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Challenges in the hydrodynamics
modelling of wave energy converters
Vincenzo Nava
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Image from the website of OPERA H2020 project,
courtesy of EVE
Screenshot from Compass
Youtube Video
Outline
Motivation
Top Five Challenges in Modelling the Hydrodynamics of WECs
2 ▌
Four Lessons Learnt
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Why Harnessing Energy From Oceans?
3 ▌
Credits the data originate from the ECMWF WAM model archive and are calibrated and corrected by Fugro OCEANOR against a global buoy and Topex satellite altimeter database).
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
How can we get Energy From Oceans?
4 ▌
Souce(s) Figure inspired from Peñalba, M., Ringwood, J. V., A Review of Wave-to-Wire Models for Wave Energy Converters, Energies 2016, 9(7), 506; doi:10.3390/en9070506
WAVE RESOURCE
ELECTRIC GENERATOR
WAVE ENERGY
CONVERTER
POWER TAKE OFF
POWER CONVERSION
GRID
ABSORPTION
FPTO
x, x , x T
ω
i
V
Pout
TRANSMISSION GENERATION CONDITIONING
Oscillating Water Column (OWC)
Overtopping Devices
Oscillating Bodies
(Point Absorbers & Surge Devices)
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
A more precise modelling of the hydrodynamics will improve:
Why numerical models are needed to simulate the FSI?
5 ▌
POWER PRODUCTION
FORCES, DISPLACEMENTS,
STRESSES AND DEFORMATIONS
OVERTOPPING
OPTIMISATION OF SHAPES, LOSSES,
ETC…
WAVE PROPAGATION
ENVIRONMENTAL IMPACTS
ECONOMIC ASSESSMENTS
SURVIVABILITY
IMPACTS ON THE
ENVIROMENT
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Top FIVE Challenges 1.
2.
3.
4.
5. WAVE PROPAGATION
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Reduction of Energy – Using TMA spectra
7 ▌
Source A. Sinha, D. Karmakar, and C. Guedes Soares, “Shallow water effects on wave energy converters with hydraulic power take-off system,” Int. J. Ocean Clim. Syst., vol. 7, no. 3, pp. 108–117, 2016.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Modeling devices with Mike 21
8 ▌
Source E. Angelelli and B. Zanuttigh, “A farm of wave activated bodies for coastal protection purposes,” Coast. Eng. Proc., vol. 1, no. 33, p. 68, 2012. M. Folley et al., “A review of numerical modelling of wave energy converter arrays,” in ASME 2012 31st International Conference on Ocean, Offshore and Arctic Engineering, 2012, pp. 535–545.
“Boussinesq models are not capable of modelling the hydrodynamics of a moving device. They may be used to model device characteristics, such as wave transmission reflection and absorption. If radiation characteristics are known, these may be included by use of an internal generation line, although this may become cumbersome when more than 1 WEC is considered and, for this reason, their use warrants caution.”
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Top FIVE Challenges 1.
2.
3.
4. VALIDATION OF MODELS
5. WAVE PROPAGATION
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
How validate and verify models?
10 ▌
• Code-to-code verification EXAMPLE: Task 10 – OES – Methodology Modeling case studies, as for example a heaving sphere (decay tests, regular
waves tests, irregular wave tests) Review the models o Model a WEC o Compare with existing test data
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
How validate and verify models?
11 ▌
• Experimental Data
Source Percher, A., Kofoed, J.P., Handbook of Ocean Wave Energy, 2016, Springer
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
12 ▌
Performance of WECs will normally be scaled using Froude similitude. Tests with large scales are recommended when scale effects are envisaged. Important factors in energy conversion that are not addressed by standard scaling procedures include, but are not limited to, the effects listed below: • The power output of devices utilising a pneumatic power take off is related to the
compressibility of the air, which is dictated by atmospheric pressure and the absolute temperature of atmosphere. Therefore, the stiffness of the air “spring” will not be scaled correctly using Froude similarity if geometric similarity is maintained.
• In small - scale model tests, viscous damping and in particular damping associated with vortex shedding from sharp edges cannot be scaled appropriately with Froude similarity and may be overestimated.
Source ITTC – Recommended Guidelines, Wave Energy Converter Model Test Experiments, 2014 Weber, J.,2007,“Representation of non-linear aero-thermodynamic effects during small scale physical modelling of OWC WECs”, Proceedings,7th European Wave and Tidal Energy Conference(EWTEC 2007), Porto,Portugal
Importance of scale – Scaling Law
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Importance of scale – Instrumentation and Indirect Measurements
13 ▌
Source C. Ruzzo, G. Failla, M. Collu, V. Nava, V. Fiamma, and F. Arena, “Operational Modal Analysis of a Spar-Type Floating Platform Using Frequency Domain Decomposition Method,” Energies, vol. 9, no. 11, p. 870, Oct. 2016.
Measurements of: • Displacements / Velocities / Accelerations • Line Tensions • Wave Characteristics • Overtopping
Use of indirect measurements, especially with data in sea. Development of algorithms for indirect measurements, also from other sectors. For example, FDD for modal characteristics (from the Structural health monitoring Sector).
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Top FIVE Challenges 1.
2.
3. MODELING ARRAY EFFECTS
4. VALIDATION OF MODELS
5. WAVE PROPAGATION
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Why arrays?
15 ▌
Deploying arrays of
ocean energy devices
would increase the
profitability of the
investment
Image courtesy of
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
How to model array effects?
16 ▌
Source H. Day et al., “Hydrodynamic modelling of marine renewable energy devices: A state of the art review,” Ocean Eng., vol. 108, pp. 46–69, Nov. 2015. M. Folley et al., “A review of numerical modelling of wave energy converter arrays,” in ASME 2012 31st International Conference on Ocean, Offshore and Arctic Engineering, 2012, pp. 535–545.
•Point-absorber method (diameters smaller than wavelenghts and interdistances - diffraction of radiated and diffracted waves from each device by others in the array is neglected)
•Plane-wave method (spacing between axisymmetric absorbers is large compared to the incident wavelength – diffraction is taken into account)
•Multiple Scattering Method (radiation and diffraction taken into account as succession of distinct scattering events)
•Direct Matrix Method (the amplitudes of all scattered waves simultaneously without the need for iteration, using a single body solution)
SPH
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Some results
17 ▌
Personal Note: when fully designing an array of heaving point absorbers, most probably the layout would depend on other subsystems rather than on the hydrodynamic impacts
Source Peñalba, M., Touzon, I., Lopez Mendia, J., Nava, V., A numerical study on the hydrodynamic impact of device slenderness and array size in wave energy farms in realistic wave climates, 2017, under review
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Some results
18 ▌
Source E. Renzi, A. Abdolali, G. Bellotti, and F. Dias, “Wave-power absorption from a finite array of oscillating wave surge converters,” Renew. Energy, vol. 63, pp. 55–68, Mar. 2014.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Development of Tools
19 ▌
DTOcean is the acronym for an European FP7
project and it stands for
Optimal Design Tools for Ocean Energy Arrays
Hydrodynamics modeled using semi-analytical methods
BEM NEMOH + DMM
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Top FIVE Challenges 1.
2. VISCOUS EFFECTS AND EXTREME LOADS
3. MODELING ARRAY EFFECTS
4. VALIDATION OF MODELS
5. WAVE PROPAGATION
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Sloshing
21 ▌
Source Y. Zhang, Q.-P. Zou, and D. Greaves, “Air–water two-phase flow modelling of hydrodynamic performance of an oscillating water column device,” Renew. Energy, vol. 41, pp. 159–170, May 2012.
It is difficult to extract energy from sloshing frequency, so they should be avoided.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Vortex Formation
22 ▌
Source Y. Zhang, Q.-P. Zou, and D. Greaves, “Air–water two-phase flow modelling of hydrodynamic performance of an oscillating water column device,” Renew. Energy, vol. 41, pp. 159–170, May 2012.
The formation of the vortex in front of and behind the front wall of the air chamber and evolution of the air vortex can be seen clearly which is a good indicator of energy loss.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Vortex Induced Vibration / Motions
23 ▌
Source Z.-S. Chen and W.-J. Kim, “Numerical investigation of vortex shedding and vortex-induced vibration for flexible riser models,” Int. J. Nav. Archit. Ocean Eng., vol. 2, no. 2, pp. 112–118, Jun. 2010.
If the structure is flexible or flexibly mounted, these vortices may cause vibrations, leading to stresses and fatigue damage. This motion of the body influences, in turn, the vortex formation process, establishing a feedback mechanism that may lead to stable or unstable dynamic equilibria. On the hull: - They increase the global loads - They may increase the offset (drag effect)
On the moorign lines: - They may affect fatigue life of the line
The disagreement between numerical and experimental observations attributes much to the underestimation of multimodal vibration state by numerical method.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Vortex Induced Vibration / Motions
24 ▌
Source M. S. Triantafyllou, R. Bourguet, J. Dahl, and Y. Modarres-Sadeghi, “Vortex-Induced Vibrations,” in Springer Handbook of Ocean Engineering, Springer, 2016, pp. 819–850.
On the hull: - They increase the global loads - They may increase the offset (drag effect)
On the moorign lines: - They may affect fatigue life of the line
Higher harmonic components of strain and acceleration are significant. Fatigue damage calculations, by taking into account the higher harmonic components, give fatigue damage values of up to one order of magnitude larger than those calculated based on only first-harmonic signals.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Extreme Wave Loads
25 ▌
Source J. Westphalen, “Extreme wave loading on offshore wave energy devices using cfd,” 2011.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Top FIVE Challenges 1. NONLINEARITIES AND COUPLING
2. VISCOUS EFFECTS AND EXTREME LOADS
3. MODELING ARRAY EFFECTS
4. VALIDATION OF MODELS
5. WAVE PROPAGATION
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Mooring Lines Effects
27 ▌
Source S.-H. Yang, J. W. Ringsberg, E. Johnson, Z. Hu, and J. Palm, “A comparison of coupled and de-coupled simulation procedures for the fatigue analysis of wave energy converter mooring lines,” Ocean Eng., vol. 117, pp. 332–345, May 2016.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Parametric Roll – Mathieu/Hill Instability
28 ▌
Source K. Tarrant and C. Meskell, “Investigation on parametrically excited motions of point absorbers in regular waves,” Ocean Eng., vol. 111, pp. 67–81, Jan. 2016.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Parametric Roll – Mathieu/Hill Instability
29 ▌
Source H. Mao and H. Yang, “Parametric pitch instability investigation of Deep Draft Semi-submersible platform in irregular waves,” Int. J. Nav. Archit. Ocean Eng., vol. 8, no. 1, pp. 13–21, Jan. 2016.
The phenomenon happens in real seas: which are numerical tools to learn more about its occurrence?
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Lessons Learnt 1) Describing the problem properly
and identifying the outcomes
Problem statement stage is important, in order to focus on the right physics to study.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Lessons Learnt 2) Importance of using the appropriate
numerical model for solving the
problem
Risks: a. Unneeded
computational burden
b. Not answering to the questions
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Lessons Learnt 3) Interpreting the results of the
experimental / numerical study in
order to “bark at the right tree”
The results of a good experimental campaign or numerical test can be jeopardised by a wrong understanding of what’s going on.
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017
Lessons Learnt 4) All the problems are important, but
some problems are more important
than others
Developers have identified different priorities for the problems to be solved through their experience. Moreover, sometimes we look at a problem from a wrong perspective.
THANKS FOR YOUR ATTENTION
Vincenzo Nava
BCAM Workshop Hydrodynamics of wave energy converters
Bilbao, 3-7 April 2017