G3 - Sessione Speciale sullo sfruttamento dell'Energie Rinnovabili Marine

Quartiere Fieristico di Ferrara, 23 Settembre 2016

SFRUTTAMENTO DELL'ENERGIA DEL MOTO ONDOSO

MEDIANTE UN DISPOSITIVO A OSCILLAZIONE DI

COLONNA D'ACQUA

Irene Simonetti

Ilaria Crema

Lorenzo CappiettiUniversità degli Studi di Firenze

Framework and aims

OWC

parametric

optimization

Evaluation of different OWC chamber shapes and

types for site-specific hydraulic efficiency

maximization.

PTO – OWC chamber interaction:

combined optimization is needed

Provide indications about the

optimal turbine damping for

specific OWC geometries

(flow-pressure characteristics)

(2013, Vannucchi)

1/11

H= 2 - 3 mT = 7 - 8 s

Adopted modelling approaches:

Analytical

Physical model tests

CFD simulations

OWC SPECIFIC ISSUES

Relevant non linear effects (extreme waves, PTO)

Turbulent flow/vortex on the OWC front lip

Development of a virtual

wave flume to model all

relevant phenomena for

OWCs

Physical

model test

CFD model validation

Measure

ment

range f

or

instr

um

ent

set up

Simplified rigid

piston

frequency

domain model

Optimization of

OWC geometry

and turbine

damping

CFD model of

the OWC

(OpenFOAM)

Methodology

2/11

Analytical Model

m: water column mass

d: chamber draught

D: back wall length

H: wave height

L: wave length

z: OWC surface elevation

Linear wave theory, complete incident

wave reflection, linear turbine, Isentropic

compression and decompression

e hystat rad Pf f f f m z

excitation force

hydrostatic force

radiation force

air pressure effect

Preliminary selection of relevant

design parameters

Measurment range selection for

instrument set up of the physical tests

3/11

RIGID PISTON MODEL

(Evans, 1982, Sarmento & Falcão, 1985)

The physical modelThe physical model is carried out according to FROUDE SIMILARITY

with a SCALE FACTOR: 1/50

MODERATE WAVE

CLIMATE:

highest annual energy

in Mediterranean sea

H= 2 - 3 m

T = 6 - 8 s

STUDIED PARAMETERS:

Front wall draught (D)

Chamber thickness (W)

Turbine damping (V)

Incident wave period (T)

Incident wave height (H)

LABIMA www.labima.unifi.it

4/11

ULTRASONIC

WAVE PROBE

HOT WIRE

ANEMOMETER

PRESSURE

TRASDUCER

The physical model

50 52 54 56 58 60 62-4

0

4

in

c [cm

]

50 52 54 56 58 60 62-4

0

4

O

WC [

cm]

50 52 54 56 58 60 62-2

0

2

p [

mB

ar]

50 52 54 56 58 60 62

-10

0

10

Time [s]

Um

ax [

m/s

]

5/11

CONVERSION EFFICIENCY

The physical model

Starting point

Most efficient

geometries from

laboratory experiments

Parameter study with

the CFD numerical

model

SEQUENTIAL OPTIMIZATION

1( ) ( )

0

testT

Powc Q t P t dtTtest

21 21

16 sinh(2 )wave

khP gH

k kh

Powc

P Bwave

6/11

27 DIFFERENT GEOMETRIES

Incompressible Navier-Stokes equations

for a single Eulerian fluid mixture of two-

phases (air-water) (interFoam)

Volume of Fluid (VOF) surface tracking

Wave generation with waves2Foam

Large Eddy Simulation (LES)

PIMPLE algorithm for pressure - velocity

coupling

Unstructured mesh, with refinements:

- free surface zone (H/cells ~ 6, L/cells ~80 )

- around the OWC structure (D/cells ~ 40)

- around the pipe (d/cells > 16)

Symmetry plane

Near pipe refinement

CFD model in OpenFOAM

7/11

CFD model Validation

8/11

Average NRMSE < 10% on all

the selected benchmark

parameters in all the considered

OWC configurations

Water Level

Air chamber pressure

Air velocity

VALIDATION WITH

EXPERIMENTAL

DATA

Different geometries

Different damping condition

Different incident waves

ηOWC Pair Uy

NRMSEAver. 7,3% 8,1% 7,8%

Max 13,2% 13,5% 12,3%

Corr.

Coeff.

Aver. 0,97 0,96 0,97

Min 0,88 0,85 0,92

Impulse

turbineQuadratic flow

pressure relation

PK

Q

Turbine damping close to the

optimal OWC chamber damping

MAXIMUM

EFFICIENCY

9 values of damping by using

orifices with different

diameter V

AIRFLOW-PRESSURE RELATIONS FOR THE CHAMBER

CFD parameter study

9/11

CFD parameter study

1( ) ( )

0

testT

Powc Q t P t dtTtest

21 2

116 sinh(2 )

wavekh

P gHk kh

DEVICE CONVERSION EFFICIENCY

Powc

P Bwave

SAME OWC GEOMETRY –DIFFERENT WAVES

highest experimental ε

effect of D

effect of Woptimal damping non linear function of kh

water depth

wave number

SAME WAVE – DIFFERENT OWC

GEOMETRIES

10/11

Conclusions

Model validation results relatively well

in agreement with experimental data

CFD MODEL CAN BE USED AS A VIRTUAL

LABORATORY

CFD model of the OWC virtual wave tank

Parameter study

Maximum efficiency of about 85%.

Efficiency strongly affected by the OWC

geometry (draught D, chamber width W,

damping K).

For waves with H=2m and T=7s HIGH

EFFICIENCY WITH RELATIVELY SMALL FRONT

WALL DRAUGHT

Laboratory tests Preliminary selection of the optimal

geometry and validation of the CFD

model

11/11

THANKS FOR YOUR

ATTENTION

Irene Simonettiirene.simonetti@dicea.unifi.it