Preliminary Verification and Validation of Wec-sim, an Open-source Wave Energy Converter Design Tool

To promote and support the wave energy industry, a wave energy converter (WEC) design tool, WEC-Sim, is being developed by Sandia National Laboratories and the National Renewable Energy Laboratory. In this paper, the WEC-Sim code is used to model a point absorber WEC designed by the U.S. Department of Energy’s reference model project. Preliminary verification was performed by comparing results of the WEC-Sim simulation through a code-to-code comparison, utilizing the commercial codes ANSYS-AQWA, WaveDyn, and OrcaFlex. A preliminary validation of the code was also performed by comparing WEC-Sim simulation results to experimental wave tank tests. INTRODUCTION Even though wave energy converters (WECs) have been conceptualized and patented for over a century, most WEC developers are in technology readiness levels (TRLs) 3 through 5, corresponding to basic research and development [1-3]. Several developers have deployed scaled devices in the open ocean, and there have been a few full-scale grid connected deployments, such as the WaveGen Limpet in Islay and the Ocean Power Technologies (OPT) PowerBuoy in Hawaii. The industry, however, has yet to reach commercial viability, corresponding to TRL 9. As a result, WECs remain a nascent technology, and are highly dependent on numerical modeling and experimental testing to develop innovative designs and advanced control strategies. For numerical modeling, wave energy developers currently depend on commercially available codes or codes developed in house to model their devices. Such software is both a financial burden on and a large time commitment for WEC developers. In addition, the codes are often limited in their ability to model the breadth of existing WEC designs. To reduce these burdens, the U.S. Department of Energy (DOE) has identified opensource code development as an area to advance WEC technology development. To meet this need, DOE has directed the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL) to develop WEC-Sim, a publicly available, low-cost, open-source WEC modeling tool that is capable of running on a standard personal computer. WEC-Sim is developed in a modular structure that enables users to easily modify the code to meet their specific modeling needs. In this way, users can model a variety of WEC archetypes. WEC-Sim’s code development is a multiyear effort, complete with verification and validation. This paper covers the initial results of verification and validation. There are many different numerical approaches to model WECs, such as frequency domain boundary element method (BEM) models, time-domain equations of motion solvers, Morrison equation solvers, and spectral models. The numerical approach taken is highly dependent on the purpose of the model. To understand potential environmental impacts of WECs, spectral codes such as SWAN, TOMAWAC, and MIKE21 are used to evaluate possible impacts of wave farms on the nearand far-field wave environment [4-6]. Spectral models account for bathymetry, and are typically best at