Dating to a crisis in the price of coal in the late 19th century, ocean waves have been an oft-examined but unrealized source of electrical energy. It is a source of energy estimated to be able to satisfy a tenth to a quarter of the World?s electrical energy needs. More recent developments in the late 20th century have inspired a new generation of wave energy converters (WECs). When ocean waves interact with these devices they produce a motion between two moving parts of the WEC. This relative motion is then use to generate electricity. One of the features to optimize in the design of a WEC is the amplitude of the relative motion which in turn will maximize the harvested energy.
The purpose of the proposed work is to construct, model, analyze, demonstrate, and improve upon a novel excitation system for WECs. This system is based on the intake and release of water during the motion of the WEC. The timing of the intake and release is governed by the motion of the WEC features an open loop control, and has the potential to increase the electrical power generated by the WEC. The model for the WEC can be considered as a parametrically excited system or a hybrid system. Incorporating the effects of the stochastic ocean wave environment and the fluid-structure interaction are non-trivial. To this end, a team of featuring analysts, mechanicians, and experimentalists from the University of California at Berkeley, the University of Illinois at Urbana-Champaign, and the United States Naval Academy will conduct the proposed research. The research features construction and testing of a scaled prototype and has several educational aspects. It is hoped that the research on the novel excitation system will result in more efficient and cost-effective methods to harvest ocean wave energy.
Ocean wave energy is a large source of renewable energy that has remained mostly untapped. In the past three decades, several devices have been proposed to harvest this energy source. Among the designs, buoy devices featuring two or more connected rigid bodies that oscillate with the incident waves have been popular. These wave energy converters (WECs) harvest ocean wave energy by converting the relative oscillatory motion of the rigid bodies into electrical energy. Given the harsh ocean environment and challenging operating conditions, any feature that can possibly improve the harvesting capabilities of a WEC is worthy of consideration. The research conducted during this NSF funded work examined the development and testing of new mass-modulation schemes that were designed to improve the energy harvesting capabilities of a WEC. The schemes take advantage of the ambient water by enabling a component of the WEC to either enclose a volume of water during its motion or change the so-called hydrodynamic added mass of a component of the WEC by actively changing its geometry. The change in geometry can be realized in a number of manners and the one considered in the research work featured flap mechanisms. It is not immediately obvious that these mass-modulation schemes will work. To explore their feasibility a set of numerical models were first developed and examined. The results from these models enabled the researchers to see which mass-modulation schemes were optimal. Some aspects of modeling and analyzing the mass-modulation scheme lie at the limits of current numerical codes. As a result, prototype testing was paramount and three mass-modulation schemes were considered by building and wave-tank testing a scaled prototype. Eventually, a scheme which proved to be optimal based on our work with numerical models also proved to be optimal in testing. Modeling the fluid-structure interaction in a WEC has multiple challenging aspects. First, some of the models we use are known as hybrid dynamical systems and featuring switching from one model to another. This switching creates complex dynamics. A second aspect of particular interest were the memory effects due to the fluid-structure interaction. These effects are modeled as a functional differential equation (FDE). Due to the fact that ocean structures experience random inputs, a careful study of FDE with noisy forcing has also been considered in this project. FDE are infinite dimensional systems and proper investigation of these equations requires one to cast these in an appropriate functional analytic setting. The major goal of this aspect of the project was to develop systematic scheme to reduce the infinite dimensional system to a lower dimensional system in the presence of noise. We also investigated the almost sure stability of linearized equations in the presence of parametric noise. The research was conducted by three research groups: Prof. Carolyn Judge at the US Naval Academy in Annapolis, Prof. N. Sri Namachchivaya at the University of Illinois at Urbana Champaign, and Prof. Oliver O'Reilly and Omer Savas at the University of California at Berkeley. The research was presented at seminars and conferences in the United States, Europe, and Asia, and disseminated in a series of 8 published journal articles with two more being considered for publication. One student at Berkeley completed his Ph.D., another completed his M.S., and a student at UIUC is currently completing their Ph.D. Portions of the research also involved a total of 5 undergraduate students at UCB. Three of these undergraduates are currently enrolled in graduate programs: two at UCB and another at UCLA.
