1、 Definition of Long Term Energy Storage

From a global perspective, there is no unified definition for long-term energy storage internationally. Different countries and institutions have provided different definitions of long-term energy storage based on energy structure, power system, policies, and market characteristics. The definition of the US Department of Energy (DOE) is an energy storage system that operates continuously (discharged) at rated power for more than 10 hours and has a service life of 15 to 20 years; Sandia National Laboratory in the United States believes that long-term energy storage technology is an energy storage technology with a continuous discharge time of not less than 4 hours; The California Energy Commission (CEC) and the Advanced Research Projects Agency (ARPA E) of the US Department of Energy define energy storage systems lasting 10 hours or more as long-term energy storage; The International Long Term Energy Storage Commission (LDESC) has proposed two definitions, namely long-term energy storage for 8-24 hours and energy storage technology for more than 24 hours.

Based on the current development status of renewable energy in the United States, it can be seen that since the Biden administration, renewable energy in the United States has developed rapidly. Multiple regions have gradually adopted 4-hour energy storage systems for peak electricity demand services, and some regions have formulated the "4-hour capacity rule", allowing energy storage systems that last for more than 4 hours to receive compensation in the capacity market or other contracts that provide capacity. Based on this rule, approximately 40% of the energy storage systems deployed in the United States from 2021 to 2022 will have a duration of 4 hours. However, due to the aging of the current power grid system and equipment in the United States, it has begun to face problems such as grid restrictions and backlogged grid connections. Faced with the continuous increase in electricity demand and the high proportion of renewable energy integration, the United States urgently needs energy storage for more than 4 hours to improve grid efficiency and reliability to accommodate renewable energy.

From the perspective of power grid structure and electricity consumption scale, the situations in China and the United States are relatively similar; However, in terms of the scale and technological level of renewable energy development, China far exceeds the current level of the United States in two key indicators: installed capacity and power generation. Therefore, based on the current situation and development trend of renewable energy and power systems in China, we believe that the definition of long-term energy storage suitable for China's national conditions is: large-scale low-cost energy storage technology that can achieve continuous discharge for 4 hours or more, or several days or months at rated power. Specifically, based on the development stage of China's new energy system and the overall demand for flexible energy storage, long-term energy storage technologies can be divided into:

(1) Medium to long-term energy storage mainly refers to energy storage systems that operate (discharge) continuously for 4-10 hours at rated power;

(2) Long term energy storage mainly refers to an energy storage system that operates (discharges) continuously for 10 hours to 1 week at rated power;

(3) Ultra long term energy storage mainly refers to energy storage systems that operate (discharge) continuously for more than one week at rated power.

2、 Main features and functions of long-term energy storage technology

At present, long-term energy storage can be divided into four main lines: physical energy storage, chemical energy storage, thermal energy storage, and hydrogen energy storage. Specific technologies include pumped storage, compressed air energy storage, gravity energy storage, flow batteries, molten salt heat storage, and various hydrogen storage technologies. With the large-scale development of renewable energy, long-term energy storage technology will be a key link in future new energy systems, and it has the following main characteristics:

One is the long time scale. Long term energy storage has the ability to charge and discharge for a long time, which can achieve long-term, cross day, cross week, cross month, and even cross season power regulation within a day. Moreover, as the proportion of renewable energy increases, the total energy storage time in the power system also increases, and the proportion of long-term energy storage increases.

Secondly, it has a large energy storage capacity. Long term energy storage has a large energy storage capacity, and power and capacity can generally be decoupled. It can store and release a large amount of electricity according to different power and capacity requirements.

Thirdly, the unit cost is low, and large-scale long-term energy storage can significantly reduce the cost of electricity per kilowatt hour. Taking pumped storage as an example, its cost per kilowatt hour is only 0.21-0.25 yuan/kWh.

The fourth is technological diversity. Long term energy storage covers multiple technological routes, each with its unique working principle and applicable scenarios, which can be selected and optimized according to actual needs. Different technologies can complement each other and together form a more comprehensive energy storage system.

It is generally believed that the increasing proportion of renewable energy installed capacity and power generation will gradually lead to fundamental changes in the physical form and technological framework of the energy system, especially the power system. In this process, the proportion of long-term energy storage will also gradually expand. The International Long Term Energy Storage Council (LDES) announced at the 26th Annual Summit of the United Nations Framework Convention on Climate Change in 2021 that long-term energy storage will become the "lowest cost flexible solution" when renewable energy sources account for 60% to 70% of electricity generation. It is predicted that by 2030, the cumulative installed capacity of long-term energy storage worldwide will reach 150-400GW, and by 2040, the cumulative installed capacity of long-term energy storage will further increase to 1.5-2.5TW.

China has become a major renewable energy country. By the end of 2023, China's installed capacity of renewable energy will exceed 1.45 billion kilowatts, accounting for over 50% of the country's total electricity installed capacity and surpassing that of thermal power, ushering in a historic moment. Long term energy storage will play a more important role in this new energy system with large-scale integration of new energy:

One is to provide long-term regulation capability for the power system and support energy structure adjustment. The use of long-term energy storage technology can support the dynamic balance of power systems across days and weeks at different spatial and temporal scales.

The second is to address the energy demand under extreme weather conditions and enhance the safety characteristics of the power grid. The use of long-term energy storage and ultra long term energy storage technology can improve the safe and stable operation of power systems under extreme weather conditions.

The third is to improve the flexibility of the power system and enhance its flexibility. Explore and enhance the value of short-term energy storage, increase the deployment of multi time scale energy storage technologies, achieve coordinated cooperation between source, grid, load, and storage, adapt to the development of new power systems, and provide support for the planning and construction of new energy systems and new power systems.

3、 Development status and main suggestions:

During the 14th Five Year Plan period, China has achieved outstanding results in the field of long-term energy storage technology. Created multiple international first demonstration projects in the fields of compressed air energy storage, flow batteries, and hydrogen energy; Single unit pumped storage units are moving towards 300MW AC excitation variable speed pumped storage units; The international first 300MW advanced compressed air energy storage technology has been connected to the grid; The thermal storage technology has achieved the commissioning of the world's first electric thermal molten salt energy storage test station and a 660MW coal-fired power unit coupled with steam molten salt thermal storage peak shaving. In new energy storage construction projects, 100MW level and medium to long-term energy storage have become the norm. It is expected that the installed capacity of long-term energy storage in China will be about 23 million kilowatts by 2030, accounting for about 20% of the total installed capacity of new energy storage during the same period; The installed capacity of ultra long term energy storage in 2060 is about 150 million kilowatts.

In terms of policies and regulations, the Chinese government has successively issued documents such as the "Guiding Opinions on Accelerating the Development of New Energy Storage" and the "Opinions on Deepening the Reform of the Power System and Accelerating the Construction of a New Power System" to promote the large-scale application of high-capacity long-term energy storage technology. The National Energy Administration pointed out in the "Blue Book on the Development of New Power Systems" that China aims to meet the demand for balanced regulation on a daily or above time scale from 2030 to 2045, and to operate multiple types of energy storage systems covering the entire cycle from 2045 to 2060.

Building a new energy system is a long-term systematic project that requires a foundation in the country's energy resource endowment and steady progress on the basis of safe and reliable substitution of new energy. China has a vast territory, and the development of renewable energy varies in different provinces and cities, with different demands for resources and electricity. For energy storage, some regions adopt advanced development models, while others choose to deploy various energy storage technologies comprehensively. Some regions focus on developing energy storage technologies of a certain type or application scenario. Therefore, for the future development of long-term energy storage technology and industry in China, it is necessary to leverage the linkage and regional coordination mechanisms between the national and local governments in policy planning, and work together to promote the healthy development of long-term energy storage technology. Specific measures are suggested as follows:

One is to adhere to top-level design and scientific planning. Scientifically carrying out long-term energy storage planning that combines short-term and long-term approaches is conducive to solving the uncertainty of stakeholders, enhancing market confidence, guiding the establishment of a long-term energy storage supply chain, and providing guarantees for the high-quality development of low-cost long-term energy storage. Accelerate the establishment of a world-class long-term energy storage national laboratory, gather resources and strength from the entire industry, rely on major scientific research infrastructure, collaborate with industry associations, research institutes and universities, establish strategic research and collaborative innovation platforms, and continue to carry out the research and development of energy storage technology. Build a standard system for long-term energy storage technology, optimize the standard system in a timely manner based on the industry's development level and emerging application scenarios, and enhance the overall technical level of the industry.

The second is to adhere to the coordinated development of renewable energy and long-term energy storage. Conduct scientific research on the scale and proportion of renewable energy allocation and storage, adapt to local conditions and times, and coordinate the layout of long-term energy storage technologies with different durations and scales. Fully leverage the linkage mechanism between national and local governments, actively guide the demonstration and industrialization of long-term energy storage technology in energy bases, and explore and improve the coordinated and integrated development model of renewable energy and long-term energy storage. At the same time, it is necessary to clarify tax credits, carbon pricing, and greenhouse gas emission reduction targets as early as possible in policies, guide the convergence of various positive factors such as the market, capital, and talent, and jointly promote development.

The third is to further improve the long-term energy storage market mechanism. Firstly, establish a comprehensive energy storage value evaluation system, explore a graded capacity bidding mechanism, and improve the electricity pricing mechanism that includes long-term energy storage network load storage projects. Secondly, establish and improve a long-term energy storage policy guarantee mechanism, gradually improving project management, technological innovation, market environment, pricing mechanism, industrial development, and other aspects. Finally, in the demonstration application stage, direct technical support and support measures can also be discussed, including technology transfer incentives, treasury bond priority support and loan guarantees, as well as direct compensation for the services and resources provided by the long-term energy storage system, so as to improve the confidence of investors in this field in deploying long-term energy storage.