The Necessity and Feasibility of Hydrogen Storage for Large-Scale, Long-Term Energy Storage in the New Power System in China

Abstract

In the process of building a new power system with new energy sources as the mainstay, wind power and photovoltaic energy enter the multiplication stage with randomness and uncertainty, and the foundation and support role of large-scale long-time energy storage is highlighted. Considering the advantages of hydrogen energy storage in large-scale, cross-seasonal and cross-regional aspects, the necessity, feasibility and economy of hydrogen energy participation in long-time energy storage under the new power system are discussed. Firstly, power supply and demand production simulations were carried out based on the characteristics of new energy generation in China. When the penetration of new energy sources in the new power system reaches 45%, long-term energy storage becomes an essential regulation tool. Secondly, by comparing the storage duration, storage scale and application scenarios of various energy storage technologies, it was determined that hydrogen storage is the most preferable choice to participate in large-scale and long-term energy storage. Three long-time hydrogen storage methods are screened out from numerous hydrogen storage technologies, including salt-cavern hydrogen storage, natural gas blending and solid-state hydrogen storage. Finally, by analyzing the development status and economy of the above three types of hydrogen storage technologies, and based on the geographical characteristics and resource endowment of China, it is pointed out that China will form a hydrogen storage system of “solid state hydrogen storage above ground and salt cavern storage underground” in the future.

1. Introduction

The discovery and use of fossil energy brought about a great leap forward in human history [1]. In the nineteenth century, the burning of coal in steam engines lit the fire of the industrial revolution and illuminated the way forward for human civilization [2]. With the continuous development of human society, the over-exploitation and use of fossil energy has caused serious environmental problems. Since the industrial revolution, fossil energy consumption has produced a total of 2.2 trillion tons of carbon dioxide, and the climate crisis has become a global, non-traditional security issue [3]. Under the climate crisis, the transformation of the global energy structure is imminent. A growing number of governments are turning “carbon neutrality” into national strategies with visions of a carbon-free future [4,5]. In addition to environmental risks, the finite nature of fossil resources is an important reason for the transition in the global energy mix [6]. Fossil fuels were formed from plants and animals in ancient times after being squeezed and heated by the earth’s crust over a time scale of several million years, and are non-renewable resources. Among the fossil energy sources, the storage and extraction ratios of coal, oil and natural gas are 100 years, 50 years and 51 years respectively. To break the contradiction between limited resources and sustainable development, the development of clean and renewable energy is inevitable [7]. In 2021, Chinese President Xi Jinping put forward the great vision of achieving “peak carbon” by 2030 and “carbon neutral” by 2060 at the ninth meeting of the Central Finance and Economics Commission. In the same year, it was pointed out that building a new power system with new energy as the mainstay is a major initiative to achieve the “double carbon” goal [8,9]. At present, the global energy structure is still dominated by fossil energy, and electricity is the largest direct use of the fossil energy industry, comprising nearly half of the carbon emissions from the power industry. The power sector, as the largest carbon-emitting sector in China, accounts for over 40% of the total [10]. The power sector is the key to carbon-emission reduction, and the power sector’s actions for carbon reduction have a significant impact on the national policy of carbon reduction. Effective use of non-fossil energy, enhancing the proportion of electricity using non-fossil energy, and improving the efficiency of power system scheduling and operation are the three main focus points of the power industry to help reduce carbon action. Considering that new energy can only be transformed into the form of electricity, the new power system will become an important carrier to promote the low-carbon transformation of the power industry. With the rising importance of new energy, a large amount of investment is concentrated in new energy, and the growth rate of investment in traditional fossil energy is declining or even negative.

As the proportion of new energy in the energy system increases, electricity will become the main energy carrier, and the power system will act as a bridge between new energy and users, balancing the supply and demand between the two. The International Commission on Long-Term Energy Storage estimates that when 60–70% of the power system comes from renewables, the power supply-demand imbalance problem may last for days or even weeks [11]. As the share of intermittent renewable energy generation increases, the power system faces more challenges, such as a seasonal imbalance of power supply and demand, lack of power system inertia, and blockage in the distribution grid [12]. Considering that the prediction of the Long-Term Energy Storage Committee for the power system is global, it is not applicable to the power system in China. Therefore, this paper carries out research from three aspects, including the necessity of long-term energy storage, the feasibility of hydrogen energy participation in long-term energy storage and the economics of hydrogen energy participation in long-term energy storage, from the perspective of ensuring the reliable energy supply of Chinese power.

2. The Need for Long-Term Energy Storage

As the process of new power systems continues to advance, a high percentage of wind power and photovoltaic power becomes an important part of the power supply. The strong uncertainty and volatility of new energy will have a great impact on the power supply and demand of the power system [13]. According to statistics, global energy trends are shown in Figure 1. Wind and PV will be the dominant energy sources in 2050, accounting for 68% of all society’s electricity generation. Since both PV and wind power have poor continuity, geographical constraints, and are prone to surplus or shortage, they will cause the security and reliability of the power system to be further threatened.

From the seasonal perspective of China’s power supply and demand, most of China’s geographical areas are divided into four seasons, and the seasonal attributes of power demand are distinct [15].

Summer cooling and winter heating make China’s electricity consumption significantly higher in summer and winter than in spring and fall. Wind and photovoltaic power generation is highly uncertain due to regional and weather effects. Wind power output is at peak in spring and autumn, and photovoltaic output is at peak in summer and autumn [16,17,18]. According to the power balance analysis, wind-PV complementarity can reduce the impact of new energy seasonality to a certain extent. However, the seasonal power distribution of new energy does not match the power demand. There is a seasonal power-balance problem in summer when the power load is high and new energy generation is low [19]. In order to guarantee the reliable supply of energy to the new power system, it is necessary to determine the timing of the deployment of long-term energy storage, i.e., what percentage of the power system is penetrated by new energy sources. Therefore, the total generated active value (hourly level) of thermal, wind power, hydropower, photovoltaic, and nuclear power in China’s power grid for 2018–2020 is used as the basis. In addition, wind power and photovoltaic power are considered as uncontrollable power, and hydropower, nuclear, and thermal power are considered as flexible controllable power. In order to attenuate the impact of weather changes on wind power and PV output as much as possible, the wind power, PV, controllable power and power load data from 2018–2020 were summed up and averaged as typical data, and the results are shown in Figure 2. As can be seen, the relationship between electricity supply and demand is largely consistent with the seasonality analyzed in the previous section.

Based on the output characteristics and load characteristics of the various types of output devices given in Figure 2, the new energy penetration growth step is set at 5% according to the development plan for wind and PV in China. The power system stochastic generation simulation method is used to simulate the power supply and demand relationship under different new energy penetration rates. [20]. The results show that when the penetration rate of new energy in the power system reaches 45%, at this time wind power, photovoltaic power and controllable power account for 32%, 13% and 55% respectively, and the relationship between power supply and demand is shown in Figure 3a. As can be seen in Figure 3, the correlation between controllable power and power demand becomes significantly worse in terms of power supply and demand throughout the year. The problem that photovoltaic power generation has only daytime output makes the stability of the power supply worse and has a significant impact on the reliability of the power system. Figure 3b shows the net electricity supply and demand. Although there is also a shortage of electricity in summer, the electricity supply is generally higher than the demand. In winter, the power supply capacity is significantly weaker due to the smaller output of new energy power. There is the problem of oversupply, and the longest power deficit is up to 166 h. Figure 3c shows the simulation results of a typical winter week. Affected by the photovoltaic output characteristics, the electricity demand can only be met when there is sufficient sunlight during the daytime. With insufficient power supply at night, long-time energy storage will become an indispensable means of regulation for the power system.

In order to avoid long-term power surplus or shortages caused by high percentage demand and large-scale new energy grid connections, energy storage technology is needed to cut the peaks and fill the valleys of the grid. Therefore, long-term energy storage technology will become a key component in building a new power system. When the penetration rate of new energy is low, only short-term energy storage is needed to provide power-regulation capacity for the system and improve the utilization rate of new energy. With the increase of new energy penetration, it is difficult to meet the regulation demand of the system by only allocating short-term energy storage. In addition, long-term energy storage should be added as a means of cross-seasonal energy regulation. Therefore, there is a need to build an energy storage system with seasonal energy regulation and short-term and long-term complements [21].

3. Feasibility Analysis of Hydrogen Participation in Long-Term Energy Storage

3.1. Energy Storage Technology Comparison

At present, the energy storage methods applied in the power system, such as “pumped storage [22], electrochemical energy storage [23], etc., mainly provide intra-day peak regulation, frequency regulation, and ramp climbing services for the power system, used to smooth out the short time-scale (seconds, minutes, hours) power fluctuations. However, it is difficult to cope with the long-time (week, month, year) renewable energy output and load demand of the power imbalance problem [24]. In order to achieve energy balance on long time scales and participate in monthly, quarterly, annual or even interannual regulation processes, long-time and large capacity energy storage technologies are required. At this time, long-term energy storage can rely on the characteristics of long-period and large storage capacity to regulate the fluctuations of new energy generation in a long time dimension. It avoids grid congestion when there is a surplus of clean energy and increases the consumption of clean energy during peak loads. In addition, long-term energy storage can guarantee the power supply in extreme weather and reduce the cost of electricity for society [25].

There are many types of energy storage technologies, and different types of energy storage technologies have different principles and different technical-economic characteristics. Overall, mechanical energy storage is easier to achieve for large-scale applications, but the efficiency is low [26]; electrochemical energy storage is more efficient, although large-scale applications need to break through the life and safety issues [27]; thermal and chemical energy storage can store large-scale energy, but the energy conversion efficiency is not high, generally not suitable for the “electricity—heat—electricity” form of energy storage [28]. The comparison of energy storage duration and capacity of various energy storage methods is shown in Figure 4. It can be seen that the ultra-short time-scale application scenario is suitable for ultra-short-time storage or short-time storage with a fast response time and a continuous discharge time of minutes or hours such as a super capacitor and electrochemical energy; the short time-scale application scenario is suitable for short-time storage with a continuous discharge time of hours such as pumped storage and electrochemical storage; the long-period scale application scenario is suitable for long-time storage with a continuous discharge time of days and above such as hydrogen storage and compressed air.

There are many types of energy storage and various technical routes, and the application scenarios also have their own focus. Many factors affecting the adaptability of energy storage cannot be quantified. Most of the relevant studies have focused on the economic or technical adaptability of energy storage. The performance of various types of energy storage technologies under different application scenarios is compared from three levels of analysis: technical, economic and safety, as shown in Figure 5. Combining Figure 4 and Figure 5, hydrogen energy storage has the advantages of high energy density, large storage scale, and the ability to cross seasons, making it the optimal solution for participating in the long-term energy storage of new power systems. In order to have good reliability and stability in the new power system when the share of new energy reaches 45%, hydrogen storage needs to be commercially promoted in the next 10–30 years, which is significant for building a new power system.

3.2. Long-Term Hydrogen Storage Technology

Hydrogen storage is more flexible in time and space dimensions, as it can be stored in solid phase in hydrogen storage materials, or in liquid or gas phase in high pressure tanks. Hydrogen storage time can be up to several weeks. It can also be transported over long distances and across regions in different storage forms, solving the problem of time and space mismatch for electricity consumption [31]. However, not all hydrogen storage technologies are suitable for long-term storage. Long-term and efficient storage of hydrogen energy is also one of the key issues in the development of hydrogen energy on a large scale and one of the constraints that limit the high price of hydrogen energy. Therefore, long-term storage of hydrogen in a safe and stable form is a prerequisite. Verification of hydrogen storage length, energy storage efficiency and cost reduction is the focus of the development priorities of hydrogen storage technology.

According to the field, eleven types of hydrogen storage have been or will be used in different applications. They can be classified as physical hydrogen storage, chemical hydrogen storage, and other hydrogen storage [32,33]. Currently, more than 70% of hydrogen is stored by compression, but it is not possible to say which technology has the exclusive advantage. Many technologies can vary greatly between laboratory and mass production, so some advanced technologies need to be tested by both time and the market [34]. Different storage technologies will be needed in the future hydrogen economy to meet different constraints and the variability of energy production and demand on different time scales (from hourly to seasonal). Table 1 gives the advantages, disadvantages, and application areas of each type of hydrogen storage technology.

By analyzing the hydrogen storage principle and hydrogen storage performance of various types of hydrogen storage technologies, the comparison of hydrogen storage capacity and hydrogen storage duration of various types of hydrogen storage technologies is shown in Figure 6. According to the analysis of the necessity of long-term energy storage, the main position of hydrogen energy in the new power system is determined as a large-scale seasonal regulation resource. Thus, the ability to achieve large-scale and seasonal storage of energy is an important criterion to judge the development prospect of hydrogen storage technology. Among the physical hydrogen storage technologies, high-pressure gaseous hydrogen storage has a small storage capacity, and the storage length is only a few days. Low-temperature liquid hydrogen storage has high bulk density and high storage capacity. However, the storage process requires a lot of energy to maintain low temperatures, and the storage process is subject to fugitive phenomena. Therefore, it is suitable for large-scale storage of only a few days or weeks. Underground hydrogen storage and natural gas blending do not require storage tanks and other devices. The scale of hydrogen storage can reach hundreds of millions of cubic meters, and can achieve weeks and months of hydrogen storage. Among the chemical hydrogen storage technologies, inorganic compound hydrogen storage has the relatively lowest hydrogen storage capacity among the existing hydrogen storage technologies and its reversibility is poor. Organic liquid hydrogen storage has a storage density of 5–10 wt%, and the hydrogen storage material is recyclable and low cost. However, its dehydrogenation process requires a certain amount of energy and is less reversible. The hydrogen storage capacity of methanol and liquid ammonia is 12.5 wt% and 17.6 wt%. After being made into methanol and liquid ammonia, they are generally used directly in chemical and fuel applications. Metal hydride hydrogen storage is the most widely used hydrogen storage material, and the storage capacity can reach 18 wt% in theory. Hydrogen storage by adsorption and hydrogen storage by hydrate are still in the laboratory research stage, and it will take a long time to apply them in practice. The amount of hydrogen storage that can be theoretically achieved by adsorption and hydrate methods is inferior to that of metal hydrogen storage, and the duration of hydrogen storage is only a few days or weeks. In summary, in order to play a role in the seasonal storage of hydrogen energy in new power systems, natural gas doping, salt-cavity hydrogen storage and metal-hydride hydrogen storage are the best choices to participate in large-scale, long-cycle energy storage.

4. Economic Analysis of the Long-Term Storage of Hydrogen

4.1. Salt-Cavern Hydrogen Storage

With the continuous progress of hydrogen storage technology, underground hydrogen storage is considered the most feasible development direction for large-scale hydrogen storage technology. There are four main types of underground hydrogen storage: depleted hydrocarbon reservoirs, salt caverns, aquifers and caverns, and a comparison of the advantages and disadvantages is shown in Table 2 [45,46,47]. Depleted hydrocarbon reservoirs account for a large proportion of underground gas storage. However, a large number of voids in the formation can lead to a large amount of residual gas and increase the amount of bedding gas [48]. Aquifer hydrogen reservoirs are made by injecting gas under the cap layer to replace water in the rock formation, with a large storage capacity but a high exploration risk and incomplete recovery of mat gas [49,50]. Cavern hydrogen reservoirs are small in capacity and prone to leaks and are rarely used [51]. Salt-cavern hydrogen storage is currently considered to be the most promising option for non-surface hydrogen storage. This is because of its high gas injection and extraction efficiency and low requirements for bedding gas volumes. In addition, the high sealing capacity of rock salt and the inertness of the salt structure prevent contamination of the stored hydrogen [52,53,54,55].

The strategic position and economic rationality of hydrogen energy in the new power system mainly lies in the demand for large-scale, long-term capacity storage and diversified end-use in the energy transition process. Salt-cavern hydrogen storage is an ideal option for storing hydrogen on a large scale and at low cost. It is effective in the construction of new power systems to take advantage of long-time energy storage. Taking a typical underground salt-cavern storage system as an example, the solubility of hydrogen in brine can be further reduced by adjusting parameters such as temperature, pressure, and brine concentration in a three-phase system of water-hydrogen-salt [56]. However, because of its low molecular weight and high degree of diffusion, hydrogen is likely to leak through the cap and interlayer to the surface or into the surrounding area [57,58]. When storing hydrogen underground, the effect of the porosity of the subsurface space on diffusion must be considered. When drawing on proven experience, the differences in storage hydrogen pressure, injection and extraction process, and other factors in practical engineering applications, should be taken into account [59,60,61].

As shown in Table 3, there are currently four operating salt-cavern hydrogen storage projects worldwide, located in the Teesside region of the UK and Texas, USA (95% hydrogen and 3–4% CO₂). These underground storage experiences show that hydrogen can be safely stored for long periods of time [55].

In addition to the above-mentioned salt-cavern hydrogen storage projects, new project plans have been released for underground hydrogen storage in all Western countries. The U.S. values the full range of salt-cavern energy storage applications for hydrogen storage in order to maintain its strategic energy reserves and international leadership [66]. The EU is actively assessing salt-cavern hydrogen storage potential and deploying salt-cavern hydrogen-storage project plans, focusing on industrial-scale salt-cavern hydrogen storage research [67]. Germany sees hydrogen as the key to a successful energy transition, and DLR has partnered with Oldenburg Energy to conduct tests on salt-cavern hydrogen storage [68]. The UK has identified underground hydrogen storage as one of the key technologies for achieving net-zero emissions and is conducting related pilot studies [69]. In addition, Canada [70], Poland, Turkey [71] and Denmark [45] have made plans for the development of hydrogen storage in salt caverns. Compared with the above-mentioned countries, the research into underground hydrogen storage in China lags behind, the research into the economics of geological hydrogen storage is insufficient, and there is no practice of underground hydrogen storage yet.

Salt cavern reservoirs have high requirements as reservoir sites, with special requirements for tectonic integrity, salt rock grade and distribution, and cap sealing [72]. Compared with foreign salt cavern reservoirs, most of the salt cavern reservoirs under construction and proposed in China are built in stratified salt formations. The low grade of salt rock and multiple interlayers lead to difficulties in controlling the cavern formation pattern and low cavern formation efficiency [73]. Figure 7 shows the distribution of salt mines in China. Salt mines suitable for building underground storage are rock salt formations, which are usually buried at a depth of 50–1700 m below ground. As can be seen from Figure 7, China’s rock salt deposits are mainly distributed in Sichuan, Chongqing, Hubei, Jiangxi, Anhui, Jiangsu, Shandong and Guangdong local areas. In contrast, the northern and northwestern regions, which are rich in photovoltaic and wind energy, lack salt rock strata suitable for the construction of underground gas storage in salt caverns. On the one hand, surplus electricity from the northwest and northern regions can be transported through the extra-high voltage transmission network to the vicinity of the salt cavern hydrogen storage reservoir for hydrogen production and storage. On the other hand, hydrogen can be produced locally and the produced hydrogen can be transported to the salt cavern hydrogen storage via a natural gas pipeline for storage through the West-East gas transmission network.

In addition, the levelized hydrogen storage costs were calculated for the four types of underground hydrogen storage methods. The operating parameters of the four types of underground hydrogen storage methods are shown in Table 4 [63,75]. Based on the parameters in Table 4, the costs of the four types of underground hydrogen storage methods are calculated, as shown in Table 5, including the costs of pad gas, reservoir development, compressors, pipelines, and wells. According to Table 5, depleted hydrocarbon reservoirs and aquifers are more competitive in terms of cost because they do not require extraction costs. Considering that hydrogen may react and migrate in depleted oil and gas reservoirs and aquifers, resulting in increased cost uncertainty, salt caverns and caverns are more suitable for long-period, large-scale hydrogen storage. Moreover, the cost of salt caverns is only slightly higher than that of depleted oil and gas reservoirs and aquifers, making them the best choice for subsurface hydrogen storage [66,76,77,78,79].

4.2. Hydrogen Blending in Natural Gas

Hydrogen blending in natural gas is characterized by high economic feasibility, low investment costs, access to many end customers, and relatively easy commercialization in the future [80]. With the maturity of hydrogen blending and hydrogen separation technologies, the production of hydrogen from cheap power resources in the northwest and its blending with natural gas is expected to lead to large-scale storage and transportation of hydrogen. It helps to solve the problem of uneven geographical distribution of energy in China and promote the large-scale and rapid development of the hydrogen energy industry. Hydrogen blending in natural gas can solve the cost problem of hydrogen mass-storage pipeline construction. It contributes to carbon reduction in the natural gas sector. Therefore, hydrogen blending in natural gas is one of the important ways to promote the development of large-scale and long-cycle storage methods for the hydrogen energy industry [81,82]. Due to the initial unclear mechanism and low public awareness, hydrogen blending in natural gas in China is discussed more in the transportation sector and more at the application level for civil use. The sensitivity to safety issues in pipeline transportation and in the civil sector has led to long experimental-validation cycles for the project. Therefore, natural gas blending has been in the demonstration stage and limited in scale, and the commercialization process has been slow to advance [83,84]. Europe has pioneered a holistic energy solution of “hydrogen production from renewable energy sources—inter-seasonal storage of hydrogen in natural gas infrastructure—hydrogen-doped natural gas combustion for power generation”, which is basically used to cope with the development of large-scale renewable energy generation and to realize the seasonal storage and application of renewable energy [85]. In China, with the acceptance and recognition of blending hydrogen into natural gas pipelines, in 2021, blended hydrogen–natural gas combustion power projects began to emerge [86]. The emergence of power generation projects with hydrogen blending in natural gas complements the technical elements of the inter-seasonal application of renewable energy. It is conducive to promoting the development of renewable energy and solving the carbon emission problem of power plant enterprises, as well as expanding the application scale of hydroelectric energy.

The hydrogen storage capacity of natural gas blending depends on the construction of a natural gas storage infrastructure, which includes three types of natural gas storage depots, LNG receiving stations, and emergency reserve peaking stations.

• 1.. Natural gas storage depots

Natural gas storage reservoirs are the cornerstone to ensure the safe and stable operation of the natural gas market. Gas storage construction in mature countries such as Europe and the United States is relatively complete. At present, there are about 720 gas storage reservoirs in the world, with a working gas capacity of over 420 10⁸ cubic meters, with Europe and America accounting for over 70% of the total. The development of underground gas storage in China is later than that of European and American countries. Since the “12th Five-Year Plan”, underground gas storage has only entered the large-scale construction stage [87,88]. By the end of 2020, China had built 14 underground gas storage reservoirs (clusters) with a total designed working gas volume (actual storage volume) of 239 × 10⁸ m³, forming an effective working gas volume of 145 × 10⁸ m³. This accounts for 4.5% of the national natural gas consumption in that year, which is still a big gap from the world average of 11.8%. The situation of the completed gas storage is shown in Table 6.

Among them, the Jintan type of gas storage is salt-cavern storage, and the others are Depleted Hydrocarbon Reservoirs storage. If a depleted oil and gas reservoir is used for hydrogen storage, the hydrogen may react with ancient microorganisms or mineral components in the reservoir. Some of the stored hydrogen will be consumed and the resulting reactants may clog the pores of the reservoir, which is not conducive to the long-term storage of hydrogen. Therefore, only Jintan gas storage is suitable for natural gas blending storage, with a storage capacity of 42.4 × 10⁸ m³ and a working capacity of 26.1 × 10⁸ m³. Driven by the policy, there are more underground gas storage projects under construction and planning in China. According to the planning of CNPC and Sinopec, the number of new gas storage reservoirs in China will reach 30 by the end of 2030. In the future, the total storage capacity of gas storage in China will exceed 100 × 10⁸ m³, with a new working capacity of 40 × 10⁸ m³. Salt-cavern gas storage reservoirs in construction, field test or pre-feasibility stage are shown in Table 7.

It will take 5–10 years for salt-cavern gas storage to be initially completed and put into operation from the start of the project. Comprehensive existing salt-cavern gas storage reservoirs in China that have been put into operation, under construction, field testing or pre-feasibility study stage, mean that as of 2030, the total capacity of salt-cavern gas storage reservoirs in China can reach 181 × 10⁸ m³, with a working capacity of 104.942 × 10⁸ m³. According to the calculation of 10% hydrogen doping ratio, China’s underground salt-cavern gas storage reservoir will be able to store 1.05 × 10⁸ m³ of hydrogen in 2030.

• 2.. LNG receiving stations

At present, there are 22 LNG receiving stations built, 14 LNG receiving stations under construction and 15 LNG receiving stations to be built. The receiving capacity of China’s existing and under-construction LNG receiving terminals totals about 100 million tons/year [91]. Taking into account the current existing, under-construction, planned and new and old project expansion plans, China’s LNG supply capacity is expected to reach 150 to 190 million tons/year in the next 5 to 10 years’ time [92]. LNG receiving terminal tanks are used for natural gas storage at low temperatures (−162 °C) in liquid form under normal pressure [93]. However, hydrogen needs to be below −253 °C to liquefy, and the natural gas blending mentioned now is done in gaseous form, so LNG receiving stations are not suitable for hydrogen blended gas storage.

• 3.. Emergency reserve peaking stations

In recent years, there have been frequent “gas shortages”, especially in the winter season. The government has to prepare in advance and even take extraordinary measures such as limiting the use of gas by enterprises to protect the supply of natural gas. The construction of urban gas storage facilities is relatively lagging behind, and the lack of peaking capacity is an important reason for the tight gas supply situation in winter [94]. With the development of the natural gas industry and advances in construction technology, a variety of gas storage methods have been developed to meet peak and emergency storage needs. Emergency reserve peaking stations are generally constructed as underground gas storage and LNG storage tanks with large gas storage capacity and strong peaking and emergency protection capabilities. Combining the difference between upstream gas supply and downstream gas consumption, when the downstream gas consumption is low, the excess natural gas supplied by the upstream is liquefied and stored. When the upstream gas supply cannot meet the downstream gas demand, the stored LNG is gasified into gaseous natural gas and supplied to customers through town gas pipelines at specific locations. The main way of managing seasonal peaking and the emergency reserve in the United States, Europe and Japan is underground gas storage and LNG storage tanks [95].

In summary, there are three main ways to store natural gas on a large scale: underground storage, LNG storage tanks and natural gas pipelines. Among them, natural gas pipelines and end storage are mainly used to solve the problem of uneven intra-day gas use and are not suitable for seasonal storage of hydrogen. LNG storage tanks store liquid natural gas and are not suitable for hydrogen blending for storage. Salt-cavern storage has good gas tightness. It is the best choice for underground large-scale storage of hydrogen-blended natural gas.

The main storage means of natural gas blending is also underground salt-cavern hydrogen storage. The difference with the above section of salt-cavern hydrogen storage is that the natural gas blending is mixed hydrogen storage. Compared to salt-cavern pure hydrogen storage, salt-cavern blended hydrogen storage reduces reservoir development, bedding costs and pipeline construction costs. Therefore, the levelized cost of natural gas–blended hydrogen storage is 0.74 USD/kg based on the cost of salt-cavern hydrogen storage in the above section.

4.3. Solid-State Hydrogen Storage

Among the existing hydrogen storage methods, solid-state hydrogen storage is the hydrogen storage technology with the highest bulk density per unit of hydrogen storage. Taking MgH2 as an example, its bulk hydrogen storage density can reach 106 kg/m³, which is 1191 times the density of hydrogen in the standard state, 2.7 times that of 70 Mpa high-pressure hydrogen storage, and 1.5 times that of liquid hydrogen [96]. It can significantly save installation space and reduce floor space, which is especially suitable for applications with strict restrictions on location, such as building/campus/household fuel cell combined heat and power supply systems, fuel cell backup power, distributed hydrogen storage systems, etc. [97]. The low operating pressure and high safety of solid-state hydrogen storage have great potential for application in power systems.

Solid-state hydrogen storage technology requires the use of certain properties of materials as hydrogen storage media, which can be mainly divided into physical adsorption materials and chemical hydride materials [98]. However, since physisorption relies on weak intermolecular forces for hydrogen storage, it can theoretically only absorb hydrogen at low temperatures [99]. Chemical hydride hydrogen storage materials absorb hydrogen in the form of metal hydrides by reacting with hydrogen, and the resulting metal hydrides release hydrogen when heated. Hydrogen storage alloys are mainly composed of two parts. One part is hydrogen-absorbing elements or elements with a strong affinity to hydrogen, which control the amount of hydrogen storage and are the key factors of hydrogen storage alloys, mainly including titanium hafnium and magnesium, etc. The other part is elements with little or no hydrogen absorption, including iron and nickel, etc. [100,101]. In the long term, the technology has great potential for development.

At present, the technical parameter index of solid-state hydrogen storage is mostly for vehicle-mounted solid state hydrogen storage power systems. Considering that the process of metal hydride hydrogen storage is a chemical reaction, the working conditions of metal hydride hydrogen storage should be adapted to the requirements of seasonal peaking of the power grid, such as the charging and discharging rate, charging pressure, etc. Combining the current development status of solid-state hydrogen storage and drawing on the technical parameters of vehicle-mounted hydrogen storage systems, the technical parameters of stationary hydrogen storage power plants suitable for new power systems are shown in Table 8 [97,102].

As we all know, solid-state hydrogen storage requires hydrogen storage materials. Among the four series of lanthanum (rare earth), titanium and iron, magnesium, and demolition, magnesium-based hydrogen storage materials have the most obvious advantages, including light quality, low price, abundant raw materials and a strong hydrogen storage capacity, which makes this the most promising hydrogen storage medium [106]. In addition, it has good reversibility, a moderate working temperature and suitable thermal system properties, and has a wide range of application prospects. The levelized cost of solid-state hydrogen storage is calculated using a magnesium-based alloy as an example. In comparison with the cost of high-pressure gaseous hydrogen storage, the results are shown in Table 9.

Comparing the levelized costs of three large-scale, long-term hydrogen storage options, storing hydrogen in natural gas storage reservoirs and blending it with hydrogen is the most competitive way of storing hydrogen. However, considering the problem of insufficient capacity of natural gas storage in China itself, it is only applicable to hydrogen storage in the short term (before 2030). With the continuous improvement of hydrogen production and the continuous development of hydrogen storage technology, the levelized cost of salt-cavity hydrogen storage can be reduced to 0.19–0.27 USD/kg, and the levelized cost of solid-state hydrogen storage can be reduced to 0.57 USD/kg. Therefore, in the medium and long term (2030–2060), salt-cavern hydrogen storage and solid-state hydrogen storage will be the main means of large-scale hydrogen storage.

5. Conclusions

Hydrogen energy storage has high energy density, low operational and maintenance costs, can be stored for a long time and can achieve a pollution-free process. It is one of the few energy storage technologies that can store more than 100 GW-h. It can be applied to both very short and very long power supplies. In addition, it is considered to be a new large-scale energy storage technology with great potential. Especially with the advancement of the process of building new power systems, the demand for long term energy storage is rising and the importance of hydrogen storage is also rising with it. Based on this, this paper analyzes the necessity of long-term energy storage, the superiority of hydrogen energy participation in long-term energy storage and the economy of long-time hydrogen energy storage. The details are as follows:

• (1). By carrying out the simulation of power supply and demand generation of the new power system, when wind power and photovoltaic power account for 32% and 13%, there is a power supply and demand imbalance that lasts for several days and weeks. At this time, the new power system needs to be equipped with long-time energy storage to solve the power balance problem of several hours, days or even across seasons.

• (2). By comparing the energy storage capacity, storage length and application scenarios of various types of energy storage means, hydrogen energy storage has the characteristics of high energy density, large storage scale and small energy-capacity cost, which makes it the optimal solution for seasonal and large-scale energy storage. Among the eleven existing hydrogen storage technologies, salt-cavern hydrogen storage, hydrogen blending of natural gas, and solid-state hydrogen storage are the best options for future hydrogen storage to participate in seasonal energy storage of new power systems.

• (3). The levelized hydrogen storage costs of natural gas–blended hydrogen, salt-cavity hydrogen storage and solid-state hydrogen storage are 0.74 USD/kg, 1.61 USD/kg, and 2.1 USD/kg, respectively. With the decreasing levelized cost of salt-cavern hydrogen storage and solid hydrogen storage, it will be more competitive in the future. Combining the resource endowment and geological conditions of each region in China, the hydrogen storage system of “solid hydrogen storage above ground and salt cavern storage below ground” will be formed.

From the above analysis, hydrogen energy, as a form of energy storage with both physical and energy characteristics, can solve the large-scale and long-time energy storage needs of new power systems. The development path of hydrogen energy to support the construction of new power systems can be roughly divided into three stages. The details are as follows:

• (1). New power system construction preparation period (current—2030). The main task of the power grid in this period is to increase the research and development of key technologies and core components in all aspects of hydrogen energy, and actively promote the pilot demonstration of hydrogen energy application scenarios in the new power system. At the same time, electrolysis hydrogen production equipment should be deployed in the new energy base on the power side to support the new energy power consumption.

• (2). New power system construction and development period (2030–2050). The main task in this period is to coordinate the construction of hydrogen energy infrastructure and advance the deployment of large-scale salt cavern and solid hydrogen energy storage centers. The government accelerates the construction of a safe, stable and efficient national hydrogen energy supply system, and gradually builds a convenient and low-cost hydrogen pipeline transmission network.

• (3). New power system construction maturity period (2050–2060). The main task in this period is to explore the synergistic development model of electricity and hydrogen in accordance with the resources and economic development patterns of each region. By playing the role of energy hub for electricity-hydrogen synergy, we can achieve the goal of carbon neutrality in each field.

Author Contributions

Project administration, H.Y.; methodology, writing—review and editing, W.Z.; supervision, J.K.; formal analysis and resources, T.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Global power structure development trends [14].

Figure 2. Basic power output characteristics, including controllable power, wind power, photovoltaic power, power load.

Figure 3. (a) Electricity supply and demand in a power system with a 45% penetration of new energy sources; (b) Net power supply and demand in a power system with a 45% penetration of new energy sources; (c) Power deficit in a typical winter week.

Figure 4. Comparison of energy storage time and capacity [29].

Figure 5. Matching energy storage technology with application scenarios [30].

Figure 6. Comparison of hydrogen storage capacity and storage duration of hydrogen storage technology [35,36,37,38,39,40,41,42,43,44].

Figure 7. Diagram of the distribution of salt caves in China [74].

Table 1

Advantages, disadvantages, and application areas of hydrogen storage technology.

Storage Method	Advantages	Disadvantages	Application Areas
High-pressure gaseous [35]	Mature technology, low cost, fast charging and discharging	Small storage capacity, high energy consumption and safety problems	Transportation, hydrogen refueling stations
Low-temperature liquid [36]	High bulk density and high hydrogen storage capacity	High requirements for conversion technology and storage materials, high costs	Aerospace, Vehicle Mounted
Organic liquid [37]	High storage density, recycling of hydrogen storage materials, low cost	High plant cost, low dehydrogenation efficiency and prone to side reactions	Chemical, Fuel
Liquid ammonia [38]	Can be used directly as fuel, mild storage conditions	Stronger corrosiveness	Industrial, combined heat and power supply
Methanol [39]	Good economy of application, easy storage and transportation	Not zero carbon emissions	Industrial, fuel, automotive
Metal Hydride [40]	High bulk density, easy to handle and transport	Low quality efficiency and immaturity	Hydrogen refueling stations, automotive
Inorganic compounds [41]	Easy activation, storage and transportation	Poor hydrogen storage capacity and reversibility	Laboratory phase
Metal adsorption [42]	High efficiency and easy dehydrogenation	High cost	Transportation
clathrate hydrates [43]	Low energy consumption, low cost, high safety	Low hydrogen storage density	Laboratory phase
Underground hydrogen storage [44]	Good physical properties, simple operation, rapid charging and discharging, low cost	Difficult to build storage depots	Seasonal Storage
Hydrogen blending of natural gas	Expanding hydrogen application scenarios and scale, relieving the tight supply of natural gas	Risk of hydrogen embrittlement, hydrogen penetration and corrosion of gas meters and burners	Fuel, combined heat and power supply, transportation

Table 2

Comparison of advantages and disadvantages of underground hydrogen storage [45,46,47].

Storage Type	Advantages	Disadvantages
depleted hydrocarbon reservoirs	large gas storage and peaking capacity for seasonal peaking and strategic reserves	high requirements for ground treatment and high mat air volume
salt caverns	large gas storage capacity, second only to depleted hydrocarbon reservoirs	low geological awareness, long construction period and high cost of building the reservoir
aquifers	high ratio of working air volume, capable of fully recovering mat air	few developable pits, and manual excavation is limited by geological conditions
caverns	good sealing, less bedding gas volume, flexible injection and extraction conversion	Small volume, slow expansion speed, high construction cost

Table 3

Salt-cavern hydrogen storage projects in operation [62,63,64,65].

Project Name	Operating Conditions/MPa	Depth/Meters	Capacity/m3
Teesside (UK)	4.5	365	210,000
Clemens (USA)	7–13.7	1000	580,000
Moss Bluff (USA)	5.5–15.2	1200	566,000
Spindletop (USA)	6.8–20.2	1340	906,000

Table 4

Parameters of four types of underground hydrogen storage methods [75].

Technical Specifications	Salt Caverns	Depleted Hydrocarbon Reservoirs	Caverns	Aquifers
Operating pressure/MPa	13.789	13.755	13.789	13.755
Volume/m3	580,000	676,941	580,000	676,941
Depth/meters	1158	1403	1158	1403
Operating capacity/ton	1912	1912	1912	1912
Air cushion ratio/%	30	50	30	50
Air cushion capacity/ton	574	956	574	956
Total reserves/ton	2486	2868	2486	2868

Table 5

Levelized cost comparison of four types of underground hydrogen storage [75,76,77,78,79].

Technical Specifications	Salt Caverns	Depleted Hydrocarbon Reservoirs	Caverns	Aquifers
Cost of gas cushion/million USD	11.228	21.492	11.228	21.492
Cost of storage construction/million USD	23.34	-	48.72	-
Cost of compression/million USD	27.539	18.36	27.539	18.36
Hydrogen injection rate/(kg/h)	2960	2487	2960	2487
Hydrogen absorption rate/(kg/h)	4920	2487	4920	2487
Compressor power/(kWh/kg)	2.2	2.2	2.2	2.2
Operating days/(days/year)	350	350	350	350
Compressor capacity factor/%	96	96	96	96
Electricity price/(USD/KW·h)	5	5	5	5
Pipeline Costs/(USD/ton)	4.39	6.26	4.39	6.26
Reservoir life/year	30	30	30	30
Discount rate/%	10	10	10	10
Total Cost/million USD	63.255	40.107	89.644	40.999
Levelized Costs/(USD/kg)	1.61	1.23	2.77	1.29

Table 6

Completed Gas Storage in China [89].

The Company	Underground Gas Storage	Capacity/108 m3	Working Capacity/108 m3
China National Petroleum Corporation	Dagang, Huabei, Banan, Suqiao, Shuang 6, Hutubi, Xiangguosi, Shaanxi 224, Jintan, Liuzhuang	408	190
China Petrochemical Corporation	Chubun original 96, Chubun original 23, Jintan	122	43
Gang Hua Gas Company	Jintan	10	6

Table 7

Completed Gas Storage in China [90].

Underground Gas Storage	Capacity/108 m3	Working Capacity/108 m3	Stage
Kunming, Yunnan	0.852	0.338	construction
Jianghan	48.09	28.04	construction
Chuzhou	31.3	18.5	construction
Huai’an	6.24	5.554	field test
Shandong Tai’an	5	2.38	field test
Shandong Heze	0.39	0.33	field test
Hunan Hengyang	19.34	7.35	pre-feasibility
Yunying, Hubei	8.67	5.78	pre-feasibility
Pingdingshan, Henan	19.17	10.57	pre-feasibility

Table 8

Technical parameters of a solid-state hydrogen storage power plant [102,103,104,105].

Technical Specifications	2020	2030	2050
Hydrogen absorption rate/(Nm3/kWh)	0.22	0.24	0.34
Hydrogen release rate/(Nm3/kWh)	0.9	1.2	1.2
Hydrogen absorption pressure/Mpa	<4	≤3	≤2
Hydrogen release pressure/Mpa	≥0.3	≥0.3	≥0.3
Hydrogen purity/%	99.95	99.97	99.99
Purity of hydrogen supply/°C	10–80	0–650	0–1000
Cycle life/times	3000	4500	6000
Hydrogen storage density/(kg/m3)	50	70	100

Table 9

Levelized cost of solid-state hydrogen storage vs. high pressure gaseous hydrogen storage [107].

Technical Specifications	High Pressure Gaseous Hydrogen Storage	Solid State Hydrogen Storage
Cycle life/times	1000	1000
Hydrogen storage capacity/(kg/times)	5.6	4
Cost of storage tanks/USD	4300	5300–6700
Hydrogen compression pressure/Mpa	70	6
Hydrogen compression costs/USD	1400	500
Total Cost/USD	5700	8000–12,100
Levelized Costs/(USD/kg/times)	1	2–3

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