## 1 Introduction  
The world's total CO\({}_{2}\) emissions have presently reached an all-time high. According to the World Energy Outlook (WEO) 2019 report published by the International Energy Agency (IEA [1]) on 13 November 2019, if countries around the world maintain their current energy policies, the energy demand is expected to increase by 1.3% each year until 2040. To meet the need for reducing CO\({}_{2}\) emissions and energy services, it is necessary to accelerate investments in cleaner and more efficient energy technologies (consider issues such as climate change and fossil fuel depletion). The US Energy Information Administration (EIA) has predicted that by 2050, renewable energy will account for approximately 28% of the total global energy consumption, and approximately 49% of the total electricity production [2]. In particular, it has been announced that wind power generation is expected to account for 20% of the power generation capacity by 2021, and solar power generation is expected to grow significantly, accounting for more than 43% of the power generation capacity by 2050. Among the various resources of renewable energy, wave energy is not the most widely used but has extremely high potential in comparison with solar energy and wind power. Wave energy not only has greater power than other renewable energy sources but is also reliable. The energy densities of wind energy and solar energy per unit area are 0.4-0.6 and 0.1-0.2 kW/m\({}^{2}\) respectively, whereas that of wave energy is 2-3 kW/m\({}^{2}\)[3]. Additionally, wave energy has a wide range of applications as a renewable energy resource. It is known that the total amount of theoretical wave energy that can be generated is approximately 3.7 TW [4]. In Japan particularly, which is surrounded by seas, an average of 36 GW of electric power can be obtained from wave energy [5]; 36 GW is a value that accounts for approximately 12% of Japan's total power generation capacity, based on the 2016 standard. When the annual residential electricity use per capita is 7480 kWh [6], the value of 36 GW amounts to 4800 times as much. In the case of wind and solar power generation equipment, it is possible to generate electricity only for approximately 20-30% of the time in a day, whereas wave energy converters (WECs) have almost no time restrictions, and can generate electricity for up to 90% of the time in a day [7]. Therefore, at night when the electricity consumption is low, it is possible to store the generated power and use secondary energy resources that can be stored and transported in combination with a hydrogen production device. Approximately 37% of the world's population lives within 90 km of a coast. As a result, mutual understanding of resource supply and demand is possible, enabling the installation of WECs in multiple locations [3]. From an environmental point of view, the negative impact of the wave energy recovery device on the environment is the lowest, thereby generating low environmental load [8]. WECs can be classified into onshore, nearshore, and offshore structures, depending on the installation locations [9]. Onshore devices are typically located in coastal regions as oscillating water column (OWC) types accompanying turbine-generators [10; 11; 12; 13; 14; 15; 16; 17]. Nearshore devices are generally installed at a water depth of 10-25 m several hundred meters away from the coast [18; 19; 20], and offshore WECs are installed in the open sea with a depth of more than 40 m as floating types and submerged sea-bed fixed types [21; 22; 23]. WECs are can also be classified into attenuators (e.g., Pelamis [23]), point absorbers (e.g., PowerBuoy [22]), and terminators (e.g., Wave Dragon in Denmark [24]) depending on the size and direction of the device with respect to the waves [3].