Renewable and Sustainable Energy Reviews 27 (2013) 413–434 Contents lists available at SciVerse ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser Review of wave energy technologies and the necessary power-equipment Iraide López a,n, Jon Andreu a, Salvador Ceballos b, Iñigo Martínez de Alegría a, Iñigo Kortabarria a a Department of Electronics Technology, University of the Basque Country UPV/EHU, Spain b Tecnalia Research and Innovation, Derio 48160, Spain article info abstract Article history: The wave energy is having more and more interest and support as a promising renewable resource to Received 13 May 2013 replace part of the energy supply, although it is still immature compared to other renewable Received in revised form technologies. This work presents a complete analysis of the wave energy technology, starting with the 4 July 2013 characterisation of this global resource in which the most suitable places to be exploited are showed, and Accepted 5 July 2013 the classification of the different types of wave energy converters in according to several features. It is Available online 31 July 2013 also described in detail each of the stages that are part in the energy conversion, that is, from the capture Keywords: of the energy from the waves to the extraction of a proper electrical signal to be injected to the grid. Wave energy Likewise, existing offshore energy transmission alternatives and possible layouts are described. Wave power & 2013 Elsevier Ltd. All rights reserved. Wave energy converter Power electronics Power transmission Contents 1. Introduction........................................................................................................ 413 2. Global wave energy resource . 414 3. Wave energy converters . 416 3.1. Location..................................................................................................... 416 3.2. Device size and directional wave characteristics . 417 3.3. Working principle . 418 4. Energy conversion stages . 420 4.1. Primary conversion . 421 4.2. Secondary conversion . 422 4.2.1. Air-turbines . 423 4.2.2. Hydraulic turbines . 423 4.2.3. High-pressure or oil-hydraulics cylinders. 424 4.2.4. Electrical generator . 424 4.3. Direct conversion. 426 4.4. Tertiary conversion . 427 5. Power transmission systems . 427 6. Conclusions . 432 Acknowledgement. 433 References.............................................................................................................433 1. Introduction n Corresponding author. Tel.: +34 946017243. The level of energy consumption is one of the most direct ways E-mail address: [email protected] (I. López). to measure the progress and welfare of society. It is estimated that 1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.rser.2013.07.009 414 I. López et al. / Renewable and Sustainable Energy Reviews 27 (2013) 413–434 global energy consumption in 2040 will be about 30% higher than could be Mutriku OWC wave-plant [8] which is located in a the one in 2010. In 2040, electricity generation will be more than dyke on shoreline, or the Wave Hub test-site [9] which is the 40% of the global energy consumption [1]. Given that tradi- located 15 km from shore. Nowadays there are not offshore tional energy sources are finite, this demand could not be supplied wave-plants installed, but there are several WECs (Wave in the near future. In this context, renewable energies play a Energy Converter) developed to be located offshore such as key role. Wave Dragon [10], Pelamis [11] or OE Buoy [12]. In the past three decades, great efforts have been made in the It has little environmental interference; and in the same way it development of solar and wind energy generation. However, there can muffle the surf in port or erodible areas [2,5,6]. is another source with high potential energy that is slowly beginning to establish itself and arouse a great interest from the On the other hand, it should not be forgotten the challenges scientific community, this is the wave energy. that this technology need to overcome to become commercially Contrary to what one might think, wave energy is not new, it competitive in the global energy market: has been discussed for centuries, but until today little importance has been given to it (financial support). Early indications were The conversion of the slow (about 0.1 Hz), random and the found in China, in the thirteenth century, where the waves were oscillatory motion of waves into useful motion to connect a already used to move the mills. However, the first patent was not generator (50 Hz) and provide a suitable output to the grid. obtained until 1799 by Girard and son in France [2]. To achieve it, some energy conversion stages are necessary, The french Praceique-Bochaux developed in 1910 one of the first firstly to convert wave energy to electricity; and secondly, to applications using wave energy to supply his home in Royan by rise WECs generated voltage levels to provide the energy electricity. He used a pneumatic system, similar to what is now known transmission from the sea to the land [2,6,13]. as OWC (Oscillating Water Column). Yoshio Masuda, from Japan, Waves vary in high and period, so their power level varies in developed the whole concept of OWC holding works since 1940 [3]. the same way. This energy vector must be converted into On the one hand, in Europe, driven by the oil crisis, Stephen smooth electrical signal, hence, some types of energy storage Salter and Kjell Budal pioneered this technology starting their systems usually help to provide a regular power output (water studies in 1973 . On the other hand, in America, Michael E. reservoirs in overtopping devices, gas accumulators in high- McCormick was the first academic to work with this technology. pressure hydraulic circuits, large electrical capacitors and fly- As mentioned before, due to the rising price of oil, universities and wheels), as well as other means of compensation [2,13]. researchers began to focus their efforts on wave technology. At the In offshore zones, wave direction varies very often, and there- end of the decade, governments as well as private entities began to fore, in order to capture as much energy as possible, the devices assist the R&D projects, but a decade later (in 80s), the decline in have to align themselves (much as the moorings leave them) oil prices caused a sharp decline in this financing [4]. This leads us with the direction. In near shore, directions can be determined to today, as before, due the energy crisis and environmental in advance [6]. awareness, this technology re-emerges stronger than ever. Another challenge relates to offshore converters. In these This work presents a complete review of the wave energy locations, the device also has to withstand extreme wave technology describing, analysing and fixing many of the concepts conditions, which leads to difficult structural engineering that are necessary to achieve an important global knowledge of challenges [13]; obviously that the maintenance operations this matter. Thus, the global dispersion of this resource as well as become very difficult. the locations with the highest potential are introduced. Also the Another important barrier is the funding [5]. Waves are an different developed devices for the extraction of this energy are energy source with great potential and with a number of classified. From a somewhat more technical point of view, the advantages that, as has been seen, make them really attractive. different energy conversion stages which are done until to get an However, it has to compete against more mature technologies appropriate electric signal at the output of the device are analysed. where the investment already is done. In this situation, In the same way, the alternatives of that electric energy transmis- investors should see a significant advantage to spend large sion, as well as the different topologies or layouts for wave parks amounts of money, as these plants require large investments. are described. Moreover, in the context of the economical crisis that we live, the investment in these technologies becomes more difficult. Therefore, while this situation lasts, the “only” thing 2. Global wave energy resource that the scientific community can do is to join the efforts in order to turn this technology into a more viable, efficient and Wave energy power is enormous and more reliable than other affordable one. renewable resources such as solar and wind energy; because its density (2–3kW/m2) is greater (wind 0.4–0.6 kW/m2; solar 0.1– 0.2 kW/m2) [5]. Other benefits are listed below: For the development of wave energy technology and the selection of suitable sites for wave farms, it is necessary the Waves can travel large distances with little energy loss. For knowledge of the available wave climate and power estimation. example the storms originated from western side of the In this regard, several papers have been published which show the Atlantic Ocean will travel to the western coast of Europe with existing wave power worldwide. Example of that, among others, little energy loss [2,6]. are the works presented in [14–18]; where the best quality wave Wave energy converters can generate power up to the 90% of model data combined with high precision satellite altimetry and the time (20–30% for wind and solar devices) [6,7]. measured buoy data are employed during 10 years to generate a Its predictive capacity is far greater than the wind one [5,6]. wave atlas, such as the one showed in Fig. 1 [14]. From all these It has good correlation between resource and demand, since works, he following conclusions can be drawn: around 37% of the population of the world lives at 90 km of the coast [5]. The most energy-rich areas are between 401 and 601 in both It is a widely available energy source because it has multiple hemispheres.