Difference between revisions of "Wave energy converters"

Revision as of 17:52, 10 January 2021

Introduction

Vast and reliable, wave power has long been considered as one of the most promising renewable energy sources. In a special IPCC 2011 report^[1] several estimates are presented for the world total potential of ocean wave energy resources. The theoretical maximum has been estimated at about 30,000 TWh/yr (3.10¹³ kWh/yr), which is about 20% of the 2019 world energy consumption. However, given technological and economic constraints, the exploitable resource is probably almost a factor 10 less.

Wave Energy Converters (WECs) convert wave power into electricity. Although attempts to utilize this resource date back to at least 1890, wave power is currently not widely employed (Miller, 2004^[2]). The operational wave power installed worldwide in 2020 totaled approximately 16 MW [4], which is about 5 orders of magnitude less than the 2-3 TW required to exploit the global wave energy potential. An important reason is the production costs per kWh, which were in 2020 about a factor 10 higher compared to offshore wind farms ^[3].

A plethora of innovational methods for wave power conversion have been invented in the last three decades, resulting in thousands of patents over recent years. At present, a number of different wave energy concepts are being investigated by companies and academic research groups around the world. Although many working designs have been developed and tested through modelling and wave tank-tests, only a few concepts have progressed to sea testing. Strong cost reductions, which are only possible with a sharp increase in global application, should enable wave plants to compete favorably with conventional power plants in the near future (Pelc and Fujita, 2002^[4]).

Wave Energy Conversion Principles

In this section the most popular current methods for wave energy conversion are briefly presented.

Oscillating Water Columns

Fig. 1. Wave energy conversion using the principle of the oscillating water column. From IPCC 2011^[1].

The principle of the oscillating water column is illustrated in Fig. 1, in this example for use in conjunction with a fixed structure (e.g., breakwater). The functioning of the oscillating water columns (OWCs) is very similar to that of a wind turbine, being based on the principle of wave induced air pressurization. The device is set upon a closed air chamber, which is placed above the water. The passage of waves changes the water level within the closed housing and the rising and falling water level increases and decreases the air pressure within the housing introducing a bidirectional air flow. By placing a turbine on top of this chamber air will pass in and out of it with the changing air pressure levels.

Fig. 2. Wells turbine. From^[5]).

There are two options to separate the bi-directional flow: a Wells turbine to create suction or alternatively, pressure generating valves (Kofoed and Frigaard, 2008^[6]). The wells turbine is constructed such that it rotates always in the same direction regardless of airflow direction, see Fig. 2. The claimed efficiency is 60-65%, a bit lower than conventional turbines, but not achievable by conventional turbines in alternating mode^[7].

Fig. 3. Wave energy buoy based on the oscillating water column principle.

Fig. 4. Spar buoy wave energy converter.

OWC devices can be moored offshore but they can also be placed near the shore where waves break. This results in significant cost savings. The disadvantage is the shallow water depth along the shore, which dampens the largest waves. See also Wave energy converters in coastal structures.

Special buoys have been developed for the application of OWC converters in deep water, according to the principle of Fig. 3. The length of the shaft determines the resonance frequency, allowing optimum energy efficiency to be achieved. An example of an offshore OWC is the Spar Buoy, Fig. 4. The original concept was invented by Yoshio Masuda (1925-2009), who developed navigation buoys powered by wave energy, equipped with an air turbine. It is circular in plane and therefore invariant to wave direction (Fig. 4). The size varies according to the target sea conditions at the deployment site but maximum dimensions were estimated at 30m diameter, 50m height and 35m draft, which could deliver up to 450 kW.

Figure 5: Wave Dragon overtopping wave energy converter.

Overtopping Devices

Another type of wave energy converter is the overtopping device, which works much like a hydroelectric dam. The "Wave Dragon" created by Wave Dragon ApS^[8] is the best known example of an offshore overtopping device (Fig. 5). Its floating arms focus waves onto a slope from which the wave overtops into a reservoir. The resulting difference in water elevation between the reservoir and the mean sea level then drives low-head hydro turbines. Proposed optimal size design of 260m width and 150m length can produce up to 4 MW. In wave climates above 33 kW/m, this technology was expected to be economically competitive with offshore wind power in the near future. After a combined cost saving and power efficiency increase, the power price could eventually be in line with costs of fossil fuel generation (Christensen et al., 2005^[9]). However, a feasibility study for a Wave Dragon deployment at the northern Spanish coast published in 2020 reported costs which were still a factor 10 higher^[10].

Near the shore, overtopping converters can be installed in front of or as part of caisson breakwaters. An example is the SeaWave Slot-Cone Generator (SSG)^[11], which collects seawater by wave overtopping over several reservoirs placed above each other, resulting in high hydraulic efficiency, see Wave energy converters in coastal structures.

Wave absorbing devices

A great number of different devices have been developed to harness wave energy directly by using wave-induced water motion. The most popular types are displayed schematically in Fig. 6.

Fig. 6. Different energy conversion principles based on absorption of wave energy. Image from ^[12] www.mdpi.com/journal/energies Creative Commons licence.

Point absorbers

Figure 7: The FO3 point absorber (above) and the Wave Star attenuator (below)

Point absorber are buoy-type wave energy converters (WECs) that harvest incoming wave-energy from all directions. They are placed offshore at the ocean surface or just below. A vertically submerged floater absorbs wave energy which is converted by a piston or linear generator into electricity. One such a point absorber WEC is the FO3 concept developed by Norwegian entrepreneur Fred Olsen. It consists of several (12 or 21) heaving floaters attached to a 36 by 36 meter rig (Fig. 7). By means of a hydraulic system, the vertical motion is converted into a rotational movement that drives the hydraulic motor. This motor in turn powers the generator that can produce up to 2,52 MW (Leirbukt and Tubaas, 2006^[13]).

The multiple point absorber type WEC "Wave Star", developed by Wave Star ApS^[14], has a number of floaters on movable arms (Fig. 7). The energy of the motion of the arms is again captured in a common hydraulic line and converted into electric current. Most noticeably, being able to raise the entire installation along its pillars, this system has a high endurance for rough storm conditions. So far, this method has not been deployed at full scale. A 1:2 scaled installation has been built at Hanstholm which turns out 600 kW. However, production is thought to be scale-able up to 6 MW (Bjerrum, 2008^[15]). A major benefit of these types of exploitation is the minimal contact with water, placing any delicate machinery and electrics out of reach of any corrosion or physical forcing of the waves.

Wave Activated Bodies (WAB)

Figure 8: DEXA concept

Wave activated bodies (WABs), also called 'wave attenuators' or 'line absorbers', are devices with moving elements that are directly activated by the cyclic oscillation of the waves. Power is extracted by converting the kinetic energy of these displacing parts into electric current. One example of such a WAB, is made by a single floater connected to a linear magnetic generator fixed to the seafloor. In other cases, only parts of the body are fully immersed and dragged by the orbital movements of the water. In order to maximally exploit this resource, the moving compounds need to be small in comparison to the wavelength and preferably they are placed half a wavelength apart. For these reasons, wave activated bodies are usually very compact and light. The main disadvantage of this type of wave energy converters is the high cost of the power generator needed to convert the irregular oscillatory flux into electricity.

The "DEXA", developed and patented by DEXA Wave Energy ApS ^[16], is an illustrative example of a WAB. The device consists of two hinged catamarans that pivot relative to the other (Fig. 8). The resulting oscillatory flux at the hinge, is harnessed by means of a water-based low pressure power transmission that restrains angular oscillations. Flux generation is optimized by placing the floaters of each catamaran half a wavelength apart. A scaled prototype (dimensions 44x16.2m^[17]) placed in the Danish part of the North Sea should generate 160 kW (Martinelli et al., 2009^[18]). Full-scale models are thought to be able to generate up to 250 kW. ^[16]

Power take-off systems (PTO)

The power take-off (PTO) system of a wave energy converter has a direct impact on the capital cost of a project by usually accounting for between 20 and 30% of the overall investment. The economic viability, efficiency and complexity of a wave energy converter depends largely on its PTO system. Maintenance at sea is a demanding and expensive task; high reliability and durability of all components of the PTO system are therefore required. This is technically challenging for systems exposed to the harsh marine environment, especially systems that consist of many moving parts that are exposed to corrosion and fouling. Several popular PTO systems have been reviewed by Ahamed et al. (2020^[19]), from which the summary below is extracted.

Fig. 9. Schematic of a linear electrical generator based on a permanent magnet generator. Image from Ahamed et al. (2020^[19]).

Oscillating water column wave converters (OWC) generally use the previously described Wells turbine as PTO system. These turbines are vulnerable due to the relatively large number of moving parts. Overtopping devices are generally equipped with hydro turbines for power take-off. Conventional hydro turbines require for high efficiency more head and flow than provided by overtopping ocean waves. Wave absorbing devices use hydraulic motor systems or direct mechanical or electrical drive PTO systems. Hydraulic motor-based PTO systems are suited for converting the low-speed oscillating motion into energy. However, the hydraulic motor-based PTO system consists of many mechanical moving parts and due to compression and decompression of the fluid there is a risk of hydraulic oil leakage. Direct mechanical drive uses linear-to-rotary conversion systems without pneumatic or hydraulic systems. The efficiency is high, but the lifetime is relatively short and maintenance costs are substantial. Direct electrical drive transmits the wave energy into electrical energy directly by coupling the mechanical energy to the moving part of a linear generator (Fig. 9). Issues are the need for a heavy structure due to the attractive forces between the stator and the translator and the complicated power transmission system due to the unequal generated voltage created by the irregular wave motion.

The 'Triboelectric nanogenerator' (TENG) and its extension, the 'Triboelectric−Electromagnetic Hybrid Nanogenerator' (TENG-EMG)^[20], are a new development that can efficiently harvest energy in any frequency range, are low-cost lightweight, easy to fabricate and easy to be scaled (size of individual units being below the micrometer scale). The nanogenerator uses a polymer-metal pair to create contact electrification (triboelectric effect) between two materials sliding against each other and to induce charge transfer between their electrodes due to electrostatic induction, either in a layer structure or in a spherical-shell structure^[21], see Fig. 10. Challenges for application in a prototype lie in the use of TENGs for power transfer to the shore, cost to scale, lifetime of the TENG materials in the ocean environment and connection methods of the thousands of TENG units that are needed.

Fig. 10. Principle of tribo-electric nanogenerators. (a) Triboelectric charges are generated on the surface of two laterally sliding dielectric films, due to frictional effects. Polarization occurring in the sliding plane drives an electron flow between the metal electrodes that generates an alternating current. (b) Soft contact rolling-mode spherical TENG (SS-TENG). When receiving external vibration from the ocean waves, the ball will roll back and forth between the electrodes to provide AC power to the external load. The soft ball maximizes the contact area. Images from Huang et al. (2020^[21]), Creative Commons licence.

Issues with wave energy converters

To be able to produce energy at competitive prices, several obstacles have to be overcome. The following factors in particular determine the high costs of wind energy.

A very robust construction made of high quality materials is required that remains intact under severe storms and withstands the demanding conditions at sea, leading to corrosion, fouling and fatigue. The classical protection measure against fouling and corrosion of steel structures is regular maintenance and repainting. But this is time-consuming and costly because of the difficult access to offshore installations, especially under harsh conditions when damage occurs. In addition, the use of antifouling paints may be detrimental to the marine environment (e.g. Tributyltin paints^[22]). Full-scale devices out of concrete may provide a valuable alternative since concrete is long-lived if properly mixed (Metha, 2001^[23]). The cost of repairs is a major component of the wave farm costs. The reliability of the components, especially due to the cost of eventual repair actions, is pivotal for the economic viability of a project^[24]. Deployments should be capable of lasting 30 years or more. This is a tall order as demonstrated by the mooring of the Wave Dragon that failed during a severe storm on the 8^th of january 2004.

The power generated by ocean waves fluctuates strongly due to the irregular wave climate, which makes connection to the electricity grid difficult^[25]. In addition, connection points to the electricity grid may not be available in areas where the conditions for wave energy generation are most favorable.

Crucial for any design is the mooring which ensures a maintained position under both normal operating loads as well as extreme storm load conditions. It should not exert excess tension loads on the electrical transmission cables and ensure the suitable safety distances between devices in multiple installations. Most commonly, a free hanging catenary configuration is used for mooring but multi-catenary systems and flexible risers are not infrequent as well. Every configuration should be sufficiently compliant to accommodate tidal variations and environmental loading while remaining sufficiently stiff to allow berthing for inspections and maintenance.

Due to the great variability of the wave climate with rare extremes, it is important to be able to test prototype designs over a long period before they are deployed on a large scale. Mathematical simulation models can be of great value here, because this is a quick and relatively inexpensive way to test a design for its effectiveness and efficiency at the long term, both in terms of capital and operational expenses, to achieve the lowest overall cost of electricity. For this purpose, the modeling tool supports the decision-maker in the pursuit of the most reliable and easy to maintain device design but also reports the trade-off between energetic yield and operational and maintenance efforts^[24].

The power take-off system is a crucial part of wave energy converters, which largely determines the cost, efficiency and reliability of the WEC. Many studies focus on optimisation of the PTOs; several challenges related to PTOs are mentioned in the previous section.

Wave energy converters combined with offshore wind

The high cost of stand-alone wave energy conversion schemes is a major obstacle for its large-scale application. However, costs can be significantly reduced by combining WECs with structures offshore or along the coast that are being built for other applications. A good example is the integration of WECs with breakwaters in the coastal zone. This is described in the article Wave energy converters in coastal structures.

Another possible combination is the integration of wind and wave energy production. This is especially interesting in areas where the conditions for optimal wind energy production do not systematically coincide with conditions for optimal wave energy production. It is also a way to make optimal use of marine space^[26]. The main advantage of integrated wind power generation is shared infrastructure costs, especially foundations and grid connections. However, with the existing WEC techniques, the costs per kWh produced are still higher with a combined wind-wave application than with wind energy alone^[27]. Synergy benefits can also be sought through improved stability of the structure, for example in the case of an OWC wave energy converter integrated into an offshore wind turbine monopile^[28]. Stability improvement can be a major benefit for designs in which the interaction between the wind and wave sub-structures is strong, as in the case of a WEC combined with a floating wind turbine^[29]. Wave energy converters can also reduce wave heights inside a wind farm, increasing in this way the weather windows to access the wind turbines^[30].

International organizations

Several organizations have been established for worldwide collaboration on ocean wave energy development involving research institutes, wave energy developers, operators and governments.

Ocean Energy Europe (OEE), a not-for-profit organization, is a network of ocean energy professionals collaborating with 120 organizations, including Europe’s top utilities, industrialists and research institutes. It aims to create a strong environment for the development of ocean energy, improve access to funding, and enhance business opportunities for its members. To this end, OEE engages with the European Institutions (Commission, Parliament, Council, EIB, etc.), and national ministries on policy issues affecting the sector. Ocean Energy Europe also organizes the annual Ocean Energy Europe Conference and Exhibition – the annual event which gathers representatives from the industry, Ministers and Commissioners.

The Ocean Energy Systems Technology Collaboration Programme (OES), established by the International Energy Agency (IEA), is an international organisation that is actively collaborating between its 29 member countries with the aim of advancing ocean energy research, development and demonstration of technologies to generate electricity from ocean energy resources (Wave, Tidal Range, Tidal and Ocean Currents, Ocean Thermal Energy Conversion (OTEC) and Salinity Gradients). The primary education and research initiatives of OES are aimed at promoting the feasibility, recognition and adoption of ocean energy systems in an environmentally acceptable manner.

The European Marine Energy Centre (EMEC) Ltd provides developer companies, wave and tidal energy converter-technology developer with certified open-sea testing services. The objective of EMEC is to decrease the time, cost, and risk related with marine energy technology development, increasing the use of EMEC’s modified facilities as well as industry knowledge and other experiences. EMEC has 13 grid connected test berths where many marine energy converters have been installed. EMEC is an independent organization that maintains relations with various developer companies, academic organizations and governing bodies and presently working with companies and researchers to broaden research programmes to solve a variety of environmental and operational challenges relevant to industry.

The European Network for marine renewable energy (WECANet) is a network of 31 partner countries to assist networking, training and collaboration in Europe. Four working groups address the following topics: numerical modelling for WECs, experimental hydrodynamic modelling including PTO systems, technology development and economical / political aspects.

OCEANERA-NET organizes funding programmes to support research and innovation between European countries and regions in the ocean energy sector.

International conferences on ocean energy are organized by

European Wave and Tidal Energy Conference (EWTEC). Selected peer-reviewed conference contributions are published in the EWTEC’s International Marine Energy Journal.
Asian Wave and Tidal Energy conference (AWTEC) addresses updates on recent global activities and initiatives with a distinctly special interest in the Asian region. The conference cooperates with the Journal of Marine Science and Engineering for publication of selected conference contributions.
International Conference on Ocean Energy (ICOE) is a biennial global marine energy event focused on the industrial development of renewable marine energy. ICOE is a collaboration with the IEA Technology Collaboration Programme – Ocean Energy Systems (OES).

External links

Ocean Energy Systems website, 2018 report Spotlight on ocean energy

References

↑ ^1.0 ^1.1 Lewis, A., Estefen, S., Huckerby, J., Musial, W., Pontes, T. and Torres-Martinez, J. 2011. Ocean Energy. In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
↑ Miller, C. 2004. A Brief History of Wave and Tidal Energy Experiments in San Francisco and Santa Cruz. [1]
↑ IRENA, 2014. Ocean Energy Technology Brief 4, www.irena.org
↑ Pelc, R. and Fujita, R.M. 2002. Renewable energy from the ocean. Marine Policy, 26: 471-479.
↑ Falcao, F.O. and Henriques, J.C.C. 2016. Oscillating-water-column wave energy converters and air turbines: A review. Renewable Energy 85:1391-1424
↑ Kofoed, J.P. and Frigaard, P., 2008. Hydraulic evaluation of the LEANCON wave energy converter. DCE Technical Report No. 45. Dep. of Civil Eng., Aalborg University, Oct. 2008. Leancon Wave Energy. Leancon website
↑ Drew, B., Plummer, A.R. and Sahinkaya, M.N. 2009. A review of wave energy converter technology. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 223(8): 887-902
↑ Wave Dragon ApS Wave Dragon website
↑ Christensen, L., Friis-Madsen, E., Kofoed, J.P., 2005. The Wave Energy Challenge: The Wave Dragon case. PowerGen 2005 Europe Conference - Wave Dragon, 20pp. [2]
↑ Castro-Santos, L., Bento, A.R. and Soares, C.G. 2020. The Economic Feasibility of Floating Offshore Wave Energy Farms in the North of Spain. Energies 13, 806; doi:10.3390/en13040806
↑ Vicinanza, D., Margheritini, L., Kofoed, J.P. and Buccino, M. 2012.The SSG Wave Energy Converter: Performance, Status and Recent Developments. Energies 5: 193-226
↑ Hansen, R.H., Kramer, M.M. and Vidal, E. 2013. Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter. Energies 6: 4001-4044; doi:10.3390/en6084001
↑ Leirbukt, A. and Tubaas, P., 2006. A wave of renewable energy. ABB Review. 3, 29-31. [3]
↑ Wave Star ApS website
↑ Bjerrum, A., 2008. Wave Energy - new unlimited source of energy. European Renewable Energy Conference presentation.
↑ ^16.0 ^16.1 Dexawave Energy ApS. Dexawave website
↑ Kofoed, J.P., 2009. Hydraulic evaluation of the DEXA wave energy converter. DCE Technical Report No. 57. Dep. of Civil Eng., Aalborg University, 23 pp.
↑ Martinelli L., Zanuttigh,B., Kofoed, J.P., 2009. Statistical analysis of power production from OWC type wave energy converters. EWTEC Conference, Uppsala, 7-11 Sept 2009, electronic format, 9 pp.
↑ ^19.0 ^19.1 Ahamed, R., McKee, K. and Howard, I. 2020. Advancements of wave energy converters based on power take off (PTO) systems: A review. Ocean Engineering 204, 107248
↑ Wen, Z., Guo, H., Zi, Y., Yeh, M.-H., Wang, X., Deng, J., Wang, J., Li, S., Hu, C. and Zhu, L. 2016. Harvesting broad frequency band blue energy by a triboelectric–electromagnetic hybrid nanogenerator. ACS Nano 10 (7): 6526–6534
↑ ^21.0 ^21.1 Huang, B., Wang, P., Wang, L., Yang, S. and Wu, D. 2020. Recent advances in ocean wave energy harvesting by triboelectric nanogenerator: An overview Nanotechnology Reviews 9(1)
↑ Paul, J.D. and Davies, I.M., Effects of copper- and tin-based anti-fouling compounds on the growth of scallops (Pecten maximus) and oysters (Crassostrea gigas).Aquaculture, 54(3),191-203.
↑ Metha, P.K., 2001. Reducing the Environmental Impact of Concrete. Concrete International,61-66.
↑ ^24.0 ^24.1 Rinaldi, G., Portillo, J.C.C., Khalid, F., Henriques, J.C.C., Thies, P.R., Gato, L.M.C. and Johanning, L. 2018. Multivariate analysis of the reliability, availability, and maintainability characterizations of a Spar–Buoy wave energy converter farm. Journal of Ocean Engineering and Marine Energy 4: 199–215
↑ Sjolte, J. 2014. Marine Renewable Energy Conversion: Grid and Off-Grid Modeling, Design and Operation. Doctoral Thesis, Norges Teknisk-Naturvitenskapelige Universitet, Trondheim, Norway
↑ Azzellino, A., Lanfredi, C., Riefolo, L., De Santis, V., Contestabile, P. and Vicinanza, D. 2019. Combined exploitation of offshore wind and wave energy in the Italian seas: a spatial planning approach. Frontiers in Energy Research 7, 42
↑ Clark, C., Miller, A. and DuPont, B. 2017. Analytical Cost Modeling for Co-Located Wind-Wave Energy Arrays. Proceedings of the 12th European Wave and Tidal Energy, Cork, Ireland, paper 871
↑ Zhou, Y., Ning, D., Shi, W., Joanning, L. and Liang, D. 2020. Hydrodynamic investigation on an OWC wave energy converter integrated into an offshore wind turbine monopile. Coastal Engineering 162, 103731
↑ Zhu, H., Hu, C., Sueyoshi, M. and Yoshida, S. 2020. Integration of a semisubmersible floating wind turbine and wave energy converters: an experimental study on motion reduction. J. Mar. Sci. Technol. 25: 667–674
↑ Perez-Collazo, C., Greaves, G. and Iglesias, G. 2015. A review of combined wave and offshore wind energy. Renewable and Sustainable Energy Reviews 42: 141-153

The main author of this article is De Rijcke, Maarten
Please note that others may also have edited the contents of this article.

Citation: De Rijcke, Maarten (2021): Wave energy converters. Available from http://www.coastalwiki.org/wiki/Wave_energy_converters [accessed on 21-11-2024]

For other articles by this author see Category:Articles by De Rijcke, Maarten
For an overview of contributions by this author see Special:Contributions/MaartenDeRijcke

The main authors of this article are Andersen, Thomas Lykke, Martinelli, Luca, Zanuttigh, Barbara, Nørgaard, Jørgen Harck, Silva, Rodolfo and Roul, Piero
Please note that others may also have edited the contents of this article.

For other articles by this author see Category:Articles by Andersen, Thomas Lykke

Article reviewed by

Job DronkersSee the discussion page

[IPCC-1] 1.0 ^1.1 Lewis, A., Estefen, S., Huckerby, J., Musial, W., Pontes, T. and Torres-Martinez, J. 2011. Ocean Energy. In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

[2] Miller, C. 2004. A Brief History of Wave and Tidal Energy Experiments in San Francisco and Santa Cruz. [1]

[3] IRENA, 2014. Ocean Energy Technology Brief 4, www.irena.org

[4] Pelc, R. and Fujita, R.M. 2002. Renewable energy from the ocean. Marine Policy, 26: 471-479.

[FH-5] Falcao, F.O. and Henriques, J.C.C. 2016. Oscillating-water-column wave energy converters and air turbines: A review. Renewable Energy 85:1391-1424

[6] Kofoed, J.P. and Frigaard, P., 2008. Hydraulic evaluation of the LEANCON wave energy converter. DCE Technical Report No. 45. Dep. of Civil Eng., Aalborg University, Oct. 2008. Leancon Wave Energy. Leancon website

[7] Drew, B., Plummer, A.R. and Sahinkaya, M.N. 2009. A review of wave energy converter technology. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 223(8): 887-902

[8] Wave Dragon ApS Wave Dragon website

[9] Christensen, L., Friis-Madsen, E., Kofoed, J.P., 2005. The Wave Energy Challenge: The Wave Dragon case. PowerGen 2005 Europe Conference - Wave Dragon, 20pp. [2]

[10] Castro-Santos, L., Bento, A.R. and Soares, C.G. 2020. The Economic Feasibility of Floating Offshore Wave Energy Farms in the North of Spain. Energies 13, 806; doi:10.3390/en13040806

[11] Vicinanza, D., Margheritini, L., Kofoed, J.P. and Buccino, M. 2012.The SSG Wave Energy Converter: Performance, Status and Recent Developments. Energies 5: 193-226

[12] Hansen, R.H., Kramer, M.M. and Vidal, E. 2013. Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter. Energies 6: 4001-4044; doi:10.3390/en6084001

[13] Leirbukt, A. and Tubaas, P., 2006. A wave of renewable energy. ABB Review. 3, 29-31. [3]

[14] Wave Star ApS website

[15] Bjerrum, A., 2008. Wave Energy - new unlimited source of energy. European Renewable Energy Conference presentation.

[DEXA-16] 16.0 ^16.1 Dexawave Energy ApS. Dexawave website

[Kofoed-17] Kofoed, J.P., 2009. Hydraulic evaluation of the DEXA wave energy converter. DCE Technical Report No. 57. Dep. of Civil Eng., Aalborg University, 23 pp.

[Martinelli-18] Martinelli L., Zanuttigh,B., Kofoed, J.P., 2009. Statistical analysis of power production from OWC type wave energy converters. EWTEC Conference, Uppsala, 7-11 Sept 2009, electronic format, 9 pp.

[A20-19] 19.0 ^19.1 Ahamed, R., McKee, K. and Howard, I. 2020. Advancements of wave energy converters based on power take off (PTO) systems: A review. Ocean Engineering 204, 107248

[20] Wen, Z., Guo, H., Zi, Y., Yeh, M.-H., Wang, X., Deng, J., Wang, J., Li, S., Hu, C. and Zhu, L. 2016. Harvesting broad frequency band blue energy by a triboelectric–electromagnetic hybrid nanogenerator. ACS Nano 10 (7): 6526–6534

[H2020-21] 21.0 ^21.1 Huang, B., Wang, P., Wang, L., Yang, S. and Wu, D. 2020. Recent advances in ocean wave energy harvesting by triboelectric nanogenerator: An overview Nanotechnology Reviews 9(1)

[22] Paul, J.D. and Davies, I.M., Effects of copper- and tin-based anti-fouling compounds on the growth of scallops (Pecten maximus) and oysters (Crassostrea gigas).Aquaculture, 54(3),191-203.

[23] Metha, P.K., 2001. Reducing the Environmental Impact of Concrete. Concrete International,61-66.

[R18-24] 24.0 ^24.1 Rinaldi, G., Portillo, J.C.C., Khalid, F., Henriques, J.C.C., Thies, P.R., Gato, L.M.C. and Johanning, L. 2018. Multivariate analysis of the reliability, availability, and maintainability characterizations of a Spar–Buoy wave energy converter farm. Journal of Ocean Engineering and Marine Energy 4: 199–215

[25] Sjolte, J. 2014. Marine Renewable Energy Conversion: Grid and Off-Grid Modeling, Design and Operation. Doctoral Thesis, Norges Teknisk-Naturvitenskapelige Universitet, Trondheim, Norway

[26] Azzellino, A., Lanfredi, C., Riefolo, L., De Santis, V., Contestabile, P. and Vicinanza, D. 2019. Combined exploitation of offshore wind and wave energy in the Italian seas: a spatial planning approach. Frontiers in Energy Research 7, 42

[27] Clark, C., Miller, A. and DuPont, B. 2017. Analytical Cost Modeling for Co-Located Wind-Wave Energy Arrays. Proceedings of the 12th European Wave and Tidal Energy, Cork, Ireland, paper 871

[28] Zhou, Y., Ning, D., Shi, W., Joanning, L. and Liang, D. 2020. Hydrodynamic investigation on an OWC wave energy converter integrated into an offshore wind turbine monopile. Coastal Engineering 162, 103731

[29] Zhu, H., Hu, C., Sueyoshi, M. and Yoshida, S. 2020. Integration of a semisubmersible floating wind turbine and wave energy converters: an experimental study on motion reduction. J. Mar. Sci. Technol. 25: 667–674

[30] Perez-Collazo, C., Greaves, G. and Iglesias, G. 2015. A review of combined wave and offshore wind energy. Renewable and Sustainable Energy Reviews 42: 141-153

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 ==Introduction==
-Vast and reliable, wave power has long been considered as one of the most promising renewable energy sources. Wave Energy Converters (WECs) convert wave power into electricity. Although attempts to utilize this resource date back to at least 1890, wave power is currently not widely employed (Miller, 2004<ref>Miller, C.,2004. A Brief History of Wave and Tidal Energy Experiments in San Francisco and Santa Cruz. [http://www.outsidelands.org/wave-tidal.php]</ref>). The plethora of innovational ideas for wave power conversion have been invented in the last three decades, resulting in thousands of patents over recent years. At present, a number of different wave energy concepts are being investigated by companies and academic research groups around the world. Although many working designs have been developed and tested through modelling and wave tank-tests, only a few concepts have progressed to sea testing. Rapidly decreasing costs however, should enable wave plants to compete favorably with conventional power plants in the near future (Pelc and Fujita, 2002<ref>Pelc, R. and Fujita, R.M., 2002. Renewable energy from the ocean. ''Marine Policy'', '''26''',471-479.</ref>).
+Vast and reliable, wave power has long been considered as one of the most promising renewable energy sources. In a special IPCC 2011 report<ref name=IPCC>Lewis, A., Estefen, S., Huckerby, J., Musial, W., Pontes, T. and Torres-Martinez, J. 2011. Ocean Energy. In IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [O. Edenhofer, R. Pichs-Madruga, Y. Sokona, K. Seyboth, P. Matschoss, S. Kadner, T. Zwickel, P. Eickemeier, G. Hansen, S. Schlömer, C. von Stechow (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.</ref> several estimates are presented for the world total potential of ocean wave energy resources. The theoretical maximum has been estimated at about 30,000 TWh/yr (3.10<sup>13</sup> kWh/yr), which is about 20% of the 2019 world energy consumption. However, given technological and economic constraints, the exploitable resource is probably almost a factor 10 less.
+Wave Energy Converters (WECs) convert wave power into electricity. Although attempts to utilize this resource date back to at least 1890, wave power is currently not widely employed (Miller, 2004<ref>Miller, C. 2004. A Brief History of Wave and Tidal Energy Experiments in San Francisco and Santa Cruz. [http://www.outsidelands.org/wave-tidal.php]</ref>). The operational wave power installed worldwide in 2020 totaled approximately 16 MW [https://www.weamec.fr/en/synthesis/ocean-energy-key-trends-and-statistics-2019/], which is about 5 orders of magnitude less than the 2-3 TW required to exploit the global wave energy potential. An important reason is the production costs per kWh, which were in 2020 about a factor 10 higher compared to offshore wind farms <ref>IRENA, 2014. Ocean Energy Technology Brief 4, www.irena.org</ref>.
-==Classifications==
+A plethora of innovational methods for wave power conversion have been invented in the last three decades, resulting in thousands of patents over recent years. At present, a number of different wave energy concepts are being investigated by companies and academic research groups around the world. Although many working designs have been developed and tested through modelling and wave tank-tests, only a few concepts have progressed to sea testing. Strong cost reductions, which are only possible with a sharp increase in global application, should enable wave plants to compete favorably with conventional power plants in the near future (Pelc and Fujita, 2002<ref>Pelc, R. and Fujita, R.M. 2002. Renewable energy from the ocean. Marine Policy, 26: 471-479.</ref>).
-[[Image:DEXA concept.jpg|thumb|right|250px|Figure 1: DEXA concept]]
-===Wave Activated Bodies===
-Wave activated bodies (WABs) are devices with moving elements that are directly activated by the cyclic oscillation of the waves. Power is extracted by converting the kinetic energy of these displacing parts into electric current. One example of such a WAB, is made by a single floater connected to a linear magnetic generator fixed to the seafloor. In other cases, only parts of the body are fully immersed and dragged by the orbital movements of the water. In order to maximally exploit this resource, the moving compounds need to be small in comparison to the [[wavelength]] and preferably they are placed half a wavelength apart. For these reasons, wave activated bodies are usually very compact and light. The main disadvantage of this type of wave energy converters is the high cost of the power generator needed to convert the irregular oscillatory flux into electricity. <br/>
-The "DEXA", developed and patented by DEXA Wave Energy ApS <ref name="DEXA">Dexawave Energy ApS. [https://wavepartnership.dk/dexa-wave Dexawave website]</ref>, is an illustrative example of a WAB. The device consists of two hinged catamarans that pivot relative to the other (Fig. 1). The resulting oscillatory flux at the hinge, is harnessed by means of a water-based low pressure power transmission that restrains angular oscillations. Flux generation is optimized by placing the floaters of each catamaran half a wavelength apart. A scaled prototype (dimensions 44x16.2m<ref name="Kofoed">Kofoed, J.P., 2009. Hydraulic evaluation of the DEXA wave energy converter. ''DCE Technical Report No. 57''. Dep. of
-Civil Eng., Aalborg University, 23 pp.</ref>). placed in the Danish part of the North Sea should generate 160 kW (Martinelli et al., 2009<ref name="Martinelli">Martinelli L., Zanuttigh,B., Kofoed, J.P., 2009. Statistical analysis of power production from OWC type wave energy converters. EWTEC Conference, Uppsala, 7-11 Sept 2009, electronic format, 9 pp.</ref>). Full-scale models are thought to be able to generate up to 250 kW. <ref name="DEXA"/>
+==Wave Energy Conversion Principles==
+In this section the most popular current methods for wave energy conversion are briefly presented.
-[[Image:OWCs examples.jpg|thumb|right|250px|Figure 2: Different concepts of oscillating water columns - Sperboy (above) and REWEC-3 (below)]]
+===Oscillating Water Columns===
-===Oscillating Water Columns===
+[[File:REWEC.jpg|thumb|left|500px|Fig. 1. Wave energy conversion using the principle of the oscillating water column. From IPCC 2011<ref name=IPCC/>.]]
-The functioning of the oscillating water columns (OWCs) is very similar to that of a wind turbine, being based on the principle of wave induced air pressurization. The device is set upon a closed air chamber, which is placed above the water. The passage of waves changes the water level within the closed housing and the rising and falling water level increases and decreases the air pressure within the housing introducing a bidirectional air flow. By placing a turbine on top of this chamber air will pass in and out of it with the changing air pressure levels. There are two options to separate the bi-directional flow: a Wells turbine to create suction or alternatively, pressure generating valves (Kofoed and Frigaard, 2008<ref>Kofoed, J.P. and Frigaard, P., 2008. Hydraulic evaluation of the LEANCON wave energy converter. ''DCE Technical Report No. 45''. Dep. of Civil Eng., Aalborg University, Oct. 2008. Leancon Wave Energy. [http://www.leancon.com/ Leancon website]</ref>). OWC devices can be moored offshore or be placed on the shoreline where waves break.<br/>
+The principle of the oscillating water column is illustrated in Fig. 1, in this example for use in conjunction with a fixed structure (e.g., breakwater). The functioning of the oscillating water columns (OWCs) is very similar to that of a wind turbine, being based on the principle of wave induced air pressurization. The device is set upon a closed air chamber, which is placed above the water. The passage of waves changes the water level within the closed housing and the rising and falling water level increases and decreases the air pressure within the housing introducing a bidirectional air flow. By placing a turbine on top of this chamber air will pass in and out of it with the changing air pressure levels.
+[[File:WellsTurbine.jpg|thumb|right|150px|Fig. 2. Wells turbine. From<ref name=FH>Falcao, F.O. and Henriques, J.C.C. 2016. Oscillating-water-column wave energy converters and air turbines: A review. Renewable Energy 85:1391-1424</ref>).]]
+There are two options to separate the bi-directional flow: a [https://en.wikipedia.org/wiki/Wells_turbine Wells turbine] to create suction or alternatively, pressure generating valves (Kofoed and Frigaard, 2008<ref>Kofoed, J.P. and Frigaard, P., 2008. Hydraulic evaluation of the LEANCON wave energy converter. ''DCE Technical Report No. 45''. Dep. of Civil Eng., Aalborg University, Oct. 2008. Leancon Wave Energy. [http://www.leancon.com/ Leancon website]</ref>). The wells turbine is constructed such that it rotates always in the same direction regardless of airflow direction, see Fig. 2. The claimed efficiency is 60-65%, a bit lower than conventional turbines, but not achievable by conventional turbines in alternating mode<ref>Drew, B., Plummer, A.R. and Sahinkaya, M.N. 2009. A review of wave energy converter technology. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 223(8): 887-902</ref>.
+{| border="0" align="left"
+|-
+| valign="top"|
+[[File:OWCbuoy.jpg|thumb|left|150px|Fig. 3. Wave energy buoy based on the oscillating water column principle.]]
+| valign="top"|
+[[File:SparBuoy.jpg|thumb|left|150px|Fig. 4. Spar buoy wave energy converter.]]
+|}
+OWC devices can be moored offshore but they can also be placed near the shore where waves break. This results in significant cost savings. The disadvantage is the shallow water depth along the shore, which dampens the largest waves. See also [[Wave energy converters in coastal structures]].
-An example of an offshore OWC is the "Sperboy", developed and patented by Embley Energy LTD <ref>Embley Energy LTD [https://openei.org/wiki/Embley_Energy_Ltd Sperboy tm website]</ref>. It is circular in plane and therefore invariant to wave direction (Fig. 2).  Its size varies according to the target sea conditions at the deployment site but maximum dimensions are set at 30m diameter, 50m height and 35m draft. Up to 450 kW mean annual output can be obtained from this concept. An inshore example is the resonant wave energy converter "REWEC-3", created by the Università degli Studi "Mediterranea" di Reggio Calabria (Fig. 2). It operates much like conventional concrete caisson breakwaters but here, each caisson is fitted with a Wells turbine. Efficiency of these devices is generally considered to be high (Boccotti, 2003<ref>Boccotti, P., 2003. On a new wave energy absorber. ''Ocean Engineering'', '''30''', 1191–1200.</ref><ref>Ricerca Italiana [https://www.sts-italia.it/energia-rinnovabile-dal-mare/ Ricerca Italiana on REWEC-3]</ref>).
+Special buoys have been developed for the application of OWC converters in deep water, according to the principle of Fig. 3. The length of the shaft determines the resonance frequency, allowing optimum energy efficiency to be achieved. An example of an offshore OWC is the Spar Buoy, Fig. 4. The original concept was invented by Yoshio Masuda (1925-2009), who developed navigation buoys powered by wave energy, equipped with an air turbine. It is circular in plane and therefore invariant to wave direction (Fig. 4).  The size varies according to the target sea conditions at the deployment site but maximum dimensions were estimated at 30m diameter, 50m height and 35m draft, which could deliver up to 450 kW.
+<br clear=all>
-[[Image:OVTs examples.jpg|thumb|right|250px|Figure 3: Different overtopping devices - Wave Dragon (Above) and SSG (below) ]]
+[[Image:WaveDragon.jpg|thumb|right|300px|Figure 5: Wave Dragon overtopping wave energy converter. ]]
 ===Overtopping Devices===
-Another type of Wave energy converter is the overtopping device, which works much like a hydroelectric dam. The "Wave Dragon" created by Wave Dragon ApS<ref>Wave Dragon ApS [http://www.wavedragon.net/ Wave Dragon website]</ref> is an example of an offshore overtopping device (Fig. 3). Its floating arms focus waves onto a slope from which the wave overtops into a reservoir. The resulting difference in water elevation between the reservoir and the mean sea level then drives low-head hydro turbines. Proposed optimal size design of 260m width and 150m length will produce 4 MW. In wave climates above 33 kW/m, this technology is expected to be economically competitive with offshore wind power in the near future. After a combined cost saving and power efficiency increase, the power price will eventually be in line with costs of fossil fuel generation (Christensen et al., 2005<ref>Christensen, L., Friis-Madsen, E., Kofoed, J.P., 2005. The Wave Energy Challenge: The Wave Dragon case. ''PowerGen 2005 Europe Conference - Wave Dragon'', 20pp. [http://ppemactron.com/pdf/Wave%20energy%20conersion%20overall%202005.pdf]</ref>). <br/>
+Another type of wave energy converter is the overtopping device, which works much like a hydroelectric dam. The "Wave Dragon" created by Wave Dragon ApS<ref>Wave Dragon ApS [http://www.wavedragon.net/ Wave Dragon website]</ref> is the best known example of an offshore overtopping device (Fig. 5). Its floating arms focus waves onto a slope from which the wave overtops into a reservoir. The resulting difference in water elevation between the reservoir and the mean sea level then drives low-head hydro turbines. Proposed optimal size design of 260m width and 150m length can produce up to 4 MW. In wave climates above 33 kW/m, this technology was expected to be economically competitive with offshore wind power in the near future. After a combined cost saving and power efficiency increase, the power price could eventually be in line with costs of fossil fuel generation (Christensen et al., 2005<ref>Christensen, L., Friis-Madsen, E., Kofoed, J.P., 2005. The Wave Energy Challenge: The Wave Dragon case. ''PowerGen 2005 Europe Conference - Wave Dragon'', 20pp. [http://ppemactron.com/pdf/Wave%20energy%20conersion%20overall%202005.pdf]</ref>). However, a feasibility study for a Wave Dragon deployment at the northern Spanish coast published in 2020 reported costs which were still a factor 10 higher<ref>Castro-Santos, L., Bento, A.R. and Soares, C.G. 2020. The Economic Feasibility of Floating Offshore Wave Energy Farms in the North of Spain. Energies 13, 806; doi:10.3390/en13040806</ref>.
+Near the shore, overtopping converters can be installed in front of or as part of caisson breakwaters. An example is the SeaWave Slot-Cone Generator (SSG)<ref>Vicinanza, D., Margheritini, L., Kofoed, J.P. and Buccino, M. 2012.The SSG Wave Energy Converter: Performance, Status and Recent Developments. Energies 5: 193-226</ref>, which collects seawater by wave overtopping over several reservoirs placed above each other, resulting in high hydraulic efficiency, see [[Wave energy converters in coastal structures]].
+===Wave absorbing devices===
+A great number of different devices have been developed to harness wave energy directly by using wave-induced water motion. The most popular types are displayed schematically in Fig. 6.
+[[File:WaveAbsorbers.jpg|thumb|center|700px|Fig. 6. Different energy conversion principles based on absorption of wave energy. Image from <ref>Hansen, R.H., Kramer, M.M. and Vidal, E. 2013. Discrete Displacement Hydraulic Power Take-Off System for the Wavestar Wave Energy Converter. Energies 6: 4001-4044; doi:10.3390/en6084001</ref> www.mdpi.com/journal/energies Creative Commons licence.]]
+====Point absorbers====
+[[Image:Point absorbers and attenuators.jpg|thumb|right|250px|Figure 7: The FO3 point absorber (above) and the Wave Star attenuator (below)]]
+Point absorber are buoy-type wave energy converters (WECs) that harvest incoming wave-energy from all directions. They are placed offshore at the ocean surface or just below. A vertically submerged floater absorbs wave energy which is converted by a piston or linear generator into electricity. One such a point absorber WEC is the FO3 concept developed by Norwegian entrepreneur Fred Olsen. It consists of several (12 or 21) heaving floaters attached to a 36 by 36 meter rig (Fig. 7). By means of a hydraulic system, the vertical motion is converted into a rotational movement that drives the hydraulic motor. This motor in turn powers the generator that can produce up to 2,52 MW (Leirbukt and Tubaas, 2006<ref>Leirbukt, A. and Tubaas, P., 2006. A wave of renewable energy. ''ABB Review.'' '''3''', 29-31. [http://www05.abb.com/global/scot/scot271.nsf/veritydisplay/1e2fadd298a58d14c12571d900412482/$file/29-31%203m646_eng72dpi.pdf]</ref>). <br/>
+The multiple point absorber type WEC "Wave Star", developed by Wave Star ApS<ref>Wave Star ApS [http://wavestarenergy.com/ website]</ref>, has a number of floaters on movable arms (Fig. 7). The energy of the motion of the arms is again captured in a common hydraulic line and converted into electric current. Most noticeably, being able to raise the entire installation along its pillars, this system has a high endurance for rough storm conditions. So far, this method has not been deployed at full scale. A 1:2 scaled installation has been built at Hanstholm which turns out 600 kW. However, production is thought to be scale-able up to 6 MW (Bjerrum, 2008<ref>Bjerrum, A., 2008. Wave Energy - new unlimited source of energy. ''European Renewable Energy Conference presentation.''</ref>).  A major benefit of these types of exploitation is the minimal contact with water, placing any delicate machinery and electrics out of reach of any corrosion or physical forcing of the waves.
+====Wave Activated Bodies (WAB)====
+[[Image:DEXA concept.jpg|thumb|right|250px|Figure 8: DEXA concept]]
+Wave activated bodies (WABs), also called 'wave attenuators' or 'line absorbers', are devices with moving elements that are directly activated by the cyclic oscillation of the waves. Power is extracted by converting the kinetic energy of these displacing parts into electric current. One example of such a WAB, is made by a single floater connected to a linear magnetic generator fixed to the seafloor. In other cases, only parts of the body are fully immersed and dragged by the orbital movements of the water. In order to maximally exploit this resource, the moving compounds need to be small in comparison to the [[wavelength]] and preferably they are placed half a wavelength apart. For these reasons, wave activated bodies are usually very compact and light. The main disadvantage of this type of wave energy converters is the high cost of the power generator needed to convert the irregular oscillatory flux into electricity. <br/>
+The "DEXA", developed and patented by DEXA Wave Energy ApS <ref name="DEXA">Dexawave Energy ApS. [https://wavepartnership.dk/dexa-wave Dexawave website]</ref>, is an illustrative example of a WAB. The device consists of two hinged catamarans that pivot relative to the other (Fig. 8). The resulting oscillatory flux at the hinge, is harnessed by means of a water-based low pressure power transmission that restrains angular oscillations. Flux generation is optimized by placing the floaters of each catamaran half a wavelength apart. A scaled prototype (dimensions 44x16.2m<ref name="Kofoed">Kofoed, J.P., 2009. Hydraulic evaluation of the DEXA wave energy converter. ''DCE Technical Report No. 57''. Dep. of
+Civil Eng., Aalborg University, 23 pp.</ref>) placed in the Danish part of the North Sea should generate 160 kW (Martinelli et al., 2009<ref name="Martinelli">Martinelli L., Zanuttigh,B., Kofoed, J.P., 2009. Statistical analysis of power production from OWC type wave energy converters. EWTEC Conference, Uppsala, 7-11 Sept 2009, electronic format, 9 pp.</ref>). Full-scale models are thought to be able to generate up to 250 kW. <ref name="DEXA"/>
+==Power take-off systems (PTO)==
+The power take-off (PTO) system of a wave energy converter has a direct impact on the capital cost of a project by usually accounting for between 20 and 30% of the overall investment. The economic viability, efficiency and complexity of a wave energy converter depends largely on its PTO system. Maintenance at sea is a demanding and expensive task; high reliability and durability of all components of the PTO system are therefore required. This is technically challenging for systems exposed to the harsh marine environment, especially systems that consist of many moving parts that are exposed to corrosion and fouling. Several popular PTO systems have been reviewed by Ahamed et al. (2020<ref name=A20>Ahamed, R., McKee, K. and Howard, I. 2020. Advancements of wave energy converters based on power take off (PTO) systems: A review. Ocean Engineering 204, 107248</ref>), from which the summary below is extracted.
+[[File:DirectElectricalDrive.jpg|thumb|150px|left|Fig. 9. Schematic of a linear electrical generator based on a permanent magnet generator. Image from Ahamed et al. (2020<ref name=A20/>).]]
+Oscillating water column wave converters (OWC) generally use the  previously described Wells turbine as PTO system. These turbines are vulnerable due to the relatively large number of moving parts.
+Overtopping devices are generally equipped with hydro turbines for power take-off. Conventional hydro turbines require for high efficiency more head and flow than provided by overtopping ocean waves.
+Wave absorbing devices use hydraulic motor systems or direct mechanical or electrical drive PTO systems. Hydraulic motor-based PTO systems are suited for converting the low-speed oscillating motion into energy. However, the hydraulic motor-based PTO system consists of many mechanical moving parts and due to compression and decompression of the fluid there is a risk of hydraulic oil leakage.
+Direct mechanical drive uses linear-to-rotary conversion systems without pneumatic or hydraulic systems. The efficiency is high, but the lifetime is relatively short and maintenance costs are substantial. Direct electrical drive transmits the wave energy into electrical energy directly by coupling the mechanical energy to the moving part of a linear generator (Fig. 9). Issues are the need for a heavy structure due to the attractive forces between the stator and the translator and the complicated power transmission system due to the unequal generated voltage created by the irregular wave motion.
-Near shore, OVTs can be installed in front of or as part of caisson breakwaters. The Norwegian company WAVEnergy<ref>Vicinanza, D., Margheritini, L., Kofoed, J.P. and Buccino, M. 2012.The SSG Wave Energy Converter: Performance, Status and Recent Developments. Energies 5: 193-226  [https://www.researchgate.net/publication/233420375_The_SSG_Wave_Energy_Converter_Performance_Status_and_Recent_Developments]</ref> is developing an integrated multi level overtopping device named the "SeaWave Slot-Cone Generator (SSG)" (Fig. 3). The SSG has the advantage of harvesting wave energy in several reservoirs placed above eachother, resulting in high hydraulic efficiency. The reservoir capacity smooths out the irregularity of incoming waves, providing a regular electricity output to the grid. Additionally, with the turbine shaft and the gates controlling the water flow, SSG is built as a robust concrete structure with few moving parts in the mechanical system. This most likely makes it a low maintenance, durable system. Other SSG designs can be deployed onshore or offshore.
+The [https://en.wikipedia.org/wiki/Nanogenerator#Triboelectric_nanogenerator 'Triboelectric nanogenerator' (TENG)] and its extension, the 'Triboelectric−Electromagnetic Hybrid Nanogenerator' (TENG-EMG)<ref>Wen, Z., Guo, H., Zi, Y., Yeh, M.-H., Wang, X., Deng, J., Wang, J., Li, S., Hu, C. and Zhu, L. 2016. Harvesting broad frequency band blue energy by a triboelectric–electromagnetic hybrid nanogenerator. ACS Nano 10 (7): 6526–6534</ref>, are a new development that can efficiently harvest energy in any frequency range, are low-cost lightweight, easy to fabricate and easy to be scaled (size of individual units being below the micrometer scale). The nanogenerator uses a polymer-metal pair to create contact electrification ([https://en.wikipedia.org/wiki/Triboelectric_effect triboelectric effect]) between two materials sliding against each other and to induce charge transfer between their electrodes due to electrostatic induction, either in a layer structure or in a spherical-shell structure<ref name=H2020>Huang, B., Wang, P., Wang, L., Yang, S. and Wu, D. 2020.  Recent advances in ocean wave energy harvesting by triboelectric nanogenerator: An overview Nanotechnology Reviews 9(1)</ref>, see Fig. 10. Challenges for application in a prototype lie in the use of TENGs for power transfer to the shore, cost to scale, lifetime of the TENG materials in the ocean environment and connection methods of the thousands of TENG units that are needed.
+[[File:TENG.jpg|thumb|center|700px|Fig. 10.  Principle of tribo-electric nanogenerators. (a) Triboelectric charges are generated on the surface of two laterally sliding dielectric films, due to frictional effects. Polarization occurring in the sliding plane drives an electron flow between the metal electrodes that generates an alternating current. (b) Soft contact rolling-mode spherical TENG (SS-TENG). When receiving external vibration from the ocean waves, the ball will roll back and forth between the electrodes to provide AC power to the external load. The soft ball maximizes the contact area. Images from Huang et al. (2020<ref name=H2020/>), Creative Commons licence.]]
-[[Image:Point absorbers and attenuators.jpg|thumb|right|250px|Figure 4: The FO3 point absorber (above) and the Wave Star attenuator (below)]]
-===Point absorbers and Attenuators===
-Point absorber are buoy-type WECs that harvest incoming wave-energy from all directions. They're placed offshore at or near the ocean surface. A vertically submerged floater absorbs wave energy which is converted by a piston or linear generator into electricity. One such a point absorber WEC is the FO3 concept developed by Norwegian entrepreneur Fred Olsen. It consists of several (12 or 21) heaving floaters attached to a 36 by 36 meter rig (Fig. 4). By means of a hydraulic system, the vertical motion is converted into a rotational movement that drives the hydraulic motor. This motor in turn powers the generator that can produce up to 2,52 MW (Leirbukt and Tubaas, 2006<ref>Leirbukt, A. and Tubaas, P., 2006. A wave of renewable energy. ''ABB Review.'' '''3''', 29-31. [http://www05.abb.com/global/scot/scot271.nsf/veritydisplay/1e2fadd298a58d14c12571d900412482/$file/29-31%203m646_eng72dpi.pdf]</ref>). <br/>
-Comparable, the attenuator type WEC "Wave Star", developed by Wave Star ApS<ref>Wave Star ApS [http://wavestarenergy.com/ website]</ref>, has a number of floaters on movable arms (Fig. 4). The energy of the motion of the arms is again captured in a common hydraulic line and converted into electric current. Most noticeably, being able to raise the entire installation along its pillars, this system has a high endurance for rough storm conditions. So far, this method has not been deployed at full scale. A 1:2 scaled installation has been built at Hanstholm which turns out 600 kW. However, production is thought to be scale-able up to 6 MW (Bjerrum, 2008<ref>Bjerrum, A., 2008. Wave Energy - new unlimited source of energy. ''European Renewable Energy Confrence presentation.''</ref>).  A major benefit of these types of exploitation is the minimal contact with water, placing any delicate machinery and electrics out of reach of any corrosion or physical forcing of the waves.
+==Issues with wave energy converters==
+To be able to produce energy at competitive prices, several obstacles have to be overcome. The following factors in particular determine the high costs of wind energy.
+A very robust construction made of high quality materials is required that remains intact under severe storms and withstands the demanding conditions at sea, leading to corrosion, fouling and fatigue. The classical protection measure against fouling and corrosion of steel structures is regular maintenance and repainting. But this is time-consuming and costly because of the difficult access to offshore installations, especially under harsh conditions when damage occurs. In addition, the use of antifouling paints may be detrimental to the marine environment (e.g. Tributyltin paints<ref>Paul, J.D. and Davies, I.M., Effects of copper- and tin-based anti-fouling compounds on the growth of scallops (Pecten maximus) and oysters (Crassostrea gigas).''Aquaculture'', '''54(3)''',191-203.</ref>). Full-scale devices out of concrete may provide a valuable alternative since concrete is long-lived if properly mixed (Metha, 2001<ref>Metha, P.K., 2001. Reducing the Environmental Impact of Concrete. ''Concrete International'',61-66.</ref>). The cost of repairs is a major component of the wave farm costs. The reliability of the components, especially due to the cost of eventual repair actions, is pivotal for the economic viability of a project<ref name=R18>Rinaldi, G., Portillo, J.C.C., Khalid, F., Henriques, J.C.C., Thies, P.R., Gato, L.M.C. and Johanning, L. 2018. Multivariate analysis of the reliability, availability, and maintainability characterizations of a Spar–Buoy wave energy converter farm. Journal of Ocean Engineering and Marine Energy 4: 199–215</ref>. Deployments should be capable of lasting 30 years or more. This is a tall order as demonstrated by the mooring of the Wave Dragon that failed during a severe storm on the 8<sup>th</sup> of january 2004.
-==Challenges==
+The power generated by ocean waves fluctuates strongly due to the irregular wave climate, which makes connection to the electricity grid difficult<ref>Sjolte, J. 2014. Marine Renewable Energy Conversion: Grid and Off-Grid Modeling, Design and Operation. Doctoral Thesis, Norges Teknisk-Naturvitenskapelige Universitet, Trondheim, Norway</ref>. In addition, connection points to the electricity grid may not be available in areas where the conditions for wave energy generation are most favorable.
-Designs are quite different from WEC to WEC, mainly due to differences in energy harvesting and subsequent conversion (Power Take-Off). Nevertheless, each design faces the same challenges. They should be optimized to effectively extract wave energy under most wave conditions while used materials are to withstand the classical problems of marine technologies i.e. corrosion, fatigue, [[biofouling]], impact loading and fractures. The classical protection measure against fouling and corrosion of steel structures is regular maintenance and repainting. But this is time-consuming and costly. In addition, the use of antifouling paints may be detrimental to the marine environment (e.g. Tributyltin paints<ref>Paul, J.D. and Davies, I.M., Effects of copper- and tin-based anti-fouling compounds on the growth of scallops (Pecten maximus) and oysters (Crassostrea gigas).''Aquaculture'', '''54(3)''',191-203.</ref>). Fullscale devices out of concrete (e.g. Dexa, WaveStar, SSG, REWEC3, Wave Dragon) may provide a valuable alternative since concrete is long-lived if properly mixed (Metha, 2001<ref>Metha, P.K., 2001. Reducing the Environmental Impact of Concrete. ''Concrete International'',61-66.</ref>). <br/>
-Crucial for any design is the mooring which ensures a maintained position under both normal operating loads as well as extreme storm load conditions. It shouldn't exert excess tension loads on the electrical transmission cables and ensure the suitable safety distances between devices in multiple installations. Most commonly, a free hanging catenary configuration is used for mooring but multi-catenary systems and flexible risers are not infrequent as well. Every configuration should be sufficiently compliant to accommodate tidal variations and environmental loading while remaining sufficiently stiff to allow berthing for inspections and maintenance. Finally, the system should be capable of lasting 30 years or more. A tall order as demonstrated by the mooring of the Wave Dragon that failed during a severe storm on the 8<sup>th</sup> of january 2004.
+Crucial for any design is the mooring which ensures a maintained position under both normal operating loads as well as extreme storm load conditions. It should not exert excess tension loads on the electrical transmission cables and ensure the suitable safety distances between devices in multiple installations. Most commonly, a free hanging catenary configuration is used for mooring but multi-catenary systems and flexible risers are not infrequent as well. Every configuration should be sufficiently compliant to accommodate tidal variations and environmental loading while remaining sufficiently stiff to allow berthing for inspections and maintenance.
+Due to the great variability of the wave climate with rare extremes, it is important to be able to test prototype designs over a long period before they are deployed on a large scale. Mathematical simulation models can be of great value here, because this is a quick and relatively inexpensive way to test a design for its effectiveness and efficiency at the long term, both in terms of capital and operational expenses, to achieve the lowest overall cost of electricity. For this purpose, the modeling tool supports the decision-maker in the pursuit of the most reliable and easy to maintain device design but also reports the trade-off between energetic yield and operational and maintenance efforts<ref name=R18/>.
-==Wave energy converters as a coastal defense technique==
+The power take-off system is a crucial part of wave energy converters, which largely determines the cost, efficiency and reliability of the WEC. Many studies focus on optimisation of the PTOs; several challenges related to PTOs are mentioned in the previous section.
-Wave Energy Converters are generally not very sensitive to sea level rise since this is expected to be small over the lifetime of most designs. The design of coastal defending WECs should however be ideally optimized to reflect and/or absorb a significant part of incident wave energy under all wave conditions, especially when presented with rough conditions. Unfortunately, the geometry of the layout which maximizes wave attenuation is yet to be determined. Existing deeply tested WECs  such as the Wave Dragon and the DEXA devices are being investigated as coastal protection measures. The DEXA devices are small and therefore might be cost effective (Kofoed, 2009<ref name="Kofoed"/>). Placement in shallow waters may reduce the transmitted energy in a differential way which could alter the coastal morphology (Ruol, 2010; Zanuttigh, 2010<ref>Ruol, P., Zanuttigh, B., Martinelli, L., Kofoed, P., Frigaard, P., 2010. Near-shore Floating Wave Energy Converters: Applications for Coastal Protection. ICCE 2010 Shanghai, Jun. 2010.</ref><ref>Zanuttigh B., Martinelli, L., Castagnetti, M., Ruol, P., Kofoed, J.P., Frigaard, P., 2010. Integration of wave energy converters into a coastal protection schemes. 3rd Int. Conf. on Ocean Energy ICOE – Bilbao, Ott. 2010.</ref>). The Wave Dragon was chosen for its large energy absorption and reflection capacities. According to numerical simulations, the wave climate behind a single Wave Dragon and an array has significantly reduced wave heights (Beels, 2009<ref>Beels, C., 2009. Optimization of the lay-out of a farm of wave energy converters in the North Sea : analysis of wave power resources, wake effects, production and cost. PhD dissertation. Academic Year 2009-2010. Ghent University, Faculty of Engineering.</ref>). However, the model was not based on validated absorption and reflection performances. More detailed knowledge on the performance of a single device needs to be generated before a reliable model can be created that, ultimately, will contribute to optimization of the array lay-out {{THESEUS}}
-==See also==
+==Wave energy converters combined with offshore wind==
+The high cost of stand-alone wave energy conversion schemes is a major obstacle for its large-scale application. However, costs can be significantly reduced by combining WECs with structures offshore or along the coast that are being built for other applications. A good example is the integration of WECs with breakwaters in the coastal zone. This is described in the article [[Wave energy converters in coastal structures]].
+Another possible combination is the integration of wind and wave energy production. This is especially interesting in areas where the conditions for optimal wind energy production do not systematically coincide with conditions for optimal wave energy production. It is also a way to make optimal use of marine space<ref>Azzellino, A., Lanfredi, C., Riefolo, L., De Santis, V., Contestabile, P. and Vicinanza, D. 2019. Combined exploitation of offshore wind and wave energy in the Italian seas: a spatial planning approach. Frontiers in Energy Research 7, 42</ref>. The main advantage of integrated wind power generation is shared infrastructure costs, especially foundations and grid connections. However, with the existing WEC techniques, the costs per kWh produced are still higher with a combined wind-wave application than with wind energy alone<ref>Clark, C., Miller, A. and DuPont, B. 2017. Analytical Cost Modeling for Co-Located Wind-Wave Energy Arrays. Proceedings of the 12th European Wave and Tidal Energy, Cork, Ireland, paper 871</ref>. Synergy benefits can also be sought through improved stability of the structure, for example in the case of an OWC wave energy converter integrated into an offshore wind turbine monopile<ref>Zhou, Y., Ning, D., Shi, W., Joanning, L. and Liang, D. 2020. Hydrodynamic investigation on an OWC wave energy converter integrated into an offshore wind turbine monopile. Coastal Engineering 162, 103731</ref>. Stability improvement can be a major benefit for designs in which the interaction between the wind and wave sub-structures is strong, as in the case of a WEC combined with a floating wind turbine<ref>Zhu, H., Hu, C., Sueyoshi, M. and Yoshida, S. 2020. Integration of a semisubmersible floating wind turbine and wave energy converters: an experimental study on motion reduction. J. Mar. Sci. Technol. 25: 667–674</ref>. Wave energy converters can also reduce wave heights inside a wind farm, increasing in this way the weather windows to access the wind turbines<ref>Perez-Collazo, C., Greaves, G. and Iglesias, G. 2015. A review of combined wave and offshore wind energy. Renewable and Sustainable Energy Reviews 42: 141-153</ref>.
+==International organizations==
+Several organizations have been established for worldwide collaboration on ocean wave energy development involving research institutes, wave energy developers, operators and governments.
+[https://www.oceanenergy-europe.eu/ocean-energy/wave-energy/  Ocean Energy Europe (OEE)], a not-for-profit organization, is a network of ocean energy professionals collaborating with 120 organizations, including Europe’s top utilities, industrialists and research institutes. It aims to create a strong environment for the development of ocean energy, improve access to funding, and enhance business opportunities for its members. To this end, OEE engages with the European Institutions (Commission, Parliament, Council, EIB, etc.), and national ministries on policy issues affecting the sector. Ocean Energy Europe also organizes the annual Ocean Energy Europe Conference and Exhibition – the annual event which gathers representatives from the industry, Ministers and Commissioners.
+The [https://www.ocean-energy-systems.org/ Ocean Energy Systems Technology Collaboration Programme (OES)], established by the International Energy Agency (IEA), is an international organisation that is actively collaborating between its 29 member countries with the aim of advancing ocean energy research, development and demonstration of technologies to generate electricity from ocean energy resources (Wave, Tidal Range, Tidal and Ocean Currents, Ocean Thermal Energy Conversion (OTEC) and Salinity Gradients). The primary education and research initiatives of OES are aimed at promoting the feasibility, recognition and adoption of ocean energy systems in an environmentally acceptable manner.
+The [http://www.emec.org.uk/ European Marine Energy Centre (EMEC) Ltd] provides developer companies, wave and tidal energy converter-technology developer with certified open-sea testing services. The objective of EMEC is to decrease the time, cost, and risk related with marine energy technology development, increasing the use of EMEC’s modified facilities as well as industry knowledge and other experiences. EMEC has 13 grid connected test berths where many marine energy converters have been installed. EMEC is an independent organization that maintains relations with various developer companies, academic organizations and governing bodies and presently working with companies and researchers to broaden research programmes to solve a variety of environmental and operational challenges relevant to industry.
+The [https://www.wecanet.eu/ European Network for marine renewable energy (WECANet)] is a network of 31 partner countries to assist networking, training and collaboration in Europe. Four working groups address the following topics: numerical modelling for WECs, experimental hydrodynamic modelling including PTO systems, technology development and economical / political aspects.
+[https://www.oceancofund.eu/ OCEANERA-NET] organizes funding programmes to support research and innovation between European countries and regions in the ocean energy sector.
+International conferences on ocean energy are organized by
+*[https://ewtec.org/ European Wave and Tidal Energy Conference (EWTEC)]. Selected peer-reviewed conference contributions are published in the EWTEC’s International Marine Energy Journal.
+*[https://www.awtec-headoffice.com/ Asian Wave and Tidal Energy conference (AWTEC)] addresses updates on recent global activities and initiatives with a distinctly special interest in the Asian region. The conference cooperates with the Journal of Marine Science and Engineering for publication of selected conference contributions.
+*[https://www.icoe-conference.com/ International Conference on Ocean Energy (ICOE)] is a biennial global marine energy event focused on the industrial development of renewable marine energy. ICOE is a collaboration with the IEA Technology Collaboration Programme – Ocean Energy Systems (OES).
+==Related articles==
 :[[Wave energy converters in coastal structures]]
+==External links==
+[https://www.ocean-energy-systems.org/documents/84169-oes-spotlight-on-ocean-energy.pdf/ Ocean Energy Systems website, 2018 report Spotlight on ocean energy]
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 [[Category:Marine energy]]
 [[Category:Coastal and marine human activities and uses‏‎]]

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