ReviewAdvancements of wave energy converters based on power take off (PTO) systems: A review
Introduction
To solve the present energy crisis and pollution problems, wave energy can play an important role because of its energy density and availability. To harness energy from ocean waves there are many research works proceeding all over the world. Research on wave energy technology started informally in the 1940s by Yoshio Masuda, a Japanese marine captain, who invented a navigation buoy based air turbine PTO system which was later named as the (floating) oscillating-water-column (OWC) (Antonio, 2010; Falcão and Henriques, 2016). Academically the research started in the 1970s when the fossil oil price increased and the Middle East restricted oil supply (Polinder and Scuotto, 2005). By the 1980s, the wave energy research slowly stopped because the price of oil again reduced and the necessary funds to continue research in this field was not available. Some researchers from Europe, especially from UK and Norway did the majority of the early work and during this time the researchers mostly focused on hydrodynamic systems and developed the point absorber type of wave energy converters and the oscillating water column type of device concepts and the fundamental theoretical understanding of ocean wave energy (Elwood et al., 2010; McArthur and Brekken, 2010). It was then several decades before the research of wave energy started again, driven by increasing energy demand, the price of conventional energy, and pollution and climate change concerns. Therefore, the academic research into ocean wave energy can be divided into two phases as in the late 1970s and at present. Up to date there are many wave energy converters (WEC) that have been developed and deployed in different countries and several hundred WEC projects around the world are still at various stages of developments. This number is continuously increasing as new concepts and technologies are developed. Day et al. (2015) summarised that since 2015, worldwide there are more than a hundred projects and more than one thousand patents that have been developed in Europe, USA, Japan, China and Asia. Different concepts, techniques, designs and working principles have been investigated using the WEC to harness energy from the ocean waves, therefore the classification of WEC depends on different aspects. Wave energy systems can be classified by different methods such as according to location, structure, working or operational principle, size and orientation, and power take-off systems (Antonio, 2010; Czech and Bauer, 2012; Falnes, 2007; Hong et al., 2014; Wang et al., 2018). The most well-known diagram of the WEC classification was presented by Falcão et al. (Antonio, 2010). Based on installation location, the WECs can be classified by three types: (1) Onshore devices which are usually designed to be installed at or to the shoreline; (2) Offshore devices that are installed in deep water (>40 m); (3) Near shore devices which are deployed in shallow water regions (water at depths less than 20m). Moreover, some new PTO technologies have recently been added to the WEC classification. The working principles of the PTO system with their classification, as developed in this paper, are shown in Fig. 1.
The current status, development and future perspectives of ocean wave energy of different countries like US, China, Europe etc. can be seen in (Clément et al., 2002; Cruz, 2007; Kofoed et al., 2006; Lehmann et al., 2017; Magagna and Uihlein, 2015; Zhang et al., 2009). Emre Ozkop and Ismail H. Altas (Ozkop and Altas, 2017) listed almost all publications in the literature based on WEC classifications, research projects, control systems, validation and generator types which have been published up to 2017. Falcao et al. (Antonio, 2010) and Johannes Falnes (2007) also presented excellent review articles of wave energy technologies. There are also some books that have been published by T W Thorpe (1999), Johannes Falnes (2002), Cruz (2007) and others (McCormick, 2013; Pecher and Kofoed, 2017).
Power take off (PTO) systems are at the heart of wave energy converters (WEC) and many academic researchers from several universities and many wave energy technology developer companies are actively working to develop and improve the PTO system (Babarit, 2013; Babarit and Clément, 2006; Babarit et al., 2009; Clément and Babarit, 2012; Cretel et al., 2011; Folley et al., 2012; Folley and Whittaker, 2009; Fusco and Ringwood, 2012; Hals et al., 2002; Saulnier et al., 2011). So far there are many concepts that have been used in PTO systems to harness maximum energy with low installation and maintenance costs, as can be seen in Fig. 1. Previously published review articles have focused either on the challenges, current status, and development of WECs or classifications based on working principles and generators, with a brief discussion on the various parts of the WEC and energy harvesting systems. However, there does not appear to have been a single article that presents the review of WECs based on the power take off (PTO) system. It is known that the economic viability, efficiency and complexity of the structure of the WEC depends on its power take off (PTO) system. Therefore, with the aim to find the success of research, advancement and deployment of power take-off (PTO) systems of the WEC, a comprehensive study is required where all past and present works are reviewed. It is anticipated that this present review work can offer novel insights into the range of PTO systems that are being used for WECs with the intention of encouraging new research activity in the wave energy field.
Section snippets
Wave energy converters based on power take off (PTO) systems
There are many researches works that have been done to develop the PTO system of the WEC, using various concepts. Among all the working methods of the PTO systems, the hydraulic motor, turbine transfer and direct mechanical and electrical drive based working methods are the very well-known and the most used methods. However, there are some new techniques such as the triboelectric nanogenerator, hybrid systems and others that have been used in the last couple of years to develop the WEC PTO
Research and development
The WECs are designed to harness energy from the low frequency motions of ocean waves. While a wide number of WECs have been designed over the last few decades, their concepts of extracting waves are identical. By using the PTO system, the absorbed energy is transformed into useable electricity. Therefore, the PTO system is very important in the WEC device because it not only directly affects how effectively the captured wave energy is converted into electricity, but also determines the WEC's
Current market of WEC
The size of the potential ocean energy market is huge. By 2050, it is projected that more than 300 GW of ocean energy installed capacity could have been deployed, capable of generating 68K direct jobs and saving 500 million tonnes of CO2 emissions (OES, 2020b). The ocean energy industry is preparing to install 100 GW of production capacity in Europe alone by 2050, meeting 10 percent of demand for electricity which would be enough to meet the daily needs of 76 million households, creating 40K
Conclusion and remarks
As studied in this paper, wave energy technologies are currently undergoing innovative research and development all over the world. Due to the significant potential of the future ocean energy resources, the improvement and progress of wave energy technologies continues to develop very fast. Most of the reported research works have been done in Europe and the current leading wave energy technology developer companies also come from either Europe or the USA. The analysis has shown that
Acknowledgments
The authors gratefully acknowledge the Australian Government Research Training Program Scholarship for Mr. Raju Ahamed's study at Curtin University, Australia.
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