Status and Challenges of Marine Current Turbines: A Global Review

Abstract

Over the past few decades, marine current energy utilization has transitioned from conceptual demonstrations to industrial-scale prototypes. This progression now approaches a crucial phase emphasizing the need for industrialization and commercialization. This paper provides an in-depth examination of the developmental status of large-scale marine current turbines and arrays, underscoring the promising prospects for marine current energy systems. Despite the significant advancements, the deployment of these systems has revealed several challenges, including blade design optimization, transmission system selection, and the establishment of appropriate test sites. Addressing these issues is essential for technological maturity and economic feasibility, which will foster the next wave of innovation in marine energy systems. Furthermore, the paper offers various perspectives for future applications.

1. Introduction

With the rapid development of socio-technological and economic facets, there has been a growing dependence on energy utilization. In modern society, nearly all activities rely on energy, whether it pertains to industrial production, transportation, or daily life, necessitating substantial quantities of electricity and fuel, as depicted in Figure 1 [1]. However, this substantial energy demand has given rise to many issues, including the depletion of conventional energy resources and severe environmental concerns, such as greenhouse gas emissions. Traditional fossil fuels, including coal, oil, and natural gas, have historically served as primary energy sources. Nevertheless, the long-term extraction and utilization of these energy resources have resulted in significant environmental problems, encompassing air pollution, climate change, and water resource contamination [2]. Greenhouse gas emissions have emerged as a paramount global ecological challenge, compelling a reevaluation of prevailing energy consumption structures.

Amidst this increasingly pressing backdrop, the quest for renewable energy sources has emerged as a pivotal avenue for addressing energy and environmental concerns. Renewable energy refers to sources of energy that are essentially inexhaustible, such as solar, wind, ocean, and geothermal [3]. These sources not only have a reduced environmental footprint but also harbor immense potential to meet the energy demands of human society. As depicted in Figure 2, the power generation shares of common energy sources in 2012 and 2022 clearly illustrate a gradual decline in the share of fossil fuels over the past decade, while the share of renewable energy has steadily increased, rising from 21.3% in 2012 to 29.9% in 2022, marking a notable increase of nearly 9%. In the future, renewable energy sources will account for a major share of the energy consumption structure.

The Earth’s surface is predominantly covered by the vast expanse of the ocean, encompassing more than 70 percent of its total area. This extensive aquatic domain is often called the “Energy Ocean” due to its remarkable potential as a source of abundant green and renewable energy resources. These resources encompass various forms of energy, including wave energy, marine energy, thermal energy, and salinity gradient energy. Despite their considerable promise, the comprehensive harnessing of these high-quality ocean energies has encountered significant challenges, primarily from technical complexities and cost-related issues [4]. Among these energy resources, marine current energy, a subset of marine energy, stands out for its exceptional attributes, characterized by high predictability, reliability, and stability, coupled with a remarkable power density [5]. Substantial is an understatement when referring to the globally estimated technically harvestable reserves for marine current energy, which impressively amount to approximately 1 TW, as depicted in Figure 3. This substantial reserve holds the potential for extensive exploration and utilization, presenting promising opportunities for the advancement of sustainable energy solutions [5,6]. Indeed, marine current energy development is facilitated by its technical congruence with wind energy [7]. Notably, marine current energy has experienced a significant and expeditious evolution over the past two decades, primarily driven by its adaptation and emulation of wind energy technologies as a reference point for its advancement [8]. To date, marine current energy conversion devices have exhibited a noteworthy progression, transitioning from power generation at the scale of tens or hundreds of kilowatts to now operating at the level of megawatts [9,10]. Concurrently, a limited number of projects featuring commercially operational turbines and array farms have been tentatively undertaken, while others remain in the phase of design and planning [11]. The marine current energy conversion devices primarily consist of two distinct configurations: horizontal-axis turbines and vertical-axis turbines. A wealth of theoretical analyses and experimental findings published in the literature consistently corroborate the superior effectiveness and reliability of the horizontal-axis turbine design, affirming its heightened potential for sustainable development and extensive commercial deployment on a large scale [12,13]. Within the scope of this study, our primary focus centers on the advancement of horizontal-axis large-scale marine current turbines (MCTs), specifically those characterized by a capacity exceeding 300 kW, and the noteworthy progress achieved in pre-commercial and commercial array projects. It is worth highlighting that these MCTs, with capacities exceeding 300 kW and designed for free-stream operation, represent an optimal solution as technology-finalized prototypes suitable for eventual large-scale commercial deployment.

This paper is organized as follows. In Section 2, a primary emphasis will be placed on delineating the development of MCTs, encompassing the classification of turbines, their operational principles, and the application of large-scale MCT systems. Section 3 will predominantly revolve around the planning of MCT arrays, explicitly focusing on elucidating the rationale, significance, design principles, and post-installation aspects such as maintenance and operation. Section 4 will center on the commercialization of MCTs, addressing factors such as commercialization trends, market prospects, and investment flows. Section 5 will spotlight the bottlenecks and challenges encountered in the development of MCTs, including aspects like blade design, resource allocation, and test site construction. Section 6 will proffer insights into prospective development trends, while Section 7 will summarize the preceding content and provide conclusive remarks.

2. The Technological Aspects of MCTs

2.1. The Operational Principles and Technical Characteristics of MCTs

Marine energy devices can be broadly categorized into two main types, horizontal-axis MCTs and vertical-axis MCTs, based on variations in their operational principles.

A horizontal-axis MCT refers to a system in which the direction of water flow is parallel to the horizontal axis of the turbine rotor, as illustrated in Figure 4 [14]. A vertical-axis MCT is characterized by the orientation of its turbine rotor’s axis of rotation being perpendicular to the direction of water flow, as depicted in Figure 5 [15].

Table 1. Comparison between horizontal-axis and vertical-axis MCTs.

Characteristics	Horizontal-Axis MCTs	Vertical-Axis MCTs
Blade structure	The blade structure is intricate [16].	The blades feature a symmetrical airfoil structure [17].
Force balance	They generate torque, necessitating a complex support structure to withstand the torsional forces [18].	They generate rotational torque, producing a relatively straightforward support structure [19].
Startup procedure	They exhibit a substantial self-starting torque, ensuring stable rotation [20].	They have a limited self-starting capability, leading to significant torque fluctuations [21].
Efficiency	They possess a high energy conversion efficiency [22].	They have a moderate energy conversion efficiency [23].
Installation and maintenance	Installation and maintenance are relatively complex, demanding more human resources and resources [24,25].	Installation and maintenance are comparatively straightforward, resulting in lower costs.

presents a comparative analysis of characteristics between horizontal-axis and vertical-axis MCTs. Due to the higher energy conversion efficiency, horizontal-axis MCTs have more significant potential for sustainable development and are better suited for large-scale commercial applications.

2.2. Developments of Large-Scale MCTs

Table 2. Developments of large-scale MCTs (free-stream, over 300 kW) over the past decades (2008–2023).

Turbine Name	Company	Location	Rotor Diameter	Rated Power	Installation Form	Status
SeaGen	Marine Current Turbines (acquired by SAE), UK	Strangford Narrows, Northern Ireland	16 m/each	2 × 600 kW at a current velocity of 2.4 m/s	Seabed bottom-supported (liftable)	Installed in 2008 and generated over 9.2 GWh (up to November 2013). Removed in 2017.
AK1000	SIMEC Atlantis Energy (the predecessor is Atlantis Resources), global	EMEC, Orkney, Scotland	18 m (two rotors)	1.0 MW	Seabed bottom-supported, piled	Deployed in 2010. Withdrawn soon after one month due to blade failure.
AR1000		As above	18 m	1.0 MW at a current velocity of 2.65 m/s	As above	Tested in 2011. Successful open ocean testing for four months.
AR1500		Globally commercial use	18 m	1.5 MW at a current velocity of 3 m/s	As above	4000 operational hours in Scotland.
SeaGen U		-	20 m	1.5 MW	As above	No data.
AR2000		-	20 m	2.0 MW at a current velocity of 3.05 m/s	As above	Currently in development.
AR500		Naru-Seto, Japan	11 m	500 kW	As above	Installed in 2021 and generated about 80,000 kWh in three months.
HS1000	Andritz Hydro Hammerfest, UK DP Energy, Irish.	Falls of Warness, Orkney, UK	21 m	1.0 MW at a water speed of 2.7 m/s	Seabed-mounted by ballast weights or pinning	Installed in 2011. Minor repairs after over 1 year’s operation, more than 1.2 GWh up to 2014. Retrieved in 2015 [8].
MK1		Pentland Firth, Scotland	26 m	1.5 MW	As above	Installed in 2016.
MK1 (new)		Nova Scotia, Canada	18 m	1.5 MW	Seabed bottom-supported, piled	Declared to be commissioned in 2023.
SR2000	Orbital Marine Power (the predecessor is Scotrenewables), Scotland	Kirkwall, Orkney, Scotland	16 m/each	2 × 1 MW at a current speed of 3 m/s	Fixed to retractable legs mounted to a floating platform	Installed in 2016. Retrieved in 2018. Produced over 3 GWh.
Orbital O2		-	20 m/each	2 × 1 MW at a current speed of 2.5 m/s	As above	The optimized version of SR2000 is in the production stage.
DeltaStream	Tidal Energy, UK	Ramsey Sound, Wales	15 m	400 kW at a current speed of 2.25 m/s	Fixed to a triangular frame pinned to the seabed	Deployed late in 2015. Stopped three months later due to faulty equipment.
Deepgen-III	Tidal Generation (acquired by Alstom) Alstom (acquired by GE)	Fall of Warness, Eday	No data	500 kW	Tripod support structure attached to the seabed	Deployed in 2010 and generated over 250 MWh (up to November 2012) [10].
Deepgen-IV		EMEC, Orkney, Scotland	18 m	1.0 MW at a current speed of 2.7 m/s	As above	Installed in 2013, generating over 1 GWh (up to November 2014). Extensive test of 18 months period [26,27].
Sabella D10	Sabella, France	Fromveur Passage, Ushant Island, France	10 m	1.0 MW (maximum)	Separable installation on a gravity-based foundation	Installed in 2015. Retrieved in 2019 for the cooling system’s defect.
Sabella D10 (new)		Fromveur Passage, Ushant Island, France	10 m	1 MW	Separable installation on a gravity-based foundation	Installed in 2022. Currently on test.
HyTide 1000	Voith Hydro (Voith Group’s subsidiary company)	EMEC, Orkney, Scotland	13 m	1.0 MW at the flow speed of 2.9 m/s	Mounted onto a monopole drilled into the seabed	Installed in 2013. Removed in 2015.
ATIR	Magallanes Renovables, Spain	EMEC, Orkney, Scotland	19 m (two rotors)	2.0 MW	Fixed to the underwater tower of a floating platform	Installed in 2019. Currently on test.
DG500	Minesto, Sweden	Holyhead Deep, west of Anglesey, North Wales	1.5 m	500 kW	Fixed to a subsea kite attached to the tether, which is connected to a bottom joint	Deployed in 2018 [28]. Currently on test.
DRAGON12		Vestmanna Sound, Faroe Islands	3.5 m	1.2 MW	A subsea kite system	Declared to be commissioned in 2023.
ZJU600	Zhejiang University, China	Zhairuoshan Island, Zhoushan China	15.45 m	650 kW at a current speed of 2.5 m/s	Installed on a liftable support structure of a floating platform	Installed and tested in 2017. Successful sea trials for 4 months.
GD300	Guo Dian United Power, China	As above	17.7 m	300 kW at a current speed of 1.9 m/s	As above	Installed in 2018. Removed in 2019. Produced about 0.5 GWh.
JH300	Hangzhou Jianghe Hydro-Electric Science & Technology, China	As above	16.5 m	300 kW at a current speed of 2 m/s	As above	Installed in 2019. Currently on test.
PLAT-I	Sustainable Marine Energy, UK	Nova Scotia, Canada	6.4 m	9 MW	Fixed to retractable legs mounted to a floating platform	Installed in 2019. Deployed in 2022. Currently on test.

enumerates the development progression of large-scale MCTs from 2008 to 2023, encompassing locations, rotor diameter, rated power, installation form, and other information.

In the 21st century, the marine and tidal energy industry and technology have experienced rapid development, driven by government subsidies and abundant marine energy resources. The UK has emerged as a leading tech-driven nation in this sector.

SeaGen [29], a 1.2 MW (2 × 600 kW) MCT, is primarily based on the SeaFlow turbine technology and is significant for its array and large-scale technological aspects, as shown in Figure 6a [30]. SeaGen features pitchable blades to optimize energy capture from bi-directional currents. This MCT system employs a seabed bottom-supported installation type, where two turbines are mounted on a liftable symmetrical beam. This unique design is pivotal in addressing critical issues related to the scalability and array configuration of MCT technology. In particular, it allows for retrieving turbines from the water in the event of faults, thus eliminating the need for underwater maintenance and simplifying upkeep procedures. SeaGen is the world’s first large-scale, grid-connected MCT system. Unfortunately, the system was decommissioned in 2017, emphasizing the importance of addressing critical technological challenges associated with the scaling up and deployment of MCT arrays.

SIMEC Atlantis Energy (SAE) Ltd. (Bristol, UK) has developed a suite of MCTs (including the AK1000, AR1000, AR1500, and SeaGen U models) [31]. The AK1000 (refer to Figure 6b) [32], a dual-rotor 1 MW turbine [33], encountered early blade failure [15]. In contrast, the AR1000 is a 1 MW turbine with three fixed-pitch blades, as shown in Figure 6c. Equipped with an active yaw system, it demonstrated success during testing at EMEC in 2011. The AR1500 (refer to Figure 6d), a 1.5 MW turbine, is designed for extended operational lifespans and employs active pitch and yaw systems. SAE Ltd. introduced the AR2000, a cutting-edge 2.0 MW MCT with a rapid and secure quick-connect wet-mate system, positioned for sale by the end of 2019. The AR500, installed in 2021, has generated electricity, producing over 90 MWh. The turbine has maintained a high availability rate, making this pilot project the first large-scale tidal energy project in Japanese waters.

Figure 6. SAE turbines, (a) SeaGen. (b) AK1000. (c) AR1000. (d) AR1500. (Reproduced from [8,34,35,36], with permission from Elsevier).

Figure 6. SAE turbines, (a) SeaGen. (b) AK1000. (c) AR1000. (d) AR1500. (Reproduced from [8,34,35,36], with permission from Elsevier).

HS1000 (as shown in Figure 7) [37] is a pre-commercial 1 MW MCT. This turbine utilizes a rotor with full-range pitch control to optimize its angle of incidence for efficiency [38]. It lacks a nacelle yaw system but employs a self-leveling device to ensure it aligns with the oncoming flow [39]. In 2013, the turbine was temporarily retrieved for minor maintenance and repairs and subsequently re-installed. By 2014, it had generated over 1.2 GWh of energy, surpassing expectations. The MK1 turbine is a 1.5 MW version [40] based on the AH1000 design and was deployed as part of the first phase of the MeyGen array, with three MK1 units installed in Pentland Firth, Scotland in 2016 [41]. The AHH turbine type features seabed-mounted installation using ballast weights or pinning mechanisms [42]. The transition from the AH1000 to the MK1 turbine for the MeyGen array represents a crucial development in commercial deployment.

SR2000, a 2 MW (2 × 1 MW) MCT, as shown in Figure 8 [43,44], adopts a twin-turbine configuration affixed to retractable legs mounted on a floating platform. This design significantly simplifies installation and maintenance operations, contributing to reduced costs during the operational phase [45,46]. Furthermore, using a floating platform offers the potential for economies of scale, especially in volume production scenarios, as demonstrated by the optimized Orbital O2 version of the SR2000. The Orbital O2, designed as a low-cost solution for broader deployment, incorporates features such as “gull-wing” style retractable legs for easier access to heavy lifting equipment and 360-degree blade pitching control [47]. These innovations enhance the turbine’s performance and enable simplified on-site maintenance on the floating superstructure. The overall cost-effectiveness of this design approach is critical for the commercialization of marine energy solutions.

A DeltaStream unit comprises three 400 kW turbines affixed to a triangular steel main base secured to the seabed. Tidal Energy Ltd. (Cardiff, UK) deployed a single DeltaStream turbine as a demonstration, as depicted in Figure 9 [48,49]. This configuration features a fixed-pitch rotor with specially designed blades and an automated hydraulic yaw mechanism to provide a consistently high average power output. Notably, the triangular unit housing three turbines, with a cumulative capacity of 1.2 MW, presents an appealing proposition for array deployment [50]. However, the operational success of this configuration was short-lived, as the device ceased operating merely three months after installation due to equipment failure [51].

DeepGen-III and DeepGen-IV [53,54], as illustrated in Figure 10, represent two versions of Alstom’s marine turbine technology, boasting nominal powers of 500 kW and 1 MW, respectively [55,56,57]. The DeepGen turbine type exhibits several advanced technical features. Firstly, it incorporates a unique nacelle buoyancy design that simplifies installation and maintenance. Secondly, thrusters are employed to rotate the nacelle, enabling precise alignment with the current for optimized utilization of bi-directional tides. Additionally, the turbines feature pitchable blades that enhance load control and power capture. Building upon the success of these implementations and the knowledge gained, the research team embarked on the design of a next-generation 1.4 MW turbine named Oceade for the 5.6 MW NEPTHYD project. Regrettably, this project was ultimately suspended by the parent company, GE, in 2017.

Sabella has developed a series of direct-drive MCTs named Dxx, specifically emphasizing the D10 turbine as the focal point [59]. D10, depicted in Figure 11, is a full-scale 1 MW turbine that evolved from Sabella’s initial prototype D03 [26,27]. A distinguishing feature of this turbine is the direct-drive permanent magnet generator solution coupled with six fixed blades [60], offering the potential to reduce structural complexity and enhance operational reliability [8]. However, the journey of the D10 turbine has not been without challenges. In 2016, after one year of sea trials, the turbine encountered cable failures, prompting two years of maintenance and optimization efforts at Brest port, culminating in the turbine’s redeployment in October 2018. Despite these efforts, the D10 turbine faced another setback in April 2019 due to a defect in the nacelle’s cooling system, leading to its retrieval.

Penobscot Bay and Cobscook Bay are the primary areas for these tidal energy tests. In Penobscot Bay, tidal currents can reach about 1.5 m per second, with an energy density ranging from 1.5 to 3 kilowatts per square meter. In Cobscook Bay, tidal currents can exceed 2.5 m per second, with an energy density potentially reaching 3 to 5 kilowatts per square meter. In 2012, the Ocean Renewable Power Company (Maine, USA) conducted the TidGen Power System testing project in Cobscook Bay, validating the performance and reliability of 150 kW MCTs under real-world conditions. These tests not only demonstrated the high efficiency of cross-flow tidal turbines in power generation but also provided valuable data and experience for the further development of tidal energy technology.

Voith Hydro introduced the HyTide 1000 horizontal-axis turbine, as depicted in Figure 12, with a 1 MW capacity [27,62]. This innovative turbine design uses symmetrical blades instead of a pitch system for capturing bi-directional energy, eliminating the need for a yaw and gearbox system [8]. The HyTide turbine is characterized by its direct-drive configuration, featuring a simple structure with a strong focus on high robustness and low maintenance requirements [63]. This period of approximately a year (installed in 2013 and removed in 2015) allowed the development team to gather essential experience related to both technical and environmental aspects. Despite this valuable experience, it is noteworthy that Voith did not pursue further development plans for the HyTide turbine.

Magallanes Renovables embarked on testing their 2 MW marine turbine named ATIR, as displayed in Figure 13. Notably, this technology received pre-certification through successfully deploying a 1:10 scale prototype in 2014 [64]. The ATIR turbine features two rotors equipped with a variable pitch system. It is affixed to the underwater tower of a floating platform, anchored to the seabed via mooring lines at the bow and stern [65]. The floating platform integrates onboard systems, simplifying installation and maintenance procedures [66]. An innovative seawater-lubricated bearing solution is adopted to ensure an extended operational lifespan. With a foundation of achieving 400 kW of power generation in tow testing, the current focus is on incrementally increasing power output to reach the rated 2 MW.

The innovative Deep Green technology introduced by Minesto addresses the challenges of harnessing energy in low-flow velocity sites. This groundbreaking approach involves the utilization of a subsea kite to host an MCT, as depicted in Figure 14 [67]. The subsea kite operates by capitalizing on hydrodynamic lift forces acting on its wing while being steered in an eight-shape trajectory with the assistance of a rudder. This unique method allows the kite to attain speeds several times greater than the actual flow velocity, consequently reducing the required size of the turbine rotor compared to conventional designs. By the end of 2019, the initial objectives of flying full subsea operational trajectories and generating electricity were successfully achieved. Subsequent phases are set to encompass more in-depth verifications, such as full power production and exploring possibilities for commercial array expansion, like Dragon 12.

The development of marine current energy utilization in China has seen significant progress, primarily attributed to government policies and financial support. Zhejiang University (ZJU) has played a pivotal role in this development by establishing a marine power station in the waters of Zhoushan in 2014. A key milestone in this endeavor was the deployment and testing of a 600 kW turbine on the ZJU marine power station in 2017, as depicted in Figure 15 [69]. This turbine is a fixed-pitch design, featuring a drive train with a half-direct drive mechanism. The drive train comprises a two-stage parallel-axis power-diffluence and confluence gear train along with a relatively low-speed generator. Notably, a modular design philosophy is adopted, rendering each component relatively independent [70]. These distinctive characteristics hold the potential to optimize the nacelle’s streamlining, minimize current blocking effects, and streamline repair and maintenance processes. This technology’s successful four-month sea trial validated its performance [69]. Notably, this turbine represents the largest horizontal-axis MCT in China to date.

Guodian United Power Ltd. (Beijing, China) has advanced the marine current energy sector by developing a 300 kW MCT, as depicted in Figure 16. This MCT is built upon the technological foundations established by ZJU and represents a significant step toward commercialization. Notable features of the GD300 turbine include using an electrical pitch system for blade adjustment within a wide range from 0 to 270 degrees, offering precise power regulation. Additionally, the turbine employs glass-carbon fiber composite materials for its blades, reducing weight and enhancing strength. Sea trials for the GD300 turbine were conducted at the ZJU marine power station in 2018. During the one-year testing period, the turbine produced approximately 0.5 GWh of electricity.

In 2018, Hangzhou Jianghe Hydro-Electric Science & Technology Ltd. (Hangzhou, China) introduced a 300 kW direct-drive MCT, as depicted in Figure 17. This MCT’s distinguishing characteristic is its utilization of a direct-drive mechanism, eliminating the need for a transmission system between the rotor and generator. Furthermore, a specially designed generator incorporating water lubrication has been employed to enhance underwater reliability. The 300 kW turbine was successfully installed at the ZJU marine power station in May 2019. The primary objective is to validate the feasibility of their technology.

The PLAT-I boasts four SCHOTTEL Hydro SIT250 marine turbines with a collective capacity of up to 420 kW, as depicted in Figure 18 [71]. This device can access sheltered lower resource sites across the globe. One notable feature is the incorporation of a mooring turret into SME’s proprietary design, enabling alignment with tide or river flow in any direction [72]. The system’s innovative swing-up turbine deployment modules (SDMs), a rarity in the industry, provide access to the turbines above the water’s surface. This, combined with the low mass of the SCHOTTEL turbines, allows for on-site blade and nacelle replacement [73]. However, concerns have arisen regarding the potential environmental impact of large-scale MCT arrays like PLAT-I. To address these concerns, platform staff have been conducting observations and reviewing operation videos since February 2019. The results of these efforts indicate that marine mammals tend to avoid turbine units. However, a minority of fish may encounter challenges in doing so.

3. MCT Array Configurations

MCT array configurations refer to the process and methodology of deploying multiple MCTs in marine or aquatic environments to maximize the capture of kinetic energy from water flow and generate electricity. Array optimization is the core problem the planners care about [74]. It is a complicated problem that involves numerous variables, such as the turbine’s number, type, size, shape, and capacity. Meanwhile, the resource characteristic of the flow field, its variation rule, and the wake effect should be taken into consideration [75]. For the coming industrial array application, maximum electricity generation can only be achieved when devices are arranged in optimum array configurations.

3.1. Influencing Factors of the MCT Array Configurations

3.1.1. Hydrological Characteristics and Site Selection

Before the deployment of large-scale marine energy systems, a paramount task is to understand the marine conditions of the target water body comprehensively. This entails the assessment of vital factors such as water flow velocity, direction, marine cycles, marine heights, and the potential energy of the water flow [76]. These pieces of information can be acquired through various means, including on-site observations, model simulations, and satellite data. While comprehending the hydrological characteristics of the water body, it is imperative to account for the variability of the water flow, considering different seasons, meteorological conditions, and marine cycle fluctuations. Additionally, the ecological environment of the water body must be considered to ensure that the deployment of the MCTs does not cause irreversible impacts on marine life and the seabed, among other aspects.

Subsequently, when selecting the geographical location for the MCTs, it is essential to choose sites abundant in water flow energy, such as straits, shallows, or marine estuaries. Moreover, the logistical convenience of the geographic location should be considered to facilitate the swift and efficient transmission of generated electricity to the power grid.

3.1.2. Array Layout

When configuring the MCTs, various arrangements can be chosen, such as linear, grid, or hybrid layouts. Linear layouts are suitable for one-dimensional water flow, whereas grid layouts are better suited for multi-directional water flows [76]. The spacing and density between MCTs are pivotal factors in array design. Appropriate spacing can reduce mutual interference between devices and enhance overall performance. The choice of density should consider water flow velocity and energy potential. In addition to the arrangement type and the spacing and density of MCTs, the devices’ orientation also significantly impacts the array [77,78]. MCTs should ideally face the direction of the water flow to enhance energy capture efficiency. Some MCT systems can also adjust their orientation to adapt to changes in water flow direction.

3.1.3. Wake Characteristics

Wake refers to the region where water flow velocity decreases and eddies increase after passing through the MCTs. This phenomenon has profound implications for the performance and mutual interactions of MCT arrays [79]. Firstly, wake characteristics directly impact the spacing and layout of devices. In the wake region, water flow velocity is reduced, necessitating appropriate device spacing to prevent mutual interference between devices and reduce energy capture efficiency. Additionally, layout choices need to consider how to minimize the adverse effects of wake regions on the overall array performance [80]. Second, wake characteristics are associated with devices’ relative orientation and arrangement. Proper device orientation can reduce wake generation, thereby enhancing energy capture efficiency. The configurations can also mitigate the impact of wake effects by reducing interference between adjacent devices. Furthermore, wake regions may induce eddies, affecting seabed disturbance and underwater ecosystems. In the array planning of marine energy devices, numerical simulations, experimental research, and on-site observations can be employed to fully understand and optimize wake characteristics, ensuring the maximization of array performance and sustainability [81].

3.2. Researches on Array Configurations

Beyond the constraints imposed by resource availability and technological challenges, optimizing MCT arrays presents clear advantages. For example, a study referenced as [82] illustrates an 18% disparity in power extraction between a conventional array layout and a staggered array configuration comprising an equal number of MCTs. Recent research endeavors in the domain of array configuration predominantly focus on hydrodynamics, encompassing the interactional wake effect, the Levelized Cost of Energy (LCOE), power efficiency, optimization methodologies and algorithms, and standardized procedures, among other aspects. The EMEC offers recommendations, suggesting that lateral spacing exceeding 2.5 times the rotor diameter and downstream spacing exceeding 10 times the rotor diameter is appropriate for a marine energy array, independent of available spatial resources. Consequently, practical considerations dictate that turbines should be positioned in a staggered arrangement to maximize power output. As per [83], it is further established that to achieve optimal power output in a staggered array, the distance between adjacent rows of turbines should exceed three times the rotor diameter. Addressing the financial aspects of marine array planning, Ref. [83] addresses the cost considerations. The study employs a specific optimization tool, ‘OpenTidalFarm’, to optimize turbine siting in the Inner Sound of the Pentland Firth, minimizing cabling costs while accounting for subsea connections and cable routing. As elucidated in [84], metaheuristic approaches represent the latest optimization techniques, primarily rooted in artificial intelligence and machine learning. Among these, evolutionary algorithms, genetic algorithms, and particle swarm optimization are the most prominent methods.

3.3. Large MCT Array Projects

As described above, individual turbine power capacity has reached the megawatt level, signifying a maturation of turbine technology attained through years of intensive technical research and development. Antecedent to the forthcoming large-scale commercial deployment, several globally prominent organizations have started the exploration of turbine array projects. Entities such as MeyGen, Nova, and Minesto, among others, have embarked on these ventures to showcase both the technical feasibility and commercial viability. A compendium of critical details about these projects is provided in

Table 3. MCT array projects of late years.

Array Project	Location	Capacity Planned	Turbine Type	Status
MeyGen	An offshore site between Scotland’s northernmost coast and the island of Stroma, UK	398 MW	MK1, AR1500 (Phase 1A)	Split into several phases: Phase 1A (total 6 MW, 4 units) [48]. Phase 1B (total 10 MW, 6 units). Phase 1C (will achieve 73.5 MW, 49 units). Phase 2 (will achieve 101.5 MW in 2027). Phase 3 (will achieve 146 MW in 2028). Phase 4 (will achieve 398 MW) Project construction commenced in January 2015. Turbine installation commenced in October 2016. Phase 1A began operations in April 2018. Phase 1B is in deployment. Until March 2023, it has generated to the grid for 6 years and generated 51 GWh of power.
Roosevelt Island Tidal Energy (RITE)	East Channel of the East River, New York Harbor, US	1 MW	Gen4, Gen5	Six Gen4 turbines, each 35 kW, were deployed in 2006. The Gen5 commercial class systems were installed and commissioned in 2020. Verdant worked with the European Marine Centre to conduct an off-site power performance assessment using international technical specifications. In 2021, Verdant Power’s RITE project delivered over 312 MWh.
OpenHydro turbine	EMEC(UK), France, and Nova Scotia (Canada)	14 MW	OpenHydro turbine	In 2008, a 250 kW turbine was installed and tested at EMEC. In 2011, a 500 kW turbine was installed at a depth of 35 m. In 2016, two turbines of 2 MW were installed at FORCE in Nova Scotia, Canada.
Enabling Future Arrays in Tidal (EnFAIT)	Bluemull Sound, Shetland, UK	600 kW	Nova Innovation M100	Three M100 turbines of 100 kW each were deployed from March 2016 to February 2017 [85]. The next phase involved installing an additional three turbines. The fourth marine turbine was installed in 2020, and the fifth and sixth marine turbines were added to the array in 2022. Up to now, EnFAIT has a 30% reduction in the cost of marine power and 95% turbine availability (exceeding the 80% target).
ZJU Marine Energy	Zhairuoshan Island, Zhoushan, China	2 MW	ZJU MCTs	They were commenced in 2014. Until now, three turbines of 60 kW, 120 kW, and 600 kW with floating platforms have been deployed. The 600 kW marine turbine was installed in 2014. The 120 kW marine turbine was installed in 2015. The 650 kW marine turbine was installed in 2017.
LHD-tech	Xiushan Island, Zhoushan, China	3.3 MW	LHD	This project began in 2016. By 2018, it had installed two 200 kW units, three 300 kW units, and one 400 kW unit. By 2022, an additional 1.6 MW unit was added.

.

The MeyGen project is a renowned multi-turbine array, as depicted in Figure 19. This ambitious project is the world’s most enormous planned marine array, boasting a formidable power capacity of 398 MW [86]. The project is divided into several phases:

• Phase 1A: A gravity support installation method is adopted. Each turbine has a dedicated subsea array cable laid directly on the seabed, and the electric power is fed into the onshore power conversion building.
• Phase 1B: Two additional AR2000 turbines were linked to a single power export cable through the new subsea hub, ultimately connecting to the National Grid via the MeyGen substation [87].
• Phase 2: The MeyGen project is granted a Contract For Difference (CFD) in Allocation Round 4 for 28 MW at an agreed strike price of £178.54 per MWh, with a targeted commissioning date set for 2027.
• Phase 3: MeyGen’s Phase 3 is poised to bring the remaining portion of the consented project to fruition. With the recent allocation of 28 MW in Allocation Round 4 (AR4), MeyGen holds an additional consent for 52 MW, making it eligible for future bidding in upcoming CFD Allocation Rounds.
• Phase 4: The MeyGen offshore lease and site resource allocation accommodate the installation of a capacity of up to 398 MW [88,89]. Phase 4 of the project is anticipated to expand existing consents and fully develop the remaining capacity potential.

In 2005, Verdant Power initiated the RITE project, which was dedicated to conducting prototype and pre-commercial testing of its Free Flow System [90,91]. In 2006, six Free Flow System turbines of the 4th generation, each with a capacity of 35 kW, were deployed for demonstration purposes. The noteworthy achievements include entire bidirectional operation, automatic control, continuous, unattended operation, and accumulating 9000 turbine hours. The primary focus of Gen5 is to minimize maintenance requirements by extending service intervals and achieving economic volume manufacturing and assembly. With the Federal Energy Regulatory Commission’s issuance of a 1 MW installation license, the commercial-class Gen5 systems were scheduled for deployment in 2020, as depicted in Figure 20.

OpenHydro has developed innovative marine turbines that have been tested and deployed in various locations, including the EMEC in Scotland, France, and Nova Scotia, as depicted in Figure 21. This technology features a unique design with multiple blades fixed between an open-center rim and the outside shell in a rim-driven configuration, integrating a permanent magnet synchronous generator into the rim shell. The journey began with a 250 kW turbine installed and connected to the UK national grid in 2008. In 2011, a 500 kW turbine was tested near Brest, France. By 2014, the project at Paimpol-Bréhat evolved to include two turbines with a total capacity of 1 MW. In 2016, the array was successfully connected to the French electrical grid via a subsea converter developed by General Electric. OpenHydro moved forward with a 4 MW grid-connected tidal array at the FORCE in Nova Scotia, Canada, which consisted of two turbines of 2 MW. The Normandy Hydro project planned to install seven OpenHydro turbines off the coast of Cherbourg in Raz Blanchard, achieving a total installed power of 14 MW, with grid connection in 2018.

Nova Innovation Ltd. (Edinburgh, Scottish) is embarking on the development of a marine energy array project called EnFAIT. This project extends to include six 100 kW M100 turbines, as depicted in Figure 22. The M500D MCT is a powerful and efficient device designed for commercial-scale MCT projects. With a rated capacity of 500 kW, it is engineered to harness significant energy from tidal currents. The turbine features a robust direct drive system, eliminating the need for a gearbox, which reduces maintenance and enhances reliability. Its design allows for deployment at various depths, making it adaptable to different tidal conditions and seabed topographies. The M500D’s efficient energy capture capabilities ensure consistent power generation, while its environmentally conscious design minimizes impact on marine ecosystems. Notably, the M500D had been successfully deployed in the Shetland Islands, demonstrating its practical application and contribution to local renewable energy grids.

ZJU Marine Energy, as depicted in Figure 23, represents a notable turbine array project in China. This array comprises three offshore platforms, featuring turbines with capacities of 60 kW, 120 kW, and 600 kW, established by ZJU in the vicinity of Zhairuoshan Island, Zhejiang Province. These twin-hull floating platforms have a portal-frame lifting mechanism, offering a bespoke design for enhanced functionality [94]. The project has undergone extensive open-sea testing, operating both in on-grid and off-grid modes, with successful outcomes. Furthermore, this marine power station serves as a reliable source of electricity for Zhairuoshan Island households, with an accumulated power generation of approximately two million kWh. A fourth megawatt-level unit is currently in development as a testament to its progress.

The project began in 2016, as depicted in Figure 24, launching two nominal power of 200 kW vertical-axis and two nominal powers of 300 kW vertical-axis marine power units that year. In 2018, a 400 kW vertical-axis marine power unit was deployed, followed by a 300 kW horizontal-axis unit at the end of the year. In February 2022, a nominal power of 1.6 MW horizontal-axis unit was deployed. However, during this process, some units, such as the 300 kW horizontal-axis marine power unit, were damaged and retrieved. Additionally, the installation method used involves a semi-enclosed dam construction on the seabed between two islands, which is not universally applicable to marine power equipment that captures energy from the open ocean.

Apart from the aforementioned few array projects that have been preliminarily implemented, there are others in the plan. SAE purchased the Sound of Islay site from Scottish Power Renewables (SPR) in 2016. But, they ultimately did not carry out this project. SAE has made a rational construction scheme and a detailed environmental impact statement. The Crown Estate awarded Minesto an agreement for lease for a 10 MW installation at West of Anglesey, North Wales, UK, in 2014. This project was named Holyhead Deep, a low-flow marine stream project using the company’s Deep Green technology [92]. The 0.5 MW DG500 prototype was installed and tested successfully in 2018. The next step is to gradually expand the site to a commercial demonstration array of up to 10 MW installed capacity. The Environmental Statement and Habitats Regulations Assessment report was commissioned to minimize all potential impacts on the local environment and wildlife in the starting stage. A couple of marine array works have been planned by Sabella. Phares, an island hybrid-architecture project of renewable energy at Ushant Island, France, led by Akuo Energy in partnership with Sabella, will involve two marine turbines, Sabella D12, of 500 kW each [96]. Additionally, Sabella has been chosen as a turbine supplier for a 3 MW marine power project by the H&WB Asia Pacific Corporation nowadays. The project will be located at Capul Island, the province of Northern Samar, Philippines, and install three Sabella D18 turbines, each with a 1 MW capacity.

In recent years, the landscape of MCT array projects has witnessed the emergence of several promising candidates. However, it is notable that only a select few of these projects have transitioned from the conceptual stages to the execution phase. The predominant factors contributing to this distinction often include the formidable costs associated with marine energy endeavors, complex technical challenges, and many other influential considerations. Consequently, the projects that have materialized typically assume the form of pre-commercial arrays characterized by an exploratory nature during their initial phases. It is evident that most of these actualized projects still maintain a certain degree of separation from achieving the desired levels of technical maturity and cost feasibility.

4. Pre-Commercialization of MCTs

MCTs have exhibited favorable developmental trends and potential in recent years, with support from various quarters [48]. Firstly, devices such as MCTs harness a significant renewable energy potential. They can capture and utilize constant natural sources, such as marine tides and currents, distinguishing them from other forms of renewable energy susceptible to seasonal and meteorological variations. This reliability renders MCTs an increasingly prominent choice in the renewable energy sector, offering the prospect of sustained and stable energy production. Secondly, industry reports and research data consistently indicate robust growth prospects in the MCTs market [97,98,99]. This suggests that the market scale is poised for substantial expansion in the coming years, presenting many opportunities for investors and businesses. Concurrently, heightened investor interest in the MCTs market has accelerated technological innovation and market development. Most importantly, the urgency of climate change has propelled the demand for the renewable energy market. Concerns have arisen due to rising global temperatures and an increase in extreme weather events. MCTs aid in reducing carbon emissions and thus garner active support from governments and international organizations. This urgency positions MCTs not only as a commercial opportunity but also as a component of addressing the challenges of climate change.

All in all, the MCTs market exhibits a positive outlook and receives multifaceted support, positioning it as a prominent domain in the renewable energy sector. It is poised for significant advancements in the foreseeable future.

4.1. Policy

The implementation of policies contributes to the regulation, encouragement, and support of the marine energy sector’s development. On the one hand, policies can establish the necessary market regulations, providing clear guidance for investors and businesses to ensure that projects adhere to legal and environmental standards [100]. Such regulatory policies enhance market transparency and predictability, reducing investment risks and thus attracting more capital. On the other hand, policies can incentivize the development of MCTs by offering measures such as tax incentives, power purchase agreements, and subsidies, thereby lowering project costs and improving returns on investment, increasing their attractiveness. These incentives assist projects in securing early stage financial support and gaining a competitive edge in the market. Policy stability is crucial for attracting long-term investments and ensuring project success. A stable, long-term policy environment builds confidence, encouraging investors to bear risks and participate in marine energy projects over the long run. Frequent policy changes or uncertainties can hinder investment and project development, highlighting the significance of policy coherence and stability.

Due to varying policies across different countries and regions, divergent impacts have emerged, as detailed in

Table 4. Policies in various countries and their impacts.

Countries	Relevant Policies	Impacts
Norway	① Power Purchase Agreements (PPA) supporting offshore turbine projects (in 2018). ② Provide tax incentives for renewable energy projects (in 2021). ③ The green electricity certificate system (in 2011). ④ Supporting research and technological innovation. ⑤ Environmental standards.	① Providing stable electricity prices and market access for projects, and reducing investment risks and attracting investors. ② Contributing to enhancing the economic attractiveness of the project and reducing costs. ③ Enhancing the project’s market competitiveness and aid in reducing carbon emissions, aligning with Norway’s sustainable development goals. ④ Aiding in improving efficiency, reducing costs, and enhancing the competitiveness of the technology. ⑤ Contributing to reducing adverse impacts on the ecosystem and enhancing the project’s sustainability.
UK	① PPA (in 2019). ② Provide a range of financial support, including subsidies, investments, and research and development funding” (in 2022, 68 million pounds sterling). ③ Green Finance Policies (in 2019). ④ Implemented a series of environmental regulations (in 2019). ⑤ Promoting technological innovation. ⑥The Contract for difference scheme (in 2023).	① Ensuring the sale of project electricity and a stable income. ② Assisting the project in securing startup funding in the early stages and reducing costs. ③ Providing low-interest loans and financial incentives to encourage investors and financial institutions to support these projects. ④ Contributing to reducing the potential impact of projects on the marine ecosystem. ⑤ By supporting research and development projects, enhance efficiency and reliability, thereby reducing costs.
France	① Enacted clean energy and renewable energy policies (in 2018). ② PPA (in 2018). ③ Supporting technological innovation. ④ Implemented a series of environmental regulations (in 2020).	① Increasing the share of renewable energy in the national energy supply, providing policy support for offshore turbine projects to drive their commercialization. ② Reducing the commercial risks of the project, attracting a significant number of investors and developers. ③ Enhancing the efficiency and reliability of the turbines, reducing costs. ④ Ensuring that projects adhere to high environmental standards, contributing to the reduction of potential impacts on the marine ecosystem.
China	① OESF fund(in May 2010,more than USD 200 million). ② The National Key Research and Development Program of “Research and development of innovative technologies for efficient utilization of ocean energy based on the resource characteristics(in 2019, about USD 2.76 million). ③ The 13-th Five-Year Plan of China (in 2016). ④ The 14-th Five-Year Plan of China (in 2021).	① Marine current and wave energies secure more than half of the investment amount. ② Several national marine current energy test sites are under construction. ③ Actively developing coastal marine energy resources. ④ Continuing to carry out marine energy demonstration projects and actively promoting the application of megawatt-scale marine energy generators.
Japan	① EIA Act(in 2024). ② Financial Incentives and Subsidies (in 2022). ③ Electricity Business Act. ④ Support for innovation and R&D.	① Protecting marine ecosystem. ② Contributing to renewable energy capacity and helping reduce reliance on fossil fuels. ③ Helping manage the intermittent nature of renewable energy, ensuring a stable and reliable power supply. ④ Helping lower the costs and improving the feasibility of tidal energy projects.

:

Through a comparative analysis of policies among these countries and regions, it becomes evident that policy stability and the effectiveness of incentive measures have profound implications for project development and investment decisions.

The key to achieving large-scale commercialization of MCTs and fostering broader global utilization of marine energy lies in the support of more favorable policies. Only in the presence of clearer and more proactive policy support can renewable energy develop more rapidly. Therefore, the pivotal role of policy cannot be underestimated; it is a crucial factor in driving the commercialization of MCTs and the realization of large-scale marine energy.

4.2. Investment

Due to their large scale, MCT projects often require substantial financial investments to achieve commercialization. Projects of this magnitude typically demand high-end technologies, complex engineering, and long-term research and development, making funding availability crucial for their success. Particularly in the field of renewable energy, marine turbines, as a reliable source of clean energy, have captured the attention of investors [101].

As depicted in Figure 25, clean energy projects have garnered significant favor from investors and businesses. Consequently, from 2015 to the present, the investment amount in this sector has grown annually. Globally, even in challenging economic environments, many countries are willing to allocate funds to clean energy projects (refer to Figure 26), underscoring the robust appeal and promising prospects within this field. For instance, according to

Table 5. Investments in large-scale MCTs by various countries in recent years.

Time	Countries	Investment Amount
2010–2019	France	69 million euros
in 2019	China	2.76 million US dollars
in 2019	UK	35 million US dollars
in 2020	USA	148 million US dollars
in 2020	China	150 million US dollars
in 2021	UK	5.18 million US dollars
in 2023	USA	35 million US dollars
in 2023	UK	10 million pounds

, significant investments have been made in MCT projects in various countries and regions, driving the expansion and acceleration of these projects. The growth in investment amounts not only contributes to the success of these projects but also advances the development and maturation of MCT technology.

Funding for MCT projects flows through various channels. On the one hand, government support programs encourage project development by offering subsidies, power purchase agreements, and tax incentives. These government support measures provide predictability and reduce project risks, thus attracting long-term investments. On the other hand, the openness of capital markets provides financing avenues for projects, including stock issuance, bond financing, and private equity. Investors and institutions can invest in MCT projects through capital markets and gain returns.

In summary, the successful commercialization of MCT projects requires substantial funding, with a pivotal role played by government support, international funding initiatives, and capital market participation. These investment dynamics reflect the appeal and growth prospects of MCT projects, offering hope for the future of the renewable energy sector. The openness of capital markets also provides investors with diversified investment opportunities, contributing to the flourishing development of renewable energy projects.

4.3. Economic Cost

Irrespective of the technical feasibility and the formulation of optimal policies, their economic cost is the pivotal impediment to the widespread commercialization and scaling of turbine devices and array projects. In essence, they must eventually achieve economic competitiveness with conventional energy resources. This paper assessed the Levelized Cost of Electricity (LCOE) as the per-unit cost of electricity generation [103]. As reported by the U.S. National Ocean Economics Program [104], the LCOE for offshore wind and marine energy systems, encompassing marine stream and marine range technologies, ranges from 0.12 to 0.21 dollars per kilowatt-hour and 0.20 to 0.56 dollars per kilowatt-hour, respectively, over their operational lifetimes. The associated cost structure is elucidated in Figure 27. It is noteworthy that deploying offshore installations, power transmission infrastructure, and the necessary maintenance procedures incur substantial expenses.

Currently, marine energy projects primarily employ two main installation methods: seabed bottom-supported and floating installations [105]. In addition to turbine manufacturing, the cost associated with seabed bottom-supported installations predominantly encompasses the construction of subsea foundations and cables, as well as the allocation of labor and materials for subsea assembly. Subsea foundation construction constitutes the most significant proportion of these costs [28]. The layout and construction of subsea cables also contribute significantly to the overall expenditure. For instance, in the case of the 5 MW wave energy project initiated by the Korea Research Institute of Ships and Ocean Engineering in 2016, the cost of submarine cables ranged between 1 to 2 million USD per kilometer. Furthermore, subsea foundation construction, cable installation, and device positioning within a continuous and robust aquatic environment necessitate specialized equipment and skilled professionals. Both from a technical and economic perspective, this undertaking presents an exceptionally formidable challenge [106].

In contrast, the floating installation method, frequently employed for prototype testing, is characterized by reduced costs [107]. Under this configuration, the turbine is typically affixed to a floating platform or hull, with the electrical and load systems housed within the platform’s cabin. The foundation construction is simplified, comprising solely the mooring system, and is devoid of the complexities associated with submarine cables or subsea installation efforts. This streamlined approach makes installing, maintaining, and removing turbines more convenient. However, it is imperative to conduct further assessments and validation to ascertain the cost-effectiveness and feasibility of large-scale and array deployment.

The maintenance of the system is associated with considerable expenses due to the challenging underwater operating conditions, which tend to result in turbine damage. As discussed in Section 2, several turbines encountered issues shortly after deployment. Furthermore, the accumulation of marine fouling and corrosion renders the turbines susceptible to deterioration. Figure 28 visually illustrates the extent of fouling and corrosion problems that manifested approximately six months following the turbines’ submersion. When juxtaposed with the design lifespan of wind turbines, which typically spans 15 to 20 years, it becomes evident that recouping the costs invested in MCTs poses a formidable challenge. It is estimated that the annual operational and maintenance expenses for ocean energy devices can escalate to approximately 3.4–5.8% of the initial capital expenditure, in contrast to the 2.3–3.7% observed for offshore wind installations [108].

5. Challenges

The commercialization and widespread implementation of large-scale marine energy device arrays, as a critical technology in marine energy, are gaining increasing attention. However, numerous technological and other challenges must be overcome to achieve this goal.

5.1. Technical Aspects

Technical challenges primarily revolve around the turbine blades and the transmission systems. The design of turbine blades and the optimization of transmission systems are pivotal for enhancing marine energy conversion efficiency. These aspects necessitate considering multiple factors, encompassing fluid dynamics, materials science, and structural engineering.

5.1.1. Design of the Blades

As a pivotal component within the context of an MCT, the rotor is a determining factor in energy harvesting capabilities and in shaping the load characteristics and overall system reliability. It assumes a critical role in the system’s performance. Nonetheless, it is worth noting that the blades of MCT rotors have been documented to exhibit a notable incidence of failure. Instances of blade faults have been reported in various existing prototypes, including but not limited to the SeaGen turbine, Verdant Power’s Gen turbine, and the ZJU 60 kW turbine (as depicted in Figure 29 and Figure 30).

Two aspects can be contemplated to minimize the blade failure rate as much as possible: optimizing the design of blade dimensions and selecting suitable blade materials.

When optimizing the blade size and structure, several considerations come into play. Firstly, complex fluid dynamics are involved, as the blades must adapt to the ever-changing marine flow conditions, necessitating continuous adjustments in the geometric shape of the blades to maximize the capture of marine energy while minimizing hydrodynamic losses [16]. Secondly, as the device dimensions increase, the blade design must exhibit scalability, requiring the determination of appropriate blade length, width, and pitch angles to accommodate various device scales and flow velocities [110]. Finally, since marine energy devices often operate under different marine conditions, blade design must demonstrate versatility to adapt to varying flow speeds, directions, and hydrodynamic characteristics [111]. Addressing these challenges requires in-depth research and engineering innovation to achieve optimal blade performance.

When contemplating the selection of blade materials, meticulous attention should be devoted to the properties of the operating medium and its power density. The focus on associated issues should be concentrated as follows: Firstly, the heightened thrust and shear forces resulting from increased medium density necessitate shorter and thicker blade profiles [112]. Therefore, a reasonable consideration of material selection and structural layout is imperative to meet the strength requirements. Furthermore, the conventional iterative optimal Blade Element Momentum (BEM) method must undergo specific enhancements, particularly in terms of blade thickness, during the design of MCT blades [113]. Secondly, choosing coating materials and surface treatments is paramount in mitigating wear and corrosion under conditions involving sandy water and marine organisms [114]. The impeller is susceptible to rapid wear without suitable measures, as exemplified in Figure 31. Conversely, the implementation of appropriate anti-corrosion coating treatments on the blade surface significantly diminishes wear resulting from operational impacts, such as sand particles, as depicted in Figure 32. Thirdly, the issue of blade cavitation phenomena assumes increasing prominence with rising unit capacity. Blade cavitation can lead to surface peeling and reduced efficiency [19,115]. According to research on MCT prototypes [116], blade cavitation is more likely to occur when the tip speed ratio surpasses 7. Thus, it is advisable to confine the designed tip speed ratio to conditions that maximize power capture while minimizing cavitation. In general, to achieve superior performance, careful consideration should be given to various aspects, including high efficiency, robust structural integrity, wear and corrosion resistance, stall delay, and cavitation prevention, among others [111].

5.1.2. Selection of the Transmission System

Marine turbines are the primary structures in marine energy systems designed to capture the kinetic energy of seawater flow and convert it into mechanical energy. The structural design of these turbines significantly influences their capacity to harness marine energy. Marine energy generator units employ three main transmission systems: gearbox transmission, hydraulic transmission, and direct drive transmission [69].

Table 6. Three common transmission systems in MCTs.

Transmission System	Advantages	Disadvantages	Examples
Gearbox transmission	① The conversion of marine energy into electrical energy is highly efficient. ② The technology is relatively mature and has found widespread commercial applications. ③ Suitable for various water flow speeds. ④ Variable speed transmission can be achieved through a multi-stage gearbox.	① Gearboxes require regular maintenance, which can lead to downtime. ② Gearboxes are susceptible to corrosion from seawater and particle damage. ③ Gearbox transmissions are typically larger, requiring more space.	ZJU Marine Energy SR 2000
Hydraulic transmission	① Suitable for high-pressure and high-torque applications, efficiently converting marine energy. ② Suitable for various water flow speeds. ③ Relatively compact mechanical structure, saving space.	① Hydraulic systems are complex and require regular maintenance. ② Hydraulic systems require a high-pressure hydraulic system, which adds complexity. ③ Due to the strong corrosiveness of seawater, special protective measures are needed.	Pulse 100 Deep Green
Direct drive transmission	① Simplified mechanical structure, reducing maintenance requirements. ② Applicable to various water flow speeds and directions. ③ Lower cost and maintenance.	① A higher starting speed is needed to ensure rotation. ② The efficiency may be relatively low, especially in low-speed water currents.	AK1000 OpenHydro

presents the primary advantages and disadvantages of these three transmission systems. It is evident that hydraulic transmission allows for overload protection and continuous variable-speed operation but exhibits lower efficiency. Gearbox transmission offers a compact design and high efficiency but has limitations in terms of fixed transmission ratios and dampening marine impacts [117,118]. Additionally, it introduces mechanical losses, high noise levels, elevated failure rates, and the need for regular maintenance, rendering it unsuitable for prolonged operation in underwater sealed environments [119]. Direct drive transmission in generator units eliminates the need for a gearbox by directly connecting the marine turbine to a low-speed synchronous generator. Although this approach results in a larger and heavier generator unit, it reduces the probability of mechanical failures and the costs associated with routine maintenance [120,121]. Simultaneously, it enhances power conversion efficiency and operational reliability, making it suitable for maintenance-free long-term operation [122].

Sealing plays a crucial role in underwater transmission systems, as any failure in the sealing can result in severe consequences [123]. Unlike static sealing, dynamic sealing is more susceptible to damage and encompasses the main shaft sealing to ensure nacelle waterproofing and the blade-root sealing to maintain hub waterproofness. Moreover, the seal system significantly impacts power generation, affecting the turbine’s startup characteristics through mechanical friction.

Currently, there is a lack of comprehensive research concerning the seal system for MCTs, which hinders the establishment of a systematic and standardized design methodology or structure. Designing a dynamic seal system involves considerations of structural layout and material selection. A study referenced as Ref. [124] introduces a primary-shaft combined rotary sealing structure, as depicted in Figure 33, which is commonly employed in underwater environments. This system employs pressurized oil and pressurized air to balance pressure differentials and prevent water ingress. The dynamic seal system encompasses three distinct ply seals, each with a specific role. The outermost ply is an anti-silt ring constructed from chemigum, designed to mitigate the intrusion of silts from seawater. The second and third components consist of skeleton oil seal rings to prevent water ingress and oil egress. In another reference, Ref. [125], a novel main-shaft seal system employing polyvinyl formal material is developed. Experimental results from a prototype test confirm that this designed seal system effectively isolates water and exhibits reduced mean frictional torque. Ref. [126] also introduces a pitch control strategy with reduced pitch variation to minimize pitch action while ensuring power generation and reducing sealing wear at the blade root. Achieving an industrial application level for the seal system in submerged MCTs is paramount, demanding increased attention and investment.

5.2. Construction of Test Sites

The establishment of large-scale test sites for MCT arrays involves a multitude of intricate and pivotal challenges. To begin with, the selection of a suitable geographical location and environment is of paramount importance. Factors such as water depth, marine conditions, and seabed geology must be meticulously considered when constructing the test site [124]. Furthermore, building a test site may have potential impacts on the surrounding ecological environment, thus necessitating earnest attention to environmental protection and the preservation of wildlife [8]. Regulations and policies also pose obstacles to the construction of test sites. The complexity of international and domestic rules, policies, and licensing requirements may lead to project delays and uncertainties. Simultaneously, changes in renewable energy support policies may have far-reaching effects on test site construction, requiring prudent planning and risk management. Challenges about funding and costs should not be underestimated. The construction of test sites requires substantial funding and resources, and fundraising and project cost control are intricate tasks. The occurrence of overruns and budget constraints can trigger significant issues, necessitating effective financial planning and management. Moreover, challenges in technology and engineering are readily apparent. The design, manufacturing, installation, and maintenance of turbines demand highly specialized knowledge in marine engineering. The operation and monitoring of turbines in harsh marine environments also encompass a range of technological and equipment-related challenges. In summary, constructing large-scale test sites for MCT arrays is a multifaceted undertaking that requires a comprehensive consideration of geographical, regulatory, financial, technological, and societal factors.

Presently, nations are rapidly advancing the research on MCTs and establishing a foundation in test site construction, offering specialized infrastructure for technological development and commercial demonstration. The establishment of these test sites significantly propels the commercialization process of marine energy development [127]. The currently established marine energy test sites are outlined in

Table 7. Main marine current energy test sites of different countries.

Test Site	Country	Completed Year	Velocity (m · s−1)	Depth (m)
EMEC [128,129]	UK	2003	1.5~4.0	30~50
Fundy Ocean Research Center for Energy (FORCE)	Canada	2009	5.0~6.0	40~50
Korea East–West Power Uldolmok Marine Energy Test Site [130]	South Korea	2015	2.0~5.0	20~32
East River Marine Energy Project [131,132]	USA	2016	2.0~3.0	10~15
MeyGen Marine Energy Project [133]	UK	2016	3.0~4.0	40~100
Verdant Power RITE Project [134]	USA	2020	1.0~3.0	10~15
Zhoushan Marine Energy Generation Demonstration Project [135]	China	2020	1.50~3.86	20~31
Goto Islands	Japan	2023	1.5~3.0	40~50

.

6. Perspective of Application

Marine energy systems hold promising prospects as a renewable energy technology with substantial potential. With the increasing demand for clean energy, the future is expected to witness a proliferation of marine energy system demonstration projects, thereby driving the rapid advancement of this field.

6.1. Power Supply for an Offshore Island with the Multi-Energy System

Due to their isolation and considerable distance from the mainland, offshore islands face challenges in ensuring a reliable energy supply [136]. Typically, these islands rely on diesel generators, which are not only noisy and polluting but also economically burdensome [137]. Given that offshore islands typically possess access to various forms of renewable energy resources, there is a compelling rationale for the development of a multi-energy system that integrates wind, wave, solar, and marine current energy sources to enhance the power supply sustainability of offshore islands [138]. The multi-energy system can provide a stable source of clean energy for the power grid. Notably, marine and ocean current energy sources are immune to meteorological influences, and this predictability renders them an ideal choice for meeting the foundational electricity demands. Such an approach aids in reducing reliance on fossil fuels, mitigating greenhouse gas emissions, and promoting the decarbonization of the energy system.

6.2. Power Supply for Deep-Sea Marine Instruments and Devices

Harnessing marine stream energy to power oceanic instruments and devices in remote and deep-sea environments presents a sustainable and efficient solution for marine operations. Marine energy, derived from the natural movements of ocean currents, offers a reliable and predictable power source essential for continuously operating equipment in hard-to-access areas. Marine energy devices convert the kinetic energy of ocean currents directly into electrical energy on-site. This setup not only aligns with the principle of using marine energy within marine settings but also eliminates the need for costly energy transportation. These devices embody an efficient and environmentally conscious approach to powering deep-sea instruments and operations by harnessing and utilizing energy exactly where it is generated.

6.3. Marine Hydrogen Industry

Large-scale arrays of MCT systems can generate stable and predictable clean electricity, thus offering an ideal power source for the maritime hydrogen industry. These systems can employ water electrolysis to dissociate water molecules from seawater into hydrogen and oxygen gases, yielding renewable hydrogen fuel [139]. Simultaneously, they can use energy storage to bridge the gap between supply and demand [140]. The stored hydrogen can be reconverted into electricity as needed to meet the demands of the power grid. This holds paramount significance for the stability and reliability of energy systems. Renewable hydrogen derived from such processes has applications across various sectors, including hydrogen fuel cell vehicles, industrial processes, power generation, and energy storage. Therefore, the integration of marine energy utilization with hydrogen production presents a promising and prospective avenue.

In recent years, endeavors have been made to explore the integration of offshore power generation and hydrogen production. Ref. [141] outlines a comprehensive hydrogen supply chain, involving the processes of electrolysis and methanation, utilizing the existing offshore gas pipeline infrastructure and powered by offshore wind-generated electricity. Since 2018, Innovate UK has provided funding for a contemporary initiative known as the ‘Hydrogen Diesel Injection in a Marine Environment’ (HyDIME) project, with the primary objective of reducing emissions within the maritime industry [142]. Under this project, renewable energy from Orkney is channeled into an electrolysis unit at the EMEC’s marine testing facility, offering an alternative means of storing and utilizing green electricity. In 2022, Zhejiang University conducted research on an off-grid hydrogen production system powered by oceanic current energy [143]. This system incorporates horizontal axis MCTs and a polymer electrolyte membrane (PEM) electrolyzer that supplies ultrapure water. The kinetic energy of oceanic currents is harnessed by the turbines and subsequently stored as hydrogen energy through electrochemical reactions within the electrolyzer. Experimental investigations about marine current-driven hydrogen production were carried out in the Zhoushan Archipelago, successfully demonstrating the principles and feasibility of the oceanic current-hydrogen system.

As a universally acknowledged clean fuel, hydrogen holds the potential to serve as the primary secondary energy vector for marine current energy. Despite the prevailing technical challenges, including issues related to reverse osmosis membrane systems, high-pressure storage, autonomous operation, and maintenance, as well as transportation logistics, the marine hydrogen industry is not yet fully mature. Nevertheless, the pursuit of green hydrogen technology warrants comprehensive research and development efforts.

7. Conclusions

This paper focuses on the current status and challenges of large-scale arrayed MCTs. It begins by providing a comprehensive overview of the categorization and the trend toward large-scale development of marine energy systems, highlighting their potential in the renewable energy domain. Subsequently, the crucial role of arraying is emphasized, covering aspects from hydrological analysis, site selection, array layout, and wake characteristics, to international case studies of array projects, revealing its pivotal role in enhancing efficiency and sustainability. National policies underscore the favorable commercial prospects of marine energy systems, as policy support and investment potential provide a robust foundation for their development. Furthermore, the necessity of addressing technical challenges and establishing test sites to tackle issues related to rotor design and transmission technology disparities is underscored, necessitating global cooperation for collective resolution. Finally, insights are shared regarding the application prospects of MCTs, including power supply for offshore islands and devices with the multi-energy system and the marine hydrogen industry, offering a glimpse into their extensive potential across various application domains in the future.