Modelling Ocean Waves and an Investigation of Ocean Wave Spectra for the Wave-to-Wire Model of Energy Harvesting

Ocean wave energy is an abundant and clean source of energy; however, its potential is largely untapped. Although the concept of energy harvesting from ocean waves is antiquated, the advances in wave energy conversion technologies are embryonic. In many major studies related to wave-to-wire technologies, ocean waves are considered to be regular waves with a fixed amplitude and frequency. However, the actual ocean waves are the sum of multiple frequencies that exhibit a particular sea state with a significant wave height and peak period. Therefore, in this paper, detailed modelling of the ocean waves is presented and different wave spectra are analyzed. The wave spectra will eventually be used for the generation of wave elevation time series. Those time series can be used for the wave-to-wire model-based studies for improved investigations into wave energy conversion mechanisms, mimicking the real ocean conditions.

Pipeline Model Fidelity for Wave Energy System Models

Ocean wave energy conversion plants that use hydraulic power take-offs (PTOs) have been configured so that the working fluid must travel a significant distance (of several hundred to a few thousand meters) from the wave energy converter (WEC) located offshore to equipment onshore. With the pulsatile flow generated by the WEC having a peak period in the range of 3 to 12 seconds, the wavelengths of the excited pressure waves approach the length of the pipelines themselves. By the standards for modeling pipelines presented in popular fluid power and related textbooks, the system models for these plants should include distributed parameter models of the pipeline dynamics that capture the pressure wave delay effects. This work tests the importance of pipeline model fidelity for wave energy conversion plants. Simulations have been conducted of a simple but representative hydraulic PTO for wave energy conversion and incorporate several common lumped and distributed parameter pipeline models for comparison. These results are used to show the degree to which model fidelity effects several design metrics that are especially useful in the preliminary design phase of system development. The pipeline models used include: 1) a short line model that includes lumped resistive effects only, 2) a medium line model that also includes lumped inertial and capacitive effects for a single pipeline segment, 3) a long line model that uses repeated, lumped parameter line segments to approximate the distributed parameters of a real pipeline, 4) a simple method of characteristics solution to the one-dimensional momentum and continuity equations assuming a fixed wave speed, and 5) a discrete free-gas cavity model augmenting the simple method of characteristic pipeline model. The results suggest a relaxed standard for modeling pipelines in the case of this type of system, in which case, the recommended model is easily implemented in variable time step solvers and CAD software such as Simscape Fluids and can be used within the WEC-Sim modeling framework developed by the National Renewable Energy Lab.

Niche Applications and Flexible Devices for Wave Energy Conversion: A Review

We review wave energy conversion technologies for niche applications, i.e., kilowatt-scale systems that allow for more agile design, faster deployment and easier operation than utility scale systems. The wave energy converters for niche markets analysed in this paper are classified into breakwater-integrated, hybrid, devices for special applications. We show that niche markets are emerging as a very vibrant landscape, with several such technologies having now achieved operational stage, and others undergoing full-scale sea trials. This review also includes flexible devices, which started as niche applications in the 1980s and are now close to commercial maturity. We discuss the strong potential of flexible devices in reducing costs and improving survivability and reliability of wave energy systems. Finally, we show that the use of WECs in niche applications is supporting the development of utility-scale projects by accumulating field experience, demonstrating success stories of grid integration and building confidence for stakeholders.