Universal access to clean and renewable electricity is one of the seventeen sustainable development goals of the United Nations Development Program (UNDP) [1]. To achieve this goal, it is necessary to decarbonize the electricity generation process by promoting an energy transition from carbon-based to renewable and clean sources. According to Smil (2010) [2], past and future shifts in the energy base are inherent processes of human evolution and are carried out through changes in technology, economy, and society. The modern transition model proposes changes in the economy through technology, with society’s commitment to transform the current energy base into one with lower carbon consumption, ensuring the conscious and sustainable use of natural resources.

Brazil’s energy matrix, with an installed capacity of 174.7 GW (2020), mainly consists of renewable energy sources (84%) [3]. The current matrix includes 62.5% from hydroelectric power plants (from large to small power plants); 21.5% from other renewable sources, such as biomass, solar, and wind; and 16% from nuclear and carbon-based sources, such as oil, coal, and natural gas [3]. Brazil ranks 12th among the countries with the highest CO₂ emissions in the world, with 441 million tons of CO₂eq, according to data from the Emissions Database for Global Atmospheric Research (EDGAR) from the European Commission [4]. Brazil ratified the Paris Agreement [5] in 2015, committing to a Nationally Determined Contribution (NDC) goal of reducing its greenhouse gas (GHG) emissions by 37% by 2025 and by 43% by 2030, compared with its levels in 2005. In 2021, Brazil further pledged to achieve a 50% reduction in GHG emissions by 2030 and to reach carbon neutrality by 2050 [6].

In Brazil, the National Energy Plan (PNE) report ^[7][8][7,8] outlines the energy planning strategies and goals for the next 30 years. The most recent document [8] considers the expansion of the energy matrix from the perspectives of the energy transition and energy matrix decarbonization. Two hypotheses are presented: the first involves increasing the use of renewable energy by 100% and not increasing the use of fossil fuel sources. In contrast, the second involves increasing the use of fossil fuel sources if carbon sequestration occurs, resulting in a 0% emission expansion. In this PNE [8], two extreme economic scenarios were established for Brazil: a pessimistic scenario, characterized by stagnant economic growth but maintenance of the current electricity demand, and an optimistic scenario, characterized by accelerated economic growth that requires a 330% increase in the electricity supply from the levels in 2020. If the economy stagnates, the energy matrix will naturally expand. However, if the economy experiences much growth, the energy matrix needs to expand quickly, presenting challenges in ensuring steady supply as consumption is forecasted to increase from 621 TWh/year, recorded in 2020 [3], to 2053 TWh/year in 2050 [8]. In the high economic growth scenario, the report encourages the use of photovoltaic and wind energy, which are renewable, clean, and non-carbon-emitting sources. However, the report also considers the possible implementation of carbon-based thermal projects and new large hydropower plant projects, including, if necessary, Amazonian plants with environmental restrictions, which may contradict the decarbonization, energy transition, and GHG emission reduction goals [8].

Ocean energy only appears in expansion plans as part of the matrix of available energy resources [8]. Even when ocean energy is specifically addressed, only the international panorama and its national theoretical potential are highlighted, emphasizing the need for studies on tidal currents energy and pointing out ocean thermal energy as a promising resource in Brazil. However, when considering the geomorphological characteristics of the Brazilian coast, it becomes apparent that there is a coastal strip of approximately 7490 km, which includes various geometries and morphologies for the shorelines, some of which are potentially suitable for the implementation of wave energy farms. Overall, a theoretical potential of the order of 114 GW (998 TWh/year) was estimated for the Brazilian coast, which was subdivided into 27 GW of tidal potential and 87 GW of wave potential [9], a value that corresponds to 1.6 times the Brazilian energy consumption in 2020 (621 TWh/year [3]). Despite this potential, in Brazil, there are only experimental projects using this type of energy, with the most relevant being the tidal project of the Bacanga dam [10] and the wave pilot project of the Pecém port [11]. The latest one, designed by the Alberto Luiz Coimbra Institute of Graduate Studies and Research in Engineering (COPPE) of the Federal University of Rio de Janeiro (UFRJ) in partnership with Tractebel-Engie S.A., had a capacity of 50 kW and was implemented in 2012 on the port’s breakwater. The project aimed to test a wave energy harvesting technology where large floats coupled to articulated mechanical arms pressurized freshwater into accumulators connected to hyperbaric chambers. The internal pressure of these chambers corresponded to 200 and 400 m in the water column and activated a Pelton turbine [11]. Despite the positive results, this project was discontinued and is currently inactive (2022) [12]. Despite the experiments developed in Brazil, there are still no legal devices or any specific legislation regarding the implementation of ocean energy parks. This is due to the fact that the country still has a reasonable amount of available hydraulic potential and has made advances in wind and solar energy, which have competitive costs.

The United States, China, and the United Kingdom, as well as other European countries, have been discussing and encouraging research and development of ocean energy as an alternative to diversify their energy matrix ^[13][14][13,14]. Since the oil crisis in 1973, these countries have been seeking alternative ways to ensure their energy security without depending on oil [13]. The pioneering work in modern studies on the use of ocean energy was carried out in Japan in 1940 by Yoshio Masuda, who developed a signaling buoy fitted with a turbine driven by wave motion. Masuda also built a barge equipped with converters of various configurations and turbines, thus expanding the research on wave energy conversion [15]. Since then, numerous researchers have dedicated themselves to studying wave energy over the past five decades (1970–2020). Salter [16] published a study on wave energy in 1974 that attracted global attention to this energy resource. McCormick [17] became the first academic author to publish a book dedicated to this topic, in 1981 [14]. Evans ^[18][19][18,19] and Falnes ^[14][20][14,20] focused on studying wave–device interactions. Falcão [13], in Portugal, studied oscillating water column (OWC) devices ^[21][22][21,22] and their interaction with self-rectifying turbines ^[23][24][25][23,24,25]. Setoguchi ^[26][27][26,27] and Raghunathan ^[28][29][28,29] worked on designing and optimizing Wells-type self-rectifying turbines. Ocean energy harvesting technology is still developing and maturing despite these efforts over the past fifty years. Although it presents potential as a significant energy resource, using this energy still involves high costs in both the implementation and production phases [30], posing a challenge to the technology’s commercial viability.

2. Ocean Energy

There are five methods for harnessing ocean energy, each with a specific converter device and a distinct exploitation area, whether onshore, nearshore, or offshore ^[12][31][12,31]. Salinity gradient energy is obtained through the difference in salinity concentration between seawater and freshwater or simply through the salinity concentration difference in seawater [32]. So far, only two techniques are viable for extracting this energy: Reverse Electrodialysis (RED), which extracts energy directly from the chemical process, and Pressure Retarded Osmosis (PRO), which uses the pressure difference resulting from osmosis [33]. Ocean thermal gradient energy is obtained through the temperature difference in seawater, which requires a minimum difference of 20 °C, which is only possible below an 800 m depth. The process of conversion into electrical energy uses the Rankine cycle, transforming thermal energy into mechanical energy [34]. In harnessing tidal current energy, the hydrokinetic energy due to the upward and downward movements of the astronomical tide is harvested, which drives a turbine generator group. This harnessed energy resembles wind and hydraulic energy [35]. Tidal range energy is the most consolidated form of ocean energy and has been used in commercial applications since the 1960s (La Rance tidal barrage, in 1966, 240 MW [36]). Its energy potential is due to the upward and downward movements of the astronomical tide, and its operating principle is similar to a hydropower plant. During upward movement of the tide, water is stored in a barrage, and when the tide recedes, the amount of flow decreases and the stored water is then used to generate electricity through a turbine ^[37][38][37,38]. Wave energy is the form of ocean energy with the highest energy density, being up to 30 times more concentrated than solar energy ^[9][14][9,14]. The wave generation process combines wind and solar energy into kinetic and potential energy, transporting it from one location to another. The heating of the atmosphere, resulting from solar radiation, creates an imbalance in pressure between the layers of air masses, causing them to move. This movement creates a thrust and suction channel, giving rise to winds. When these winds reach the ocean surface, they cause shear stress, resulting in surface deformation of the water: the stronger and more continuous the wind, the greater the deformation amplitude. Once generated, waves can travel long distances without significant energy losses. In a wave, kinetic energy is derived from the horizontal movement of the water mass, and potential energy originates from the circular orbital motion of the water particles in the displaced mass ^[31][37][31,37]. Wave energy is the one that arouses the greatest scientific interest among the five forms of ocean energy, whether due to its apparent form (wave motion) or its magnitude scale (tsunamis). However, this energy also has the disadvantage of being inconsistent; waves, similar to the winds that generate them, have significant variability and randomness, which can vary consecutively from one wave to another [13]. This fact directly affects the energy production efficiency. As a relatively new, disruptive, and emerging form of energy exploitation, wave energy requires more studies, research, and development to improve the performance of its converter devices and to adjust its implementation, and operation and maintenance costs for commercial viability ^[30][39][30,39]. Among the technologies and devices developed for harvesting wave energy into electricity are overtopping devices ^[40][41][40,41], submerged pressure differential devices [42], attenuator devices ^[43][44][43,44], point absorber devices [45], rotation mass devices [42], oscillating body devices [46], and oscillating water column devices ^[13][15][13,15]. Although tidal energy conversion technology is the most consolidated among ocean energies, benefiting from the maturity of classical hydraulic generation, wave energy has greater applicability due to the variety of converter devices and exploration fields: onshore, nearshore, and offshore. According to Lin et al. (2015) [47], installing a converter device on the coastline is advantageous in operation and maintenance, being easily accessible and keeping the generator group away from the water. Onshore installations also lead to easy connection to the electrical grid, eliminating the need for submarine cables. Therefore, based on these facts and an exploration of the coastlines near Brazil’s major energy consumption centers, this study was limited to an analysis of the wave energy on the coastline.

3. Ocean Energies Resources

According to 2020 data from the International Renewable Energy Agency (IRENA) (2020) ^[48][49][48,49], global theoretical potential estimates for ocean energy harnessing are on the order of 76,350 TWh/year, excluding tidal range energy. When the global assessment for tidal range energy, which according to Neill et al. (2018) [36] is 25,880 TWh/year, is included, the total estimated potential increases to 102,230 TWh/year (Figure 1b). In terms of local potential, a 2013 study conducted by COPPE in collaboration with Seahorse Wave Energy (SWE) [9] estimated the ocean potential of the Brazilian coast to be approximately 114 GW, divided between tidal range and wave energy (Figure 1a). The COPPE/SWE study detailed the potential for each coastal state of Brazil, the values of which are presented in Table 1. In 2020, Ocean Energy Systems (OES) (2020) [50] reported that the installed and operational ocean energy capacity worldwide was 534.69 MW, distributed as follows: (i) tidal range (barrage), 521.50 MW; (ii) tidal current, 10.60 MW; (iii) wave, 2.31 MW; (iv) ocean thermal gradient, 0.23 MW; and (v) salinity gradient, 0.05 MW. Furthermore, the International Renewable Energy Agency (IRENA) (2020) [48] predicts that an additional 1907 MW of tidal current energy and 149.7 MW of wave energy will be added in the short term, resulting in a total capacity of 2591.36 MW by 2025, and estimates a total installed power of 10 GW by 2030.

4. Oscillating Water Column Device

Studied since 1940, the oscillating water column (OWC) device is one of the most suitable ocean energy conversion devices, having even achieved commercial status. These devices have an arrangement inspired by natural resonant cavities that form in rocky slopes and expel seawater as a geyser (blowholes). It comprises primarily a hydropneumatic chamber filled with air, in which an opening facing the ocean allows the lifting and lowering action of the wave to exert a pressurizing and depressurizing force on the chamber, forcing the displaced air to drive a turbine at the device outlet ^[13][51][13,51]. According to Rosati et al. (2022) [52], oscillating water column (OWC) devices have advantages over other converter devices. In a typical OWC, the moving parts are limited to the turbine-generator group, and this electromechanical set is located above the water surface, improving device reliability and simplifying maintenance. According to Ilyas et al. (2014) [53] and Contestabile et al. (2020) [40], these devices can have positive effects on reducing coastal erosion. However, other authors mentioned in their studies that there are negative effects that should be further investigated in order to have a more accurate overview of environmental impacts [44]. The shoreline installation of the converter is considered a positive point, as it allows for easy access during construction, operation, and maintenance, as well as easy connection to the power grid, dispensing maritime electrical wiring installation, which directly reduces the costs involved ^[13][30][13,30]. Some authors mentioned that, regarding clean and renewable energy, the use of OWC technology has almost no impact on the environment ^[21][54][21,54], with some of the significant environmental impacts observed being the noise emitted by the turbine, the visual impact on the landscape, and bird and fish collisions with the structure. However, using OWC technology for electricity generation does not emit GHG, and around 4660 tons of CO₂ equivalent per MW can be decarbonized in one year [55]. According to Falcão apud Zhang (2021) [56], the whole process of wave energy conversion consists of three stages. In the case of an OWC device, the first stage consists of absorbing the hydrodynamic energy of the wave (kinetic and potential) in the form of pneumatic compression of a volume of air. The second stage consists of the power take-off (PTO), harnessing the useful mechanical energy from the air displacement in the chamber and converting it into torque energy in the turbine. The third stage involves the generator converting this mechanical energy into electricity. Falcão and Henriques (2016) [15] explored, in their work, the main characteristics and evolution of OWC devices. The authors emphasized that the best turbine option for equipping this converter is a self-rectifying axial turbine, such as the Wells turbine and the action turbine, which maintain the direction of rotation independently of the direction of air flow (bidirectional). The disadvantage of wave energy is its variability and randomness, which leads to greater complexity in designing a PTO, as the turbine is subject to oscillations in air pressure and flow rate. However, what matters in terms of aerodynamic performance is the average efficiency and not the maximum efficiency [15]. Raghunathan and Tan (1982) (1983) ^[29][57][29,57] studied the performance of the Wells turbine, which is a reversible low-pressure axial flow turbine. Its blades have a symmetrical airfoil in relation to its rotation plane and perpendicular to the air flow. The tangential force of the air flow acts on the blade, exerting a rotational torque independent of the direction of flow. Its efficiency is lower than that of an axial turbine with asymmetric blades because it has a higher drag coefficient than the asymmetric ones, even when working under ideal conditions ^[29][57][29,57]. Regarding the electric generator, Falcão et al. (2020) [21] stated that it is possible to use a standard synchronous generator. However, a more complex control system is required due to the variability and randomness of the waves. The safest choice to equip an OWC is a variable speed generator, which responds more efficiently to this range of variations. A Doubly Fed Induction Generator (DFIG) has been a popular choice for variable rotational speed wind energy conversion systems interfacing with the electrical grid and is perfectly applicable to an OWC. The greatest challenge facing this technology is its currently high implementation cost. It is estimated to range from USD 2700 to 9100 per installed kW [8]. According to Callaghan (2006) [30], who analyzed the costs and competitiveness of wave and tidal current energies, the cost of a wave energy prototype can range from 7869 USD/kW to 16,470 USD/kW. In Andres et al. (2017) [39], the authors assumed a linearized cost of energy (LCOE) of 0.18 USD/kWh and arrived at an average implementation cost of 3241 USD/kW. Edenhofer et al. (2012) [54] presented a chart of the implementation evolution cost per kilowatt (USD/kW) and predicted an 11% reduction with every doubling of installed capacity. It is estimated that, by 2030, the cost will reach values between USD 4000 and 6000 per installed kW, which is like the implementation cost of a small hydroelectric plant [8]. Successful prototypes and pilot projects of OWC have been developed worldwide. These devices are pioneers and considered first-generation, and their experiences have led to technological advances in new projects ^[58][59][58,59]. In recent years, several advancements have been made in OWC technology, including using advanced materials and incorporating new designs and control systems. These advancements have increased the efficiency and reliability of the device, making it more attractive for commercial use [13].