12 Tidal Current Energy and the Sabella Tur

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Wasserbaukolloquium

2009: Wasserkraft im

Zeichen

des

Klimawandels

Dresdener

Wasserbauliche

Mitteilingen

Hef t 39

Tidal Current

Energy

and

the Sabella Turbine

Jacques

Ruer

The

tide

along

the

European

coast creates

powerful

currents

which can

be har-

nessed

to

produce

electdcity by

underwater turbines. The

machines

mus t be

designed

to

take into account the

wave

action and

the

worst environmental

con-

dit ions in the

open

sea.

SAIPEM

participated

in the

design

and

the

development

of

a

new

tidal turbine

named Sabella. The

turbine

concept

has been

simplified

as

fa r as

possible

in order to l imit the

needs fo r maintenance. A

first

step

of the

development

was

the

study,

construction

and test

a t

se a

of

a

Sm diameter demon-

stration

unit.

1 Introduction

The tide

along

th e

European

coast creates

powerful

currents.

Their

intensity

can

be well

predicted,

but

vary

along

th e

time

following

th e

astronomical

lunar

and

solar

cycles.

The

evaluation

o f

the

peak

kinetic

energy

exceeds

30GW.

A

part

o f

this

resource

could

be harnessed

with

tidal

turbines

installed

at

suitable

loca-

tions. The

request

to

increase the share o f renewable

energies

raised

th e

interest

for

the

development

o f

tidal

current

energy.

Some

French

companies

decided

to

join

efforts

to

develop

a

tidal turbine

specially

suited fo r

the

local

conditions

1,2).

A

first

unit

was

designed

in order

to

demonstrate

the

validity

o f

th e

turbine

concept.

The

demonstration

project

was

named Sabella.

2 The

energy

resource

Figure

1

show s the

map

of

th e

maximum

cur rent

velocities in

th e

English

Channel.

The

maximum

current

velocity during

spring

tides is

typically

2

to

3

m/s,

although stronger

current

exist

on a

fe w

zones,

l ike th e

Alderney

Race.

97



98

Tidal C e n t

Energy

an d ihe

Sabella yjirbine

On these

sites,

th e

water

depth

remains

relatively

shallow,

limited to

less than

50

m

in

m o s t

cases.

Figure

1

Map

of

the tidal currents in the

English

ChaDnel. Maximum

velocity

duiing

a

m e a n

spring

tide

Because

of th e

shallow

depth,

the

most

severe

waves

are

l im ited

to

less

than

about

13

m

.Storms

must

however be taken

into

account

fo r the

design,

as

they

are

responsible

fo r th e

extreme

mechanical

loads.

The

interesting

sites

a re

located around

capes

or

islands,

where the tida l flow is

restricted

by

the

topography.

The

coast

is

never

fa r

away,

so

that

the

cable

length

to shore

is

only

a

fe w

k ilomete rs . On

the

other

hand

these

areas

are

heavily

frequented by

surface

vessels.

They

are

also

places

of

high

biological

interest,

a

fact

to

be

carefully

considered

in

every

project.

Table

1

Typical

characteristics of

potential

sites

Nominal

current

velocity

Water

depth

Stonn

waves

Soi l

conditions

2

to

3

nols

(

4

to

6

knots)

20

m

50

m

Hmax

=

13

m

Tp

=

11

s

Gravel -

consolidated

sand

rock

5m-llm

Typical

3 km

10 km

'

:3.0

2.5

han

..

:2.0

0 essa

/1.5

Velocity

in mis

1.0

i

0.5

/0.0

Tida l

range

Distance

to

land



Wasserbaukolloquium


Zeichen

de s

Klimawandels

Dresdener Wasserbauliche

Mitteitungen

Heft

39

3 Theoretical

design

of

t idal turbines

The

kinetic

energy

resource

o f

a

current

flowing

with

a

velocity

V is

given by

th e

equation

[l]:

[1 ]

W=f.p.Cp·

02·V3

Where

p

is the

water

density

(1024

kg/m )

Cp

th e

turbine

efficiency

D

th e

rotor

diameter

V

the

water

velocity.

This

equation

shows that the

current

velocity

and the

rotor diameter

are more

important

parameters

than th e

rotor

efficiency.

Table

2

compares

th e

rotor

dia-

meters

of turbines

required

to

obtain 200 kW

following

th e

nominal current

velocity.

It

can

be

seen

that

th e

turbine is

relatively

small in

high

speed

currents,

but

have

to

be

unacceptably large

w h ere th e

current

is

slow.

Keeping

in mind

that

a

200 kW wind turbine

has

a rotor

diameter

o f

25

m ,

it

is

obvious that

tidal

turbines

are not

attractive if

the

current

does

not

exc eed 2 m/s.

Table

2

Typical

sizes

of

a

200

kW

tidal turbine

(Cp

=

0.35)

Cun·ent

velocity

Rotor

diameter

1 m/s

37.7

m

2 m/s

13.3m

3 m /s

7.3

m

4

m/s

4.7

m

The

water

velocity

varies with the

depth, being

maximal

at

the sur face

and

zero

at

the

bottom.

The

most

widely

accepted relationship

is :

[2]

I/2

-Vs·

(z/d)1/7

Where Vs

is

th e

surface

velocity,

d

th e

water

depth,

z

th e

altitude above

th e

sea

bottom.

Moreover,

the

open

sea

is

agitated by

waves.

In the

shallow

areas

considered,

the

wave

action

can

be

felt

down

to

the

sea

floor,

in

particular during

storm

events when

long

period

waves are

observed.

99



Tidal

Current

Energy

an d

th e

Sabella Turbine

i

0

2

-5

m

-10

9 15

*

40

0

25

a

30

Crest

passage

-Trough

passage

No wave

i...

.7

1))

-

JJ

1.5 2 2.5

3 3.5 4

Water

velocity

(mts)

Figure

2

Velocity

profile

with

waves

following

current

Waves

:

Hs

=

2

m-

Tp

=

9 s

Figure

2 makes it

clear

that

rotor

blades

undergo cyclic

efforts

at

each

revo-

lution. This must

be taken into

account

in the

design

in

order

to

avoid

a

premature

fatigue.

4

Design

of the Sabella turbine

The machines

are

working

completely

immerged

in

a

hostile environment.

It

is

therefore

a

prime

concern to

minimize

the

risk

o f failure

and

th e need fo r

maintenance. In order

to

fulfill this

objective,

the turbine

is

designed

as

a

simple

heavy

duty equipment:

•

The

energy

capture

device is

a

horizontal

axis

rotor

•

The rotation

sense

is reversed with

the current

and the

rotor

has

symme-

trical

blades

•

The rotation

speed

varies

with

the

current

velocity

•

The

turbines

are

installed at

or

below water

mid-depth,

away

f rom

the

most

severe

wave

action

zone

•

The

rotor

ha s

f ixed

blades

solidly

attached

to

th e shaft.

The

blades

have

no

pitch

adjusting

system

•

The b lade

tips

a re

l inked

by

a circular

ring

which

restricts

oscillations

in

the

rotor

plane

and out

o f

the

rotor

plane

directions

The

hydrodynamic

forces

on

the

structure

should be

minimized,

because an

oversize

increases

the

cost.

The overall volume

of the

structure

and

th e

size

of

100

0 0.5 1



Wasserbaukolloquium

2009:

Wasserkraft

im Zeichen

de s Klimawandels

Dresdener

Wasserbauliche

Mitteilungen

Heft

39

the

members are

reduced

as

fa r

as

possible.

This

has

a

twofold

benefit

on

th e

cost,

because

less

material is util ized

and

lower efforts have

to

be

resisted.

In

addition,

a

lighter

structure

requires

a

more

economical

installation

barge.

The

number and

th e

shape

o f

th e b la de s

are

selected

thanks

to a

hydrodynamic

calculation

model. The blade section is

an

el pse

with

a thickness

equal

to

15

o f the co rd .

In

order

to

avoid th e

onset

of

cavitation,

the

tip

speed

velocity

is

l imited

to

less

than 10

m/s.

The

f inal

design incorporates

6

blades.

The model makes

it

possible

to

draw the

theoretical

power

characteristics

o f the

turbine. A

typical

example

is shown

on

Figure

3 fo r

a

rotor

with

a

diameter

of

10

m.

Figure

3

Typical

relationship

between

water

velocity,

rotation

speed

and

rotor

power

The

current

speed

has a

very

important

influence

on

the

power,

as

could

be

expected

from the

equation

[1].

For a

given

water

velocity

there is

an

optimum

rotation

speed.

Beyond

th e

maximum,

the

power

curve

drops

to

zero

fo r

a

rotation

speed

which

corresponds

to

th e

free

running

condition,

when

no

power

is extracted

by

the

generator.

The foundation

design

must

be

selected

according

to

th e

soil characteristics.

There

is

no

one-fits-all

solution. On

most

sites

where

strong

current are

ob-

served,

th e soil is

often

composed

o f

hard

sand

and

gravel,

although moving

sand

dunes

are

reported.

In the

following,

a

gravity

base

structure

is

considered,

but it should

not

be

concluded

this choice is inherent

to

the

technology.

Other

types

of

foundations will

be

adopted

in

the future

if

this is

required

by

th e local

soil conditions.

101

500

-

450

400

O.Bm/s

350

6

300

1.2m/s

- -

1.Sm/s

250

-2.4mis

2

200

0

150

-4-3rn/s

al

100

Generator

50

0

0 5 10

15

20

Rotation

speed (rpm)



10 2

Tidal

Current

Energy

and the

Sabe

la Turbine

The

structure

includes

2

parts:

•

a base

which

rests

on

the seafloor

•

tile

turbine

itself

(rotor

and

nacelle)

attached

to a

support

f rame

The base

is held

on

th e

bottom

by

appropriate

weights

(gravity

base).

2 vertical

tubes

guide

th e

turbine

during

the

handling operations.

The

special shapes

of

th e

2

parts

o f

the

structure

match

together, allowing

an

easy

installation.

-

*-18

A

B

4.4 .1

971

l t l ll

-A,Vir..

r----7:

//a

44

8

J*

31

:46 :8**Af'r<

p<Ef .1 .'<Af;*:**

--a-

.

2.

t

/

Figure

4

Installation

sequence

of

a

Sabella

tidal turbine

No

divers

are

needed.

The

barge

crane

is

equipped

with

cameras

and

attitude

control

propellers

which

make

it

possible

fo r the

operator

on

board o f

th e vessel

to control

th e

operations.

In turbid

waters,

acoustic

devices

can

also

be used

to

facilitate the

approach.

Maintenance is

greatly simplified

and

can

be

performed

even when the

current

is

not

zero.

.Indeed,

th e

current

can

be

exploited

to

help

th e

operations.

./,I



Wasserbaukolioquium


Zeichen des

Klimawandels

Dresdener

Wasserbauliche

Mitteitungen

Hef t

39

5

The

Sabella

demonstration

project

Following

the

R&D

phase

described

in

the

above,

the

consortium

felt

th e

need

to

test

th e

concept

a t

sea.

Budgetary

reasons

led

to

the

decision

to make

f irst

a

demonstration unit. It

would

incorporate

all the

basic elements

of

the tidal

turbine,

bu t the

dimensions

would be scaled d o wn

in

order

to

l im it

the

expenses.

The

local

authorities

showed

a

strong

interest

fo r

the

idea

and offered

to

support

the Sabella

Project.

For

administrative

reasons,

th e

turbine

is

not

fitted with

a

power

cable

to

land.

The

energy

produced

is

consumed

by

resistors

installed

on

th e

machine.

The

objectives

of the

project

can

be summarized

as

follows:

•

Test of

the

overall

concept

(fixed

bidirectional

rotor,

support

structure,

installation and

handling

procedure)

•

Survey

th e

influence

o f th e

turbine

on

the

soil

stability

•

Observe th e

behavior

o f th e

fauna a round the

unit

•

Measure

th e

noise

emission

•

Evaluate

th e

acceptance

f rom

the other

users o f the

sea

and o f

the

public

•

Discover what

only

tests

at

sea

can

teach

The

demonstration uni t

has

the

following

characteristics:

•

Rotor

diameter

is

3

meters

•

The

blades

are in

glass

f iber

reinforced

plastic

with

a

steel

skeleton

The

generator

is

a

d irect d riv e

permanent

magnets

oil

filled

altemator. Its

nominal

power

is

10kW

although

it

was not

expected

to

obtain

the full

power

on

this site.

•

The

structure

is

made

of mild

steel.

•

Corrosion

protection

is

provided by coating

and

sacrificial zinc

anodes

103



Tidal Current

Energy

and

the

Sabella

Turbine

Figure

5

View

of

the Sabella turbine

The

lessons

learnt

during

th e

project

can

be

summarized as

follows:

•

The

concept

allows

th e

easy

installation

and removal.

•

The

vessels

were

not

fitted

with

dynamic

positioning

system.

It

was

made

c lear that it is

possible

to

util ize the

current.

The vessel

is

anchored

ahead

o f the

final

location and

allowed

to

drift

slightly

until the

right position

is

obtained.

Dynamic positioning

is therefore

not

absolutely

required,

al-

though

it

will

be

considered

in th e industrial

phase

to

reduce

the time

required

fo r tile

operations

and decrease the

maintenance

costs

when

many

turbines will

have

to

be serviced.

•

The rotation

speed

an d the

power

delivery

are

in

good agreement

with the

predicted

values. Accurate

comparison

was

hindered

by

the

difficulty

to

assess

precisely

the

instantaneous

water

velocity.

The noise

was

measured

during

a

spring

t ide

period.

The noise

generated by

the

turbine could

hardly

be

detected

among

th e

background

level.

104



Wasserbaukolloquium

2009:

Wasserkraft

im Zeichen des

Klimawandels

Dresdener

Wasserbautiche

Mitteilungen

Heft

39

'*.C-=, / '* /.fi.='.,-

.4/t

#lilwillillill 'llfilll lillill/6...

Aill

'

'•mill/:Il/, .1

-6--

4

-LI.

4.I-=

.-.-..

4.Irl

dj

4

14/I'll-

4 .*

AILIF

95..

\-

.---

..

-...

f.9/

.

-:

* Igg i

/-·

*6 I-

Figure

6 Fish

school around the Sabella

turbine

The turbine

was

regularly

inspected by

divers. Fish schools

were

systematically

observed

near

the installation.

The

rotor

which is

painted

with

an

anti-fouling coating

remained

clean

during

th e

whole

period.

O n

th e

static

parts

which

were not

protected against

fouling,

the marine

growth

is

noticeable

and

representative

of

the

local marine

life. The

fouling

is

not

regarded

as

a

problem

so

far.

6 Next

development phase

The

further

development

o f

the

technology

includes

th e

construction

and

test

at

sea

of

a

10

m

diam

eter

uni t . The 200

kW turbine will

be connected

to

th e

on-

shore

grid,

in

order

to

complete

th e

engineering

and the

testing

o f

the

electrical

part

of the t idal turbine.

Financial

support

The

Sabella

Project

is

th e

first

embodiment of th e

Marenergie project

supported

by

the P6le

Compdtitivitd

Mer in Brest. It

received

the f inancial

support

from

the

R6gion

Bretagne,

Conseil

Gantal

du

Finistate,

Brest

M6tropole

Ocdane,

Quimper

Communautd,

A D E ME .

The

partners

o f

the

project

thank

all

these

institutions.

105



106

Tida l Current

Energy

an d the Sa l elIa Turbine

References

[1]

Divers

aspects

de

1'exploitation

de

1'6nergie

des

courants

marins

Daviau

Majastre

Guana

Ru e r

Seatechweek

October

2004

Bres t

http://www.i

fremer.fi·/dtmsi/colloques/seatech04/mp/enerizies proceedin

e.htm

[2]

Design

and

Operational

Features

of

a

Tidal

Stream Turbine

Bischoff

Guana

Daviau

Majastre

Ruer

Tartivel

OW E ME S

2006

Civitavecchia

April

2006

Author

Dipl.

Ing.

Jacques

Ruer

SAIPEM

SA

1/7 Avenue

San Fernando

F-78884

Saint

Quentin

en

Yvelines

Tel : +33 1 61 37 87 53

Fax:

+33161378380

[email protected]

Documents

12 Tidal Current Energy and the Sabella Tur