Overview of Wave to Wire Modelling and Experimental Studies

Transcription

1 Overview of Wave to Wire Modelling and Experimental Studies Morten Kramer & Kim Nielsen 3 rd SDWED Symposium: Wave to Wire modelling Aalborg University, 3 June 2014

2 Energy production by wave energy converters / 10 Wave climate Hanstholm site. Wave probability [%] Wave period T 0,2 [s] Wave height H m0 [m] All All Wave period T 0,2 [s] * Wavestar. Predicted energy production at Hanstholm [MWh/year] Wave height H m0 [m] All All Power matrix Wavestar C float, 70 % PTO, Storm protection H m0 = 3.5 m. Electrical power [kw] Wave period T 0,2 [s] Wave height H m0 [m] Yearly production: 1650 MWh (Excluding: Periods out of operation due to maintenance or faults, and background consumption when in idle mode)

3 Wave to Wire (W2W) models Software can be downloaded from the web page: Detailed descriptions are available in: SDWED D4.10 Overview of Wave to Wire Models Extended abstract from current presentation 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

4 Examples of complete W2W codes Examples of complete W2W codes, note that the capabilities and features are very different: Commercial codes which are solving time domain hydrodynamics online Commercial codes based on pre processed frequency domain BEM hydrodynamics Geometry is defined using CAD drawings. Examples with Wavestar (left) and Dexa (right): Freeware Name of code DNV GL Sesam HydroD Wasim Compassis SeaFEM ANSYS DNV GL WaveDyn Do It Yourself with freeware tools Total price (*1000 ) Details HydroD+Wasim: NOK SeaFEM+GID USB: 7380 ANSYS Structural: ANSYS AQWA: Base module: 25000, BEM interface: 3000, WAMIT: $24000 Matlab: 2000 Simulink: 3000

5 Mathematical equation Traditionally 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 a hydrodynamic 1 st order code. The output are coefficients of: Hydrostatic stiffness coefficient Radiation coefficients: Added mass and damping Wave excitation force coefficient

6 Examples of hydrodynamic 1 st order codes Frequency domain codes Time domain code ANSYS OceanWave3D Name of code WAMIT Nemoh ShipBEM Aqwa SDWED2D Price for a single user license ~ 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) Computational time (1 = fast, 10 = slow) Note: Complexity to use and Computational time are subjectively estimated.

7 Example: Radiation by a heaving hemisphere Frequency domain BEM code where only the structure surface is discretised with elements Heave motion Added mass Damping Heave force, Time domain code where the whole fluid domain is discretised with elements OceanWave3D SDWED2D

8 Added mass A 33 /( V) Damping B 33 /( V) Example: Radiation by a heaving hemisphere Deep water, h/a (h is water depth, a is radius) WAMIT Nemoh ANSYS Aqwa ShipBEM OceanWave3D-SDWED2D Analytic by Hulme Wave number ka Shallow water h/a = a Wave number ka h

9 PTOs in experimental studies Realistic 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. Rotating system Torque transducer Controllable motor Translational system Force transducer Controllable linear actuator 02/06/20 14

10 Experimental studies Experimental 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. Small scale production tests Aalborg University Large scale extreme tests Plymouth University

11 Conclusions Model flexibility is important There 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 tools Software 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 initiatives New 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 future Usually 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 1 st order codes. 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.

12 The International Research Alliance Funded by