Marine Energy Making Waves on Both Sides of the Pond

WEC-Sim

Last year, the U.S. Department of Energy (DOE) funded NREL and SANDIA to work on a three-year collaborative project to develop a simulation tool for the wave energy sector.  A little over a year into the project, the team has already released the first version of the Wave Energy Converter Simulator (WEC-Sim), a customisable open-source numerical modeling tool designed to help the wave energy community to analyse and optimise wave-energy converters (WECs) and meet device-specific modeling needs.

WEC-Sim initiative/tool

As Kelley Ruehl, Technical Staff Member at the Water Power Technologies Department at Sandia National Laboratories, and one of the Principal WEC-Sim Developers, explains, the tool is capable of modeling wave energy devices that are comprised of «rigid bodies, power-take-off systems, and mooring systems» with simulations «performed in the time-domain by solving the governing WEC equations of motion in 6 degrees-of-freedom.»

«The WEC industry relies heavily on numerical modeling tools during the device design and optimisation process. The existing WEC modeling tools are closed-source and cannot be customised to meet device specific modeling needs. WEC-Sim provides the WEC community with an open-source tool that will allow the industry to develop new and innovative WEC devices,» she says.

Programming Code

The code that underpins the tool is developed with a combination of the MATLAB (Matrix Laboratory) programming language and the Simulink data flow graphical programming language tool — using a «multi-body dynamics solver» known as SimMechanics.  According to Ruehl, the code solves the «governing WEC equations of motion in 6 degrees-of-freedom» using what is known as the Cummins impulse response formulation — the equation of motion most commonly used to model the dynamics of a body in water, including ships and WECs.

Throughout 2014, NREL and SANDIA have worked to develop the first version (v1.0) of the code and have tested its capabilities through loose collaboration with several U.S. industry members. Ruehl also reveals that, in 2015, the WEC-Sim team plans to work with several device developers to demonstrate and verify the performance of the tool — and will participate in «an international code-to-code comparison and validation effort.»

«In order to ensure WEC-Sim is meeting the needs of industry, the team has reached out to several US industry members to determine features for future development of the WEC-Sim code.  Based on this feedback, and the WEC-Sim Questionnaire, the WEC-Sim team has focused its code development,» she says.

Open-Source

For Ruehl, the key advantage of the WEC-Sim tool in the development of innovative wave energy technologies is the fact that the code is open source and allows users to «modify the code to meet their specific modeling needs.»  In her view, this feature is critical for the WEC industry because the «wide range of existing technologies’ makes it ‘difficult to create a code capable of modeling them all.»

«These challenges will be overcome by opening development of future WEC-Sim features using the public development platform GitHub in order to accommodate the diversity of existing devices,» she adds.

Looking ahead, the SANDIA-NREL team plans to continue the development of the WEC-Sim code using user feedback provided via direct interaction and an online questionnaire.  Ruehl says that the team will also perform experimental wave tank texts for WEC-Sim code validation in 2015.

«In 2015, we are also considering the advantages of moving to a completely open-source coding language.  Although WEC-Sim is open source, it is implemented in the MATLAB environment and we are considering moving the code to a Python-based environment.  This will [provide] researchers who do not have access to MATLAB [with] the ability to use the WEC-Sim tool,» she adds.

Open-WARP Challenge

In recognition of the central role that such numerical tools will continue to play in the WEC design and analysis process, the DOE has also launched the Open Wave Analysis and Response Program (Open-WARP) Challenge to further improve the WEC-Sim tool.

As Alison Labonte, Marine and Hydrokinetic Technology Manager at the DOE explains, one «critical piece» of WEC-Sim is the boundary element method (BEM) module that «provides hydrodynamic coefficients that are needed for time-domain WEC-Sim simulations.»

«The objective of the Open-WARP project is to create a boundary element method module.  To date, several competitions have been completed.  The objectives of these competitions have been to create a mesh generation capability and a Graphical User Interface (GUI).  Upcoming competitions will be to improve existing open-source BEM codes so they meet the requirements of WEC-Sim,» she adds.

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