitational force:

2

2

d

mmGRm em

e = (2-3)

2

2

d

mGR m= (2-4)

Therefore given the centrifugal force is constant over the entire earth:

2d

mGF mc = (2-5)

So the tidal generating force on the surface of the earth, directly under the moon can be

found in the vertical direction:

( )

=

22 d

mG

ad

mGF mmTG (2-6)

a = radius of the earth

= Tidal generating forceTGF

FhF


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3

2

=

d

a

m

mgFe

m

TG (2-7)

( 2sin23

3

gd

a

m

mF

e

mh

= ) (2-8)

( 1cos3 23

= gd

a

m

mF

e

m

v ) (2-9)

The resulting effect is to generate a tidal bulge on both sides of the earth, on the moon

side due to the gravitational attraction of the moon and on the far side due to the

centrifugal force of the earth and moon orbiting a common centre of gravity. It is now

necessary to add the earths spin, which is anti-clockwise looking down from the north

pole. So an island would be pulled though the two tidal bulges, demonstrating a simi-

diurnal tide. However, because the earth is offset to the moon by 23o, one tidal bulge

will seem greater than the other (see Figure 2-3), displaying a diurnal tide.

Moon

N

S

Figure 2-3.The tidal bulge established at 23oin relation to earths tilted axis.

The gravitational and centrifugal forces between the earth and the moon must be added

to those of the earth-sun system. Although the tidal generating force of the earth-moon

system is 2.17 times greater than that of the earth-sun system, the effect of the earth-sun

system is still very significant in driving the spring / neep cycle.

The moon revolves around the earth over a period of one lunar month (27.32 days),

during which the superposition of the lunar and solar tidal generating forces can be

constructive, resulting in large spring tides, and destructive, resulting in smaller neep

tides (see Figure 2-4).


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Sun Sun Sun

New moon

Half moon

Force exerted

by moon

Force exerted

by sun

Neep tide Spring tide Spring tide

Full moon

Figure 2-4. The formation of spring and neep tides through the superposition of the

lunar and solar tidal generating forces.

The earth takes 24.0 hours to complete one full rotation in relation to the sun, therefore

the simi-diurnal solar tidal constituent S2 will have a period of 12.0 hours, due to there

being a tidal bulge on each side of the earth. However the moon is orbiting the earth, so

in the 24 hours taken for the earth to complete one rotation the moon will have moved

forward in its orbit slightly so the earth will have to rotate for a further 0.84 hours to

complete one rotation in relation to the moon. Therefore the simi-diurnal lunar tidal

constituent M2 will have a period of 12.42 hours (see Table 1-1).

The equilibrium tide can be explained through the superposition of a number of these

tidal constituents (harmonics based around the lunar day, the sidereal month, and the

tropical year, see table 2-1). The M2and the S2can be considered as being the major

tidal constituents used in representing the equilibrium tidal forcing.


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Tidal

constituent

Period

(hours)

Origin Name

M2 12.42 2 1 Principal Lunar

S2 12.00 2( 1+ 2+ 3) Principal Solar

N2 12.66 2 1- 2+4Larger Lunar

Elliptic

Simi

diurnal

K2 11.97 2( 1+ 2)Lunar-solar

semi-diurnal

K1 23.93 1+ 2+ 3Lunar-solar

diurnal

O1 25.82 1- 2 Larger LunarDiurnal

P1 24.07 1+ 2-3 Larger Solar

Where

Frequency Period (solar days) Name

1 1.035 Lunar Day

2 27.32 Sidereal Month

3 365.24 Tropical Year4 6797.3 Moons Node

Table 2-1. The major tidal constituents recognised in the equilibrium tidal forcing

(adapted from lecture notes, School of Ocean Science, University of Wales, Bangor).

The equilibrium tidal theory ignores the effect of flow around landmasses, the frictional

effect of the bed on the flow and the establishment of harmonic amplification. The tidal

range in many lakes and basins is observed and calculated to be very small, in the orderof a few centimetres, while in shelf seas the tidal range is observed to be in the order of

metres. The reason for this discrepancy is that most of the tidal range experienced in

shelf seas is forced from the tidal wave generated in the deep ocean.

The establishment of a sea surface gradient due to the tidal generating forces results in a

current flow due to the pressure gradient at all depths down the water column. Such a

current flow in the deep ocean results in a large net flux of water in the horizontal plane.


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When the water depth is reduced the same flux of current must flow therefore current

speed increases and the tidal range increases (see Figure 2-5) (Simpson, 1998).

Current

Velocity

Tidal

Ocean

Shelf Sea

Figure 2-5.a.The amplification of tidal range and current velocity due to the oceanic

forcing of shelf seas.

If the shelf sea is longer than the wavelength of the tidal wave then multiple standing

waves are set up with nodal points of zero tidal range (see Figure 2-5.b.).

.

Figure 2-5.b.The propagation of an oceanic forced tide in a shelf sea longer than one

tidal wave length.

Such a theory can be applied to the tidal forcing in the North Sea, as demonstrated in

Figure 2-8a, to show entire lines across the North Sea, which should experience zero

tidal range.

The effect of the earths rotation on any object in motion or fluid flow is described well

by Pond & Pickard (1983) Consider the following hypothetical situation. A long-range

gun mounted at the North Pole is aimed along a meridian directly at a target fixed on

earth and some distance to the south. In plane view, a projectile fired from the gun will

travel in a plane fixed relative to the fixed stars but the target will be carried to the east

by the rotating earth during the flight of the projectile. From the point of view of the


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gunner, also rotating on the earth, the projectile will appear to curve to the left. See

Figure 2-6.

Actual path Targeted path

Target point

Earths rotation

Figure 2-6.An object travelling from the North Pole to the Equator.

To represent this effect an imaginary force is used termed the Coriolis force, which

acts anti-clockwise of any object or fluid moving in the Northern Hemisphere and to the

left in the Southern Hemisphere.

Figure 2-7a.The effect of Coriolis force on moving particle (image is from wikipedia,

website).

The effect of the Coriolis force acting to the right of the current flow in a shelf sea or

gulf means that the current is forced to one side, depending on which way it is flowing,

as shown in Figure 2-8.b. This results in a sea surface slope on the nodal line

(amphidrome) at 90oto the current flow (Pugh, 1981; Pugh,1987). Therefore when one

side of the nodal line is experiencing high water (to the right of the current flow) the

opposite side is experiencing low water. So the only point that retains a tidal range of

zero along the nodal line is the point in the centre around which the tidal wave can be


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described as rotating. This point is known as the amphidromic point. The effect of

friction on the current flow means there is a weaker current reflected back from the

basin than entering the basin. Therefore the current flow on each side of the

amphidrome is not equal and so the amphidrome is displaced left of the current entering

the basin, as shown in Figure 2-8.d.

Figure 2-8.a. The theoreticalestablishment of a standing wave in the

North Sea (Doodson & Warburg, 1941).

Figure 2-8.b. The theoreticalestablishment of a tidal current in the

North Sea (Doodson & Warburg, 1941).


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Figure 2-8.c. The theoretical relative

times of high water around amphidromic

points in the North Sea (Doodson &

Warburg, 1941).

Figure 2-8.d. The measured relative

times of high water around

amphidromic points in the North Sea

demonstrating the effect of friction in

terms of amphidromic displacement

(Doodson & Warburg, 1941).

The current velocity in the vertical plane is acted upon by the frictional resistance of the

seabed (see Figure 2-9), causing the currents near to the bed to decrease in velocityaccording to the relationship shown in Eqn. 1-2 (Department of Energy, 1990).

Figure 2-9.The current velocity profile approaching the seabed.


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( ) uh

zU z

7

1

32.0

= for 0 < z < 0.5h (2-10)

( ) uU z 07.1= for 0.5 < z < h (2-11)

=u depth mean current velocity

z = height above sea bed

water depth=h

current velocity at height above the bed z( ) =zU

These give results accurate to within +15%. However, they do not apply well to

the bed layer. The bed layer extends a few centimetres from the sea bed, the

flow within this layer will be smooth if the Roughness Reynolds Number is less

than 3.5 or turbulent if it is greater than 68. Therefore within the bed layer the

following relationships should be utilized.

( )

( )( ) hzzzu

Uob

ob

z2/2/ln

/ln

= for 5.0 zzob (2-12)

( )

( )( ) hz

zuU

ob

ob

z2/2/ln

2/ln

= for hz5.0 (2-13)

= seabed roughness length, determined by the nature of the sea bedobz

= thickness of the boundary layer.

The energy removed from tidal motion in the form of friction occurs to a large extent in

shelf seas, where the water depth is relatively shallow (~200m compared with ~3,000m)

and the current speeds are high. Therefore the shelf seas account for 74% or 2.6 TW

(Munk & Wunsch, 1998) of global tidal friction, which acts to reduce the speed of the

tidal wave and slow the rotational speed of the earth. The action of reducing the speed

of the tidal wave causes the tidal bulge to lag, as shown in Figure 2-10. The phase lag of

the tides causes the gravitational pull of the tidal bulge to occur ahead of the moon in its

lunar orbit (see Figure 2-10). This transmits some of the energy lost from the rotational

speed of the earth into the angular momentum of the earth moon system. The increase in

angular momentum of the earth moon system increases the orbital distance from the

common centre of gravity (Lambeck, 1980).


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Friction presentNo friction present

Figure 2-10.The effect of tidal friction.

2.1.2 The concentration of tidal energy on shelf seas

Tidal energy is amplified on shelf seas. However, there are areas on shelf seas where

this energy is concentrated even further. Such areas are of extreme interest to the

developers of TEDs (Tidal Energy Devices), because the greater the concentration of

the energy, the lower the capital cost per kW of power extracted. Such areas include

narrow channels and the entrances to gulfs, estuaries, loughs and seas. To raise the sea

level of the area contained within the gulf, estuaries, etc., there must be a much higher

flux of water at the entrance than at the landward end of the sea, as shown in Figure 2-

11. However, not all estuaries experience this phenomenon; estuaries that have been

exposed to long periods of erosion have undergone sedimentation inland and erosion at

the mouth of the estuary, leading to a more uniform flux along the estuary.

Flux

Sea

Land

Figure 2-11. High current velocities at the entrances to gulfs, estuaries, Loughs and

seas.

The tidal streams are also concentrated off headlands due to the establishment of a

coastal current. This current is set up due to the effect of the Coriolis force. As the sea

level drops, the coastal current is forced to the right and during flood it is forced to the

left. When current is forced against a head it is driven offshore, regardless of whether

the tide is flooding or ebbing, as shown in Figure 2-12.


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Offshore current during ebb tide Offshore current during a flood tide

Figure 2-12. The establishment of an offshore current from a headland.

HeadlandHeadland

Such areas may appear to be excellent areas to establish TEDs. However, headlands

also act as focal points for wave action and so must be considered with great care. Thisis because the shallow water waves (waves in water depth less than their wave length)

travel faster in deeper water than in shallow water and so bend towards areas of shallow

water (see Figure 2-13).

Headland

Wave propagation

Increasing

seabed

de th

Figure 2-13.The propagation of waves onto a headland.

Waves involve the movement of the water particles under them in a circular motion

which decreases in size with depth (see Figure 2-14). In shallow water the circular

motion becomes elliptical near the seabed. The passage of waves over and through a


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tidal turbine would result in a net increase of energy from the turbine because of the

increase in velocity. However, it would increase the velocity gradient across the turbine

and would impose serious structural strain that would drive up the cost per MW.

Figure 2-14.The motion of particles under a surface water wave in shallow water.

2.2 Generation from tidal streams

People have been harnessing the power of the tides for millennia; a good example of

this in Ireland can be seen on Mahee Island in Strangford Lough where the early

monastic community of Nendrum constructed a tidal barrage system to drive wooden

turbines that powered millstones (see Figure 2-15) for milling grain from 618AD

(McErlean et al., 2002). The flood tide would fill man-made lagoons, then during the

ebb tide the water would run out through turbines driving the millstones.

Figure 2-15. Artists representation of what a tidal mill would have looked like at

Nendrum (McErlean et al., 2002).

In the last century tidal power has been used for the generation of electric power using

barrage schemes such as La Rance, in France (Frau, 1993). These harness tidal energy


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by capturing high water behind a barrage and then exploiting its potential energy by

allowing it to flow out during low water. However, this method prolongs the period of

high water in the estuary, thus endangering the original estuarine ecosystem. It is also

only able to generate power during low water; otherwise the estuary would fill up with

silt. The capital cost of developing a barrage system is very high, forcing investors into

an all-or-nothing gamble. Hence in the last few years there has been a focus on

generation from tidal streams, which would remove the need for a barrage, high capital

costs and the environmental impacts that barrage schemes incur. This report will

therefore focus on tidal stream rather than tidal barrage schemes.

2.2.1 Tidal Energy Devices (TEDs) currently in development

The development of TED systems would appear to be taking a similar route to that of

wind turbine development, with a large number of initial concepts being reduced in time

to produce a few viable solutions. Therefore the TED systems in development have

been studied and divided into four classes:

Class of TED development Stage of development

Class A Development and installation of a full sized

prototype, close to being market ready.

Class B Development and installation of a full sized

prototype, not close to being market ready.

Class C Development of a scale prototype.

Class D System patented but only in the design

phase.

Table 2-2.Ranking method used during assessment of TED systems.

In classes C and D no distinction has been made between projects with ongoing

research and those which are in financial difficulty, because it has been recognized that

systems in the early stages of development may have difficulty attracting funding and

therefore go through periods of inactivity.


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Class Company developing

TED

Concept

Class A Marine Current Turbines Twin 2 bladed horizontal axis turbine, pile

mounted (Marine Current Turbines, website).

Class B The Engineering Business Undulating wing mounted on the seabed (The

Engineering Business, website; Trapp, 2004).

Class B Hammerfest Stromas 3 bladed horizontal axis turbine, mounted on

the seabed (Hammerfest Stromas, 2002a;

Hammerfest Stromas, 2002b; Hammerfest

Stromas, website).

Class C Open Hydro Constrained flow system with horizontal

blades mounted from the circumference,

leaving a hole in the center. The blades have a

permanent magnet around them so they can act

as a rotor and the constraint funnel as a stator

(OpenHydro, website).

Class C Blue Energy Limited Enclosed vertical axis turbine (Blue Energy

Ltd., website).

Class C JA Consult Tidemill Twin 2 bladed horizontal axis turbine, mounted

on a hinged pile (J.A. Consultants, 2004).

Class C Hydro Venturi Constrained flow system uses the pressure

change on the primary flow to pull in either air

or water from a secondary flow, which is used

for generation (Hydroventuri, website).

Class C SMD Hydrovision Twin 2 bladed horizontal axis turbine, mounted

on a buoyant support chained to the seabed

(SMD Hydrovision, 2004).

Class D Van den Noort

Innovations BV

Enclosed horizontal axis turbine with many

fine blades (Van den Noort Innovations BV,

website).

Class D Hydraulic Current

Turbines Ltd.

3 bladed horizontal axis turbine, mounted on

the seabed, using hydraulic power aggregation

(Hydraulic Current Turbines, 2003).


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Class D Ocean Tecs Twin Savonius rotors, mounted on a pile or

existing wind turbine (OceanTecs, 2004).

Class D Sea Energy Ireland Constrained flow vertical axis turbine

(Callaghan, 2003).

Class D Hydrohelix Energies Constrained flow horizontal axis turbine,

mounted on the seabed (Hydrohelix Energies,

website).

Class D Edinburgh University Vertical axis turbine, mounted from a floating,

tethered ring.

Class D Lunar Energy Limited Constrained flow horizontal axis turbine,

mounted on the seabed (Lunar Energy,

website).

Class D Verdant Power Horizontal axis turbine, mounted on the

seabed, with a wire screen to protect sea life

(Verdant Power, website).

Table 2-3.Companies developing TED systems.

The only company developing a system which could be described as being close to

market ready is Marine Current Turbines. Whilst a report has been prepared containing

a detailed description of all the systems listed in Table 2-3 (Bryans, 2004), this thesis

will only describe the system being developed by MCT.

2.2.2 Marine Current Turbines

MCT (Marine Current Turbines) are developing twin horizontal axis 2 bladed turbines

mounted on a monopile in such a way that they can be jacked up for servicing (see

Figure 2-16). Currently MCT has limited the feasible installation sites to have spring

peak current velocity of > 2 m/s and with a depth of 20 m to 40 m. MCT have

successfully installed a 300 kW prototype off the coast of Lynmouth which has been in

operation since 16/6/03, dumping power into a load bank. MCT are currently expected

to be the first UK company to provide a full-sized (1.2 MW peak) grid connected TED

during 2006 / 2007. Funding has been secured for this system, planning permission has

been granted in the Strangford Narrows, and Northern Ireland Electricity (NIE) have

offered grid connection options for it. Following successful installation and operation in


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Strangford, the next stage will be a semi-commercial venture with the installation of

about 10 turbines at a site which has not yet been announced.

Figure 2-16. The current system design for a ~1.2 MW system to be located in

Strangford Lough, utilizing a 15 m blade diameter (Wright, 2004).

The 300 kW prototype off the coast of Lynmouth was found to produce better energy

conversion efficiency than expected (Wright, 2004). The model used to predict energy

output was based on a wind turbine model which has a maximum theoretical efficiency

of 0.59 known as the Betz limit. The Betz limit is dependent on the velocity difference

between the front and rear of the turbine (see Eqn. 1-14 - 23).

The energy contained in the flow (wind or tidal) is:

2

2

1VEarea =

2

2

1mVEk = (1-14)

Where:

=areaE Energy per unit area

= Kinetic energy per unit volumekE

= massm

= velocity before the turbineV

= density


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Figure 2-18. The surface elevation along the centre of the estuary with no turbines

installed and with three rows installed during mid flood.

These results (shown in Figure 2-18) demonstrate the establishment of a head of water

across the turbine, the reduction of the tidal range across the turbine and the reduction in

peak current speed. Similar findings have been made by Bryden (2003).

Previous work (Bryden, 2003) demonstrated that in a channel with a constant flow, the

velocity after the turbine would be greater than the velocity before the turbine. The

reason for this is, the water depth beyond the turbine is reduced and to maintain the

same mass transport the velocity must increase. To review the work of Bryden (2003)

the experiment described above was repeated. However, the beach was removed and

one end was set to have a surface elevation of zero, whilst the other end was elevated

above zero to induce a constant flow along the channel. One row of turbines was placed

in the channel at 16 km from the mouth. Figure 2-19 supports the work of Bryden

(2003) demonstrating the depth to decrease behind the row of turbines, demonstrating a

head of water is setup across the row causing the velocity increases behind the turbines.


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Figure 2-19.The effect of a row of turbines completely across a uniform channel with a

constant stream speed.

The study described above does not permit water to flow around the turbines. However,

in reality, as the sea level rises in front of the turbine, the increased pressure gradient

would also act to force the water around the turbine. Therefore installing the turbines in

long rows at 90oto the current flow will reduce the flow around the turbines and would

increase efficiency. The distance between each row is dependent on two factors: the

turbulent wake from the previous row; and the distance required to recover the peak

velocity (in areas where the turbine rows are being installed at the edge or over a small

section of the main flow) through the velocity shear stress set up between the main flowand the flow behind the turbine (see Figure 2-20). Some models have attempted to

represent the velocity and turbulent wakes (Thomson, 2004). However, until actual

measurements are taken from the turbine in Strangford Lough, these models cannot be

calibrated or verified.

Figure 2-20.The formation of velocity and turbulent wakes behind turbine rows.

The economic predictions made by MCT indicate that tidal energy will become more

financially attractive than off-shore wind (see Figure 2-21). However, when looking at


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these Figures it must also be remembered that the power extracted from the tide and

hence the financial return is a cubic function of the velocity.

Figure 2-21.The economic predictions made by MCT (Wright, 2004).

Whilst MCT are devoting much of their efforts towards developing a system capable of

harnessing tidal stream in the most financially attractive areas (water depths of 20

40m with peak current speeds of greater than 2 m/s), they have also considered somedesigns for future deeper water systems once the most viable areas are developed. One

of these designs includes mounting a number of large turbines on a horizontal support

beam which is lowered onto multiple mono-piles at each end, and can be raised for

servicing, which may be feasible for depths up to 50 m. A second design aimed at

accessing the resource in depths up to 100 m is based around mounting multiple

turbines above large submerged buoys. However another developer (SMD Hydrovision)

has a design which may surpass MCTs 50 - 100 m system. This is a class D system

(see Table 2-3) and comprises of a twin 2 - bladed horizontal axis turbine, mounted on a

buoyant support chained to the seabed (SMD Hydrovision, 2004). The big technical

challenge developers face in scaling up blade size is constructing a gearbox to convert

very low rotational speeds to a usable speed for generators. However, better gearboxes

are being designed for wind turbines and multi-pole synchronous generators capable of

operating at lower speeds are also being developed.


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The choice of a 2-bladed turbine is in keeping with the strategy of developing a system

for operation in areas of high current speed. Research done on wind farms indicates that

2-bladed turbines are more efficient at high current speeds than 3-bladed turbines (see

Figure 2-22). Therefore in the future when developing large turbines for deep water in

areas of lower current speed it may be more efficient to switch to a 3-bladed design.

Figure 2-22.The efficiency in relation to the tip speed ratio found for wind farms

(Twidell & Weir, 1986).

2.3 Impact of tidal energy extraction

In recent years man has come to understand that the earths climate depends on a

delicate balance between a number of equally opposing forces. It is therefore important

to check the significance which tidal power extraction has for the energy balance as well

as the environment as a whole.

2.3.1 Effect on tidal regime

Extracting energy from the tidal stream in effect increases the amount of seabed friction.

The effect on the tidal regime of increasing seabed friction can be seen in Figure 2-8,

which depicts the movement of the amphidromic point (point of zero tidal range). It will


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also result in a reduction of the tidal range down-steam of the turbine and a reduction of

the peak tidal stream speed.

2.3.2 Effect on the local eco-system

The impact of TEDs on the eco-system can be considered during the installation and

during the operation of the TED.

During installation there is first a need for a jack-up barge to drill a number of sample

bore holes to asses the geophysics of the seabed to be certain the bed is capable of

holding the pile, and therefore ascertain the depth the pile needs to be driven or drilled

in by. The second jack-up barge is then used at a later date to drive or drill the pile into

the seabed and to install the turbine on to the pile. The action of a jack-up barge

lowering its pile feet on to the seabed will destroy anything under them, whilst the

drilling will result in a discharge of suspended particulate material into the water

column. Therefore the jack-up barge coming in will crush any coral, sponges, rock

features, etc. and those near to the drilling point will almost certainly be smothered by

particles settling on them. However much greater damage is caused to the benthic

environment through fishing nets being trailed over the seabed (often illegally), and

after the installation of the turbine this area will become physically protected from such

action. The discharge of the particulate material from the drilling process is not thought

to be a major environmental risk because it can be pumped to the surface where the

water current speeds are greatest (bear in mind these turbines will only be installed in

areas of high current speed), allowing the particles to be dispersed to such a

concentration that the resulting settling rate on the benthic organisms is negligible when

compared to an influx from a river following heavy rainfall.

During operation the current behind the turbine will be mixed so the current speed at the

seabed will be greater that before. Therefore it is probable the native benthic species

will be replaced with a gradient of benthic species approaching the turbine, in the same

way that a gradient of coral types are found across a coral reef, with the organisms

favouring very high current speeds next to the turbine and those favouring lower current

speeds further away from the turbine. The monopile used by the turbines is known

(from its use in offshore wind turbines) to cause seabed scaring in areas of soft sediment

such as sand banks. To prevent this the area around the base of the turbine often has to


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be filled with rocks. The impact on the organisms that live or pass through the water

column is through to be very small because of the slow rotational speed of the

turbines. The tip speed of a 15 m 2-bladed device in Strangford Lough is estimated to

reach ~30 knots (~15 m/s). When this is compared to the speed of a mackerel or a

diving bird, which can reach speeds of about 35 knots (~18 m/s), it is seen that not only

can fish such as mackerel swim though the turbine but, technically speaking, they could

actually swim around in front of the blades. Slower fish such as cod may not be able to

swim in front of the blades, but could certainly swim between the blades. Mammals are

larger and therefore the window available for a marine mammal to swim through is

much smaller. It is much more likely that marine mammals will avoid the structure due

to their higher level of intelligence and acoustic awareness. Should it be found to the

contrary, it is possible to install an acoustic warning device on the turbine developed for

fishing nets to scare away marine mammals. However, sick or injured animals may

stray into the turbine in a confused and disoriented state. Concern has, however, been

raised about seal populations, and whether seals may try to swim through turbines.

Experts have advised that it is unlikely, but that the situation should be monitored to

confirm this.

2.3.3 Effect on human activities

Tidal stream turbines are expected to have a minimal effect on human activities. The

visual impact is relatively small compared to that of a wind turbine (see Figure 2-23).

However, the installation will require the establishment of an exclusion zone around the

turbine for all maritime use. Diving and drift diving (a sport which involves a diver

using strong currents to cover large distances) near the turbines would also be

prohibited.


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Figure 2-23.Scaled comparison of a tidal turbine and a wind turbine (Whittaker et al.,2003).

2.3.4 Global scale

On the larger scale the installation of TEDs will result in the earth moon distance

increasing at a greater rate (~1 cm per year per 1 TW year extracted). Much larger

deviations in the rate of the earth moon separation are believed to have occurred in

nature due to the formation of polar ice caps, and the geological movement of

continents and coast lines.


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3. Resource assessment

3.1 Introduction

To study the effect tidal generation may have on a power system it is essential tounderstand the resource magnitude, the nature of the variability the resource may bring

on the system and the location at which it may connect to the system. To gain an

understanding of the resource, an oceanographic model was developed, the output of

which was combined with data from wave buoys to populate an oceanographic database

(see Figure 3-1). The oceanographic database was interrogated by specifying the

feasible range of each parameter to determine the viable areas. The total resource

magnitude was calculated using each of these feasible areas and superimposing the

results to give the entire resource magnitude and profile.

Chart of tidal

constituents

Chart of sea-

bed depth

Mike 21

Shelf sea model

Current flux Water depthSeabed depth

Seabed slope

Max. spring

current velocityTidal Phase

Distance from

Ireland

Wave height

Feasible resource

Input data Ocean model Oceanographic database

Max. neep

current velocity

Data from

wave buoys

Figure 3-1. Method of resource assessment.

The following chapter explains the setup and development of each step detailed in

Figure 3-1, and presents the resulting feasible resource according to the operational

range of the MCT system.


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3.2 Shelf sea models

Models aimed at reproducing the observed flows in coastal waters are extremely

challenging due to the sharp gradients in physical parameters such as water depth,

velocities, turbulence and salinity. For these reasons, and in combination with thelimitation of computing resources, modellers make simplifications to the relationships

defining coastal currents. However, as computing power increases the simplifications

which have been made in the past can be reduced to produce more realistic and accurate

predictions (Haidvogel and Beckmann, 1998).

Models of coastal waters function by describing the problem both spatially and in time.

Each of these divisions can be represented in a number of different ways. The grid

which divides the area up geographically in the x and y directions, can take the form

of structured regular grid cells known as an Arakawa-C grid (see Figure 3-2) (e.g.

DieCAST, GBM, GFDLM, HAMSON and GHERM: referenced in Table 3-1) or

structured curvilinear grid cells (SCRUM and SPEM: referenced in Table 3-1). The grid

cells can also be created in an unstructured format, in the form of triangles (QUODDY:

referenced in Table 3-1) or quadrilaterals (SEOM: referenced in Table 3-1), which

produces a better representation of the sharp depth gradients seen in coastal waters.

Some models only use two dimensions (the x and y) (Flather, 1993; Flather & Heaps

1975; and Falconer & Owens, 1987) whilst other models divide the grid further in the

z direction. The z dimension can either be at intervals of constant geopotential (e.g.

DieCAST, GBM, GFDLM and HAMSON: referenced in Table 3-1), or it can follow the

bottom topography, with a constant number of divisions in each grid square (GHERM,

POM, SCRUM, QUODDY and SPEM: referenced in Table 3-1). The latter approach in

particular can be difficult to implement in areas of sharp depth gradients because the

vertical layers may not line up with their counterparts in neighbouring grid squares. This

problem is greatly exaggerated as the depth nears zero (which happens when

introducing drying banks).


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The governing equations (Flather & Heaps, 1975).

Continuity equation:

( ) ( ) 0=

+

+

Hv

yHu

xt

(3-1)

Equations of motion:

( )0

2

122

=

+

++

+

+

xg

H

vukufv

y

uv

x

uu

t

u (3-2)

( )0

2

122

=

+

+++

+

+

yg

H

vukvfu

y

uv

x

vu

t

v (3-3)

were:

u = the current velocity in the x direction.

v = the current direction in the y direction.

f = the Coriolis parameter.

k = drag coefficient of bottom friction.

H = total water depth h+ .

g = the acceleration due to gravity.

= the sea surface elevation.

The continuity equation states that if the volume of water entering an area is different to

that exiting the area then the volume of the water contained in that area will change,

resulting in a change in sea surface height.

The equations of motion state that acceleration is proportional to the barotrophic effect

of the sea surface slope, the friction, the Coriolis force and the rate of change in velocity

in the x and y directions.

On starting the research two models existed of the shelf sea surrounding Ireland the first

was a three dimensional model developed commercially by the Metoffice, in

conjunction with the Proudman Oceanographic Laboratory and further refined by the

Marine Institute (Marine Institute, website), however this remained inaccessible. The

second was a 2 dimensional model (Mike-21) developed by DHI (Danish

Hydrographical Institute) which had been implemented in previous studies by KirkMcClure Morton Ltd. to include; the Irish Sea, the North coast and the West coast of


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Ireland. Kirk McClure Morton Ltd. kindly agreed to make the results of these models

available, leaving only the southeast coast of Ireland uncovered. DHI provided a 1-year

license for Mike-21 enabling the development of a model to cover the southeast coast of

Ireland. Following the development of the southeast model, Kirk McClure Morton Ltd.

was awarded a contract to develop an ocean model for all the sea around Ireland (Kirk

McClure Morton et al., 2004), and agreed to make these results available. These

compared well to the model developed for the southeast and offered a set of results for

the entire island, mitigating the problem of joining data sets from different models.

Therefore the results from the Kirk McClure Morton Ltd. model were used to populate

the oceanographic database, whilst the development of the southeast coast model has

been presented in the following section as an example of methodology.

The development of the model for the southern coast is broken up into a number of

different stages, these include:

The definition of closed boundaries and land cells

The setting of grid cell depths

Selecting a time step

The forcing of open boundaries

Selecting a bed friction parameter

Selecting an eddy viscosity parameter

3.2.1 The definition of closed boundaries and land cells

All the data regarding the land sea boundary was input from 1:50,000 scale digital

charts (produced by C-Map Norway) apart from a short stretch of coast line on the south

of Ireland for which there was no 1:50,000 scale chart held (highlighted in Figure 3-3).

Therefore this section of coastline was covered at 1:100,000 scale. The data from digital

charts is output to file in a format of latitude and longitude with the use the software

package Mike C-Map (access to both Mike C-Map and the C-Map Norway data base

was available under the terms of the software license of Kirk McClure Morton Ltd.).


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Figure 3-3 The area of coastline covered at the lower resolution of 1:100,000

highlighted against the total study area.

The land sea boundary is defended with a series of polygons which overlap each other

(Figure 3-4) to form the complex outline of the coastline, seen on the final map (Figure

3-5). The areas between the polygons, on the landside have to be covered with either

one large polygon or a series of larger polygons, in order that land can be defined as

being completely covered in polygons. A graphical user interface is provided within the

Mike Zero software for this.

Figure 3-4.Land / sea boundary defined by a series of overlapping polygons.


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Figure 3-5Resulting land / sea boundary translated from the polygons in Figure 3-4.

3.2.2 Selecting the Grid Scale Size

Choosing the grid scale to use with a model is a balance between providing sufficient

resolution to define the nature of the flow in the area against the time taken to process

the model. With these considerations and with the maximum charted depth resolution ofthe area being to 1:25,000 the scale was set to a 3600 m x 3600 m grid scale for a coarse

model of the entire area. This grid will be used to enable the bad data generated next

to boundaries to dissipate before forcing a nested finer grid of 1200 m x 1200 m which

in turn forces an even finer grid of 400 m x 400 m (see Figure 3-6).


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3.2.3 The setting of grid cell depths

The water depth in chart datum (the lowest predicted water level) is also provided in the

C-Map Norway database with irregular spacing. Unlike the data points defining the land

sea interface the bathymetry data points can be superimposed on data points from charts

of different scales. Therefore to obtain as accurate a representation of the bathymetry as

possible data was output from scales ranging from 1:1,000,000 to 1:20,000 of selected

areas. The accuracy of the data was maintained in coastal areas by limiting the bilinear

search radius. Therefore, it was necessary to provide a few manually interpolated data

points in the sparser areas.

The Mike C-Map software also provides the difference between chart datum and MSL

(mean sea level). Correction grids were generated within Mike-21 to correct the data to

mean sea level at the same resolution as each of the bathymetry grids, using the same

method as that used to generate the bathymetry grids (see Figure 3-8).

Figure 3-8.Correction grid used to account for the difference between chart datum and

mean sea level for the 3600 m x 3600 m bathymetry grid.

The water depth data was superimposed in the Mike Zero software with the land sea

interface and bilinearly interpolated to provide a grid of Mean Sea Level (MSL) and

land squares (given the value of 10 m above MSL).


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3.2.4 Selecting a Time Step

The time step taken between each cycle was determined using the CFL (Courant

Friedrichs Lewy) criterion, which states the maximum time step that can be chosen for

the model to remain stable.

The method used to determine the maximum time step.

gD

xt

< (Richtmyer, 1967) (3-4)

where:

=t the time step.

=x the grid spacing (smallest of the two directions).

g = acceleration due to gravity (9.81 ms-2).

D = the grid square depth (m).

To be certain that stability is maintained within the model it is common practice to

divide the maximum possible time step by 4 (see Eqn. 3-5).

For example:

20081.94

400


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The open boundary surface elevations where forced using tidal height predictions for

each open boundary cell at every time step. The tidal prediction was performed at five

points along each open boundary or as near to the boundary as could be accurately

measured on the contour plot of tidal constituents (Howarth, 1990). The values between

these points where interpolated using a matlab routine developed for the purpose of

achieving a smooth curve along the boundary (see Figure 3-9) rather than a linear

interpolation which was found to cause instabilities in the model. These tidal

predictions were generated in Mike 21 from the M2, S2, K1 and O1 tidal constituents

using the admiralty method (Doodson & Warburg, 1941). The amplitude and phase of

each tidal constituent was obtained from Howarth (1990) which provides contour maps

of the phase and elevation for the British Isles based on tidal analysis of data from

buoys and ships.

Figure 3-9.An example of sea surface elevation determined along a boundary with theuse of curved interpolation.

The depths of the grid squares on the open boundaries and those in the adjacent row /

column were set to be greater than 10 meters in depth (chart datum) in order to prevent

these squares from drying out. In doing this is must be noted that in these boundary

areas of shallow water there will be unrealistically high current velocities. However the

boundaries have been set sufficiently far outside the area of interest to account for this

effect and for the effect of not permitting along boundary flow. Again as long as the


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g

HC

n

d

=

3

1

(3-6)

(lecture notes, School of Ocean Science,

Where: University of Wales, Bangor).

n = the Manning number

Cd= the drag coefficient

H = water column depth

g = acceleration due to gravity (9.81)

3.2.8 Eddy viscosity

The eddy viscosity was determined using the relationship given in Eqn. 3-7.

( )dt

dxEv

2

04.0= (3-7)

where

= grid square widthdx

= time stepdt

eddy viscosity=Ev

3.2.9 Model Calibration

The model was calibrated against both locally measured sea surface elevation data

where possible (see Figure 3-6), and predictions generated from local tidal harmonics

(see Figure 3-7). When using the locally measured data, it is important to realise that

there will be deviations due to the effect of weather, and at low water the tidal gauge

often runs dry so the values for low water must be extrapolated from the rest of the

curve. To calibrate the model to match these points as best as possible the phase and

amplitudes of the tidal constituents used to generate the boundary conditions were

slightly altered, until the best comparison was achieved.

A detailed description of how each record compared to the expected value has been

given in Bryans (2004).

The model has been calibrated to be within tolerable limits, further and finer calibrationwould have been possible, by altering the boundary conditions and by creating a grid of


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the bed friction and adjusting it. However the data which the model is being calibrated

against is currently not believed to be sufficiently accurate to warrant undertaking this

work. The Irish Geological Survey have indicated concern over the accuracy of the data

from which the local tidal predictions are made in the Republic of Ireland, creating a

major problem in model calibration. They are sufficiently worried about this data that

they have commissioned a complete resurvey of tidal constituents around southern

Ireland.

The velocity profile at a few locations over the model was inspected to confirm there

are no large velocity peaks. A few small peaks in velocity were observed in areas where

there are large areas of drying and flooding such as the Bristol Channel and the Welsh

coast. This can be expected in drying and flooding areas and will be confined locally to

those areas (Bryans, 2003).

3.3 Development of an oceanographic database

The oceanographic database was established to provide a source of data for each factor

which may restrict the development of a site for tidal generation. The database was

structured to store each of the limiting factors in individual grids comprised of grid

squares the same size as the all-island Mike-21 model (405 x 405 m). This database was

populated using the outputs from the Mike-21 shelf sea model, and data from wave

buoys. Each of the limiting factors listed below were calculated and stored in the

appropriate gird. These grids could then be tested with limitations to determine the

feasible areas of tidal resource.

Seabed depth

Seabed slope

Maximum spring tidal current velocity

Maximum neep tidal current velocity

Wave height

Tidal Phase

Distance from Ireland


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3.3.1 Seabed depth

The seabed depth data was taken directly from the Mike 21 model in grid form as the

uncorrected chart datum (the lowest predicted tide) as shown in Figure 3-10. The depth

within the model has been limited to a minimum of 300m to reduce the calculation

time within the model.

Depth (m)

Figure 3-10. Depth of the seabed in chart datum (lowest predicted tidal level).

3.3.2 Seabed slope

The seabed slope was calculated according to the height difference between the adjacent

grid square depths using the least squares method. The maximum slope experienced

around the point was taken to be the value of the seabed slope (see Figure 3-11).


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(degrees)

Figure 3-11. The calculated slope of each grid square within the study area.

3.3.3 Maximum spring tidal current velocity

The current velocity in the X and Y directions was determined from the Mike21 model

output of current flux (in m3s-1m-1) between grid squares according to eqn. 3-8.

+=d

FV (3-8)

where

F = current flux

d = depth

V =current velocity

= surface elevation

The velocity vector was then converted to a scalar measure of speed. This process was

repeated for each 15-minute model time step around a spring tide to find the maximum

value for each grid square (see Figure 3-12).


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pBAHs += (3-9)

where

A,B= modal frequency parametersp= probability of the wave

Hs= the significant wave height

The data provided by the BODC was in the format of significant wave height. Each data

set was positioned on a grid of resolution 16.2 km and the rest of the grid was

interpolated using a linear interpolation in Mike21. Three grids were produced the 50

year significant wave height (Figure 3-13a) which demonstrates the wave height the

structure must be capable of withstanding, the 1 year significant wave (Figure 3-13b)

which demonstrates the wave height the turbine must be capable of operating during

and the significant wave height (Figure 3-4c) which indicates the normal effect waves

can be expected to have on the turbine.

(m)

Figure 3-13a. The 50 year significant wave height interpolated to a 16.2 km grid

resolution.


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(m)

Figure 3-13b. The 1 year significant wave height interpolated to a 16.2 km grid

resolution.

(m)

Figure 3-13c. The significant wave height interpolated to a 16.2 km grid resolution.


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3.3.6 Tidal Phase

The time of peak current speed in each grid square was identified by asking if the speed

both before and after each time step was less than at the current time step, indicating the

point of maximum current speed. Upon identifying the time of maximum current speed

a note was made that it had already been found and to exclude the grid square from

further searches. The final result would have given an answer ranging from 0 to 6.2

hours for each grid square however it is more conventional to give the tidal phase in the

range from 3.1 hours to +3.1 hours, therefore this value was subtracted from the array.

Figure 3-14. The tidal phase patterned around Ireland.

3.3.7 Distance from Ireland

To calculate the distance from Ireland it was first necessary to distinguish between

Ireland and other land. This was done manually by selecting the land within manually

specified grids and setting its depth to a greater value than the remaining land. Two

approaches were identified to calculate the distance from Ireland:


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3.3.8 Database integration

To determine the feasible areas with the study area, each grid representing a limiting

factor was tested to determine the grid squares that were within the feasible. This was

repeated for all the limiting factors and only the squares which remain viable in all of

them have been identified as being viable.

The database was tested against MCTs current design assuming the feasible ranges for

development of a site given in table 3-1.

Limiting factor Feasible range for development of site

Seabed depth Between 20 m and 40 m.

Seabed slope N/A (only used in gravity mounted TEDs)

Max. spring tidal current speed 2.25 m/s

Significant wave height Less than 2 m.

Distance from Ireland Less than 15 km

Table 3-2.The conditions required to assign a site as being feasible for development by

MCTs 1stgeneration of TED.

Figure 3-16.The areas identified as having a current velocity >2.25 m/s and a depth

between 20 and 40m.


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It is believed by the author that following technology advances the number of feasible

areas will increase. This statement was made based on the fact that larger turbines in

deeper water will be capable of harnessing greater amounts of energy for a lower

current velocity as shown in Figure 3-17. Therefore to provide cases that may anticpate

the resource that may be accessible by 2ndand 3rdgeneration turbines the database has

also been tested for peak spring current speed of 2.0 m/s and 1.8 m/s, whilst the depth

range has been increased to 20 to 50 m (Figure 3-18) and 20 to 70 m (Figure 8-13).

Figure 3-17.The power output in relation to current velocity and blade diameter, based

on a horizontal axis turbine with an efficiency of 40%.

Figure 3-18. The areas identified as having a current velocity >1.8 m/s and a depth

between 20 and 50 m.


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Figure 3-18. The areas identified as having a current velocity >1.8 m/s and a depth

between 20 and 70 m.

The interrogation of the database found the majority of feasible sites to be off the north

coast of Ireland (see Figure 3-16 to 3-18). There are also a few sites off the east coast of

Ireland on the Arklow and Codling banks (see Figure 3-16 to 3-18) and a few possible

sites off the southwest headlands (see areas of high current speed in Figure 3-12). Thesites off the southwest headlands would almost certainly experience extreme wave

climates for reasons explained in chapter 2 and would therefore not be feasible to

develop. There are also a few small sites, which lie outside the grid resolution of this

model. Such sites may include the Shannon estuary, Strangford Lough and areas on the

west coast sheltered from wave action behind Islands. There was also some resource

seen to open up off Malin Head when the feasible depth range was set to 20 to 70 m

(Figure 3-18).


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3.4 Determination of Power Output

According to the literature the general power available per square meter can be

determined according to Eqn. 3-10.

The power available per square meter of sea surface (Fraenkel, 2002).

3

2

1VkkhkP efns= (3-10)

Where:

h= water depth

= Daily availability factor (0.424)sk

n

k = neap / spring availability factor (0.57)

= efficiencyefk

V= Max. Current velocity

Including and will give the average power output over both spring neap cycle and

over a daily cycle. The values of and given in 3-8 were presented in Fraenkel

(2002). The efficiency of the TED can also be included to appreciate the total amount

of energy available at any given location. However the TEDs cannot be installed

infinitely close behind and in front of each other because there is a need to enable the

tidal current to both recover in velocity behind them and return to a laminar flow (see

chapter 2). In chapter 2 it is theorized that the most efficient method of installing the

turbines would be in rows at right angles to the direction of current flow, with the

turbines in these rows being infinitely close to each other (side by side). The area seen

by each row can therefore be determined according to eqn. 3-11.

sk nk

sk nk

The area seen by each turbine row.

( )xch

4 (3-11)

Where:

h= water depth

c= blade clearance depth

=x width of the grid square, assumed length of the row


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output would peak twice, once in each direction. The tidal velocity also varies according

to the spring neap cycle over a period of 14.75 days. To determine the power at a given

time the availability factors were removed from equation 3-8 and the velocity at the

given time was approximated from the maximum velocity during a spring tide and the

maximum velocity at a neap tide by superimposing a spring-neap sin function to

account for this velocity range on a second semi-diurnal sin function to account for

daily variation (see Figure 3-19).

The method of determining an approximate tidal velocity for each square at any given

time.

( ) ( )

+

= 75.14

242

sin

T

VVV nssn

(3-15)

( ) ( )

+=

4224.12

2sin

TVVV snst (3-16)

Where

Documents

Garth Bryans - Impacts of Tidal Stream Devices on Electrical Power Systems