Offshore Wind Power—Seawater Electrolysis—Salt Cavern Hydrogen Storage Coupling System: Potential and Challenges

Abstract

Offshore wind power construction has seen significant development due to the high density of offshore wind energy and the minimal terrain restrictions for offshore wind farms. However, integrating this energy into the grid remains a challenge. The scientific community is increasingly focusing on hydrogen as a means to enhance the integration of these fluctuating renewable energy sources. This paper reviews the research on renewable energy power generation, water electrolysis for hydrogen production, and large-scale hydrogen storage. By integrating the latest advancements, we propose a system that couples offshore wind power generation, seawater electrolysis (SWE) for hydrogen production, and salt cavern hydrogen storage. This coupling system aims to address practical issues such as the grid integration of offshore wind power and large-scale hydrogen storage. Regarding the application potential of this coupling system, this paper details the advantages of developing renewable energy and hydrogen energy in Jiangsu using this system. While there are still some challenges in the application of this system, it undeniably offers a new pathway for coastal cities to advance renewable energy development and sets a new direction for hydrogen energy progress.

1. Introduction

One of the greatest threats of this century is climate change, which has already had significant impacts on many regions of the world. Increasing greenhouse gas emission is one of the major sources of climate change [1]. Developing renewable and clean energy as a substitute for fossil fuels is an effective strategy to reduce carbon emissions [2]. As a result, renewable energy demonstrates greater potential than ever before, whether as a direct energy source or as a raw material. In September 2020, President Xi proposed at the 75th United Nations General Assembly that China aims to peak its carbon dioxide emissions before 2030 and strives to achieve carbon neutrality by 2060. Renewable energy has become a vital force for accelerating the achievement of China’s “dual carbon goals” and advancing the energy transition.

Currently, renewable energy technologies have become dominant in the global energy market for sustainable energy production. By effectively utilizing natural resources such as solar, wind, and geothermal energy, it is possible to reduce the reliance on fossil fuels, decrease greenhouse gas emissions, and achieve a harmonious coexistence between long-term energy supply and ecological preservation. Among the existing renewable energy sources, solar and wind energy technologies are the most mature and are rapidly developing [2]. However, the intermittency and variability of renewable energy to some extent limit its development. In order to improve the utilization rate of renewable energy, many scholars have turned their attention to hydrogen energy, and extensive research has been conducted. Some researchers have analyzed the feasibility of combining hydrogen energy with renewable energy. Dansoh [3] focused on two case studies in Newhaven and Massawa, and the results showed that hydrogen storage not only reduced renewable energy waste but also played a significant role in seawater desalination. Uyar and Beşikci [4] introduced the concept of 100% renewable energy, detailing the importance, necessity, and feasibility of electricity and hydrogen as clean energy carriers in the energy transition process. On this basis, some scholars have conducted research on hydrogen production from renewable energy. Ishaq and Dincer [5] analyzed hydrogen production systems based on solar, biomass, and geothermal energy, comparing the energy efficiency of each system. Chi et al. [6] studied the application of renewable energy in hydrogen production, providing a detailed comparison of different electrolysis technologies in terms of principles, energy consumption, and other aspects, and discussed the prospects of hydrogen production from renewable energy. Furthermore, the establishment of energy systems is also a research hotspot. Marocco et al. [7] analyzed different energy storage configurations to optimize energy system design, pointing out that hydrogen-based storage schemes are crucial for the economical and efficient construction of 100% renewable energy systems. Wu et al. [8] developed a wind-photovoltaic-salt cavern-hydrogen storage utilization system and used the example of Qianjiang to analyze the advantages of this system in improving the utilization of renewable energy and reducing carbon emissions. Thanks to its high energy density, high calorific value, and zero emissions, hydrogen energy has become one of the best solutions for renewable energy conversion and storage applications [9]. On the one hand, through the flexible conversion of “electricity-hydrogen-electricity”, green hydrogen generated using renewable energy has already been used to smooth the peaks and valleys of the power grid [10,11], addressing the energy consumption problem. On the other hand, hydrogen energy has wide-ranging applications in sectors such as transportation, construction, and industry. Currently, the global demand for hydrogen is growing rapidly, with consumption increasing by 6% annually [12]. Therefore, the combination of renewable energy and hydrogen energy will provide a powerful pathway to promote the clean, low-carbon, and efficient utilization of energy and realize the cross-regional and cross-seasonal optimization of energy allocation.

China possesses abundant wind energy resources, and over the past decade, the wind energy industry has experienced explosive growth [13] (as shown in Figure 1). Despite the tremendous potential and the rapid development of wind power, it is important to note that several issues remain inevitable in its practical application. Currently, the amount of wind power curtailment in China is still substantial (as shown in Figure 2), reaching an astonishing 121 × 10⁸ kW in 2022. On the one hand, the inherent randomness and the intermittency of wind energy are significant factors contributing to its difficult integration into the power grid. On the other hand, wind energy resources in China are primarily located in the northwest, whereas energy consumption is concentrated in the eastern coastal regions, leading to a significant supply–demand mismatch that causes much of the wind power curtailment. Therefore, for the eastern coastal regions, the utilization of nearshore and offshore wind energy resources is an urgent priority. In addition, in 2022, China released the “Medium and Long-Term Plan for Hydrogen Energy Industry Development (2021–2035)”, which clearly defined the role of hydrogen energy in China’s energy system and emphasized the active development of China’s hydrogen energy industry. For coastal cities, the integration of offshore wind power and hydrogen energy provides a comprehensive solution for clean energy supply, carbon emission reduction, and sustainable economic development.

However, although significant progress has been made in the research of hydrogen energy in the field of renewable energy utilization, there are still gaps in the integration of offshore wind power and hydrogen energy. First, the grid connection of offshore wind power has always been a limiting factor, with high technical complexity and costs, resulting in low efficiency in utilizing offshore wind resources [14]. This calls for the exploration of new paths for wind power utilization. Secondly, hydrogen storage remains an unavoidable issue in the hydrogen energy industry. Hydrogen has an extremely high energy density per unit mass, but its energy density per unit volume is very low. As a result, hydrogen often requires processes such as compression, liquefaction, or conversion to achieve large-scale storage [15]. Traditional hydrogen storage technologies face limitations in terms of small storage capacity, short storage duration, and safety concerns [16,17,18,19], making it necessary to explore large-scale hydrogen storage solutions.

Based on this, this paper provides an extensive review of the latest research on renewable energy utilization, water electrolysis for hydrogen production, and hydrogen energy storage. It innovatively proposes the integration of offshore wind energy, seawater hydrogen production, and underground salt cavern hydrogen storage to form a comprehensive hydrogen production and storage system. The goal of constructing this system is to address the challenges of offshore wind power grid integration and improve wind power utilization efficiency. By replacing electricity transmission with hydrogen transport, it avoids the construction costs associated with subsea cables and enhances the economic feasibility of wind power utilization. The integration of hydrogen production and storage effectively addresses supply–demand fluctuations and improves the security and stability of energy supply. This system offers a new pathway for the development of renewable energy in coastal cities and sets a new direction for the integrated development of the hydrogen energy industry.

Section 2 of this paper provides an overview of the current status of offshore wind power technology in China and the challenges faced in offshore wind power grid integration. Section 3 discusses the research status of water electrolysis for hydrogen production and the cutting-edge technology of in situ hydrogen production through seawater electrolysis. Section 4 focuses on the advantages and feasibility of large-scale underground hydrogen storage using salt caverns. Based on the reviews presented in Section 2, Section 3, Section 4 and Section 5, we propose an integrated system that combines offshore wind power, hydrogen production, and hydrogen storage, and we analyze the feasibility and application potential of this system using Jiangsu Province as a case study. Section 6 highlights the current challenges of the system and suggests solutions. Finally, the conclusion of this paper is summarized.

2. Analysis of Offshore Wind Power Technology

2.1. Status of Offshore Wind Power in China

China has a vast territory and abundant wind resources, but the distribution of wind energy shows significant regional characteristics. Wind energy is more abundant in the northwestern regions, while it is relatively lower in the southeastern regions (as shown in Figure 3a) [20]. The majority of regions in Xinjiang, Inner Mongolia, and northern Gansu possess abundant wind energy resources. This has resulted in the majority of China’s onshore wind projects being located in the northwestern part of the country, while energy consumption is mainly concentrated on the eastern coast, which leads to long-distance power transmission and reduced efficiency. However, although the onshore wind energy resources in the southeastern region of China are relatively low, it possesses excellent offshore wind energy resources. Offshore wind technology in coastal areas has great potential for development (as shown in Figure 3b). China’s coastline is approximately 18,000 km long, with more than 6000 islands, and the duration of wind speeds exceeding 6 m/s reaches up to 4000 h annually. The vast potential for offshore wind power development highlights the significant practical importance of harnessing offshore wind energy resources.

Compared with onshore wind power generation, an offshore wind farm, due to the strong stability of wind resources, low turbulence intensity, and strong wind energy, can reduce the occupation of land resources, has low noise pollution, has a higher power generation efficiency, allows for more turbine units in large-scale operations, and has many other advantages, which have gradually captured the attention of various countries [1,21]. Due to the limitations of land resources and wind resources, large wind farms are mostly located in remote areas far from power nodes or offshore. Currently, the development of onshore wind power in some countries is saturated, and offshore wind power is the main development direction of the wind power industry in the future [22,23].

The development of offshore wind power plays a critical role in China’s transition to a green, low-carbon energy system and in the establishment of a new energy infrastructure [24]. In 2007, the China National Offshore Oil Corporation (CNOOC) installed China’s first fixed offshore wind turbine at the Suizhong 36-1 site in the Bohai Sea, featuring a Goldwind 1.5 MW wind turbine. During the 12th Five-Year Plan (2011–2015), the development of offshore wind power in China was slow, with an installed capacity of less than 1 MW. However, during the 13th Five-Year Plan (2016–2020), China’s offshore wind power industry experienced rapid growth, with a total of 11 MW of new offshore wind capacity installed, representing 50.5% of the global increase in offshore wind power during the same period. By the end of 2023, the installed offshore wind power capacity in China reached 37.3 GW, ranking first globally and accounting for 51.0% of the world’s total installed capacity. Furthermore, China’s offshore wind power industry chain has developed into a mature system that supports an annual development scale of 3000 MW of offshore wind power [1,25,26]. Nevertheless, the technically exploitable offshore wind energy resources in China, at a height of 150 m, within 200 km offshore, and at depths of less than 100 m, amount to 2.78 billion kilowatts. However, the current utilization rate of the installed capacity is still less than 4% for nearshore areas and 0.9% for deep-water offshore areas, indicating that there is still a significant potential for future development. With the depletion of nearshore resources and the maturation of offshore wind technology, the development of the offshore wind power industry is expected to gradually shift toward deep-water areas in the future.

2.2. Challenges of Offshore Wind Power Grid Integration

As the pace of offshore wind power construction accelerates globally, technical and management challenges related to its grid-connected operation are gradually emerging. These challenges have attracted significant attention in the industry and have emerged as key issues in both the wind energy sector and related research fields. Compared to onshore wind power, the integration of offshore wind power into the grid presents greater difficulties, becoming one of the primary bottlenecks which prevents its further development. For instance, Germany initially aimed to achieve 800 MW of offshore wind power generation by 2012, but the actual installed capacity only reached 280 MW, just one-third of the original target, underscoring the considerable challenges associated with offshore wind power grid integration.

Current mainstream grid connection technologies for offshore wind power include high-voltage alternating current (HVAC), high-voltage direct current (HVDC), and fractional frequency transmission systems (FFTSs) [27]. HVAC offshore wind power system adopts industrial frequency alternating current (AC) transmission and does not need to convert power to DC or other frequencies; this transmission scheme is simple in structure and low in cost, but it is only applicable to small-scale offshore wind farms. HVDC offshore wind power system refers to the conversion of wind power AC energy into DC energy for transmission; this method is suitable for power transmission over long distances, but its cost is an important factor restricting its development. FFTS offshore wind power system adopts the frequency division transmission technology to achieve the purpose of increasing the transmission capacity and the transmission distance, which is a kind of transmission technology between HVAC and HVDC.

With the offshore wind power industry expanding into deeper waters, the distance between wind farms and the coast continues to increase, resulting in longer grid connection lines [14]. This trend is an inevitable characteristic of all current grid connection technologies and imposes higher demands on the power grid. Traditional offshore wind farms require the construction of offshore substation platforms, which are connected to the mainland via submarine cables (as illustrated in Figure 4). Transformers, which serve as the central component for electrical energy transmission within the offshore wind power system, play a vital role in this energy transmission process. Transformers raise the low voltage power generated by the generator to a high voltage suitable for transmission through the grid, thus ensuring a stable and efficient transmission of power over long distances. Submarine cables are an indispensable part of offshore wind power systems, responsible for connecting electrical equipment and systems. Due to the harsh offshore environment, these cables must have excellent waterproof, anti-corrosion, and salt mist resistance properties [24]. Compared to onshore wind farms, the construction costs of offshore wind farms are significantly higher. Offshore substations account for about 5% of the construction costs, while subsea cables account for about 15% [28]. Additionally, the technical complexity of installation, operation, and maintenance (O&M) is another major obstacle affecting the development of offshore wind power [23].

3. Hydrogen Production from Seawater

3.1. Status of Water Electrolysis Technology for Hydrogen Production

Hydrogen production methods are diverse, primarily including steam reforming, dry methane reforming, thermal cracking of hydrocarbons, petroleum hydrocarbon oxidation, and coal gasification, all of which rely on fossil fuels [29,30]. Therefore, hydrogen production from the electrolysis of water is considered to be the most feasible and important clean hydrogen production technology [13], where water can be decomposed into high-purity H₂ and O₂ using electricity generated from waste heat or renewable intermittent energy sources [31]. This approach achieves near-zero carbon emissions in hydrogen production and effectively facilitates the large-scale and efficient utilization of renewable energy. Additionally, it helps to mitigate the issues of curtailment due to the intermittency and variability of renewable energy sources.

The primary focus of water electrolysis-based hydrogen production technologies is the conversion of electricity into hydrogen. Currently, the main routes of hydrogen production from water electrolysis include alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE), solid oxide electrolysis (SOE), and anion exchange membrane water electrolysis (AEMWE) [32,33,34,35]. While SOE and AEMWE are still under research and development, AWE and PEMWE are more advanced in terms of commercialization. Among these, alkaline water electrolysis, being the first industrialized technology, remains the dominant and most mature method for water electrolysis-based hydrogen production. A detailed comparison of these electrolysis technologies is provided in

Table 1. Comparison of different water electrolysis hydrogen production technologies [29,36,37].

Technology	AWE	PEMWE	AEMWE	SOE
H2 production rate (Nm3 H2/h)	0.97	0.5 or 1.0	—	—
Current density/(A·cm–2)	<0.8	1~4	1~2	0.2~0.4
Power consumption (kWh/Nm3 H2)	4.5~5.5	4.0~5.0	4.2~4.6	>3
Efficiency	60~70%	60~80%	60~75%	85~100%
Operating temperature/°C	≤90	≤80	≤60	≥800
Service life/kh	60~90	30~90	10~30	10~30
Hydrogen purity (%)	≥99.8	≥99.99	≥99.99%	—
Technology maturity	Fully commercialized	Initial commercialization	Initial commercialization	Early demonstration
Advantages	Significant economies of scale; low hydrogen production material costs	High hydrogen production efficiency; large current; quick start-stop capability; strong adaptability to renewable energy fluctuations	Relatively low cost; quick response; no pollution	High electrolytic efficiency
Disadvantages	Poor compatibility with renewable energy; corrosive strong alkali; product requires maintenance; low working current	High cost due to use of precious metals	Material technology requires further breakthroughs; still in the research and development stage	High operating temperature; high energy consumption; still under development and validation

.

However, electrolytic hydrogen currently accounts for only about 4% of the global hydrogen production [38], with this proportion even lower in China, at just 1% [39]. A larger share of hydrogen production still comes from steam reforming and coal gasification, processes that not only consume non-renewable fossil fuels but also emit large amounts of greenhouse gasses, pollute the air, and contribute to global warming. In addition to the relatively low maturity of this technology, the cost remains as one of the main reasons preventing the widespread adoption of water electrolyzers in the market. At present, the cost of hydrogen production via water electrolysis in China is approximately 30 RMB/kg, far less competitive than hydrogen production from coal (10 RMB/kg), natural gas (15 RMB/kg), and industrial by-products (9–22 RMB/kg) [39]. Therefore, to meet the ambitious target set by the International Energy Agency (IEA), which forecasts electrolyzer installations to reach several hundred gigawatts by mid-century [5], breakthroughs are urgently needed not only in achieving economies of scale in manufacturing but also in materials, electrolyzer design, and management systems [38].

3.2. Electrolytic Hydrogen Production from Seawater

In the context of electrolysis materials, the availability of freshwater is a major limiting factor in the widespread adoption of electrolysis-based hydrogen production [38,40]. Traditional low-temperature electrolyzers typically use freshwater or purified water as feedstock, requiring 10.0 kg to 22.4 kg of freshwater to produce 1 kg of hydrogen [41]. However, freshwater is a limited resource, whereas seawater, which constitutes 96.5% of the Earth’s total water, is widely available. Utilizing seawater as a feedstock for hydrogen production via electrolysis can alleviate the pressure on freshwater resources [40]. Against this backdrop, seawater electrolysis (SWE), including direct SWE (directly utilizing untreated seawater as the electrolyte) and alkaline SWE (electrolysis of seawater with the addition of alkali or other auxiliary electrolytes) [41,42], has garnered significant attention from both the academic and industrial communities.

However, the complex composition of seawater presents a significant challenge to the large-scale implementation of SWE. The diverse ionic components of seawater, along with unwanted competitive reactions, hinder both the efficiency and long-term sustainability of hydrogen production through SWE [43]. In natural seawater, the high concentration of chloride ions (approximately 0.5 M) leads to competition between the oxygen evolution reaction (OER) and the chlorine evolution reaction (CER), which significantly lowers the overall energy conversion efficiency [40,44]. Additionally, these reactions can cause the corrosion of electrolysis cell components over time [11], ultimately reducing the operational lifespan of the electrolyzers [38]. To overcome these challenges, considerable research [45,46,47] efforts have been directed towards the development of electrocatalysts that exhibit high activity, corrosion resistance, and long-term durability.

As mentioned in the previous section, the increasing scale of offshore wind power presents challenges in transmitting renewable electricity to the mainland due to significant transmission losses and high costs. Several studies [48,49,50,51] have suggested that combining offshore wind with SWE technologies may offer some advantages. These studies focus on developing a coupled model of the two that utilizes abundant offshore wind and flexible hydrogen to meet energy security and climate goals. In situ direct electrolysis of seawater with wind power as the energy input could provide an effective way to achieve convenient, efficient, and economical energy conversion.

In the research on integrating offshore wind power with SWE, Liu et al. [52] first proposed a large-scale floating platform that utilizes offshore wind energy for direct SWE on-site. This platform is capable of operating under fluctuating marine conditions with a hydrogen production rate of 1.2 Nm³ h⁻¹. Despite the complex and dynamic environmental conditions, the concentration of impurity ions in the electrolyte remains low and stable over extended periods. The electrolysis process consumes 5 kWh Nm⁻³ H₂, and its stable operation can last for over 240 h. The hydrogen produced has a purity greater than 99.9%, comparable to that of land-based water electrolysis systems. The system integrates three seawater electrolyzers and is equipped with energy storage, current conversion, hydrogen detection, and transmission functions (as shown in Figure 5). This approach eliminates the need for complex seawater desalination processes, additional platform space, or the energy consumption typically associated with desalination pretreatment. In terms of hydrogen production rate, the current hydrogen production capacity of PEM electrolyzers is limited to 10 kg per day [6], while the SWE platform can only reach 2.6 kg per day. Although there is a significant gap, a higher hydrogen production rate can be achieved by scaling up.

In a word, the successful operation of the SWE floating platform provides a novel solution to the offshore wind power grid integration problem—on-site SWE hydrogen production using offshore wind power. On the one hand, using SWE for on-site hydrogen production can reduce the construction costs associated with offshore wind farms, particularly those related to the need for offshore substation platforms and submarine cable installations. On the other hand, this system does not consume freshwater resources, and the hydrogen produced can be converted back into electricity via fuel cells, generating water in the process, which contributes to seawater desalination. The electrolysis platform operates reliably in the dynamic marine environment, with energy supplied by offshore wind turbines. In the future, on-site SWE powered by renewable energy could enhance the economic feasibility of such systems. This hydrogen production method, which does not require pure water and demonstrates excellent performance, is expected to become increasingly attractive.

4. Hydrogen Storage in Caverns

4.1. Status of Hydrogen Storage Technology

As the world’s largest producer of hydrogen, China has made the development and utilization of hydrogen energy a critical component of its national energy strategy. In this context, hydrogen storage plays a key role in addressing supply and demand fluctuations in the hydrogen energy sector. However, the large-scale application of hydrogen energy is faced with storage and transportation challenges. Due to its low storage density, flammability, and explosiveness, hydrogen has put forward extremely high requirements for storage technology.

Currently, the main existing hydrogen storage methods in the field of hydrogen storage include the following four types: solid hydrogen storage [53], high-pressure gaseous hydrogen storage [54], low-temperature liquid hydrogen storage [55], and organic liquid hydrogen storage [56]. However, these traditional methods have notable limitations. Solid and high-pressure gaseous hydrogen storage are both costly, severely restricting their scalability for large-scale storage. Low-temperature liquid hydrogen storage requires maintaining low temperatures over long periods, resulting in high energy consumption, and organic liquid hydrogen storage suffers from side reactions that lower the hydrogen’s purity, coupled with low dehydrogenation efficiency and high energy demands, preventing its large-scale implementation. Due to these limitations in cost and scalability, traditional methods are not suitable for large-scale hydrogen storage. Therefore, exploring new storage technologies, especially safe, large-scale, and cost-effective solutions, has become an urgent need for the hydrogen energy industry.

He et al. [57] studied the capacity planning models of three energy storage methods—traditional electrochemical energy storage (EES), high-pressure hydrogen storage tanks (HHS), and salt cavern hydrogen storage (SCHS)—in power systems based on cost minimization. The capacity of these three methods and the carbon emission results for the power system under different renewable energy penetration rates are shown in Figure 6 and Figure 7. Taking an 80% penetration rate as an example, the installed capacity of SCHS is approximately 1.6 times that of HHS and 2.1 times that of EES. The higher the capacity, the stronger the system’s peak-shaving and valley-filling abilities. Correspondingly, in the SCHS scenario, the carbon emissions of the power system are 5.8% lower than those in the HHS scenario and 24.4% lower than those in the EES scenario. This demonstrates the advantages of salt cavern hydrogen storage in renewable energy utilization. Currently, salt cavern hydrogen storage, as a new hydrogen storage method, is gradually standing out due to its large storage capacity, favorable geological conditions, and long storage periods, making it a research hotspot in the field of hydrogen energy storage.

4.2. Hydrogen Storage in Salt Caverns

Salt cavern hydrogen storage is achieved through solution mining, where caverns are created in thick underground salt formations to store the gas (as shown in Figure 8). These caverns are typically located at depths of 500 to 1500 m, with individual chambers having capacities greater than 100,000 m³, providing substantial storage potential [58]. Geological hydrogen storage offers significant advantages, such as large-scale capacity, long storage durations, and the ability to store energy across seasons, making it a promising direction for large-scale hydrogen storage development. Among these methods, UHS in salt caverns has the following unique advantages:

(1)

The rock salt is dense, with extremely low permeability, low porosity, and self-healing properties [35,36], making salt caverns ideal for storing gas, oil, and hazardous waste [37,59];

(2)

Salt caverns offer more favorable engineering conditions compared to other storage reservoirs. For example, the space of salt caverns typically ranges from 10 to 10 × 10⁵ m³, with depths ranging from 600 to 2000 m, making them highly suitable and economically efficient for the storage of high-pressure hydrogen gas. Salt cavern hydrogen storage has the ability to rapidly inject and extract hydrogen, allowing it to quickly respond to changes in market demand [60]. This characteristic gives it a significant advantage in energy peak shaving and emergency supply;

(3)

The costs of salt cavern UHS are lower than those of depleted gas reservoirs or aquifers [38,39], as salt caverns require only one-third of the total gas volume as buffer gas, compared to 50–80% [40] for depleted gas reservoirs or aquifers [61]. From the viewpoint of foreign practice, developed countries such as the UK and the USA have taken an early lead in salt cavern hydrogen storage projects, and the technology for salt cavern hydrogen storage is now well established.

Table 2. Salt cavern hydrogen storage in operation worldwide [39].

Country	Hydrogen Storage Facility	Commissioning Year/Year	Hydrogen Purity/%	$\mathbf{Depth}/\mathbf{m}$	$\mathbf{Operating}\mathbf{Pressure}/\mathbf{MPa}$	Salt Cavern Volume/(105 m3)	Stored Energy/(GW·h)
UK	Teesside	1972	95%	365	4.5	3 × 0.70	25
USA	Clemens	1983	95%	1000	7.0~13.5	5.80	92
USA	Moss Bluff	2007	95%	1200	5.5~15.2	5.66	120
USA	Spindletop	2014	95%	1340	6.8~20.2	>5.80	>120

presents basic information on salt cavern hydrogen storage facilities currently in operation worldwide [62,63,64]. Currently, there are four hydrogen storage facilities in operation worldwide, all of which are salt cavern storage types. These facilities have demonstrated excellent hydrogen storage performance and safety. The earliest salt cavern hydrogen storage facility in the UK has been operating safely for nearly 50 years. These successful practical cases not only prove the technical feasibility of salt cavern hydrogen storage but also provide valuable experience and reference for the development of salt cavern hydrogen storage technology in China. Meanwhile, in recent years, many countries around the world have formulated and implemented their own hydrogen energy development strategies. In this context, the site selection research and experimental validation activities for salt caverns as hydrogen storage facilities are accelerating. This proves that salt caverns have good hydrogen storage capacity, both theoretically and practically.

The geological conditions of China’s salt rock formations differ significantly from those in other countries, characterized by thinner salt layers, frequent interlayers, and higher impurity content. These features pose substantial challenges to the implementation of salt cavern hydrogen storage technology. Previous studies [13,31,32,33,34,35] indicate that the permeability of salt rocks typically ranges from 1.0 × 10⁻²¹ m² to 1.0 × 10⁻¹⁵ m², with impure salt layers and interlayers exhibiting significantly higher permeability than pure salt layers. Notably, compared to methane, hydrogen has a lower dynamic viscosity, a smaller molecular weight, and a smaller kinetic diameter, which enhance its permeability within salt rock formations [38]. These factors may prevent the permeability of China’s salt layers from reaching the desired low levels, leading to challenges in site selection and evaluation, sealing safety, and other critical aspects of hydrogen storage [9,39].

Under these conditions, conducting in-depth research on the applicability and safety of China’s layered salt rock formations for hydrogen storage is of critical importance. Song et al. [9] used numerical simulation methods to investigate the sealing performance of hydrogen storage in layered salt caverns, analyzing the mechanisms governing hydrogen flow and offering recommendations on injection-production frequency, brine saturation, and interlayer permeability. Zhang et al. [65] conducted laboratory experiments, theoretical studies, and numerical simulations using representative layered salt from China to verify the sealing performance of salt cavern hydrogen storage. Their findings revealed that interlayer regions are the primary pathways for hydrogen leakage, emphasizing the need to consider porosity and permeability when selecting storage caverns. Liu et al. [61] assessed the feasibility of utilizing salt caverns in Jiangsu’s Jintan region for UHS, focusing on critical factors such as salt layer sealing and caprock density. Using numerical simulations, they analyzed the stability and the sealing performance of ultra-high-pressure caverns, concluding that the Jintan salt mine meets the requirements for constructing ultra-high-pressure hydrogen storage facilities.

At present, a large number of research results have shown that, despite the challenges faced by the implementation of salt cavern hydrogen storage technology in China, it is feasible from a technical point of view. In fact, a demonstration project for hydrogen storage in salt caverns has already been launched in Pingdingshan, China. The implementation of this project will accelerate the research process of large-scale geologic hydrogen storage technology, which will help to fill the technological gaps in this field in China, and lead the direction of the development of large-scale geologic hydrogen storage technology.

5. OWP—SWE—SCHS Coupling System

5.1. System Description

An offshore wind power—seawater electrolysis—salt cavern hydrogen storage (OWP—SWE—SCHS) coupling system is shown in Figure 9. This system primarily generates electricity from offshore wind farms to power the electrolysis of water for hydrogen production. Hydrogen is produced through SWE and stored in salt caverns, enabling large-scale hydrogen storage and long-term energy scheduling. By integrating the advantages of wind energy, hydrogen energy, and underground energy storage, this system effectively addresses the intermittency issues associated with renewable energy. Moreover, it offers a sustainable, economical, and environmentally friendly energy solution for a future low-carbon society [66]. The system is composed of three core modules: an offshore wind power generation module, a SWE hydrogen production module, and a salt cavern hydrogen storage module.

The offshore wind power generation module, as the core power source of the entire coupled system, primarily harnesses the strong offshore winds to drive wind turbine units, converting mechanical energy into electrical energy. A portion of this electricity is directly integrated into the power grid to supply electricity for various applications. Given the fluctuating nature of offshore wind energy, the surplus electricity is utilized in hydrogen production systems, aiming to fully exploit the offshore wind energy potential.

The SWE hydrogen production module offers distinct advantages by using seawater as a raw material, thereby avoiding the consumption of freshwater resources. This hydrogen production process is characterized by low carbon emissions and provides a stable supply of hydrogen. The SWE floating platform, installed near offshore wind farms, is highly adaptive, converting the delivered electricity into transportable hydrogen, thereby addressing the challenges of offshore wind power utilization.

The salt cavern hydrogen storage module enables large-scale hydrogen storage, ensuring both the safety and stability of the stored hydrogen. Additionally, it effectively mitigates fluctuations in energy supply and demand. Through rapid hydrogen injection and extraction scheduling, it supports grid peak shaving, enhancing the stability of power supply. Simultaneously, hydrogen can be transported to various sectors such as transportation, construction, and industry, facilitating the adjustment of hydrogen energy supply and demand across multiple applications.

In summary, the advantages of the OWP-SWE-SCHS coupled system are as follows:

(1)

It achieves efficient utilization of offshore renewable energy, overcomes the challenges of integrating offshore wind power into the grid, and significantly enhances the utilization rate of offshore wind power;

(2)

By utilizing offshore wind power for in situ hydrogen production, it reduces freshwater resource consumption, avoids occupying land resources, and contributes to seawater desalination while utilizing hydrogen energy;

(3)

Transitioning from onshore to offshore hydrogen production, it converts electricity transmission into hydrogen transportation, saving costs associated with submarine cables and offshore substations, thereby offering greater economic benefits;

Integrating hydrogen production and storage enhances the peak-shaving capability of the power system, buffers hydrogen supply and demand fluctuations, and significantly improves the security and stability of energy supply.

5.2. Application Potential in China

Despite all the advantages of this coupled system, it should be emphasized that the construction of this system is strictly geographically limited. First, coastal cities with abundant offshore wind resources will be prioritized. High-quality wind resources are a prerequisite for establishing this system, as they can provide sufficient electricity for SWE hydrogen production. Second, the surrounding area of the system’s construction site should possess abundant salt mine resources. Although converting electricity transmission to hydrogen transportation can help reduce the costs, the unavoidable involvement of ships or pipelines in transporting hydrogen from production to storage necessitates shorter transportation routes to minimize expenses. Finally, economically developed regions are recommended to prioritize pilot projects. On the one hand, the system involves many projects, a large scale, and large investments in the initial stage of construction. On the other hand, the market demand for hydrogen energy is strong in economically developed regions, and the industrial chain is mature, which makes it easy to form the advantage of industrial agglomeration. Combined with the above analysis, Jiangsu Province, with its better economic development and unique advantages in offshore wind power and hydrogen storage in salt caverns, has a strong potential for its application.

(1)

Advantages of Offshore Wind Power in Jiangsu

Jiangsu Province possesses abundant onshore and eastern coastal wind resources, providing exceptional conditions for the development of offshore wind power. With a coastline stretching 954 km along the western Pacific Ocean, Jiangsu boasts substantial wind power potential. The average offshore wind speed in this region ranges from 6 to 9 m/s, while the wind power density at a height of 80 m is concentrated between 328 and 500 W/m² (as shown in Figure 10) [61]. If fully harnessed, the offshore wind energy in Jiangsu Province could generate approximately 2 × 10³ MWh annually, equivalent to four times the province’s total electricity demand [67].

Jiangsu Province has experienced robust economic development, and its coastal construction conditions are favorable. As early as the “13th Five-Year Plan” period, China proposed targets for Jiangsu’s offshore wind power by 2020, which aimed for an installed capacity of 4.5 MW and a cumulative grid-connected capacity of 3 MW [68]. As of 2019, the cumulative installed wind power capacity in Jiangsu reached 10.41 MW, including 4.23 MW of offshore wind power, which accounted for 7.8% of the total power generation capacity. The cumulative installed capacity of offshore wind power in Jiangsu accounted for 71.5% of the national total, maintaining its position as the leader at a nationwide scale for several consecutive years [67]. In August 2021, the Jiangsu Provincial Development and Reform Commission announced the “14th Five-Year Plan for Renewable Energy Development in Jiangsu Province” [69], which included 28 offshore wind power project sites with a planned capacity of 9.09 MW and a total planned area of 1444 square kilometers to further develop offshore wind power. By 2024, Jiangsu’s offshore wind power development had further advanced, incorporating several major provincial projects, including Guoxin Dafeng 850 MW offshore wind power, Yancheng Three Gorges 800 MW offshore wind power, and Yancheng Longyuan 1000 MW offshore wind power.

In the future, Jiangsu Province will continue to promote the large-scale development of offshore wind power and steadily carry out the demonstration of deep and distant sea wind power. The vigorous vitality of offshore wind power and the government’s strong support for the development of offshore wind power provide a solid foundation for the establishment of the OWP—SWE—SCHS coupling system.

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Advantages of hydrogen storage in salt caverns in Jiangsu

Jiangsu Province possesses abundant salt mineral resources and serves as one of China’s primary sea salt production regions. The province also produces significant quantities of well salts, with major deposits located in areas such as Jintan and Huai’an. The Jintan salt basin encompasses an exploration area of 64 km² and represents a giant rock salt deposit characterized by a simple composition, high grade, and few intercalations. The NaCl reserves are estimated at 12.538 billion tons, with an average purity ranging between 73% and 85%. The salt layers are buried at depths of 860 m to 1300 m, with thicknesses varying from 80 m to 260 m. The maximum single-layer thickness reaches 52.91 m, while the average thickness is approximately 161.91 m. These layers are stratified into five distinct units with thin and unevenly distributed interlayers, typically less than 5 m in thickness. No significant fractures were found within the salt basin. The Huai’an area is rich in salt mineral resources, primarily located in the Zhangxing mining area of the Huai’an salt basin and the Zhaoji mining area of the Hongze salt basin. The total area covers 330 km², with salt layers buried at depths ranging from 1300 m to 2200 m and thicknesses between 100 m and 150 m. The salt chemical industry is well developed in this region, and the production of salt chemicals and brine annually creates an effective salt cavern volume of 3.5 million m³, providing significant potential for the construction of salt caverns.

In addition, Jiangsu Province has extensive experience in constructing and operating salt cavern storage facilities. Currently, there are established or planned gas storage facilities in Jintan, Zhangxing, Huai’an, and Chuzhou [70]. In 2007, the CNPC Jintan gas storage reservoir in Jiangsu Province was completed, which became the first salt cavern gas storage reservoir built in Asia. Currently, 28 salt caverns have been put into operation, with a total storage capacity of 10 × 10⁸ Nm³ and a working gas volume of 7 × 10⁸ Nm³ [39]. Since then, the construction techniques and technologies for layered salt rock formations in China have gradually matured, accelerating the development of salt cavern gas storage projects. In 2012, Sinopec initiated the construction of the Jintan salt cavern gas storage facility, achieving a storage capacity of 1 × 10⁸ Nm³ and a working gas volume of 0.6 × 10⁸ Nm³. The Ganghua Jintan gas storage project commenced in 2014, with a designed storage capacity of 10 × 10⁸ Nm³ and a working gas volume of 6 × 10⁸ Nm³. Subsequently, to address the challenge of high impurity content in China’s salt deposits, the world’s first gas storage facility employing sediment void storage technology was commissioned in Huaian in October 2023 [71].

Among the operational salt cavern gas storage facilities in China, all facilities except the Jianghan gas storage are located in Jiangsu Province (as shown in

Table 3. Information on commissioned cavern gas storage reservoirs in China [39].

Gas Storage Facility	Location	Design Capacity/(108 Nm3)	Working Gas Volume/(108 Nm3)	Operating Entity
Jintan Gas Storage	Jintan, Jiangsu	26.40	17.10	PipeChina
Jintan Gas Storage	Jintan, Jiangsu	11.80	7.20	Sinopec
Jintan Gas Storage	Jintan, Jiangsu	10.00	6.00	Towngas
Zhangxing Gas Storage	Huaian, Jiangsu	31.26	18.47	Jiangsu Guoneng
Jianghan Gas Storage	Qianjiang, Hubei	48.09	28.04	Sinopec

). Considering the conversion of existing gas storage reservoirs into hydrogen storage reservoirs can save a large amount of costs related to underground engineering. In addition, the existing experience of building and operating the reservoirs will also provide an important reference for the implementation of the salt cavern hydrogen storage technology and the OWP-SWE-SCHS coupling system.

In summary, Jiangsu Province may be one of the most promising options for the implementation of this coupling system in China.

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System Implementation Analysis

The energy conversion efficiency of the entire system varies depending on the final product delivered (electricity or hydrogen). When the final product is hydrogen, the main energy conversion processes include seawater electrolysis efficiency (conversion of electrical energy to hydrogen energy), hydrogen transport efficiency, and salt cavern hydrogen storage efficiency. When the final product is electricity, the efficiency of the fuel cell (conversion of hydrogen energy to electrical energy) must also be considered. The overall energy conversion efficiency is calculated as shown in the following equation.

$${\eta }_{total}=\left\{\begin{array}{c}{\eta }_{SWE}\times {\eta }_{transport}\times {\eta }_{SCHS}\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{s}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}\\ {\eta }_{SWE}\times {\eta }_{transport}\times {\eta }_{SCHS}\times {\eta }_{FC}\mathrm{T}\mathrm{h}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}.\end{array}\right.$$

where ${\eta }_{SWE}$ is the seawater electrolysis efficiency for hydrogen production; ${\eta }_{transport}$ is the hydrogen transport efficiency, typically ranging from 85% to 90%; ${\eta }_{SCHS}$ is the salt cavern hydrogen storage efficiency; and ${\eta }_{FC}$ is the fuel cell efficiency, generally ranging from 40% to 65% [72], with a value of 55% used for the calculation.

The seawater electrolysis efficiency for hydrogen production is calculated as follows:

$${\eta }_{SWE}={\displaystyle \frac{P_{out}}{P_{in}}}\times 100\%$$

$$P_{out}={\rho LHV}_{H2}$$

where $P_{in}$ is the input energy required to generate one unit volume of hydrogen in standard conditions; $P_{out}$ is the output energy of the system, i.e., the energy per unit of hydrogen in standard conditions; $\rho$ is the hydrogen density in standard conditions; and ${LHV}_{H2}$ is the lower heating value of hydrogen. Based on this calculation, the efficiency of the seawater electrolysis floating platform is 59.3%, which is close to the efficiency of current electrolysis technologies such as AWE and PEM.

The salt cavern hydrogen storage efficiency is calculated as follows:

$${\eta }_{SCHS}={\eta }_{injection}\times {\eta }_{loss}\times {\eta }_{extraction}$$

where ${\eta }_{injection}$ is the compression efficiency during hydrogen injection into the salt cavern, which can be taken as 85% based on compressor efficiency [73]; ${\eta }_{loss}$ is the long-term storage efficiency of hydrogen, which can be taken as 98% due to the very low leakage rate in salt caverns; and ${\eta }_{extraction}$ is the efficiency of hydrogen extraction from the salt cavern, which can be taken as 90%. Therefore, the salt cavern hydrogen storage efficiency is approximately 75%.

The annual electricity generation from offshore wind energy in Jiangsu Province is approximately 2.0 × 10⁶ GWh [68]. Based on the wind curtailment rate of 3.2% in China in 2022, the hydrogen or electricity that can be produced by this system using curtailed wind power is calculated, and the results are shown in

Table 4. System efficiency and the utilization of curtailment to produce value.

Category	Value
System efficiency	20.8% (when the final product is electricity) 37.8% (when the final product is hydrogen)
Hydrogen produced from curtailment/t	3.56 × 104
Electricity produced from curtailment/GWh	4.16 × 105

.

The calculation results indicate that when the final product is hydrogen, the system efficiency is higher. However, when the final product is electricity, the efficiency is lower due to an additional fuel cell process. Therefore, from an energy efficiency perspective, it is recommended that the system be applied to industries with high hydrogen consumption, such as the industrial, construction, and transportation sectors. In addition to efficiency, we must also consider the role of the OWP-SWE-SCHS coupled system in peak shaving and buffering hydrogen supply and demand fluctuations, as shown in Figure 8. On the other hand, when relying solely on curtailed wind, this system can produce a large scale of green electricity and green hydrogen. With the further scaling up of offshore wind power development, the OWP-SWE-SCHS coupled system is expected to play a more significant role in reducing carbon emissions and addressing energy demand peaks and valleys.

6. Challenges and Solutions

Section 5.2 has explored in detail the various advantages of implementing the OWP—SWE—SCHS coupling system in Jiangsu. However, the current discussion is mainly based on assumptions, and there are still some challenges regarding the overall feasibility of this system, and a large number of details need to be considered.

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Specific Feasibility

The cyclical fluctuations in offshore wind power generation capacity and hydrogen demand in Jiangsu Province should be thoroughly investigated, as these are critical indicators for determining the scale of system construction. A detailed study of the periodic variability of offshore wind power and its grid integration in Jiangsu is necessary. By estimating the surplus electricity, the construction scale of offshore hydrogen production floating platforms can be determined. Hydrogen energy can be used not only for grid peak shaving but also in industrial sectors. Investigating the demand for hydrogen energy provides a reference for determining the scale of salt cavern hydrogen storage facilities and developing related injection and extraction strategies to meet the spatial- and temporal-matching needs from hydrogen production to utilization.

At the same time, demonstration projects for hydrogen storage in salt caverns and work related to risk assessment should be carried out as soon as possible. Although the world’s first salt cavern hydrogen storage project has been put into operation for more than 50 years, there is no case of salt cavern hydrogen storage in China, and only demonstration projects have been carried out in Jintan and Pingdingshan. The implementation of demonstration projects will greatly promote the construction of salt cavern hydrogen storage projects in China and lay the foundation for the implementation of the OWP—SWE—SCHS coupling system. In addition, hydrogen is highly flammable, explosive, and prone to leakage. Therefore, the sealing integrity of salt caverns, the selection of hydrogen-resistant materials, risk assessment, and post-disaster management measures should all be key areas of research. While salt cavern hydrogen storage has significant potential, strict safety measures must be implemented.

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Applicability in other regions

Leveraging its strengths in offshore wind power, salt mine resources, and economic and technological development, Jiangsu stands out as the most promising region in China for the implementation of the OWP—SWE—SCHS coupling system. Nevertheless, the system’s stringent requirements in terms of geographic, geological, and economic factors necessitate further investigation into its applicability in other regions. After evaluating the applicability in other regions of China, it is found that Shandong, Hebei, and Guangzhou, except Jiangsu, have some potential for its construction. All three locations are coastal provinces, and all of them have built or are building salt cavern gas storage reservoirs, such as the Shandong Tai’an gas storage reservoir, the Hebei Ningjin gas storage reservoir, and the Guangdong Sanshui gas storage reservoir. However, the methodologies to carry out the construction of these provinces according to local conditions needs to be followed up with detailed research, and this paper will not conduct that research.

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Overall Economics

Undoubtedly, whether a project can eventually come to fruition and be popularized on a large scale depends to a large extent on its economics. On the one hand, from the perspective of the whole system, the construction of a floating platform for SWE largely saves the cost of grid connection of offshore wind farms. On the other hand, the conversion of existing gas storage facilities can reduce the cost associated with hydrogen storage. But from the technology itself, due to the difficulties of installation, operation, and maintenance in the harsh marine environment, the cost of offshore wind farm installations is higher than onshore wind power installations. And with SWE floating platforms currently only available as demonstration projects, their construction can be expected to be expensive. If it is extended to commercialization, it will also need to be optimized and controlled in terms of construction costs.

The OWP—SWE—SCHS coupling system involves a large investment, and the economic advantages in the short term are not obvious, but the long-term returns should be considered. This is one of the reasons why this paper suggests prioritizing pilot projects in economically developed regions. Taking offshore wind technology as an example, the global weighted average levelized cost of electricity (LCOE) for offshore wind technology decreased from 0.162 USD/kWh to 0.084 USD/kWh from 2010 to 2020, which is a 48% decrease in 10 years [1]. This trend indicates that with the standardization and scale development of offshore wind and SWE technologies, as well as the maturity of the industry, policy support, market demand, supply chain optimization, and technological advances, the construction cost of the relevant systems is expected to be further reduced, enhancing their economics and feasibility. The return on investment will become more significant, especially in the context of the rapid development of renewable energy applications.

In summary, the challenges and solutions faced by the OWP-SWE-SCHS coupled system can be summarized in

Table 5. Challenges and solutions faced by the OWP-SWE-SCHS coupled system.

Category	Challenge	Solution
Specific feasibility	The fluctuation of offshore wind power generation and hydrogen demand cycles has not been thoroughly studied.	Investigate the cyclical fluctuations of offshore wind power and grid connection capacity in Jiangsu Province, estimate excess electricity, and determine the scale of the hydrogen production platform; investigate the demand for hydrogen in grid peak shaving and industrial sectors, and develop the scale and injection/extraction strategy for salt cavern hydrogen storage.
	Lack of domestic cases for salt cavern hydrogen storage engineering, making risk assessment difficult.	Accelerate the development of a salt cavern hydrogen storage demonstration project, focusing on salt cavern sealing properties, hydrogen material selection, risk assessment, and post-disaster measures to ensure safety.
Applicability to other regions	The system has high geographical, geological, and economic development requirements, limiting its applicability.	Conduct feasibility studies in other coastal regions (such as Shandong, Hebei, and Guangdong) and develop site-specific construction plans.
Overall economic viability	High system construction costs, making commercial promotion difficult.	Prioritize pilot projects in economically developed areas to accumulate experience and promote technological standardization and scaling, reducing construction and operational costs; make full use of existing gas storage facilities for renovation, reducing overall system construction costs
	Harsh marine environment leads to increased installation and operational challenges and costs.	Improve equipment durability and corrosion resistance; optimize offshore installation, remote operation, and maintenance technologies; and reduce operational costs.
	Technology is still in the demonstration phase, and the potential for large-scale development has not been fully realized.	Increase investment in technology research and development and policy support to enhance the maturity of relevant technologies and accelerate the transition from demonstration projects to commercial ventures.

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7. Conclusions

This paper discusses in detail the current situation and development trend of offshore wind power in China. A coupling system integrating offshore wind power, hydrogen production from electrolyzed seawater, and hydrogen storage in salt caverns is proposed to address the difficulties of offshore wind power grid connection. The system takes hydrogen energy as a carrier, realizes the utilization of renewable energy and the integration of hydrogen energy production and storage, offers a new pathway for coastal cities to advance renewable energy development, and sets a new direction for hydrogen energy progress.

The OWP—SWE—SCHS coupling system provides significant technical and environmental benefits. By utilizing offshore wind power for in situ hydrogen production and replacing electricity transmission with hydrogen transportation, this system addresses the grid integration challenges of offshore wind power, enhances its utilization efficiency, and minimizes the need for land-based infrastructure. Additionally, it reduces freshwater consumption and simultaneously contributes to seawater desalination, offering a dual advantage for resource conservation. Furthermore, the integration of hydrogen production and storage can effectively cope with fluctuations in the supply and demand of the power grid and hydrogen energy, significantly improving the security and stability of energy supply.

In addition, this paper screens out the most promising region in China, Jiangsu, for the construction of the OWP—SWE—SCHS coupling system. Thanks to the good development of offshore wind power in Jiangsu, the rich salt resources, and the experience of construction of salt cavern gas storage, Jiangsu has a unique development advantage. At the same time, Jiangsu Province has a developed economy and a high demand for hydrogen energy, which makes it easy to form an industrial agglomeration advantage.

Inevitably, the system has certain challenges in terms of implementation. For example, the specific feasibility of the details of the system, its applicability in other regions, and the overall economics need to be further studied. We suggest that policy support and more flexible financing methods be adopted at the early stage of the system’s implementation to compensate for its early economic disadvantages. In addition, relevant standards for hydrogen energy, offshore wind power, hydrogen production from electrolyzed seawater, and hydrogen storage in salt caverns should be improved as soon as possible to ensure the sustainable development of the entire renewable energy industry.