A review of the technologies for wave energy extraction

Clean Energy, 2018, Vol. 2, No. 1, 10–19 doi: 10.1093/ce/zky003 Advance Access Publication Date: 7 March 2018 Homepage: https://academic.oup.com/ce Review Article A review of the technologies for wave energy extraction Eugen Rusu and Florin Onea* Department of Mechanical Engineering, ‘‘Dunarea de Jos’’ University of Galati, Domneasca Street, 47, Galati 800008, Romania *Corresponding author. E-mail: Abstract The main objective of this article is to provide a comprehensive picture of existing wave technologies being used for wave energy extraction. The overview will explain their potential and also the challenges wave technologies face. The article will also briefly discuss the benefits of combined offshore wind-wave projects, also known as hybrids. Key factors and impacts on relevant existing wave technologies will be outlined, including capacity factor and capture width. Finally the levelized cost of energy (LCOE) targets for the most promising technologies will be discussed. Keywords: wave power; wave energy converters; capacity factor; capture width; hybrid solutions Introduction In order to reduce greenhouse gas emissions and to secure a sustainable future for all countries, it is clear that renewable energy sources will play a key role. According to the Renewables 2016 Global Status Report [1], globally, fossil fuel consumption is ~78.3% of the total share of energy consumption, followed by renewable energy sources with 19.2%. Traditional biomass accounts for 8.9%, while modern renewable energy has a percentage of 10.3%, dominated by solar and wind. The gap between fossil fuel consumption and the renewable market can be closed in the near future if we take into account recent progress from the renewable energy sector. Globally, the renewable energy sector between 2004 and 2013 (excluding hydropower) increased from 85 to ~560 GW. Leading the sector was the wind industry with growth from 48 to 318 GW, followed by the photovoltaic sector from 2.6 to 139 GW. The growth in the renewable sector was due to a number of factors including politicial support, financial incentives and reduction in the costs of technology making renewable energy cost competitive [2]. Marine energy technology is at an early stage of development, especially in the case of wave power. Wave power needs specific environmental conditions to be created. The energy is equally divided between: (i) the potential energy component, where the water is forced against gravity from the wave trough and crests and (ii) the kinetic energy component, that is, thewater oscillating velocity [3]. To use this power it is important to design a structure that can efficiently capture and harvest the energy transmitted by the waves. A further key factor is that the structure must be able to survive the marine environment, in particular, storm events wherein the wave power significantly increases. One means to convert the wave energy into mechanical energy is by using a generator that is fixed (on the sea bottom or shoreline) with parts of this system in motion. During recent decades, floating systems were introduced that are capable of being deployed offshore. The systems can be designed and targeted to take advantage of both potential and kinetic energy, individually or at the same time [4]. Received: 15 September, 2017; Accepted: 20 January, 2018 © The Author(s) 2018. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, the original work is properly cited. For commercial re-use, please contact Downloaded provided from https://academic.oup.com/ce/article-abstract/2/1/10/4924611 by guest on 25 July 2018 10 Rusu and Onea The potential of the Global Ocean’s resources is significant when considering the combinations possible between large water surfaces and marine natural resource diversity. There are a wide variety of energy extraction options, including waves, tidal and ocean currents, ocean thermal energy, salinity gradients, marine biomass and submarine geothermal energy [5]. A successful example of using the marine environment is the offshore wind industry. The European wind market currently has 81 offshore grid-connected projects shared by 10 European countries capable of generating a total of 12.6 GW [6]. Under current trends, we estimate that by 2020 the total capacity will be close to 24.6 GW, based on statistics reported for 2016. Several technological developments have contributed to this prediction: the average offshore wind turbine is 4.8 MW; the first 8-MW turbine has been connected to the grid; the average size of a wind farm is 380 MW (+12% from 2015); the average water depth is 29 m; and the average distance to shore is 44 km. The offshore wind energy sector has continually expanded since 2000 with larger size wind farms, turbines and distances from shore. In 2015 almost €18bn was invested in transmission assets and new offshore wind projects [6, 7]. The wave energy sector could potentially equal and even exceed the offshore wind sector, if we take into account that waves are a concentrated form of wind energy capable of travelling large distances with minimal losses. There are two categories of waves: wind seas (waves generated locally) and swell (waves generated by distant winds). The swell wave is more important for the wave energy converter (WEC) industry as the energy density is more consistent. The worldwide potential of wave power is around 29 500 TWh/yr, from which currently only a small fraction is efficiently extracted near ocean coastlines, islands or semienclosed basins defined by local ‘hotspots’ [8, 9]. In general, a hotspot is a site that reveals the best balance between wave energy potential and other relevant factors, such as distance to the shore, water depth or investment costs. In recent years, various onshore and offshore projects have been developed, including the Islay plant (Scotland) and the Pico Island plant (Portugal). The Islay project involved the construction and testing of the LIMPET (Land Installed Marine Power Energy Transmitter) system, which has a generating capacity of 500 kW. This unit was installed in 2000 on an island off the western coast of Scotland, and includes three water columns made from concrete and inclined horizontally at 40o. The water columns’ motion is converted into electricity throughout two counter-rotating Wells turbines operating at 700–1500 rpm [10]. The Pico plant is located in the Azores, with an installed capacity of 400 kW and was built between 1995 and 1998, under the supervision of the Instituto Superior Técnico (IST), Lisbon. Various problems emerged during this time due to the plant configuration and equipment. In 2005 the project was re (...truncated)