G3 - Sessione Speciale sullo sfruttamento dell'energie Rinnovabili Marine Quartiere Fieristico di Ferrara, 23 Settembre 2016

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1 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 Università degli Studi di Firenze Irene Simonetti Ilaria Crema Lorenzo Cappietti

2 1/11 Framework and aims H= 2-3 m T = 7-8 s OWC parametric optimization PTO OWC chamber interaction: combined optimization is needed (2013, Vannucchi) Evaluation of different OWC chamber shapes and types for site-specific hydraulic efficiency maximization. Provide indications about the optimal turbine damping for specific OWC geometries (flow-pressure characteristics)

3 Measurement range for instrument set up 2/11 Adopted modelling approaches: Analytical Physical model tests CFD simulations Methodology Simplified rigid piston frequency domain model Optimization of OWC geometry and turbine damping Development of a virtual wave flume to model all relevant phenomena for OWCs Physical model test CFD model validation CFD model of the OWC (OpenFOAM) OWC SPECIFIC ISSUES Relevant non linear effects (extreme waves, PTO) Turbulent flow/vortex on the OWC front lip

4 RIGID PISTON MODEL (Evans, 1982, Sarmento & Falcão, 1985) Analytical Model 3/11 Linear wave theory, complete incident wave reflection, linear turbine, Isentropic compression and decompression m: water column mass d: chamber draught D: back wall length H: wave height L: wave length z: OWC surface elevation excitation Preliminary force selection radiation of force relevant design fe parameters fhystat frad fp m z Measurment range selection for hydrostatic force air pressure effect instrument set up of the physical tests

5 4/11 The physical model The physical model is carried out according to FROUDE SIMILARITY with a SCALE FACTOR: 1/50 LABIMA STUDIED PARAMETERS: Front wall draught (D) Chamber thickness (W) Turbine damping (V) Incident wave period (T) Incident wave height (H) MODERATE WAVE CLIMATE: highest annual energy in Mediterranean sea H= 2-3 m T = 6-8 s

6 The physical model 4 ULTRASONIC WAVE PROBE U max [m/s] p [mbar] 5/11 HOT WIRE ANEMOMETER PRESSURE TRASDUCER inc [cm] OWC [cm] Time [s]

7 6/11 The physical model 27 DIFFERENT GEOMETRIES Powc 1 T test T test 0 Q( t) P( t) dt CONVERSION EFFICIENCY P owc Pwave B 1 2 2kh Pwave gh 1 16 k sinh(2 kh ) SEQUENTIAL OPTIMIZATION Starting point Most efficient geometries from laboratory experiments Parameter study with the CFD numerical model

8 7/11 CFD model in OpenFOAM Incompressible Navier-Stokes equations for a single Eulerian fluid mixture of twophases (air-water) (interfoam) Volume of Fluid (VOF) surface tracking Wave generation with waves2foam Near pipe refinement 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

9 8/11 CFD model Validation Water Level Air chamber pressure Air velocity VALIDATION WITH EXPERIMENTAL DATA Different geometries Different damping condition Different incident waves NRMSE Corr. Coeff. η OWC P air Uy Aver. 7,3% 8,1% 7,8% Max 13,2% 13,5% 12,3% Aver. 0,97 0,96 0,97 Min 0,88 0,85 0,92 Average NRMSE < 10% on all the selected benchmark parameters in all the considered OWC configurations

10 9/11 CFD parameter study Impulse turbine Quadratic flow pressure relation P Q K MAXIMUM EFFICIENCY Turbine damping close to the optimal OWC chamber damping 9 values of damping by using orifices with different diameter V AIRFLOW-PRESSURE RELATIONS FOR THE CHAMBER

11 10/11 CFD parameter study Powc 1 T test T test 0 Q( t) P( t) dt DEVICE CONVERSION EFFICIENCY P owc Pwave B 1 2 2kh Pwave gh 1 16 k sinh(2 kh ) effect of W effect of D optimal damping non linear function of kh highest experimental ε wave number water depth SAME WAVE DIFFERENT OWC GEOMETRIES SAME OWC GEOMETRY DIFFERENT WAVES

12 11/11 Conclusions Laboratory tests CFD model of the OWC virtual wave tank Preliminary selection of the optimal geometry and validation of the CFD model Model validation results relatively well in agreement with experimental data CFD MODEL CAN BE USED AS A VIRTUAL LABORATORY Maximum efficiency of about 85%. Parameter study 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

13 THANKS FOR YOUR ATTENTION Irene Simonetti