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large-scale

hydrodynamic

effects

of

tidal

turbine

arraysA

tidal

current

turbine

converts

kinetic

energy

from

a

tidal

current

into

electricity.

A

horizontal

axis

tidal

turbine

works

in

a

similar

way

as

a

horizontal

axis

wind

turbine.

Kinetic

energy

of

the

flow

is

transformed

in

shaft

mechanical

energy

through

the

blades

of

the

turbine.

This

shaft

mechanical

energy

is

further

transformed

into

electricity.

In

order

to

be

as

efficient

as

possible

the

turbines

are

dimensioned

as

big

as

possible.

However

due

to

negative

effects

of

surface

waves

and

the

steeper

part

of

the

boundary

layer

at

the

seabed,

a

clearance

of

8

m

is

reserved

at

the

top

and

a

clearance

of

25%

of

the

depth

is

reserved

at

the

bottom.

In

a

water

depth

of

30

m

this

would

result

in

a

tidal

turbine

with

a

diameter

of

15

m.

For

economic

reasons

tidal

current

turbines

are

expected

to

be

applied

in

array

formations,

similar

to

wind

farms.

A

tidal

farm

differs

from

a

wind

farms

in

the

fact

that

the

boundaries

(free

surface

and

sea

bed)

are

relatively

close.

One

can

imagine

that

a

turbine

in

the

wake

of

another

turbine

can

experiences

negative

effects

due

to

the

wake.

By

knowing

the

characteristics

of

the

wake

behind

the

horizontal

axis

tidal

current

turbine

the

farm

design

can

be

optimized

to

produce

as

much

power

as

possible.

Research

on

horizontal

axis

tidal

current

turbines

is

mainly

focused

on

optimizing

the

design

of

the

turbines,

scale

model

tests

and

CFD

tools

are

used

to

assess

the

hydrodynamic

characteristics

of

the

turbines.

The

optimization

of

the

tidal

turbine

farm

is

not

the

only

point

of

interest,

the

impact

on

the

environment

is

important

as

well.

How

does

the

farm

influence

the

general

flow,

sediment

transport,

water

quality

etc.

are

all

important

environmental

aspects.

This

can

be

an

opportunity

for

Delf3D

-

FLOW,

as

Delft3D-FLOW

is

used

to

solve

(far-field)

flow

problems

in

large

water

bodies

with

a

free

surface.

The

objective

of

the

project

is

to

develop

a

(far-field)

modeling

approach,

based

on

the

Delft3D

open

source

hydrodynamic

software.

The

focus

is

on

the

large

scale

hydrodynamic

effects

of

the

tidal

current

turbines.

The

goal

is

to

come

up

with

a

practical

tool

for

the

development

of

a

tidal

farm,

which

implies

the

interest

in

the

far

field.

The

main

criteria

which

are

of

interest:

• Is

• the

• wake

• (turbulence)

• modeled

• accurately

• enough?

• Is

• the

• interaction

• of

• multiple

• wakes

• modeled

• accurately?

• Is

• the

• resistance

• represented

• accurately?

• And

• finally

• the

• energy

• production

• of

• the

• turbine

• should

• be

• modeled

• sufficiently

• as

• well.

These

criteria

raise

the

important

question

how

accurate

is

accurate

enough?

Numerical

models

used

as

practical

tools

for

wind

farms

show

accuracies

of

5

to

15%

,

this

will

also

be

the

desired

accuracy

for

this

thesis.

Another

important

issue

concerns

the

definition

of

large

scale.

Because

of

practical

reasons

horizontal

axis

tidal

current

turbines

will

be

placed

at

least

4

rotor

diameters

away

from

each

other,

which

means

there

influence

should

be

modeled

accurately

enough

starting

from

this

distance.

Studies

have

shown

that

this

region

can

be

classified

as

the

far

wake

region

and

this

study

focuses

on

modeling

the

far

wake

region

as

accurate

as

possible.