OCEAN WAVE ENERGY CONVERSION CONCEPTS Dr. Michael E. McCormick zyxwv U. zyxwvutsrqp S. Naval Academy Annapolis, Maryland 21402 Consultant Gibbs and Cox, Inc. and Arlington, Virginia 22202 Abstract Nine ocean wave energyconversiontechniques are described and discussed. These techniques in- clude the use of heaving and pitching bodies, cavityresonators, wave focusing,pressuredevices surging devices, paddles, outriggers and combi - nation devices. Examples of each technique are presented, and required subsystems are described. Finally a comparison study is performed based on efficiency, operational practicality and cost. 1. Introduction From t h e earliest historical recordings, one learns that man has attempted to use the energy of water waves t o his advantage. Some of these attempts have been moderately successful; however, most have been failures. Because of the Arab o i l embargo several years ago, new attempts to exploit wave energy have been made. Although several new concepts have bee? developed and operated with moderate success, new problems involving systems components have appeared which must be overcome before wave energy conversion is a reality. Naval architects and ocean engineersdesigning ships, other ocean vehicles and ocean structures try to achieve minimum motions in allseas. Ship and marine vehicle motions includethesixdegrees of freedom: heave, pitch, roll, yaw, sway, and surge, while ocean structural motions arevibratory in nature. The wave energy conversion designer, on the other hand, normally seeks maximum motions in a sea. This goal can be achieved only after many severe system component problems are overcome. These problems are in three categories: mooring,,’ anchoring, materials and alignment. In fact, the wave energyconversion program of the United Kingdom has recently been re-directed due to the unexpected high costs of mooring. In this paper, the basic energy conversion techniques are described. The advantases and disadvantages of each theenergyconversion 2. Basic Wave Energy technique are diicussed, and system components described. Conversion Techniques In a recent paper [l], this author listed nine catagories of wave energy conversion devices. These are the following: Heaving Bodies Pitching and/or Roll ing Bodies Pneumatic or Cavity Resonators Focusing Systems Pressure Devices Surging Systems Flaps and Paddles Rotating Outriggers Combinations of the Above In the paragraphs to follow, these devices are described and discussed. A. Heaving Bodies One of the most popular wave energyconversion techniques is that which utilized a heaving body. Referring to figure zyxw 1 for notation, the body wilT have a natural heaving frequency described by where : is the weight density of water, zy A, is the waterplane area , is mass of the heavinnody and zy mw is the added-mass. Values of II$.J can be obtained =om the results presented in thereport of Wendel [2]. The response of the heaving body i s rather of narrow-band as can be seen inFigure 2 where t h e magnificationfactor, zyxw z, is shown as a function of the ratio of the wave frequency, f , and the natural frequency fz, obtained from EquatTon (1). As can be seen in Figure 2, thehigher system damping, zy A, results in a lower system response [3]. The optimal design is that which results in resonance, i.e. f/fz = 1. Since the response zyxw is narrow-band, the performance of a heaving body in a wind- generatedsea wi 11 be far less than that desired. Theie are three subsystems that can be used with heaving bodies to convert the heaving kinetic energy into electrical energy: the inductance system, piezo-electric crystals and protonic con- duction membranes. These are discussed in Section3 zy 553 U.S. Government work not protected by US. copyright.