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Overview of Wave to Wire Modelling and Experimental Studies

Morten Kramer & Kim Nielsen

3rd SDWED Symposium: Wave‐to‐Wire modellingAalborg University, 3 June 2014

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Energy production by wave energy convertersWave climate Power matrix

∗24 ∗ 365 /

10 ⇒

2-3 3-4 4-5 5-6 6-7 7-8 8-9 All2.5 3.5 4.5 5.5 6.5 7.5 8.5

0.0 - 0.5 0.25 2.65 8.18 1.84 0.38 0.14 0.03 0.01 13.230.5 - 1.0 0.75 1.22 19.22 11.44 2.21 0.18 0.06 0.02 34.351.0 - 1.5 1.25 0.00 6.84 13.07 2.96 0.30 0.04 0.00 23.211.5 - 2.0 1.75 0.00 0.33 9.59 3.05 0.29 0.04 0.00 13.302.0 - 2.5 2.25 0.00 0.02 3.34 4.60 0.20 0.04 0.00 8.202.5 - 3.0 2.75 0.00 0.01 0.22 3.89 0.21 0.01 0.01 4.353.0 - 3.5 3.25 0.00 0.00 0.00 1.39 0.51 0.01 0.01 1.923.5 - 4.0 3.75 0.00 0.00 0.00 0.17 0.58 0.02 0.01 0.784.0 - 4.5 4.25 0.00 0.00 0.00 0.00 0.25 0.07 0.00 0.32

4.5 - 4.75 0.00 0.00 0.00 0.00 0.07 0.21 0.05 0.34All 3.87 34.60 39.50 18.65 2.73 0.53 0.11 100.00

Hanstholm site. Wave probability [%]

Wave heightH m0 [m]

Wave period T 0,2 [s]2-3 3-4 4-5 5-6 6-7 7-8 8-9

2.5 3.5 4.5 5.5 6.5 7.5 8.50.0 - 0.5 0.25 0 0 11 13 13 13 130.5 - 1.0 0.75 19 46 71 85 89 88 841.0 - 1.5 1.25 50 121 182 212 215 206 1921.5 - 2.0 1.75 95 231 339 381 374 350 3222.0 - 2.5 2.25 154 375 535 579 554 511 4662.5 - 3.0 2.75 228 552 600 600 600 600 6003.0 - 3.5 3.25 319 600 600 600 600 600 6003.5 - 4.0 3.75 0 0 0 0 0 0 04.0 - 4.5 4.25 0 0 0 0 0 0 0

4.5 - 4.75 0 0 0 0 0 0 0

Wavestar C6-600 20 float, 70 % PTO, Storm protection H m0 = 3.5 m. Electrical power [kW]

Wave heightH m0 [m]

Wave period T 0,2 [s]

2-3 3-4 4-5 5-6 6-7 7-8 8-9 All2.5 3.5 4.5 5.5 6.5 7.5 8.5

0.0 - 0.5 0.25 0 0 2 0 0 0 0 20.5 - 1.0 0.75 2 78 71 16 1 0 0 1691.0 - 1.5 1.25 0 72 209 55 6 1 0 3421.5 - 2.0 1.75 0 7 285 102 9 1 0 4042.0 - 2.5 2.25 0 1 156 233 10 2 0 4022.5 - 3.0 2.75 0 0 12 204 11 1 1 2293.0 - 3.5 3.25 0 0 0 73 27 1 1 1013.5 - 4.0 3.75 0 0 0 0 0 0 0 04.0 - 4.5 4.25 0 0 0 0 0 0 0 0

4.5 - 4.75 0 0 0 0 0 0 0 0All 2 158 734 684 64 5 1 1650

Wave heightH m0 [m]

Wave period T 0,2 [s]Wavestar. Predicted energy production at Hanstholm [MWh/year]

*

Yearly production: 1650 MWh

(Excluding: Periods out of operation due to maintenance or faults, and background 

consumption when in idle mode)

StructuralDesign of Wave

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Wave to Wire (W2W) models

A W2W model typically produces the following: • Time series of the power output • Time series of structure motions and mooring 

line forces• Fatigue loads on structural components which 

are exposed to high cyclic loading

Software can be downloaded from the web‐page: http://www.sdwed.civil.aau.dk/Software/

Detailed descriptions are available in:‐ SDWED D4.10 “Overview of Wave to Wire Models”‐ Extended abstract from current presentation

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Examples of complete W2W codesExamples of complete W2W codes, note that the capabilities and features are very different:

Geometry is defined using CAD drawings. Examples with Wavestar (left) and Dexa (right):

Commercial codes which are solving time domain hydrodynamics online

Commercial codes based on pre‐processed frequency domain BEM hydrodynamics Freeware

Name of code DNV GL Sesam HydroDWasim Compassis SeaFEM ANSYS DNV GL WaveDyn Do It Yourself with freeware tools

Total price (*1000 €) 15 7 91 47 5

Details HydroD+Wasim: NOK 118680 SeaFEM+GID USB: 7380€

ANSYS Structural: € 37400ANSYS AQWA: € 53900

Base module: £ 25000, BEM interface: £ 3000, WAMIT: $24000

Matlab: € 2000Simulink: € 3000

StructuralDesign of Wave

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Mathematical equationTraditionally models are based on superposing forces where the equation of

motion is based on Newton’s second law:

⇔

⇔

Normally all the hydrodynamic forces are linearized and pre-calculated by ahydrodynamic 1st order code. The output are coefficients of:

• Hydrostatic stiffness coefficient• Radiation coefficients: Added mass and damping• Wave excitation force coefficient

StructuralDesign of Wave

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Examples of hydrodynamic 1st order codesFrequency domain codes Time domain code

Name of code WAMIT  ANSYS Aqwa Nemoh ShipBEM OceanWave3D‐

SDWED2DPrice for a single user license ~€17000 €53900 Free N/A Free

Multi body (max structures) (inf) (50) (inf) (1) (‐)

Graphical interface ‐ ‐ ‐

Pre‐ and post processing plotting ‐ ‐ ‐ ‐

Generalized modes ‐ ‐ ‐

Irregular frequency removal ‐ ‐

Added mass at infinite frequency ‐ ‐ ‐ ‐

Symmetry x and y ‐ ‐ ‐

Infinite water depth ‐ ‐ ‐

Complexity to use (1 = easy, 10 = difficult) 7 3 6 5 9

Computational time (1 = fast, 10 = slow) 3 7 7 5 ‐

Note: “Complexity to use” and “Computational time” are subjectively estimated.

StructuralDesign of Wave

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Example: Radiation by a heaving hemisphere

Time domain code where the whole fluid domain is discretised with elements

OceanWave3D‐SDWED2D

Frequency domain BEM code where only the structure surface is discretised with elements

,

Added mass DampingHeave motion

Heave force

StructuralDesign of Wave

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0 2 4 6 8 100

0.2

0.4

0.6

0.8

1

Example: Radiation by a heaving hemisphereShallow water

h/a = 2

0 2 4 6 8 100

0.2

0.4

0.6

Wave number ka

0 2 4 6 8 100

0.2

0.4

0.6

0.8

1

Add

ed m

ass

A33

/( V

)

0 2 4 6 8 100

0.2

0.4

0.6

Wave number ka

Dam

ping

B33

/( V

)

Deep water, h/a → ∞(h is water depth, a is radius)

WAMITNemohANSYS AqwaShipBEMOceanWave3D-SDWED2DAnalytic by Hulme 1982

h

2a

StructuralDesign of Wave

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PTOs in experimental studiesRealistic PTOs cannot be scaled down for direct use in small‐scale model tests. A solution is to equip the device with a mechanism that imitates the behaviour of the real PTO. Two systems for WECs which have been developed and tested in the wave tank facilities at Aalborg University are shown below.

02/06/2014

Torque transducer

Controllable motor

Rotating system Translational system

Force transducer

Controllable linear actuator

StructuralDesign of Wave

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Experimental studiesExperimental testing in wave tanks of small‐scale WECs can provide valuable knowledge about the hydrodynamic behaviour of the device. The Froude model law provides good accuracy for scaling the waves, forces and motions up to real scale.

Large scale extreme testsPlymouth University

Small scale production testsAalborg University

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Conclusions• Model flexibility is importantThere are many wave energy converters under development of different geometry, using different operating principles, PTO’s and mooring systems – and thus a W2W tools has to be very flexible, or composed of blocks with focus on part systems such as Power Take Off design, mooring design, structural design, or array interaction effects. 

• Free VS Commercial toolsSoftware tools to assist in modelling WECs are commercially available, but such software is rather expensive and cannot be modified by the user for specific needs. Case studies in the SDWED project on different WEC systems, have demonstrated how “low‐cost” numerical modelling and testing can be completed depending on device configuration and purpose. 

• New/upcoming initiativesNew initiatives promise open‐source codes to become available sometime in the future, for example the U.S. Department of Energy has initiated the so‐called “Water Power Program” which purpose is to develop open‐source software to simulate the generated electric power of different wave energy converter designs.

• Coupled models might be the new standard in the futureUsually park effects are evaluated in a simplified manner. As pointed out by Stratigaki (2014), recent research explore benefits in combining wave propagation codes based on the mild‐slope equations and traditional potential 1st order codes.

02‐06‐2014

Figure by Stratigaki (2014)Stratigaki, V. (2014). Experimental Study and Numerical Modelling of Intra‐Array Interactions and Extra‐Array Effects of Wave Energy Converter Arrays. Ph.D. thesis, Department of Civil Engineering, Ghent University.

Funded by

The International Research Alliance