energies Review Review on Power Performance and Efficiency of Wave Energy Converters Tunde Aderinto 1 and Hua Li 2,* 1 Sustainable Energy Systems Engineering, Texas A&M University-Kingsville, Kingsville, TX 78363, USA; [email protected] 2 Mechanical and Industrial Engineering Department, Texas A&M University-Kingsville, Kingsville, TX 78363, USA * Correspondence: [email protected]; Tel.: +1-361-593-4057 Received: 22 October 2019; Accepted: 6 November 2019; Published: 13 November 2019 Abstract: The level of awareness about ocean wave energy as a viable source of useful energy has been increasing recently. Different concepts and methods have been suggested by many researchers to harvest ocean wave energy. This paper reviews and compares the efficiencies and power performance of different wave energy converters. The types of analyses used in deriving the reported efficiencies are identified, and the stage of the power conversion processes at which the efficiencies were determined is also identified. In order to find a common way to compare the efficiencies of different technologies, the hydrodynamic efficiency in relation to the characteristic width of the wave energy converters and the wave resource potential are chosen in this paper. The results show that the oscillating body systems have the highest ratio in terms of the efficiency per characteristic width, and overtopping devices have the lowest. In addition, with better understanding of the devices’ dynamics, the efficiencies of the newer oscillating water column and body systems would increase as the potential wave energy level increases, which shows that those newer designs could be suitable for more potential locations with large variations in wave energy potentials. At last, discussion about the cost of ocean wave energy is presented as well. Keywords: wave energy converters; power performance; efficiency 1. Introduction The potential of using ocean wave energy as a useful and reliable energy source has been generally accepted. Ocean wave energy resources have been assessed at global [1–3] and regional levels [4–7], and the amount of available ocean wave energy is substantial enough to generate serious interest in exploiting it. The general consensus is that the amount of available ocean wave energy is capable of contributing significantly to the energy needs of different countries. Countries with a considerable amount of exposure to the ocean along and within their borders would benefit from the successful exploitation of the ocean wave energy. Countries in Europe, the United States of America, China, and India are at the forefront of developing strategies to add ocean wave energy into their energy mix [8]. Governmental agencies in these countries have commissioned studies to perform resource assessments [9] and provided laboratories and test sites for the design and testing of wave energy converters (WECs) [10–12]. Apart from government commissioned studies, some studies are being performed by private organizations [13], universities [14], research centers [15], and individuals, which are all participating in the quest to bring ocean wave energy into the mainstream of the renewable energy industry. As a way of encouraging research into ocean wave energy systems, the United States Department of Energy sponsored a public design–build–test competition for organizations involved in wave energy converter development [16]. Top prizes were awarded to the devices with the best Energies 2019, 12, 4329; doi:10.3390/en12224329 www.mdpi.com/journal/energies Energies 2019, 12, 4329 2 of 24 energy harvest efficiency. The winner of the competition was “AquaHarmonics”, which used fivefold technology to improve its performance [16]. Back in 2002, there were already more than 1000 different patents filed on wave energy systems around the world, as stated in [17]. Different designers, scientists, and engineers have claimed the uniqueness of their equipment in terms of the advancement of performance. However, despite these numerous attempts, the commercial exploitation of the wave energy resource has been very slow compared toEnergies renewable 2019, 12, x energyFOR PEER REVIEW sources such as wind and solar. Many issues have been2 of 24 identified as factors affecting theBack low in 2002, penetration there were already of ocean more wavethan 1000 energy. different Challenges patents filed on ranging wave energy from systems the variability of the ocean waves’around properties the world, as andstated the in [17]. survivability Different designers, of the scientists, wave energyand engineers converters have claimed in the the harsh ocean environmentuniqueness have been of their identified equipment by in di termsfferent of the studies advancement [18, 19of performance.]. However, despite these numerous attempts, the commercial exploitation of the wave energy resource has been very slow Many conceptscompared to and renewable designs energy of sources wave such energy as wind converters and solar. Many have issues been have developed been identified over as the years. Those conceptsfactors and affecting designs the low that penetration have cropped of ocean wave up inenergy. the Challenges wave energy ranging industry from the variability can be categorized into three mainof the categories, ocean waves’ as properties shown and in the Figure survivability1: (1) oscillatingof the wave energy water converters columns in the (OWCs), harsh ocean (2) oscillating environment have been identified by different studies [18,19]. body systems, andMany (3) concepts overtopping and designs systems. of wave Theseenergy converters devices have can been be either developed placed over alongthe years. the coastlines as fixed structuresThose concepts or used and asdesigns floating that have structures cropped up in in otheff shorewave energy areas. industry This can main be categorized characterization of wave energyinto converters three main iscategories, based as on shown different in Figure types 1: (1) ofoscillating interaction water columns between (OWCs), the (2) devices oscillating and the ocean body systems, and (3) overtopping systems. These devices can be either placed along the coastlines waves. Thoseas conceptsfixed structures and or designsused as floating aforementioned structures in offshore are areas. based This on main combinations characterization andof wave modifications of the three mainenergycategories. converters is based The on growth different oftypes the of interaction wave energy between industry the devices is and also the ocean hampered waves. by the lack of standard proceduresThose concepts andand designs criteria aforementioned related to choosingare based on suitable combinations technology and modifications among of the three main three main categories. The growth of the wave energy industry is also hampered by the lack of categories tostandard harvest procedures wave energy and criteria at di ffrelatederent to locations. choosing suitable It is alsotechnology difficult among to quantifythe three main the progress of the advancementcategories in wave to harvest energy-harvesting wave energy at different techniques, locations. It evenis also thoughdifficult to many quantify researchers the progress of have claimed to improve existingthe advancement concepts in wave and energy-harvesting designs. techniques, even though many researchers have claimed to improve existing concepts and designs. Figure 1. ClassificationFigure 1. Classification of wave of energywave energy converter converter (WEC)(WEC) extraction extraction technology: technology: (a) oscillating (a) oscillatingwater water column, (b) overtopping devices, and (c) oscillating bodies. column, (b) overtopping devices, and (c) oscillating bodies. The first recorded patent filed on wave energy harvest was in 1799 using the oscillating water The firstcolumn recorded technology patent [20]. filed The device on wave was reported energy to harvestsupply a home was with in 1799about using1 kW of theelectricity. oscillating water column technologyBetween this [20 period]. The and device the early was 1960s, reported no serious to activity supply on wave a home energy with systems about was recorded. 1 kW of electricity. The WEC system put in place by Y. Masuda [21] was used to provide power for ocean buoys in the Between thislate period 1960s to and early the 1970s. early A larger 1960s, device no named serious Kaimei activity was developed on wave later energyby Masuda. systems This device was recorded. The WEC systemhoused putand tested in place several by OWCs Y. Masudaequipped with [21 ]different was usedtypes of to air provide turbines. However, power its for power ocean buoys in the late 1960soutput to early performance 1970s. was A not larger good devicesince it was named still at an Kaimei early stage was when developed the theoretical later knowledge by Masuda. This of wave energy absorption was in its infancy. With the increase in the theoretical knowledge of wave– device housedstructure and interactions, tested several improvements OWCs were equipped made on Masuda’s with di basicfferent design, types which of led air to the turbines. design However, its power output performance was not good since it was still at an early stage when the theoretical knowledge of wave energy absorption was in its infancy. With the increase in the theoretical knowledge of wave–structure interactions, improvements were made on Masuda’s basic design, which led to the design and installation of two full-size devices in Norway with rated capacities of 350 kW and Energies