The discovery and use of fossil energy brought about a great leap forward in human history 
. 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 
. 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 
. 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 
. In addition to environmental risks, the finite nature of fossil resources is an important reason for the transition in the global energy mix 
. 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 
. 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 
. 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 
. 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.
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 
. 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.
Global power structure development trends 
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 
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 
. 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 
. 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.
Figure 2. Basic power output characteristics, including controllable power, wind power, photovoltaic power, power load.
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. 
. 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 3
a. 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 3
b 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 3
c 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.
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.
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 
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 
, electrochemical energy storage 
, 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 
. 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 
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 
; electrochemical energy storage is more efficient, although large-scale applications need to break through the life and safety issues 
; 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 
. 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.
Comparison of energy storage time and capacity 
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.
Matching energy storage technology with application scenarios 
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 
. 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 
. 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 
. 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.
Table 1. Advantages, disadvantages, and application areas of hydrogen storage technology.
|High-pressure gaseous 
||Mature technology, low cost, fast charging and discharging
||Small storage capacity, high energy consumption and safety problems
||Transportation, hydrogen refueling stations
|Low-temperature liquid 
||High bulk density and high hydrogen storage capacity
||High requirements for conversion technology and storage materials, high costs
||Aerospace, Vehicle Mounted
|Organic liquid 
||High storage density, recycling of hydrogen storage materials, low cost
||High plant cost, low dehydrogenation efficiency and prone to side reactions
|Liquid ammonia 
||Can be used directly as fuel, mild storage conditions
||Industrial, combined heat and power supply
||Good economy of application, easy storage and transportation
||Not zero carbon emissions
||Industrial, fuel, automotive
|Metal Hydride 
||High bulk density, easy to handle and transport
||Low quality efficiency and immaturity
||Hydrogen refueling stations, automotive
|Inorganic compounds 
||Easy activation, storage and transportation
||Poor hydrogen storage capacity and reversibility
|Metal adsorption 
||High efficiency and easy dehydrogenation
|clathrate hydrates 
||Low energy consumption, low cost, high safety
||Low hydrogen storage density
|Underground hydrogen storage 
||Good physical properties, simple operation, rapid charging and discharging, low cost
||Difficult to build storage depots
|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
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.
Comparison of hydrogen storage capacity and storage duration of hydrogen storage technology