A Critical Review of Power Take-Off Wave Energy Technology Leading to the Conceptual Design of a Novel Wave-Plus-Photon Energy Harvester for Island/Coastal Communities’ Energy Needs

Abstract

As the global interest in renewable energy generation continues, the need to develop new and innovative solutions is being explored every day throughout the world by researchers and innovators. Hybrid renewable energy innovations are gaining progressive interest not only because of the threat of climate change but also due to the technological advancements seen in renewables. Ocean waves have immense potential as a renewable energy source, and related technologies have advanced continuously over the past few decades. In response, this paper extensively studies wave energy converters (WECs) based on the power take-off (PTO) technique, and presents a novel hybrid wave-plus-photon energy (HWPE) harvester called Wavevoltaics, based on wave and solar energy capture systems for coastal communities’ power needs, in line with decarbonization measures. The HWPE harvester uses a simple rack-and-pinion mechanism in combination with solar cell technology to convert the wave energy into usable electrical energy in a water column structural design. This novel HWPE device can be used to provide power for lighting and gadgets for coastal communities that rely heavily on fossil fuels for their lighting and electrical needs. Later in the paper, the challenges faced in hybrid wave energy development are presented.

1. Introduction

With the increasing energy demand trend, fossil fuels (FFs) have an immense negative impact on a global scale. FFs impact environmental sustainability—from extraction, to processing, to final consumption—by producing local, regional, and global pollutants. Carbon dioxide (CO₂) is the most prominent pollutant, but water pollution and local particle emissions are also common side effects of FF extraction and combustion of renewable sources like biomass [1,2,3]. Such pollutants affect water and air quality, and have been named as a cause of increasing global warming and carbon emissions. Besides this, FFs—primarily gas, oil, and coal—are still seen as the better alternative for meeting the current global energy demand due to their capacity and cost-effectiveness. Even so, as the global population grows, so does the world’s energy consumption, and the continued extraction of FFs could contribute to our planet’s demise [1].

In 2019, FF accounted for almost two-thirds (63.3%) of global electricity generation sources. Low-carbon sources accounted for 36.7%, of which 26.3% were generated by renewables and 10.4% were generated from nuclear energy, as given in Figure 1 [2]. Disappointingly, the energy mix of low-carbon and FFs has been quite stagnant for decades. However, in order to combat the increasing energy demand, renewable sources of energy are being researched and implemented. Considering this, renewable energies are being systematically studied and introduced as a way to combat the ever-increasing global energy demand and to reduce CO₂.

Attractively, owing to their non-polluting nature and eco-friendliness, solar, wind, hydropower, biomass, and geothermal are some examples of easily available renewable energy resources (RERs) that are being used to harvest green and clean energy [3]. According to the Global Status Report [3], in 2020, more than 256 GW of renewable power generation capacity was added, and the global installed renewable power capacity grew by almost 10% to reach 2839 GW by the end of the year, as seen in Figure 2 [3]. Despite the effects of the COVID-19 pandemic, renewable energy established a new high for new power capacity in 2020, and was the only form of electricity generation to increase its overall capacity by a net amount. For the third year in a row, investment in renewable power capacity increased marginally, and companies continued to set new records for obtaining renewable energy. Solar PV and wind power, as in previous years, accounted for the majority of new renewable energy additions. Around 139 GW of solar PV capacity was added, accounting for more than half of the total renewable capacity increases, with 93 GW of the installed wind generating capacity accounting for 36%. Nearly 20 GW of hydropower capacity was added, with the rest coming from biopower, along with ocean, geothermal, and concentrating solar thermal power (CSP) to achieve these RE capacities, China topped the world in new capacity additions for the second year in a row, accounting for over half of all new installations and dominating the worldwide markets for biopower, CSP, hydropower, solar PV, and wind power. China added more capacity to the grid in 2020 than the rest of the world did in 2013, and it nearly quadrupled its own additions from the previous year. Outside of China, capacity additions totaled about 140 GW in 2019, which is an increase of 5% from the previous year, with the United States (36 GW) and Vietnam (24 GW) leading the way (11 GW). At the conclusion of the year, China led the world in its total renewable energy capacity (908 GW), followed by the US (313 GW), Brazil (150 GW), India (142 GW), and Germany (132 GW) [3].

Despite solar, wind and hydro being prime active contributors as sources of RE, oceanic wave energy (OWE) receives less attention than other types of RERs [4]. While ocean energy has immense resource potential, the technologies are in the growth phase [3], and the main focus of development areas is tidal and wave energy. Apart from this method, ocean thermal energy conversion (OTEC) and the salinity gradient are still in the initial stages, with only a few pilot projects already underway. The ocean bed is heavily used for floating solar PV systems and off-shore wind turbines. Some of the common offshore RERs are given in Figure 3.

As shown in Figure 3, wave energy technology (WET) has long been explored as a primary alternative to the use of FFs. Wave Energy Converters (WECs) convert energy from ocean surface waves—that is, the kinetic and potential energy of waves—into another type of energy [5]. These waves, which are primarily created by wind flowing across the ocean surface (ripples), can spread over deep water with little energy loss, and can combine and gain energy from the wind over long stretches of open ocean (leading to swells). Even though air–sea dynamics and energy transfer processes remain complex, the wind speed, its length, and the fetch are the most important factors in the creation of ocean surface waves. As shown in Figure 4, the most energetic wave conditions are located mainly between 30° and 60° latitudes.

WECs exist in a variety of shapes and sizes, and they can be classed according to where they are mounted. In most situations, however, WECs can be deployed in three regions, as shown in Figure 5 [7], where the average and exploitable amounts are indicated as a percentage of the maximum wave power. Onshore locations are coastal zones with a water depth of 10–15 m and a maximum wave height of 7.8 m, as the wave trough is ready to hit the seabed [8]. The sea level can reach up to 15–25 m [8] at nearshore locations with intermediate water zones. It can be categorized as deep water, intermediate water, or shallow water depending on the depth of the water. Offshore locations are in the deep-water zone, with water depths exceeding 50 m and wave heights of up to 30 m or more [8]. Onshore and nearshore wave power devices are less dense than offshore wave power devices because wave energy loses its power density as it approaches the coast. As a result, the construction of offshore devices is easier than the construction of onshore or nearshore devices. It can be considered that due to the strong wave breaking, building any significant structure in the intermediate and shallow sea sections is extremely difficult. Furthermore, these are the areas where the waves come dangerously close to colliding with the seabed. Even though they are most likely nonlinear, coastal buildings in intermediate and shallow water may be more prone to collapse than those in the deep ocean, where the sea is calm.

So far, the wave-energy-powered navigation buoy is the most efficacious mechanism in wave power utilization; it is an innovation by a Japanese inventor named Yoshio Masuda, as mentioned in [9]. The innovative navigation buoys are the best method for supplying power to buoy batteries, preventing interlude battery replacement problems in remote oceans. The established navigation buoys were quite prosperous: in Japan, 700 buoys were used, whilst the other 500 were sold to various nations, as well as 20 for the United States [10]. While they are very efficient in the use of wave energy, they vary significantly from the output of massive wave energy as we foresee it as a complementary power supply to the current power generation system.

Reportedly, more complex and diversified WECs are being designed in order to improve their energy conversion concepts, add efficiency, and provide the flexibility to be integrated with the conversational energy network and a variety of different devices [11]. Substantively, a considerable number of WECs have been established to the deployment stage or advanced to higher levels of technology. Some of the common WECs include: (i) oscillating water columns such as LIMPET [12], PICO [13], Mutriku plant [14] and OE Buoy [15]; (ii) point absorbers including CorPower [16], CETO [17], and Ocean Power Technology (OPT) [18]; (iii) attenuator technology such as Pelamis [19,20]; and (iv) Oyster [21,22] as an oscillating surging device, and Tapchan [23] and WaveDragon [24] as functional overtopping devices. As the technologies evolve, more advanced concepts and economical and efficient designs will add millstone in RERs.

Technology convergence is a real challenge in WET, and there is not yet a mature solution. This is radically different from other green energies that are advanced (or comparatively mature), such as solar, wind, and tidal energy, etc. The WET convergence is a challenge for collaboration or cooperation, e.g., due to technology intellectual property (IP) protection issues. The WET engineers and developers are not able to share their failure or success stories as a consequence of IP safeguards, but are utilizing the limited accessible resources and working on their own methods to enhance the WET. The outcome of such a strategy is that only technical innovations can be pursued for suboptimal solutions [25]. An additional challenge is that there are no well-accepted standards available for the evaluation and standardization of diversified WETs, but often requests are made by developers to assess their wave convertor technologies and principles. The key reason is that they will be in a good position to receive funding support for their technology if their technology is better than others. International efforts have recently been made to develop marine energy converter standards [26], and some technical specifications (TS) have been published by the International Electrotechnical Commission (IEC). It can, however, be advised that it will also take time for the technical specifications to evolve to the standards, and it may take time for the standards to be accepted. The global initiative also involves the reference model package for both wave and tidal energy [27]. These reference models will be of considerable value to the industry for both research work and the standardization of technology development [28,29], as there are no IP restrictions with these reference models. Besides this, one of the ultimate technical challenges is to significantly escalate the efficiency of wave energy conversion to make wave energy economically comparable with other RERs or even industrial energy generation, provided that it is directly linked to the reduction of the overall wave energy generation costs. In order to accomplish this, the efficiency of wave energy conversion must be raised as high as possible at each energy conversion stage, in which the hydrodynamic performance of WEC can play a critical role in the overall efficiency of energy absorption.

The integration of RERs will play a vital role in the future of energy systems. The hybridization of RERs will combine the benefits of the utilization of different renewable sources to enhance the space usage, energy production and efficiency altogether. The solar PV and offshore wind industries have grown rapidly in recent decades, making this technology a significant participant in the marine sector. In a worldwide situation of climate change and rising challenges to the marine environment, the long-term growth of the offshore wind and solar PV sectors is critical. Multipurpose platforms have been proposed as a sustainable way to harvest various marine resources and integrate their usage on a single platform in this context. Hybrid wave–solar systems are a sort of multifunctional platform that integrates the use of solar and wave energy on a single platform.

This paper reports a detailed, current, state-of-the-art review on various wave energy systems and hybrid systems. This article also focuses on a novel hybrid wave–photon energy harvester that combines an oscillating water column structure utilizing the rack and pinion system wave energy converter with a solar PV panel on the top surface. The primary goal of this article is to critically evaluate the current technologies, and suggest a hybrid wave–solar energy converter that is best suited for island nations and coastal communities.

2. Progress in Wave Energy Harvesters

Power can be extracted from waves in a variety of ways, notably pneumatically, hydraulically, and mechanically. All of these methods of acquiring energy are commonly referred to as “Take-Off” systems, and they can be classified into multiple stages based on the various conversions that occur before generating the necessary output for grid/load injection. The classified stages can be separated into the primary, secondary and tertiary energy conversion stages.

Primary conversion stage: The primary stage is where the wave motion is converted to body movement as hydraulic and mechanical systems, and as an air or water flow through pneumatic systems. In this stage, the low-frequency wave oscillations are converted to rapid motion. This forms a crucial stage whereby wave oscillation could be exploited by means of mechanical or pneumatic systems.

Secondary conversion stage: This stage entails the conversion of the energy created by the working medium in the preceding stage into usable form of energy, such as electricity. This is accomplished by hydraulic turbines, and pneumatic and electrical equipment. These intermediary components allow low rotation velocities or reciprocal motions to be converted to a higher velocity (1500 rpm) [1,12]. Some of these components will be discussed in detail in the sections that follow.

• Air turbine-based energy conversion equipage utilizes the flow of air induced by the oscillating wave motion to turn rotational motion into electrical energy via a linked electrical generator. Figure 6 displays the three different air turbines, namely Wells, Denniss-Auld, and the impulse turbine. A brief description of the three turbine is given in

  Table 1. Summary of some of the air turbines.

  Turbine Type	Description
  Wells air turbine	Self-rectifying axial flow turbine Torque not affected by direction of air flow High rotation speed with low velocity air flow Good peak efficiency and low cost Available in several different versions
  Denniss-Auld air turbine	Self-rectifying turbine Similar to variable pitch well turbine Larger pitching range then wells turbine Good efficiency
  Impulse air turbine	Self-rectifying axial flow turbine Several different versions available Quite efficient

  .
• Hydro-turbine technology is well entrenched, and has been in utilization for several decades in hydro-power generation. This technology has also been employed in several wave energy devices, as it can transform the energy in waves into the rotational inertia needed for electricity generation. The two major types are discussed below, and examples are depicted in Figure 7 and Figure 8.
• Hydraulic systems are another very commonly used mechanism to convert wave energy into mechanical power for electricity production. The slow oscillation caused by the waves in the power capture unit is converted into hydraulic pressure by means of hydraulic cylinders, and the hydraulic energy is then transformed into rotational energy for electrical generators by the aid of hydraulically driven rotors. The hydraulic systems contain special accumulators to supply a constant power output.
• The electrical generator forms a critical part of any wave energy conversion system, as it responsible for the conversion of the mechanical energy produced via power capture systems, and for its transformation into usable electrical energy. The use of rotational generators is very common in wave energy systems, though linear generators have been developed for some wave energy systems.

  Table 2. List of WECs based on Hydro-turbine PTO.

  Rotational Generator	Linear Generator
  Doubly Fed Induction Generator Squirrel Cage Induction Generator Permanent Magnet Synchronous Generator Field Wound Synchronous Generator	Longitudinal Flux Permanent Magnet Generator Variable Reluctance Permanent Magnet Generator Tubular Air-cored Permanent Magnet Generators

  lists the various common types of rotational and linear generators which are put to use by different wave energy producers. The use of rotational generators is more prevalent in terms of offshore suitability, high energy efficiency and low cost, whereas linear generators require large systems that are costly and provide low energy production.

Tertiary conversion stage: This is the stage at which the power is converted into the desired signal for transmission to the targeted load or grid. At this stage, power electronics converters with various topologies are used that were designed and developed for specific use. The technology used at this stage is important because it can be applied to a vast range of systems, such as transportation, electricity, mining, and manufacturing, etc. In the case of WECs, this stage provides the interface between the wave energy system and the power system or grid system. The electrical generator types only classify a wave energy device according to the generator specifications. There are several ways to actually classify a wave energy device. Studies have revealed that there are a wide range of WECs, and they have been designed, developed, deployed, and patent filed for different unique types of wave conversion devices; however, these devices can be classified in four different ways.

• It is identified with wave capture technology, which is better known as the working principle.
• Its orientation is based in part of the wavelength interacting with the wave capture device.
• It depends on the location of the device from the shore, or by the depth it is located at.
• Power take-off (PTO) technology.

A graphical representation is given in Figure 9. The next sub-section provides an extensive study on PTO-based systems for WEC.

2.1. Wave Energy Converter Based on PTO Systems

Apparently, many designs have been developed to enhance PTO technology for WECs. The concept of PTO is well established, and the technologies that have been explored are listed in Figure 10: The most common technologies are listed below:

• hydraulic motor drive,
• hydro-turbine drive
• pneumatic turbine drive,
• direct electrical, and
• direct mechanical drive-based systems.

However, some emerging technologies in WECs, such as hybrid-based systems, have been in development in the latter decade. An abridged review will be provided in this section for various PTO concepts and hybrid wave energy systems.

2.1.1. Hydro-Turbine-Based PTO System

Captivatingly, the hydro-turbine PTO-based system is very similar to the air-based PTO system. The difference is that the hydro-turbine uses fluid flow or seawater to drive the turbine, which directly drives the electric generator. The flow is due to the potential energy of water that is achieved as waves travel up a ramp into a reservoir, where it drains through the hydro-turbine. This type of WEC is generally known as an overtopping device, and illustrated in Figure 11a.

In 2003, a Danish company designed a WEC which was installed in Denmark, which was named Wave Dragon; the concept of overtopping was utilized in designing an offshore floating WEC device [30,31]. Many countries jointly supported the development and deployment of the Wave Dragon project. This device is generally a large device consisting of two arms that face the direction of the waves and guide the water into a reservoir via a semi-submerged ramp. The stored water in the reservoir is higher than the surface level of the sea, which is used as the potential energy of the water, which is channeled through a turbine back into the sea in order to convert the energy into electrical energy. Similarly, with the same concept as Wave Dragon, another overtopping device was developed named the WaveCat [32].

Interestingly, a CETO-based hydro PTO WEC was developed and deployed in Western Australia as the world’s first wave energy project at a commercial scale; it started in 2010 and was commissioned in 2015 as a grid-connected system, energizing 3500 households. The deployed system has a peak designed capacity of 5 MW, and is capable of producing electrical energy and desalinated water at the same time [33,34]. The design of CETO technology consists of a series of buoys to harness energy from ocean waves; the pressure difference incurred by the buoys is transferred through hydraulic pistons to force water through an underwater pipeline to a hydro generator for electricity generation and desalinated water production by the process of reverse osmosis. Aquabuoy is an offshore point absorber-type WEC that utilizes the pumped water concept to drive a Pelton wheel turbine-powered electrical generator [35,36]. Table 3 provides a summary of the proposed or prototyped hydro-based PTO system WEC projects in the seas in different countries.

Table 3. List of WECs based on hydro-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
1985	TAPCHAN	Onshore (Norway)	The TAPCHAN is an overtopping device in which the waves are directed through a ramped channel into a reservoir. From the reservoir the water travel through a hydro turbine back to sea, the hydro turbine produces usable electrical energy.	350 kW	11b	[37]
2003	Wave Dragon	Offshore (Denmark)	The wave dragon is a gigantic offshore floating WEC. The wave dragon uses the kinetic energy of waves to travel up a ramp into a basin where the potential energy of water is used to drive the hydro turbine.	7 MW	11c	[38]
2003	AquaBuoy	Offshore (Scotland, Canada, Ireland)	This device is a floating-point absorber system. The principle operation is similar to that of a hydraulic based point absorber but this device directly converts hydraulic pressure to electricity used a hydro generator.	2.5 MW	11e	[35]
2004	SSG	Onshore (Norway)	The SSG wave energy utilizes a 3-stage ramp system to convert the kinetic energy of waves to stored potential energy. The potential energy flow of water drives the hydro turbine to produce electricity.	150 kW	11d	[39]
2006	Cyclonical WEC	Offshore (USA)	Unlike the overtopping devices, this device is an underwater system that utilizes two hydrofoils rotating around a shaft driving an electric generator for electricity production.	5 MW	11f	[40]
2008	Anaconda	Offshore (UK)	Anaconda is essentially a closed rubber tube filled with water. The head is anchored to the sea bottom. Bulge waves are generated in the tube by pressure variation from external sea waves. These bulge waves are used to produce power through a power take of a device.	1 MW	11g	[41,42]
2008	Power Buoy	Offshore (Spain-2008, Scotland-2009, USA-2011, UK-2019)	Is an offshore two-body heaving wave energy system. The floating disc-shaped body reacts with the large horizontal damper plate to increase inertia by added mass of surrounding water. The heaving motion between two bodies is used to generate electricity by hydro PTO.	40–150 kW	11j	[38,43]
2012	Oyster	Nearshore (Scotland)	The oyster utilizes water pressure to drive a Pelton wheel to generate electric current. The technology behind is the huge flaps that oscillate creating water pressure via a piston-cylinder system.	800 kW	11h	[38,44]
2013	Wavepiston	Offshore (Denmark)	Wavepiston is a submerged offshore device also known as oscillating wave surge converter as it extracts kinetic energy available in orbitally moving water particles. The device has multiple working plates placed parallel and inline to incoming waves. The plates translate enabling a PTO.	250 kW	11k	[45]
2014	Vigor	Offshore (Sweden)	Vigor is an offshore semi-submerged floating device. It consists of a long rubber flat tube that resonates with the ocean waves. Water is propelled from the resonating waves to the hydro PTO which converts energy to electrical energy.	12 MW	11m	[46,47]
2015	CETO	Nearshore (Australia)	This device consists of submerged buoys that are connected to pumps. As the buoys experience wave disturbance, they expel fluid at high pressure through pumps. The pressurized water travels to the shore where it can be used to generate electricity.	1 MW	11l	[17,34]
2017	Crown	Offshore (China)	Crown is a circular overtopping device. It consists of a circular ramp that allows waves to come from all directly into the basin where the hydro operates to generate electrical current.	N/A	11o	[48]
N/A	WaveCat	Offshore (Spain)	The device consists of two hulls like a catamaran. Unlike a catamaran, the hulls are converging or not parallel. The waves propagate between the hull and overtops in the reservoir. It goes back through a hydro turbine to produce power.	N/A	11i	[32]
N/A	Waveplane	Nearshore (Denmark)	Waveplane is an overtopping device similar to the wave dragon design but it multiple chambers with hydro generators to convert the energy into smooth electrical output.	200 kW	11n	[40,49]
N/A	PowerGin	Offshore (US)	This device makes use of the overtopping technology of wave energy conversion to rotate a dual rotor system to achieve continuous rotary motion to generate electricity. The rotation is achieved via mini buckets lined up at an angle to produce rotation by mass of water.	2 MW	11p	[50]

Table 3. List of WECs based on hydro-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
1985	TAPCHAN	Onshore (Norway)	The TAPCHAN is an overtopping device in which the waves are directed through a ramped channel into a reservoir. From the reservoir the water travel through a hydro turbine back to sea, the hydro turbine produces usable electrical energy.	350 kW	11b	[37]
2003	Wave Dragon	Offshore (Denmark)	The wave dragon is a gigantic offshore floating WEC. The wave dragon uses the kinetic energy of waves to travel up a ramp into a basin where the potential energy of water is used to drive the hydro turbine.	7 MW	11c	[38]
2003	AquaBuoy	Offshore (Scotland, Canada, Ireland)	This device is a floating-point absorber system. The principle operation is similar to that of a hydraulic based point absorber but this device directly converts hydraulic pressure to electricity used a hydro generator.	2.5 MW	11e	[35]
2004	SSG	Onshore (Norway)	The SSG wave energy utilizes a 3-stage ramp system to convert the kinetic energy of waves to stored potential energy. The potential energy flow of water drives the hydro turbine to produce electricity.	150 kW	11d	[39]
2006	Cyclonical WEC	Offshore (USA)	Unlike the overtopping devices, this device is an underwater system that utilizes two hydrofoils rotating around a shaft driving an electric generator for electricity production.	5 MW	11f	[40]
2008	Anaconda	Offshore (UK)	Anaconda is essentially a closed rubber tube filled with water. The head is anchored to the sea bottom. Bulge waves are generated in the tube by pressure variation from external sea waves. These bulge waves are used to produce power through a power take of a device.	1 MW	11g	[41,42]
2008	Power Buoy	Offshore (Spain-2008, Scotland-2009, USA-2011, UK-2019)	Is an offshore two-body heaving wave energy system. The floating disc-shaped body reacts with the large horizontal damper plate to increase inertia by added mass of surrounding water. The heaving motion between two bodies is used to generate electricity by hydro PTO.	40–150 kW	11j	[38,43]
2012	Oyster	Nearshore (Scotland)	The oyster utilizes water pressure to drive a Pelton wheel to generate electric current. The technology behind is the huge flaps that oscillate creating water pressure via a piston-cylinder system.	800 kW	11h	[38,44]
2013	Wavepiston	Offshore (Denmark)	Wavepiston is a submerged offshore device also known as oscillating wave surge converter as it extracts kinetic energy available in orbitally moving water particles. The device has multiple working plates placed parallel and inline to incoming waves. The plates translate enabling a PTO.	250 kW	11k	[45]
2014	Vigor	Offshore (Sweden)	Vigor is an offshore semi-submerged floating device. It consists of a long rubber flat tube that resonates with the ocean waves. Water is propelled from the resonating waves to the hydro PTO which converts energy to electrical energy.	12 MW	11m	[46,47]
2015	CETO	Nearshore (Australia)	This device consists of submerged buoys that are connected to pumps. As the buoys experience wave disturbance, they expel fluid at high pressure through pumps. The pressurized water travels to the shore where it can be used to generate electricity.	1 MW	11l	[17,34]
2017	Crown	Offshore (China)	Crown is a circular overtopping device. It consists of a circular ramp that allows waves to come from all directly into the basin where the hydro operates to generate electrical current.	N/A	11o	[48]
N/A	WaveCat	Offshore (Spain)	The device consists of two hulls like a catamaran. Unlike a catamaran, the hulls are converging or not parallel. The waves propagate between the hull and overtops in the reservoir. It goes back through a hydro turbine to produce power.	N/A	11i	[32]
N/A	Waveplane	Nearshore (Denmark)	Waveplane is an overtopping device similar to the wave dragon design but it multiple chambers with hydro generators to convert the energy into smooth electrical output.	200 kW	11n	[40,49]
N/A	PowerGin	Offshore (US)	This device makes use of the overtopping technology of wave energy conversion to rotate a dual rotor system to achieve continuous rotary motion to generate electricity. The rotation is achieved via mini buckets lined up at an angle to produce rotation by mass of water.	2 MW	11p	[50]

Figure 11. Hydro PTO-based WECs: (a) a general overview of a hydro-based PTO overtopping device [1], (b) a concept of a basic TAPCHAN device [37], (c) an illustration of a Wave Dragon device [51], (d) the design of an SSG wave energy device [52], (e) AquaBuoy energy harvesting [53], (f) Cyclonical WEC concept graphic [54], (g) Anaconda concept overview [55], (h) Wave energy device—Oyster design [56], (i) WaveCat energy concept [57], (j) a Power Buoy concept illustration [1], (k) Wavepiston device design [58], (l) a CETO concept [59], (m) vigor design [60], (n) a Deployed Waveplane system [61], (o) the Crown wave energy working principle [48], (p) the PowerGin system [62].

Figure 11. Hydro PTO-based WECs: (a) a general overview of a hydro-based PTO overtopping device [1], (b) a concept of a basic TAPCHAN device [37], (c) an illustration of a Wave Dragon device [51], (d) the design of an SSG wave energy device [52], (e) AquaBuoy energy harvesting [53], (f) Cyclonical WEC concept graphic [54], (g) Anaconda concept overview [55], (h) Wave energy device—Oyster design [56], (i) WaveCat energy concept [57], (j) a Power Buoy concept illustration [1], (k) Wavepiston device design [58], (l) a CETO concept [59], (m) vigor design [60], (n) a Deployed Waveplane system [61], (o) the Crown wave energy working principle [48], (p) the PowerGin system [62].

2.1.2. Direct Linear Electrical Output-Based PTO Systems

Direct linear electrical output-based PTO has evolved to overcome the working design complexity of the other PTO-based WEC systems [63,64]. The use of a linear generator concept is based on the relationship between the stator and the translator unit, in which energy is produced when the translator moves relative to the stator, or vice versa. This is possible because the translator contains permanent magnets, while the stator is comprised of the winding coils. A floating buoy is connected to either the stator or the translator, while the other remains fixed. The floating buoy heaves up and down due to the oscillating ocean waves, causing the translator to move up and down between the stator windings, producing an electrical current in the windings. The schematics of the linear PTO systems are shown in Figure 12.

Various prototypes were designed and deployed by the Oregon State University, and the testing of these prototypes commenced in 1998 for over a decade [65,66]. The early-stage device consisted of a spar, which was moored, and the float, which corresponded with the up-and-down wave motion. The spar was of the central cylindrical three-phase armature design, and connected to the float was the outside cylinder, housing 960 magnets. This arrangement enabled electromagnetic flux to be induced in the armature winding by the up-and-down motion of the waves, producing electromotive force [67]. A wave research site was established in 2002 by the Uppsala University, where around 14 prototypes of WEC were installed and tested [66,68,69,70,71,72]. The WEC developed by the university contained a translator, a stator, and a buoy as a wave capture component. The translator moved up and down due to the buoy inside the stator, which was fixed to the seabed. The current was produced in the stator winding coils as the translator containing the magnets moved up-and-down [72].

Another overwhelming device was developed by the SINN power company; it was named the SINN Power WEC system, and it used a linear electric generator-based PTO. The WEC system consisted of multiple buoys that heaved up and down to produce an electrical current. The buoys’ movement enabled the movement of the translator inside a stator. This device was put to test in 2015 in Greece [73]. A list of linear electric drive-based WEC systems is cataloged in Table 4, which indicates that these devices range from onshore to offshore applications, with a reasonable amount of power output.

Figure 12. Electrical PTO-based WECs: (a) a Lysekil WEC concept [74], (b) the AWS device [75], (c) the design of the Oregon device [76], (d) a picture of the DCEM device [77], (e) the deployed SeaRay WEC [78], (f) a working UNDIGEN energy converter [79], (g) the Seabased design concept [80], (h) the installed SINN power device [81], (i) the StingRay device concept [82], and (j) a graphical representation of a Brandl generator [83].

Figure 12. Electrical PTO-based WECs: (a) a Lysekil WEC concept [74], (b) the AWS device [75], (c) the design of the Oregon device [76], (d) a picture of the DCEM device [77], (e) the deployed SeaRay WEC [78], (f) a working UNDIGEN energy converter [79], (g) the Seabased design concept [80], (h) the installed SINN power device [81], (i) the StingRay device concept [82], and (j) a graphical representation of a Brandl generator [83].

Table 4. List of WECs based on hydro-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
2002	Lysekil	Offshore (Sweden)	This device is a simple wave energy conversion system that utilizes the heave motion of waves to generate electrical current. The oscillations caused by the waves enable a linear electric generator to operate.	10 kW	12a	[84]
2004	AWS	Offshore (Portugal)	This wave energy converter also utilizes a linear electric generator to produce electricity. The fully submerged device with an oscillating upper unit and bottom fixed lower part when incident to waves provide oscillations to the linear generator to activate.	2 MW	12b	[43]
2008	Oregon L10	Offshore (USA)	This is a linear wave energy conversion technology that utilizes the concept of a point absorber with a saucer-shaped float. The PTO is submerged in an enclosed float and converts energy from the incident waves.	10 kW	12c	[65]
2008	DCEM	Nearshore (UK)	This system is similar to the previously designed system that comprises a buoy to capture the energy from the waves and convert it into electrical energy via an integrated linear generator.	100 kW	12d	[40]
2011	SeaRay	Offshore (USA)	The SeaRay consists of three rigid bodies. It is a non-symmetric point attenuator prototyped to function in heave, surge, and pitch modes of motion to produce electrical energy.	1 MW	12e	[85,86]
2014	UNDIGEN	Offshore (Spain)	Undigen was a project that involved two few industries and research centers to develop a two-body point absorber consisting of a direct drive PTO based on switched reluctance linear generator to produce electricity.	200 kW	12f	[87,88]
2015	Seabased	Offshore (Sweden)	This system makes use of 3-phase permanent magnet linear generator technology to produce usable energy. The generator is mounted on the sea bottom and enabled by a floating buoy on the surface via wave disturbance.	10 kW	12g	[40]
2015	SINN Power	Nearshore (Greece)	This company supplies wave energy conversion modules that float and follow the principle of heaving point absorbers. It has linear generators and can be connected in arrays to increase its capacity.	3 kW	12h	[87]
2019	StingRay	Offshore (USA)	The stingray is a 3-body wane energy converter. The Stingray technology uses different float shapes for the front and rear floats. The shapes drastically affect the movement with an incident wave to improve the power output of the device.	500 kW	12i	[88]
N/A	Brandl Generator	Offshore (Germany)	The brandl generator is a linear generator that uses the up or down movement of the shaft exerted by the waves to produce electrical energy. The energy can later be fed to the grid.	1 kW	12j	[89]

Table 4. List of WECs based on hydro-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
2002	Lysekil	Offshore (Sweden)	This device is a simple wave energy conversion system that utilizes the heave motion of waves to generate electrical current. The oscillations caused by the waves enable a linear electric generator to operate.	10 kW	12a	[84]
2004	AWS	Offshore (Portugal)	This wave energy converter also utilizes a linear electric generator to produce electricity. The fully submerged device with an oscillating upper unit and bottom fixed lower part when incident to waves provide oscillations to the linear generator to activate.	2 MW	12b	[43]
2008	Oregon L10	Offshore (USA)	This is a linear wave energy conversion technology that utilizes the concept of a point absorber with a saucer-shaped float. The PTO is submerged in an enclosed float and converts energy from the incident waves.	10 kW	12c	[65]
2008	DCEM	Nearshore (UK)	This system is similar to the previously designed system that comprises a buoy to capture the energy from the waves and convert it into electrical energy via an integrated linear generator.	100 kW	12d	[40]
2011	SeaRay	Offshore (USA)	The SeaRay consists of three rigid bodies. It is a non-symmetric point attenuator prototyped to function in heave, surge, and pitch modes of motion to produce electrical energy.	1 MW	12e	[85,86]
2014	UNDIGEN	Offshore (Spain)	Undigen was a project that involved two few industries and research centers to develop a two-body point absorber consisting of a direct drive PTO based on switched reluctance linear generator to produce electricity.	200 kW	12f	[87,88]
2015	Seabased	Offshore (Sweden)	This system makes use of 3-phase permanent magnet linear generator technology to produce usable energy. The generator is mounted on the sea bottom and enabled by a floating buoy on the surface via wave disturbance.	10 kW	12g	[40]
2015	SINN Power	Nearshore (Greece)	This company supplies wave energy conversion modules that float and follow the principle of heaving point absorbers. It has linear generators and can be connected in arrays to increase its capacity.	3 kW	12h	[87]
2019	StingRay	Offshore (USA)	The stingray is a 3-body wane energy converter. The Stingray technology uses different float shapes for the front and rear floats. The shapes drastically affect the movement with an incident wave to improve the power output of the device.	500 kW	12i	[88]
N/A	Brandl Generator	Offshore (Germany)	The brandl generator is a linear generator that uses the up or down movement of the shaft exerted by the waves to produce electrical energy. The energy can later be fed to the grid.	1 kW	12j	[89]

2.1.3. Direct Mechanical Drive Systems

Direct mechanical drive-based PTO systems convert wave energy directly to electrical energy via a mechanical drive system. The mechanical drive system is usually a gear system that is powered by the waves, which drives the electrical generator that is coupled to the gear mechanism. Many countries have designed and developed WECs prototypes based on a direct mechanical drive PTO mechanism to convert ocean energy into usable electrical current.

A WEC named “the Penguin” was developed and trialed by Wello Ltd; it used a mechanically based PTO to generate electricity [90]. The technique used in this device is the rotating mass concept, which drives an electrical generator. The Manchester Bobber is a heaving point absorber device in which the floater provides oscillatory shaft motion which is then converted to a unidirectional motion via a freewheel/clutch conversion system to produce energy [91]. Another recent WEC—known as CECO—based on a mechanical-drive PTO system, was designed and developed, and is currently undergoing testing in Portugal [92,93].

The working principle for energy conversion in this device is the use of a rack-and-pinion drive shaft to drive the electrical generation unit. The shaft is driven by a mechanical system that consists of two floating buoys, which enable the capture of kinetic and potential energy from the waves simultaneously. A list of WECs based on direct mechanical drive-based PTO is given in

Table 5. List of WECs based on mechanical-drive PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
2004	Manchester Bobber	Offshore (UK)	The Manchester Bobber was a heaving point absorber device designed where the floater provided oscillatory shaft motion which is then converted to a unidirectional motion via a freewheel/clutch conversion system. Unidirectional motion is used to produce energy.	5 MW	13a	[91,94]
2010	BOLT Lifesaver	Offshore (UK-2010 USA-2018)	The device was a self-powered energy test site that included a self-monitoring system. The device was power entirely by wave for all operations but was not connected to the grid supply. The PTO involved a novel system from WiBotic for electrical energy conversion.	84 kW	13b	[95,96]
2011	Wave Rider	Offshore (Australia)	The Wave Rider is a floating offshore device and has a truss-like structure to which numerous buoyancy pontoons are connected to keep it afloat. The system has a series of underwater buoys that heave up or down to drive an axle connected via chains to generate electricity.	1 MW	13c	[97]
2014	WaveSurfer	Offshore (USA)	The is an offshore point absorber system. The main power generation system is submerged which makes it very susceptible to harsh weather conditions.	1.5 MW	13d	[98]
2016	Inertial Sea WEC	Offshore	The Inertial Sea wave energy conversion device is an offshore floating device that uses inertia as the name suggests to produce electricity. A specially designed gyroscope is encapsulated in the system to generate inertia from the incident waves to power a generator.	100 kW	13e	[99]
2017	Penguin	Offshore (2017)	The Penguin is a floating offshore device that comprises a novel rotating mass-energy conversion system that enables a PTO device to function. The device captures the kinetic energy of waves while riding on them to produce power.	0.5–1 MW	13f	[90]
2018	CorPower WEC	Offshore (Scotland)	This device is a floating-point absorber structure for offshore ocean energy production. The device PTO is inbuilt in the buoy. It utilizes a direct motor that drove the system to convert wave energy to usable power.	10 MW	13g	[100]
2019	NEMOS	Offshore (Germany)	This wave energy converter utilizes absorbed incoming wave energy by a floating body to energize a generator via a spring-loaded belt drive system.	N/A	13h	[101]
2019	LAM WEC	Offshore (Scotland)	The device is a point absorber having four degrees of freedom (surge, sway, pitch, and roll) to convert the wave’s energy into electrical energy via mooring lines which are linked to electrical generators.	200 kW	13i	[102]
2019	AMOG	Offshore (UK)	The AMOG has installed about 450 units in Australia alone. This is a floating device with a damped pendulum and the working principle is based on dynamic vibration absorbers.	1 MW	13j	[103]
N/A	CECO	Offshore (Portugal)	The CECO uses an inclined PTO system that responds to the incoming waves. It consists of a rack and pinion system to harness the oscillation into rotational motion to generate electricity.	N/A	13k	[93]

; the majority of the prototypes were installed offshore.

The basic illustration of the direct mechanical system is given in Figure 13.

2.1.4. Hydraulic Motor System

One of the distinguished PTO systems is hydraulic motor-based technology, which is used for the conversion of wave energy in WECs [25]. A detailed mathematical and numerical model based on a hydraulic transfer system has been represented for WEC PTO concept implementation [114].

Hydraulic PTO, as the name suggests, uses fluid as a basic driving mechanism. It generally consists of a hydraulic cylinder sometimes known as a ram, a hydraulic motor, an accumulator, and a generation unit. The hydraulic motor is driven by the fluid power provided by the hydraulic cylinder mechanism, which is initiated by the wave’s oscillations. The accumulator acts as the control mechanism between the hydraulic motor and the cylinder. The rotary generation unit is propelled by the hydraulic motor to generate usable electrical energy [25]. The control strategy used for the PTO force is the key influence to enhance the efficacy of a WEC, and many control systems have been proposed by researchers, such as latching and declutching control [115,116]. The idiosyncrasies of hydraulic-based PTO systems are suitable for WECs; hence, many WECs have been designed and developed using this concept [19,117]. Numerous pieces of research have been carried out to ameliorate the designs of hydraulic motors or pumps and their control strategies in order to enhance the efficiency of WECs [118,119,120,121,122,123].

As far as hydraulic PTO-based WECs are concerned, the Pelamis, Duck, and Waveroller designs are well known, and were amongst the first devices to be developed and deployed [20,124,125,126]. The Pelamis WEC was developed by a Scottish company, Pelamis Wave Power, and was installed offshore to harness power (750 kW) by converting wave motion energy [127]. The Duck WEC was designed by a university professor in 1974 [128,129]. The Duck WEC works in such a way that the buoy pitches around a shaft instead of oscillating up and down to create hydrodynamic pressure [130,131,132]. The Duck WEC was enhanced through the use of multi-level models of hydraulic systems in 2013, and a prototype was tested in China, with a generation capacity of 100 kW [38]. The WEC Waveroller was developed by AW-Energy Ltd, and was deployed in Portugal for performance testing, with a rated output of 300 kW [38,125,133].

Table 6. List of WECs based on hydraulic PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
1983	Pendulor	Shoreline (Japan)	A bottom-fixed shoreline WEC prototype was designed with a rectangular box and it was directly actuated by the waves. The hinged-over creates an opening whereby the incident wave action causes a back and forth oscillation of the pendulum. Hence, this motion is then used to power a high-pressure hydraulic pump and an electrical generator.	5 kW	14a	[134]
1984	Kaiyo	Offshore (Japan)	The Kaiyo consisted of an inner floating structure that was allowed to oscillate relative to the outer structure. The power from the oscillating motion was transmitted via linkage arms to the hydraulic system driving a 240 V generator.	10 kW	N/A	[135]
1996	McCabe Wave Pump	Offshore (Ireland)	The McCabe Wave pump uses the pitching motion of the waves to harness the energy. The device consists of three pontoons that move relative to each other in waves powering hydraulics pumps to produce electricity or transport water.	140 kW	14b	[136]
2003	EB Frond	Offshore (UK)	The device is located at the seabed whereby the collector fin is at the top of the arm pivoted at the base below replicating an inverted pendulum. The movement of the arm drives a hydraulics pump propels a hydraulic motor to generate electricity.	263 kW	14c	[137]
2005	PS Frog	Offshore (UK)	Is a huge buoyant paddle where the PTO is achieved through the internal sliding mass. The wave disturbance allows mechanical motion within the device where electricity is produced via hydraulic drives.	2 MW	14i	[138]
2006	SEAREV	Offshore (France)	Searev is a floating hull consisting of a heavy axis cylinder. The off-centered center of gravity of the cylinder makes it behave like a pendulum. The motion of the hull relative to the cylinder activates the hydraulic power take-off device to produce an electrical current.	0.5 MW	14j	[139]
2006	Onshore oscillating buoy	Onshore (China)	The system consists of a buoy that slides on an onshore built sideway. The buoy oscillates along the sideway driving hydraulic pumps connected via chains to run hydraulic generators to produce electricity.	50 kW	14m	[140,141]
2006	WEC (FO3)	Nearshore (Norway)	The device consists serval buoys which are attached to a platform/deck through sliding guides. The motion induced by the waves via the buoy transformed into electrical energy by the hydraulic machinery in the platform.		14e	[142]
2007	Wavebob	Offshore Ireland	Wavebob consists of two bodies namely a torus and centrals spar. The torus is the wave follower and the central spar behaves as a source of reference for the torus motion. The hydraulic linkage between torus and spar transforms the wave energy into electric energy.	1 MW	14n	[38,143]
2009	Pelamis	Offshore (UK)	The pelamis is a raft WEC system. It consists of floating structures interconnected by hinged joints. Within the joints, hydraulic pumps are present which convert the movement into a fluid flow for hydraulic motors to produce energy.	750 kW	14o	[38]
2009	Wave Star	Offshore (Denmark)	The Wave Star is a multiple absorber concept. Multiple hemisphere-shaped floating buoys are attached to a platform. The induced motion of floats is converted into a steady power supply to the grid through a novel hydraulic power take-off system.	600 kW	14f	[130]
2010	SDE Energy	Shoreline (Israel)	This is a nearshore device that consists of floating buoys. The vertical motion of the buoys creates hydraulic pressure which results in the production of electricity through a system of generators.	40 kW	14d	[37]
2011	Langlee Wave Powe	Offshore (Denmark)	Langlee wave power is a semi-submerged wave surge converter. It contains two pairs of working flaps or water wings which extract energy from the surge motion of waves.	132 kW	14g	[144]
2011	SyncWave Power Resonator	Nearshore (Canada)	This is a point absorber device that converts ocean swells into electrical energy. The concept utilizes a submerged, relatively stationary secondary body that is the reference for primary float. The motion from the buoyant forces enables power generation.	25 kW	14h	[50]
2012	WaveNet	Offshore (Scotland)	The WaveNet Squid was designed for use as an array of devices. It is a heave buoy point absorber. It uses hydraulics from the arrays to generate smooth electrical output.	7.5 kW	14k	[145]
2012	Wave Roller	Offshore (Portugal)	The wave roller followed the design of an inverted pendulum. The kinetic energy of the waves swings the pendulum which converts the energy to electricity through hydraulic systems.	300 kW	14l	[38]
2013	Duck	Offshore (China)	The shape and operation of this device were comparable to a duck. Instead of an up or down movement, the buoy pitched around a shaft. The pitching movement provides hydrodynamic pressure for energy production.	100 kW	14p	[38]
2015	CCell	Nearshore & Offshore (UK)	The device is an oscillating surge wave energy converter and it utilizes curved geometry for efficient power output. The energy is converted through its hydraulic systems.	20 kW	14r	[146]
2015	Sharp Eagle	Nearshore (China)	The device consists of a floating structure that is semi-submerged. The wave absorbing floats interact with waves to transform wave energy to mechanical energy. The mechanical energy is converted to electrical energy through a hydraulic energy conversion system.	100 kW	14q	[147]
2015	BioWave	Offshore (Australia)	A huge device intended to sway back and forth and is located below the surface of the ocean. Energy is produced by the oscillating motion via a hydraulic conversion system.	250 kW	14s	[148]
2016	Triton	Offshore (US)	Is a point absorber compromising of an optimized surface float connected to a ring-shaped heave plate. This device captures energy in all degrees of freedom that as heave, pitch, surge, roll, and yaw. The captured energy is then transformed into electrical energy through hydraulic systems.	600 kW	14t	[149,150]
2018	Azura	Nearshore (USA)	This device utilizes wave heave or vertical and surge or horizontal motion to generate electricity from the relative motion induced between the hull and the float.	20 kW	14u	[151]
N/A	DEXA WEC	Nearshore (Denmark)	The device had two hinged catamarans pivoted relative to the other. This resulted in oscillatory flux at the hinge from the waves. The energy was harvested from a low-pressure power transmission system.	160 kW	14v	[152]

shows a summary of WECs based on hydraulic PTO systems that have been deployed around the world.

The hydraulic motor-based PTO system, as shown in Figure 14, is specifically for the wave-activated-bodies wave energy conversion system; it is the most appropriate device for the production of usable electricity from wave energy. This is due to the hydraulic motor-based PTO taking advantage of both the rotational and translation energy of waves for energy conversion [113].

2.1.5. Pneumatic Air Turbine-Based WEC Systems

WECs based on pneumatic air energy conversion PTO systems are also widely prototyped and installed in the oceans. Technically, the working principle of this PTO involves compressed air-driven air turbines which directly transfer energy to a generator in order to generate electricity. Figure 15 shows the schematic of an air turbine-based PTO. The air turbine-based PTO is usually integrated with oscillating water column (OWC)-type devices.

The seawater in oscillating water column WECs builds pressure in the air column, and this pressurized air is used to propel the air turbine which is directly coupled to the electrical generation unit in order to produce usable energy. Many different types of air turbines have been specially designed by various researchers for use in different WEC to harness energy from ocean waves, such as the Wells turbine, the axial flow impulse turbine, radial flow turbines, and others with or without fixed or without fixed—or with variable—guide turbine blades [169,170,171,172,173]. Table 7 provides a list of WEC devices that have been installed around the globe, all of which utilize the air turbine PTO system for energy generation. Many different control technologies have also been discussed for the various air turbine types in order to improve their efficiency and reduce energy loss [169,171,173].

Limpet OWC is quite well known amongst the WEC because it was first commercially installed as an onshore WEC in 2000, with a rated power of 500 kW [169,174]. Other OWCs installed with air-turbine PTO include Sakata and Mutriku, rated 60 kW and 300 kW, respectively. An offshore floating device known as the Mighty Whale was installed in Japan, with a rated capacity of 110 kW. These devices convert the up-and-down wave oscillation into pressurized air to drive the air turbine in order to generate electrical energy [174,175,176].

Table 7. List of WECs based on air-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
1984	Sanze shoreline gully	Onshore (Japan)	Naturally tapered channel that channel water through a gully forcing air out via air turbine producing electrical energy. This device was built primarily for the testing purpose of pneumatic turbines.	40 kW	15a	[177,178]
1985	Kaimei	Offshore (Japan)	Kaimei was built as a large articulated floating structure. The Kaimei consisted of an impulse turbine, wells turbine, and McCormick turbine for testing and development purposes.	60 to 125 kW	15b	[135]
1987	Multiresonant OWC	Onshore (Norway)	This design emerged from the harbour concept where a pair of walls protruding from the front absorber, thereby partially enclosing a rectangular basin. In the basin, the phenomenon of harbour resonance occurs. Air turbine is used as power take-off in this particular device.	500 kW	15c	[179]
1990	Bottom Standing OWC	Nearshore (India)	Bottom standing OWC is very similar to the Multiresonant OWC but the difference is the placement which in this case is nearshore. The waves travel in columns creating air pressure to drive the turbine.	125 kW	15d	[180]
1991	Vizhinjam	Onshore (India)	This is an oscillating water column plant that features a unidirectional impulse turbine. The output of the impulse turbine is directly connected to the grid supply.	150 kW	15e	[169,181]
1992	Sakata	Nearshore (Japan)	This was an oscillating water column device integrated into the breakwater to generate electricity. The energy harvester utilized a wells turbine to generate electrical energy.	60 kW	15f	[169]
1995	Osprey	Nearshore (UK)	One of the largest power energy nearshore bottom standing energy devices. The plant consisted of two wells turbine operating simultaneously to produce energy. Later it was destroyed in a natural disaster.	1 MW	15g	[169]
2000	Mighty Whale	Offshore (Japan)	The mighty whale uses several air chambers for oscillating water columns and mounted air turbines to generate air pressure created by the waves. Mighty Whale is a floating device.	110 kW	15h	[182]
2001	Limpet	Onshore (Scotland)	The Limpet is an onshore device consisting of a single air chamber that exhales and inhales air induced by the waves through a Wells turbine to produce power.	500 kW	15i	[121]
2001	Shoreline OWC	Onshore (China)	The plant was set up as a research and development program to study grid-connected wave energy systems. The device converts pneumatic power to electricity via a unidirectional turbine.	100 kW	15j	[169,183]
2005	Port Kembla OWC	Nearshore (Australia)	The power station was set up as an oscillating wave energy prototype. The project remained in operation till 2009.	500 kW	15k	[184]
2005	Pico	Nearshore (Portugal)	The device was built as a nearshore oscillating water column equipped with Wells turbine for electricity generation. The device stands on the sea-bottom.	400 kW	15l	[169]
2008	OE buoy	Offshore (Ireland)	This is a floating device converting ocean surge current to electricity through pneumatic turbines.	1 MW	15m	[169]
2008	Oceantec	Offshore (Spain)	This is an offshore floating device where the energy capture principle is inertial motion created by the gyroscopic system converting the wave energy to electricity. Another deployed in 2016—30 kW.	500 kW	15n	[40,185]
2010	Archimedes Wave Swing-iii	Offshore (Scotland)	The AWS-iii is the advanced version of previous AWS systems. The technology behind this is the multi-cell array of flexible absorbers. These absorbers convert wave power to pneumatic power for electricity generation.	2.5 MW	15o	[35]
2011	Mutriku	Nearshore (Spain)	This was the first nearshore multi turbine device to be installed in breakwater for energy harvesting using a Wells turbine. The device also marked the world’s first commercial project.	300 kW	15p	[14]
2012	Resonant Wave Energy Converter 3	Onshore (Italy)	Unlike traditional OWC, the inner chamber of REWEC3 is connected to both the ocean wave and air chamber via the tiny vertical duct. This configuration greatly affects the performance of the device.	N/A	15q	[186]
2012	Vert Labs	Offshore (Scotland)	This device transforms the kinetic energy of sea waves to pneumatic energy in the form of compressed air. The compressed air turbine is used to generate electricity.	35 kW	15r	[40]
2013	Oceanlinx	Offshore (Australia)	The oceanlinx was a singular OWC aimed at variable water depths and wave conditions. A prototype was launched, however, the deployment was not completed due to some incident.	1 MW	15s	[187]
2013	Offshore Wave Energy Ltd	Offshore (UK)	The device has a long duct where the waves enter and trap the air against the top of the duct. The narrow duct shape at the rear allows air to compress as waves travel forward. The compressed air turbine produces energy.	12 MW	15t	[40]
2015	Bombora	Nearshore (Australia)	The bombora utilizes large flexible membranes or diaphragms as an energy capture interface. As waves pass, the air pressurizes and travels through special ducts to air turbines for electricity production.	1.5 MW	15v	[188]
2015	LEANCON	Offshore (Denmark)	Is multi-chamber nearshore or offshore floating device. It has a V-shaped structure with 2 arms. The arms are equipped with cylindrical chambers which create pneumatic pressure deferential to incident waves.	600 kW	15w	[189]
2016	MARMOK-A-5	Offshore (Spain)	This device is a vertical OWC utilizing a biradial turbine for energy production.	30 kW	15u	[190]
2017	Yongsoo	Nearshore (Korea)	The plant is equipped with an impulse turbine and is grid-connected. The device is under trial run.	500 kW	15x	[191]
2019	OE35	Offshore (USA)	This concept is a technologically advanced version of the OWC utilizing an air flow turbine to generate power. The water column enclosed by the hull behaves like a piston to create airflow.	500 kW	N/A	[192]
2019	Wave Swell	Nearshore (Australia)	Wave Swell was a project where the principle of natural blowhole was explored and the OWC energy converter was deployed for harnessing electrical energy.	200 kW	15y	[193]
N/A	MRC 1000	Offshore (UK)	The device is under research and development phase. But no clear evidence about deployment for the device is provided.	1 MW	N/A	[127]

Table 7. List of WECs based on air-turbine PTO.

Year	WEC Name	Location and Deployed Place	Description	Output Power	Figure	Ref.
1984	Sanze shoreline gully	Onshore (Japan)	Naturally tapered channel that channel water through a gully forcing air out via air turbine producing electrical energy. This device was built primarily for the testing purpose of pneumatic turbines.	40 kW	15a	[177,178]
1985	Kaimei	Offshore (Japan)	Kaimei was built as a large articulated floating structure. The Kaimei consisted of an impulse turbine, wells turbine, and McCormick turbine for testing and development purposes.	60 to 125 kW	15b	[135]
1987	Multiresonant OWC	Onshore (Norway)	This design emerged from the harbour concept where a pair of walls protruding from the front absorber, thereby partially enclosing a rectangular basin. In the basin, the phenomenon of harbour resonance occurs. Air turbine is used as power take-off in this particular device.	500 kW	15c	[179]
1990	Bottom Standing OWC	Nearshore (India)	Bottom standing OWC is very similar to the Multiresonant OWC but the difference is the placement which in this case is nearshore. The waves travel in columns creating air pressure to drive the turbine.	125 kW	15d	[180]
1991	Vizhinjam	Onshore (India)	This is an oscillating water column plant that features a unidirectional impulse turbine. The output of the impulse turbine is directly connected to the grid supply.	150 kW	15e	[169,181]
1992	Sakata	Nearshore (Japan)	This was an oscillating water column device integrated into the breakwater to generate electricity. The energy harvester utilized a wells turbine to generate electrical energy.	60 kW	15f	[169]
1995	Osprey	Nearshore (UK)	One of the largest power energy nearshore bottom standing energy devices. The plant consisted of two wells turbine operating simultaneously to produce energy. Later it was destroyed in a natural disaster.	1 MW	15g	[169]
2000	Mighty Whale	Offshore (Japan)	The mighty whale uses several air chambers for oscillating water columns and mounted air turbines to generate air pressure created by the waves. Mighty Whale is a floating device.	110 kW	15h	[182]
2001	Limpet	Onshore (Scotland)	The Limpet is an onshore device consisting of a single air chamber that exhales and inhales air induced by the waves through a Wells turbine to produce power.	500 kW	15i	[121]
2001	Shoreline OWC	Onshore (China)	The plant was set up as a research and development program to study grid-connected wave energy systems. The device converts pneumatic power to electricity via a unidirectional turbine.	100 kW	15j	[169,183]
2005	Port Kembla OWC	Nearshore (Australia)	The power station was set up as an oscillating wave energy prototype. The project remained in operation till 2009.	500 kW	15k	[184]
2005	Pico	Nearshore (Portugal)	The device was built as a nearshore oscillating water column equipped with Wells turbine for electricity generation. The device stands on the sea-bottom.	400 kW	15l	[169]
2008	OE buoy	Offshore (Ireland)	This is a floating device converting ocean surge current to electricity through pneumatic turbines.	1 MW	15m	[169]
2008	Oceantec	Offshore (Spain)	This is an offshore floating device where the energy capture principle is inertial motion created by the gyroscopic system converting the wave energy to electricity. Another deployed in 2016—30 kW.	500 kW	15n	[40,185]
2010	Archimedes Wave Swing-iii	Offshore (Scotland)	The AWS-iii is the advanced version of previous AWS systems. The technology behind this is the multi-cell array of flexible absorbers. These absorbers convert wave power to pneumatic power for electricity generation.	2.5 MW	15o	[35]
2011	Mutriku	Nearshore (Spain)	This was the first nearshore multi turbine device to be installed in breakwater for energy harvesting using a Wells turbine. The device also marked the world’s first commercial project.	300 kW	15p	[14]
2012	Resonant Wave Energy Converter 3	Onshore (Italy)	Unlike traditional OWC, the inner chamber of REWEC3 is connected to both the ocean wave and air chamber via the tiny vertical duct. This configuration greatly affects the performance of the device.	N/A	15q	[186]
2012	Vert Labs	Offshore (Scotland)	This device transforms the kinetic energy of sea waves to pneumatic energy in the form of compressed air. The compressed air turbine is used to generate electricity.	35 kW	15r	[40]
2013	Oceanlinx	Offshore (Australia)	The oceanlinx was a singular OWC aimed at variable water depths and wave conditions. A prototype was launched, however, the deployment was not completed due to some incident.	1 MW	15s	[187]
2013	Offshore Wave Energy Ltd	Offshore (UK)	The device has a long duct where the waves enter and trap the air against the top of the duct. The narrow duct shape at the rear allows air to compress as waves travel forward. The compressed air turbine produces energy.	12 MW	15t	[40]
2015	Bombora	Nearshore (Australia)	The bombora utilizes large flexible membranes or diaphragms as an energy capture interface. As waves pass, the air pressurizes and travels through special ducts to air turbines for electricity production.	1.5 MW	15v	[188]
2015	LEANCON	Offshore (Denmark)	Is multi-chamber nearshore or offshore floating device. It has a V-shaped structure with 2 arms. The arms are equipped with cylindrical chambers which create pneumatic pressure deferential to incident waves.	600 kW	15w	[189]
2016	MARMOK-A-5	Offshore (Spain)	This device is a vertical OWC utilizing a biradial turbine for energy production.	30 kW	15u	[190]
2017	Yongsoo	Nearshore (Korea)	The plant is equipped with an impulse turbine and is grid-connected. The device is under trial run.	500 kW	15x	[191]
2019	OE35	Offshore (USA)	This concept is a technologically advanced version of the OWC utilizing an air flow turbine to generate power. The water column enclosed by the hull behaves like a piston to create airflow.	500 kW	N/A	[192]
2019	Wave Swell	Nearshore (Australia)	Wave Swell was a project where the principle of natural blowhole was explored and the OWC energy converter was deployed for harnessing electrical energy.	200 kW	15y	[193]
N/A	MRC 1000	Offshore (UK)	The device is under research and development phase. But no clear evidence about deployment for the device is provided.	1 MW	N/A	[127]

Figure 15. WECs based on pneumatic PTO systems: (a) a picture of the Sanze shoreline gully [169], (b) Edifice Kaimei WEC at sea [169], (c) a multiresonant OWC device [169], (d) the Botting Standing OWC system [169], (e) the Vizhinjam WEC structure [194], (f) the deployed Sakata device [169], (g) the Osprey WEC being deployed [195], (h) the Mighty whale equipment at sea [196], (i) a conceptual model of the Limpet [197], (j) the structure of a shoreline OWC [198], (k) OceanLinx installed at Port Kembla [169], (l) a Pico device at work [169], (m) an OE buoy floating at sea [199], (n) semi-submergerd Oceantec WEC [200], (o) an overview of the Archimedes Wave Swing [201], (p) a construted Mutriku device [14], (q) the OWC resonant wave energy converter 3 [202], (r) Vert Labs Energy concept [203], (s) Oceanlinx WEC design [204], (t) the OWEL design concept [205], (u) the MARMOK-A-5 system at sea [206], (v) the Bombora WEC concept [207], (w) a visualization of the LEANCON concept [169], (x) Yongsoo technology [208], and (y) Wave Swell device testing [209].

Figure 15. WECs based on pneumatic PTO systems: (a) a picture of the Sanze shoreline gully [169], (b) Edifice Kaimei WEC at sea [169], (c) a multiresonant OWC device [169], (d) the Botting Standing OWC system [169], (e) the Vizhinjam WEC structure [194], (f) the deployed Sakata device [169], (g) the Osprey WEC being deployed [195], (h) the Mighty whale equipment at sea [196], (i) a conceptual model of the Limpet [197], (j) the structure of a shoreline OWC [198], (k) OceanLinx installed at Port Kembla [169], (l) a Pico device at work [169], (m) an OE buoy floating at sea [199], (n) semi-submergerd Oceantec WEC [200], (o) an overview of the Archimedes Wave Swing [201], (p) a construted Mutriku device [14], (q) the OWC resonant wave energy converter 3 [202], (r) Vert Labs Energy concept [203], (s) Oceanlinx WEC design [204], (t) the OWEL design concept [205], (u) the MARMOK-A-5 system at sea [206], (v) the Bombora WEC concept [207], (w) a visualization of the LEANCON concept [169], (x) Yongsoo technology [208], and (y) Wave Swell device testing [209].

2.2. Hybrid Wave Systems

Generally, a lot of advancement has been made in hybrid energy systems in which two or more different ways of generating energy are merged together. Moreover, offshore, wind, and PV systems are commonly integrated in hybrid systems. The hybrid topology is very new, and is still a developing conception in the field of wave energy integration. Many researchers have designed and deployed hybrid WECs, but there are a limited number of articles which discuss the concept of hybrid WETs. Hence, in this section, some common hybrid modes will be discussed.

2.2.1. Wave and Wind

Hybrid wind–wave systems are specially designed energy harnessing devices that use wind and wave energy to produce electrical output. Hybrid wind–wave systems fall into three main types, namely floating offshore wind platforms, which are ballast stabilized; semisubmersible, buoyancy-stabilized platforms; and tension leg platforms that are mooring stabilized. Much of the research and development of hybrid wave–wind systems has included buoyancy-stabilized and ballast-stabilized platforms [210]. The most outstanding hybrid wind and wave energy systems were developed by Pelagic Power and the Floating Power Plant [211,212,213,214].

The hybrid energy harnessing system developed by Pelagic Power, named W2Power Wind and Wave system, was a semisubmersible offshore wind turbine compromised of multiple oscillating bodies and two wind turbines [213,214]. The W2Power was designed to output about 10 MW of energy through its two wind turbines and wave converters. A buoyancy stabilized platform—the Poseidon Wave and Wind system developed by Floating Power Plant—consisted of wind turbines and multiple water column WECs [211]. The Poseidon scale model was tested with an energy capacity of 63 kW, and the full-scale model had a designed capacity of 7 MW [212]. An illustration of the two devices is shown in Figure 16. The energy generated by wind and wave systems offers some benefits because the same marine space is used for the structures, which would improve the levelised cost of energy production, enhance the power generation output, and reduce the structural loading for offshore platforms.

Furthermore, a new concept that has been studied is giving hope for more hybrid wave energy systems [215]. The substructure of the platform is of the semi-submersible type, consisting of columns, pontoons and brace members. The column spacing is designed to minimize the wake effect between the turbines. The wind–wave hybrid platform has four 3 MW wind turbines at the top side of the main columns, and 24 WECs (six per side) along the four platform sides, as can be seen in Figure 17.

2.2.2. Wave and PV

A new concept called Wavevoltaics has been proposed, integrating the photovoltaic (PV) cells over the vacant open-sky surface of wave devices [216,217]. This new concept, called Wavevoltaics—shown in Figure 18—is formed by integrating the PV cells over the vacant surfaces of wave devices. These devices can float on the ocean surface or by submerged in deep waters, which is quite different from Floatovoltaics.

2.2.3. Wave, Wind, and Solar

Another interesting concept uses multiple generation sources on one platform to generate energy. The design was developed using wave, wind, and PV systems, and can supposedly withstand waves up to six meters high. It was designed to give coastal regions easy access to clean energy solutions. Each module of the Ocean Hybrid Platform by SINN Power has the capacity of four WEC of 0.75 MW, 20 kW of solar energy, and four wind turbines of 6 kWp [218]. The concept design is show in Figure 19.

2.3. Control Strategies

The wave energy sector’s control challenge does not meet the traditional definition of control in other industries, which involves using feedback (open and closed loop, set-point tracking) and limiting system variables as a constant. WEC control, on the other hand, tries to maximize the harvested energy while using feedforward control to obtain the optimum system velocity or PTO force set points, as seen in Figure 20.

Optimal computation involves the following enactment function:

$$J={{\displaystyle \int }}_0^{T}v\left(t\right)f_{PTO}\left(t\right)dt$$

(1)

where v(t) is the device velocity and f_PTO(t) is the exerted PTO force. A simple analysis of the mean absorbed and optimal control is given to express the ways in which control maximizes the energy capture [219,220]. The idea of resonance is a useful initial qualitative attempt to understand how to optimize the absorbed power, and resonance describes a system that is stimulated at its natural frequency. The response amplitude is at its maximum while operating in resonance. Resonance may not typically occur naturally for wave energy converters with natural frequencies higher than the power-rich frequency components of a typical wave spectrum; therefore, the system is tricked into resonance by optimizing the PTO damping and stiffness as needed.

2.4. Numerical Modeling

The total force operating on a WEC balance is the inertial force. For example, the force exerted by the wave is divided into external loads—i.e., wave interaction such as hydrostatic force, the excitation load, and radiation force—while the reaction force is one of the most common categories which is caused by PTO, the end-stop mechanism, and mooring. The WEC (floater)’s interaction with ocean waves is a high-order nonlinear process that may be simplified to linear equations for waves and small-amplitude device oscillation motions, which are suitable throughout the device’s operational domain. The superposition principle is therefore applicable [221]. Commonly, the PTO system is observed to be a complex nonlinear dynamic behavior; the PTO forces must be linearized in order to maintain the superposition concept’s validity. In order to be linear, the two common contributions are [222]:

• a force proportional to velocity (damper), and
• a force proportional to the displacement (spring).

A linear relationship of captor displacement and mooring spring stiffness is frequently used to characterize mooring systems. Given the complexity of a nonlinear methodology for wave energy conversion, end-stop mechanisms and other constraints (velocity or PTO force operational limits) are sudden nonlinear forces that are generally overlooked. However, increasing the PTO damping until the body reaches the maximum allowable displacement is the best way to achieve an acceptable displacement amplitude [223]. Alves analyzed the mean absorbed power for a heave motion wave energy converter in [224], assuming linearity and sinusoidal waves:

$$\left.\begin{array}{c}{P}_a=\frac{1}{2}\left[\frac{B_{pto}{\omega }^2{\left|{\widehat{F}}_e\right|}^2}{{\left(-{\omega }^2\left(m+A\right)+G+K_{pto}+K_m\right)}^2-{\omega }^2{\left(R+B_{pto}\right)}^2}\right]\\ P_a=\frac{1}{2}\frac{B_{pto}{\omega }^2{\left|\widehat{{\widehat{F}}_e}\right|}^2}{{\left|Z_i+Z_{pto}\right|}^2}\end{array}\right\}$$

(2)

where

• ω is the wave frequency,
• ${\widehat{F}}_e$ is the excitation force,
• m is the total inertia of the captor,
• A is the added mass,
• G is the hydrostatic spring stiffness,
• K_pto is the PTO mechanical spring,
• K_m is the mooring spring stiffness,
• R is the radiation damping,
• B_pto is the PTO damping,
• Z_i is the intrinsic impedance, and
• Z_pto is the PTO impedance.

An opposite, but similar, approach addresses the frequency domain force-to-velocity model of a WEC, as follows:

$$\frac{1}{Z_i\left(\omega \right)}=\frac{V\left(\omega \right)}{F_{ex}\left(\omega \right)+F_u\left(\omega \right)}$$

(3)

where V(ω), F_ex(ω), and F_u(ω) represent the Fourier transform of the velocity v(t), excitation force F_ex(t) and control force f_pto(t), respectively. Z_i(ω) is the intrinsic impedance in the frequency domain of the system:

$$Z_i\left(\omega \right)=B_r\left(\omega \right)+\omega [M+M_a\left(\omega \right)-\frac{K_b}{{\omega }^2}$$

(4)

where B_r(ω) is the radiation damping (real and even), and M_a(ω) is the frequency-dependent added mass, which is often replaced by its high-frequency asymptote m∞.

2.5. Damping Control

The linear damping of the PTO, also known as passive loading or resistive damping, is a well-researched way of avoiding issues in the implementation of the feedback control of the WECs [225,226]. It can be said that, sub-optimally, the instantaneous PTO force is proportional linearly to the oscillating body speed:

$$f_{pto}\left(t\right)=-B_{pto}v\left(\mathrm{t}\right)$$

(5)

where B_pto is the PTO damping coefficient, and is more than zero. This approach does not need the prediction of the excitation force, making it an easy technique to adopt. In fact, it is the one found in most demonstrations or pre-commercial devices which are now in use throughout the world. Conventionally, it is sufficient to know the instantaneous value of the PTO velocity, for which measurement devices are commonly available on the market.

Damping control, on the other hand, absorbs much less power than other strategies, such as reactive control [227], as we will see in the following section, and the linear relationship between the speed and force of the PTO, even when it is a simple relationship, may be difficult to execute using no feedback control. Furthermore, for regular waves, the best value of the PTO damping, which is the value of B_pto that optimizes the instantaneous power absorbed, can be easily determined. In reality, however, if the incoming wave is irregular (as defined by the wave spectrum), B_pto is much more difficult to compute due to variations in the spectral components of the incident wave that are not consistent over time, necessitating real-time feedback control for a time-varying damping value. As a result, we can tell the difference between a true time-varying damping control and a constant or passive damping control. The first generation of WEC control is realized on damping techniques with constant B_pto values. This method is still widely used in contemporary WEC prototypes developed by technology developers due to the simplicity of its implementation.

2.6. Reactive Control

This control solution often entails tuning both PTO resistance and reactance (B_pto and K_pto) while taking into consideration constraints such as the PTO power rating or displacement restrictions, and regulating PTO resistance to prevent non-linear approaches [123].

$$f_{pto}\left(t\right)=-B_{pto}v\left(t\right)-k_{pto}x\left(t\right)$$

(6)

where K_pto is the stiffness coefficient and x(t) is the value of the displacement. When implemented in demo prototypes, this form of control typically adopts a tabular method to ease the processing constraints which are necessary to determine the optimum values in real time. As a result, sub-optimal damping and stiffness coefficients are pre-calculated using an optimization technique and kept in tables. Hence, this methodology is prone to modeling mistakes, necessitating a reanalysis of the constant values after a specified testing time.

2.7. Latching/Unlatching Control

The latching control, first proposed in [118], is based on attaining the resonance of the WEC using a clamping mechanism, which fixes the device for a certain portion of the wave oscillation cycle [228]. When the device is disengaged, its control is normally managed by linear damping, as described in the earlier paragraph. As a result, the device exhibits resonance operating without the requirement for reactive power regulation. When the device velocity is zero, however, some energy must be extracted from an external source in order to activate the clamping system. The computation of the latching-unlatching time intervals is crucial for this control approach. Latching control eliminates the two-way energy transition and the accompanying energy dissipation that define reactive control, allowing for the adoption of a broader range of PTO systems that operate solely in generator mode. With the passive damping control technique in [229] as the baseline, there is a performance increase when the latching control technique is used. The findings reveal that the ideal damping coefficient is reduced by 60% while the capture width rises by 70%.

2.8. Model Predictive Control

Model predictive control (MPC) is a frequently used and studied method in the industry [230], and it should not be different for WECs due to its capacity to cope with linear and non-linear models, as well as system constraints and the real-time evaluation of future behavior. MPC methods can deal with the physical limits that any WEC technology has, as well as the non-causal optimum control solution.

However, the challenge of maximizing WEC energy necessitates a significant change in the criterion of the MPC, leading to a possibly non-convex optimization issue. Given the advantages and expanding knowledge of these algorithms, this method has emerged as the most popular control study subject in recent years. MPC optimizes energy absorption by applying the optimal force at each time step to produce resonance across a future time horizon, as initially stated in [231]. A study [232] showed the findings of a comparison of MPC control and classical (complex-conjugate control) approaches for a Linear Permanent Magnet (LPMG) PTO operated by a machine side back-to-back actuator. It was determined that complex-conjugate control is ineffective in the optimization of power absorption from ocean waves when applied to real-world systems. A predictive technique is explored in [233] as an enhancement of reactive control, in which a neural network trained with machine learning is used to forecast future waves (in terms of height and period), altering the WEC and adjusting the appropriate parameters (concisely, the PTO stiffness coefficient and PTO damping coefficient) for wave energy absorption in real time. In terms of absorbed power, the algorithm does not outperform equivalent state-of-the-art reactive control systems, but it overcomes related control imperfections from laboratory calibration, and allows the control system to be adaptive to changes in machine responsiveness caused by ageing. In [234], a neural network is used to estimate the short-term wave height and period for a heaving point absorber in order to achieve real-time adaptive latching control. This paper compared the variations in the absorbed energy for a specific wave situation with and without control. Similarly, [235] proposes a new MPC solution. It is known as robust model predictive control (R-MPC) because it combines a predictive controller that takes PTO constraints into account in order to ensure the maximum power absorption while remaining realistic, and a novel model to overcome some parametric uncertainties and model mismatches. Furthermore, [236] presents a hybrid MPC technique in which constraints are imposed to PTO damping and damping force for a two-body WEC. In order to attain the highest power absorption, a Mixed-integer Quadratic Programming (MIQP) problem is given. This problem’s answers are compared to other MPC solutions and classical models for an irregular wave situation. Interestingly, one piece of research [237] employs future wave frequency prediction with a Fuzzy Logic controller to select the optimal PTO damping and stiffness coefficients in real time. The suggested solution combines several traditional tuning approaches with a novel slow tuning process. Lastly, [226] examines fatigue, reliability, and survival, as they are managed by MPC. The results demonstrate a trade-off between the maximum electrical output and the size required for the WEC to withstand high loads and fatigue durations. These findings are also compared to traditional reactive management, with MPC improving the average yearly energy generation by 29%.

2.9. Benefits and Challenges of WECs

All of the various PTO-based WECs have their own benefits, as they are usually designed for specific use. The possible benefits and challenges of the devices when compared to the others are discussed in

Table 8. Summaries of the benefits and challenges of PTO-based wave energy harvesting.

Types	Benefits	Challenges
Hydro-turbine based PTO system	Efficiency of about 90% Well adapted to low-frequency larger power waves. Low maintenance required No environmental problems due to oil leakages.	Cavitation can occur on the turbine Components susceptible to damages due to unpredictable conditions
Direct linear electrical based PTO systems	Wave to electrical direct energy transfer Avoids system losses which occurs in other PTO systems Reduced maintenance of the system High efficiency rates	Low linear velocity Low power to weight ratio Complex power electronics involved as irregular power output
Direct Mechanical Drive Systems	High conversion ratio due to reduced friction Up to 97% efficiency 3-stage energy conversion	System reliability is an issue Short lifetime as it experiences high load cycles Higher maintenance required High maintenance cost in case of failure High cost involved the gear mechanism
Hydraulic Motor System	Suitable for low frequency operation High conversion efficiency (69–80%) Hydraulic system can also be used to control the WEC Hydraulic system components are readily available	Susceptible to hydraulic fluid leakage posing threat to marine environment Hydraulic system requires regular maintenance which could costly and risky Hydraulic end-stop problem which could damage the system due to extreme conditions
Pneumatic Air turbine based WEC systems	Convert waves slow velocity to air flow Has no environmental impact as it does not utilize harmful fluids for operation. Air turbine can be located away from the corrosive sea water and damaging sea waves Easy access for maintenance 60% to 65% efficiency	Conventional turbines not suitable because of bidirectional flow Non-return airflow system is complicated and hard to maintain Wells turbine is most popular but it has self-starting issue Needs external source to rotate the turbine initially High axial thrust and noise issue.
Hybrid Wave energy Harvesting [PV and wind]	Due to hybrid nature it has cost saving structure for energy harvesting. Reduces the installation and mooring cost. Limits the land use for PV system when integrated with other platform such WECs or floating PV system. Hybrid nature also allows the grid-connected system. In this, it increases efficiency, lowers the logistic cost, and utilizes same infrastructure. It also combines the benefits of each RERs. Maximize the power output.	More advance control is required. During unfavorable weather condition more work is required in terms of installing and uninstalling of hybrid systems.

. Though the benefits and challenges may vary according to the individual prototype or device, a generalized summary is presented.

3. A Novel Hybrid Wave and Photon Energy (HWPE) Harvester

In this section, a new concept HWPE harvester is presented that was developed by the authors of this paper. The PTO technology utilized is based on a direct mechanical drive system, whereby the wave capture is transferred through a rack-and-pinion system in a water column-like structure to an electrical energy constituent in order to generate electricity. The device is also equipped with solar cells to supplement the wave energy generation system and increase the productivity of the device.

3.1. Opportunities for a Hybrid Wave and Photon Energy (HWPE) Harvester

Nations around the world rely heavily on fossil fuels or renewable energy for energy production. The energy needs of the urban areas are usually satisfied by grid systems, but many remote and coastal communities in the outer islands are left out. This means that the unprivileged communities either rely on fossil fuel or solar systems, which comes at a huge initial cost. Additionally, the limited access to adequate energy results in poor lighting, which narrows education and health opportunities. Furthermore, the people living in these societies are left out by the fast-growing world outside in terms of the application and usage of smart gadgets to enhance the living standard. The designed HWPE harvester will not only provide electricity to the coastal communities in the outer islands but will also help to reduce the reliance on diesel- and fossil-powered devices for lighting and electricity needs. This device will reduce the burden of purchasing fuel, and will reduce the cost of electricity production. As the WEC uses locally available materials, it is cheaper to construct locally, and can be made available to communities at a low cost. The device is not very huge in terms of structure; therefore, it is portable, which means that it can be stored away when unfavorable weather is forecast. The device utilizes both solar and wave energy to produce electricity, which increases its reliability and boosts its performance to deliver electricity for a much longer period when compared to any one of them. Above all, this device is a carbon-free energy producer, which will help to reduce carbon emissions and the use of fossil-powered devices for electricity supply. Apart from these, the proposed system will be able to promote renewable energy not only in urban centers but also in remote outer islands, where access to resources is very limited. This will provide the opportunity for the islanders to benefit from the HWPE and other sectors for an enhanced productivity level, as pictured in Figure 21.

The proposed device will not only provide renewable electricity to the coastal community but will also become a promising move in the provision of renewable energy to coastal and maritime communities around the globe through the capture of wave energy. This will allow communities living without electrical grid systems to relish a moment that may have been impossible, or at least not possible in the near future. Having access to electricity means being able to connect to the rest of the world, and being able to gain and share experience. This will further improve education and health systems.

3.2. Conceptual Design of the HWPE Harvester

The proposed novel HWPE Harvester is given in Figure 22. The proposed WEC consists of several main components: a spherical floating buoy, rack-and-pinion mechanism, a solar energy system, a battery, control devices, and an electric generator. The floating buoy is forced to oscillate upward and downward under the corresponding water level in the water column due to the incident wave. The movement of the floating buoy activates the rack-and-pinion system, resulting in the rotational movement of the pinion. The pinion acts as the input shaft of the gearbox, which consists of a unidirectional mechanism and a speed multiplier system with a flywheel concept. Ultimately, energy is produced by an electric generator that is powered by the gearbox output shaft. An external load or a storage system receives the output energy.

The conceptual HWPE harvester is a semi-submerged floating device which will capture the incoming waves via an oscillating water column structure. Figure 23 shows how the structure will look when viewed from above. The two large floats on either side will keep the device afloat with the change in tides, while the incoming waves will be able to generate electricity through the designed system. The structure will be moored to the seabed, preventing it from drifting away due to wind or large waves.

The HWPE harvester is designed to harness the energy from the waves and the sun simultaneously, or whichever is available at any given time. Figure 24 shows the flow diagram of the combined energy production device. The energy produced from the wave and the solar cells is optimized by a controller to effectively and efficiently deliver the power to the storage unit. The storage unit can be made from lead-acid cells, lithium ion technology, or used battery cells, e.g., used batteries from hybrid cars, which are usually discarded but contain some good cells which can be used to make a low-cost device. The stored energy can be used to power both DC and AC low-power loads, or devices such as phones, laptops, radio, Wi-Fi routers and led lights.

The designed HWPE harvester will be able to capture wave energy throughout the entire day, while solar energy will be harnessed during daylight. Based on these assumptions, the device will be able to produce 146 kwh of energy from solar energy and 438 kwh of energy from wave energy, combining it to produce 584 kwh of energy annually.

3.3. Technology Support for the Design of the HWPE Harvester

3.3.1. Electro-Mechanical Components of Wave Energy Harvesters

The designed HWPE harvester components are shown in

Table 9. HWPE system components.

Component	Description
Oscillating water column design	The oscillating water column design is adapted to effectively convert the incoming wave to up and down movement in the water column which will push the buoy to generate electricity.
Solar panel	The solar panel is placed on top of the device to harness energy from the sun and to provide a shelter for the wave energy conversion mechanism
Buoy	The buoy will capture the water movement and transfer the energy to the rack and pinion system.
Rack and pinion system	The rack and pinion system is responsible for converting the up and down movement into rotational motion.
Unidirectional gearbox	The unidirectional gearbox is utilized to extract only the upward motion and convert it in a one direction rotational motion to drive the DC generator
DC generator	This device is used to convert the rotational motion into direct current or usable electrical energy
Control circuit	The circuitry is utilized to efficiently combine the solar energy and the wave energy to charge the battery bank and also protect the battery bank from overcharging or discharging.
Battery Bank	The battery will store the produced energy from solar and wave which will be utilized to power gadgets and lights
Inverter	An inverter is used to invert the direct current from the battery to alternating current to power AC loads

, and a short description of each component is given. The cost of the conceptual design is about $3000 (FJD), or $1400 (USD), as it is made from locally available materials for the structure, and used materials like the battery and buoy.

3.3.2. Solar Cell Technologies

Solar cells are, interestingly, a widely accepted renewable energy technology that has formed a foundation of renewable energy generation for rural, coastal and maritime communities [5]. This is largely due to its ease of access and deployment. Many communities are able to utilize electrical energy through the use of solar systems to light their house and power gadgets like radios and phones, even where a grid is impossible or economically infeasible. Solar energy systems are a hope for people in remote areas and coastal communities around the world. The HWPE harvester takes the opportunity to integrate the traditional solar renewable energy system within its design, thus exploiting the advantages of the hybridized device. The output energy from the solar cell per the solar constant, or the sun’s intensity, provides the efficiency of the solar cell. Figure 25 shows various solar cell technologies, along with their efficiencies. Crystalline Si cells are very common in terms of the balance of efficiency and cost.

The HWPE harvester utilizes solar cell technology in the design to increase the generation capacity of the device within the same structure without much added cost. Crystalline Si or monocrystalline cells will be utilized in the HWPE design due to their low cost and availability, though they do not have the highest efficiency. The chosen solar cell is also best suited for an ocean environment because it is made of single crystal of silicon, and will perform better in the harsh oceanic surroundings. The solar panels will be placed on top of the wave energy harvester to take the full advantage of the sun’s irradiation, and will also protect the device from harsh sun rays and rain. Furthermore, interesting work has been presented [216,217] which talks about rooftop solar panels and the utilization of different panels for specific uses and environments. This research work will help us to attain a better efficiency and longer service life of the system in future for the conceptual design.

3.3.3. Energy Storage Systems

Electrical Energy Storage (EES) is defined as the process of transforming electrical energy from an energy source into a form that can, when required, be retained for conversion back into electrical energy. Such a process makes it possible to save electricity at times of either low demand or a lower cost of generation, or from intermittent energy sources, and to utilise it at times of high demand, a high cost of generation, or when no other means of generation is available [239]. ESS has been used in numerous applications, such as transportation, portable devices, and remote community electrification, but in recent decades, it is becoming a major part of the power generation, distribution and transition in renewable energy systems.

Table 10. Different energy storage systems by type.

Storage Type	Storage Theory	Storage Device
Electrical energy storage	Electrostatic energy storage	Capacitors and supercapacitors
	Magnetic/current energy storage	Superconducting Magnetic Energy Storage System
Mechanical energy storage	Kinetic energy storage	Flywheels
	Potential energy storage	Pumped hydroelectric storage and compressed air energy storage
Chemical energy storage	Electrochemical energy storage	Conventional batteries such as lead-acid, nickel metal hydride, lithium ion and flow-cell batteries such as zinc bromine and vanadium redox
	chemical energy storage	fuel cells, molten-carbonate fuel cells—molten-carbonate fuel cells and Metal-Air batteries
	thermochemical energy storage	solar hydrogen, solar metal, solar ammonia dissociation–recombination and solar methane dissociation–recombination
Thermal energy storage	Low temperature energy storage	Aquiferous cold energy storage, cryogenic energy storage
	High temperature energy storage	sensible heat systems such as steam or hot water accumulators, graphite, hot rocks and concrete, latent heat systems such as phase change materials

shows different storage types, along with the theory and storage device. Among these listed ESSs, only the battery storage system is used in our study. The other types of ESS could also be used, but with the support of additional infrastructure, e.g., support structures like those we saw in the recent project announced by the Danish Ministry of Climate, Energy and Utilities [240].

Energy storage is a vital part of the designed HWPE harvester, and it acts as the core of the power management system of the device. As the power generation from the wave source is inconsistent and varies in every wave cycle, the continuous supply of energy is unanticipated; hence, the energy produced cannot be directly used to power electrical devices. Therefore, the energy produced from the wave energy needs to be stored in some form before it can be utilized. Similarly, the energy produced from the solar technology also varies with time, requiring it to be stored for a later use. Hence, a battery storage system is a reliable solution to effectively store the energy from the two generation sources; it can be used to power a DC electrical load or even AC loads through the use of DC to AC inverter systems. The storage system being considered for the HWPE harvester is a battery storage system, in which the generated electricity is stored before being utilized. The battery will be obtained from old hybrid or electric vehicles, and will be load tested to acquire healthy cells to be used in the proposed system. This will not only reduce the cost of the system, but at the same time promote re-usage and reduce environmental impacts that would be caused by discarded batteries.

3.3.4. Power Transmission Systems

The transmission of generated power from the HWPE device can be a hurdle, as the device is located over the sea. The HWPE can transmit the electrical power through marine-grade cables over the given distance. The cables can be either buried under the seabed or supported by floats near the sea surface. The float cable system is more preferable, as it can be easily maintained and repaired, and can be pulled back to shore in times of natural disasters such as cyclones. The transmission voltage can range between low-, medium- and high-voltage AC or DC systems.

As seen in Figure 26, the energy will be stored in the form of direct current in the batteries, which then has to be converted into a desirable signal before it can flow in submarine cables to the shore. Low-voltage AC is achieved through a DC-to-AC converter before it can be transmitted to the shore. Medium- and high-voltage AC systems can be used, as the voltage drop is not significant within a 5 km range. In order to transmit dc power, a DC-to-DC converter is utilized to convert the voltage to about 200–400 v DC, whereby it can be transmitted with minimal loss. DC-to-DC converters are more complex, and most electrical devices today run on an AC power source; therefore, the HWPE harvester will employ low-voltage AC transmission through submarine cables. Figure 25 shows a graphical representation of the transmission system.

4. Challenges in Designing the HWPE Harvester

Though the proposed system offers multiple benefits, there were a few challenges in designing the HWPE harvester. These challenges are related to the system components, such as the wave device, photovoltaic integration, and mooring device. For the individual components, a few challenges were explored by researchers. For instance, Clement et al. [137] described several problems facing wave power converters as early as 2002, with the most significant ones being as follows:

• Irregular wave amplitude, phase and trend, meaning that the optimized efficiency of a system over the spectrum of excitation frequencies is difficult to preserve.
• In the event of severe weather conditions, the structural load can be as high as 100 times the normal load.
• The coupling mechanism of slow and irregular waves (frequency ∼0.1 Hz and period ∼10 s) to electrical generators normally requires 50 to 60 Hz.

The critical challenge for WEC is to generate electricity at a cost comparable to other renewables, or even traditional power generation. This simply means that, in the operating environments, wave energy must be transformed efficiently and reliably to usable energy; thus, the technology must be capable of surviving in extreme waves. These are mainly the problems of wave energy convertor performance, i.e., wave energy converter reliability and survivability. In particular, the latter two issues currently create the key obstacles to most real-time wave energy technologies. The wave energy system can lose its energy production capacity if it has a reliability issue; if it has a survivability issue, the device is broken or lost. The reliability and survivability of the system are more critical given the potential unplanned costly maintenance/repairs and the energy generation downtime. The efficiency of wave energy conversion should be improved during the learning/research process.

Apart from the challenges faced within the design of the WEC, there are some challenges presented by the wave source environment and the impact of the WEC construction and installation on the ocean environment. The majority of the impact on the environment is usually eventuated during the erection and establishment phase of the WET, and has to be considered in order to prevent harmful effects on the marine ecosystem. This creates a further hurdle for the WEC designers and producers. Furthermore, ocean wave conditions are not easy to predict, and produce intermittent electrical output. The output energy of the electrical generator is dependent on the incoming wave height and period, which increases the challenge of the prediction of power outputs. The other major challenge lies in the energy transmission system, as the wave energy devices are conventionally located away from the grid systems, and infrastructure for the transmission system is an additional cost. One of the significant challenges faced in wave energy design and deployment is the monetary needs to fund the project design and deployment, as the final outcome is not yet known for most of the newly designed systems in terms of the return on the investment and the working life of the system to actually cater for the maintenance and repair cost of the device. Another issue that might be of concern is the aesthetic effect of the installed device, especially for near-shore and shoreline devices. Another issue is the solar PV cells’ integration and mooring system, as the system weight is increased.

5. Potential Environmental Impacts of the HWPE Harvester

It is challenging to generalize regarding the possible negative consequences of wave power devices because WECs differ so much in terms of their technology and size, and site differ in respect to the water column and distance from the shore [60]. In this context, tropical nations have biological and socioeconomic traits in common, such as high biodiversity and unequal economic growth [241], which the marine energy business must consider. Due to the enormous diversity of birds, fish, turtles, and corals, and the fact that this biodiversity is already threatened, limiting or mitigating the possible negative consequences of WECs in the Tropics is a serious concern. WEC projects might have cumulative effects on marine and coastal ecosystems, compromising their health and resilience [242,243]. As tropical coastal populations rely heavily on ecosystems for commodities and services like fisheries and beach protection, any changes to these ecosystems might have major social implications [244]. Similarly, WECs may have environmental benefits, such as the establishment of fishing exclusion zones, where dwindling fish populations can recover [245]. In addition to their biological and socioeconomic complexity, the absence of long-term data and varied environmental consenting processes might threaten support for maritime energy projects in tropical nations. The ability to pay for maritime energy deployment, for example, is strongly tied to the expectation of moving towards a sustainable energy supply and the reduction of the energy sector’s environmental imprint. However, the absence of long-term data makes it impossible to create a baseline, which is essential for a thorough Environmental Impact Assessment (EIA) of any project. As a result, it is critical to identify viable locations for energy production, to prioritize efforts and resources, and to develop monitoring mechanisms [246].

The potential environmental impacts are given below:

• Device Construction: Anchoring these devices may have an influence during installation. Pilings, concrete blocks, anchors, and chains are used to secure or tie several wave energy devices to the ocean floor. The dredging and scouring of the seabed may be required for site preparation in order to lay electrical wires. The number of devices deployed and the mooring mechanisms used would determine the degree of ocean-bottom disturbance.
• Environmental: Although wave energy does not emit any greenhouse gases or other pollutants when it generates power, emissions do occur throughout the life cycle of the technology. There are potential consequences of hydraulic fluid leakage and discharge into the surrounding waters for hydraulic rams, power trains, lubricating oils and fluids, and anti-corrosion and biofouling paints and coatings.
• Fishing Industry: Exclusion zones around offshore devices may have a negative influence on nearby fishing grounds. Anchor lines, tethers, and power cables prevent nets from being used, while floating devices can create protected areas that benefit some marine species and ecosystems by restricting access and fishing at the location. Fishing activity may grow just beyond the installation’s boundaries, as it does in maritime reserves.
• Marine Ecosystem: The floating constructions may be dangerous to marine mammals, or they may operate as obstacles to marine circulation and migration, altering the wildlife and vegetation on the seabed. The majority of offshore wave energy devices are tied directly to the seabed, and mooring lines might entangle certain species, particularly bigger whales. Seabirds may be enticed to utilize floating wave energy devices as temporary roosts.
• Navigational Hazards: Due of their low profile, WECs may be difficult to spot visually or using a ship’s radar, which might pose a navigational threat to shipping. WECs that are not lit at night or whose moorings break away during storms might have an influence on shipping. The water quality might also be harmed as a result of possible oil spills caused by increased boat activity in the region for maintenance and repair.
• Noise Pollution: The continual noise from wave capture devices, particularly in harsh weather, may have an effect on whales and dolphins that hunt via echolocation. The operational noise levels of shoreline and nearshore devices may be a nuisance locally on the beach or shoreline. When fully functioning, however, any device-generated noise will most likely be drowned out by the natural sounds of the wind and waves.
• Recreational Activities: Offshore and nearshore floating devices might have an influence on recreational swimming and water sports in the area. Sub-aqua diving and water skiing may benefit from the protection provided by these devices, while sailing and wind surfing may suffer. Furthermore, while nearshore devices may only require a few hundred yards of water depth, there is an aesthetic impact of large-scale installations on tourism.
• Sedimentary Flow: Onshore and nearshore wave energy facilities—such as device platforms, anchors, and cables—may alter the flow of water and sands directly surrounding the structures. Sediment transport, coastal erosion, and the deposition of coarse sediments such as pebbles or boulders will all be affected by changes in water velocity. Sediment will be deposited more readily if water currents are slowed or limited.

Consequently, combining ocean wave energy and ocean-designed equipment to generate power is a potential option to assist is lowering the existing reliance on non-renewable energy supplies. However, technological obstacles and the lack of understanding of the environmental implications of wave energy in comparison to more traditional energy sources must be addressed. In general, some of the above-mentioned environmental consequences will be reduced for floating offshore devices while increasing for near- and shore-based devices.

6. Conclusions

In this article, a detailed review was carried out on the wave energy devices based on power take-off technology. Different PTO technologies were explored and explained, with the majority of the devices based on such systems being detailed in Table 1, Table 2, Table 3, Table 4 and Table 5. This paper also presents an innovative hybrid wave–photon energy device that is capable of harnessing wave energy and the sun’s energy simultaneously to produce electrical energy in a much-enhanced and productive way. The HWPE device uses a rack-and-pinion system to capture wave energy with the ministration of the water column structure of the device. The designed hybrid system will provide hope for remote coastal communities with consistent energy requirements for lighting needs and the powering of gadgets such as phones and radios. This will reduce the reliance on diesel-powered generators and fossil fuel-powered lanterns, decreasing the carbon footprint incurred by the outer-island communities. Having said that, there are a few challenges to be faced while working with wave energy, which are related to the unpredictable nature of waves, the economics of wave energy production, and the environmental impacts in the process of the construction and installation of the wave energy system.