Energy is a vital material foundation for human survival and social development, and a crucial cornerstone for national economy, national security, and sustainable development. With the development of human society, the demand for energy is increasing daily, while the ecological environment continues to deteriorate, particularly due to increasingly severe global climate change caused by greenhouse gas emissions. This contradiction has become more acute in recent years. Currently, my country has become a major global energy producer and consumer, and its energy demand continues to grow. Therefore, adjusting the energy structure is urgently needed: on the one hand, developing new energy sources to meet demand; on the other hand, utilizing renewable energy sources rationally and effectively.
Renewable energy sources include wind, solar, biomass, ocean energy, and small hydropower, which are primary energy sources typically converted into electricity. Energy storage technology plays a crucial role in the development and utilization of renewable energy. As is well known, wind and solar energy are discontinuous and unstable during use, requiring energy storage systems for stabilization before grid connection. Energy storage systems are also essential for wind turbines operating in off-grid mode. Furthermore, uneven energy consumption can be addressed through peak shaving and valley filling, improving energy efficiency. Researching energy storage technology is of significant economic and social importance for promoting large-scale renewable energy generation, improving the efficiency of alternative energy power plants, and safeguarding national energy security. Developed industrialized countries highly value the research and development of large-scale energy storage systems; for example, Japan's "New Sunshine Project," the United States' "DOE Project," and the European Union's "Framework Plan" all prioritize energy storage technology in their research. China also attaches great importance to the development of energy storage technology. High-efficiency energy conversion and energy storage technology has been listed as a priority development area in the National Torch Program. The China Energy Storage Battery Industry-University-Research Technology Innovation Alliance was established in November 2009. Furthermore, according to the "Key Points of the Development Plan for New Energy and Renewable Energy Industries (2000-2015)" issued by the State Economic and Trade Commission, by 2015, the annual production capacity of small wind turbine generators will reach 50,000 units, with a total cumulative output of 340,000 units, a total installed capacity of 105,000 kW, and a total output value of approximately 900 million yuan. Sales of energy storage systems should exceed 100 million yuan, which will undoubtedly drive the development of the energy storage industry.
Driven by strong social development demands and a huge potential market, new energy storage systems based on new concepts, materials, and technologies are constantly emerging. Energy storage technology is developing towards large-scale, high-efficiency, long-life, low-cost, and pollution-free directions.
I. Classification and Development Trends of Energy Storage Technologies
To date, various energy storage technologies have been proposed and developed to meet different applications and needs. Global energy storage technologies mainly fall into several categories, including physical energy storage, chemical energy storage (such as sodium-sulfur batteries, vanadium redox flow batteries, lead-acid batteries, lithium-ion batteries, supercapacitors, etc.), electromagnetic energy storage, and phase change energy storage.
1. Physical energy storage
Physical energy storage technologies mainly include pumped-storage hydroelectric power, compressed-air energy storage, and flywheel energy storage. Compared to chemical energy storage, physical energy storage is more environmentally friendly and greener, utilizing natural resources. Pumped-storage hydroelectric power stations ( PSHs ) are equipped with upstream and downstream reservoirs. During periods of low load, the equipment operates as a motor, pumping water from the downstream reservoir to the upstream reservoir for storage. During periods of high load, the equipment operates as a generator, using the water stored in the upstream reservoir to generate electricity (see Figure 1). Due to its mature technology, pumped-storage hydroelectric power stations have become the most widely used energy storage technology in power systems. Currently, China has approximately 11,400 MW of installed capacity under construction for pumped-storage hydroelectric power stations, and it is projected that the total installed capacity will reach approximately 17,500 MW by the end of 2010.
Compressed air energy storage ( CAES) is a type of gas turbine power plant used for peak shaving. It utilizes surplus electricity generated during off-peak hours to compress air, which is then stored in a high-pressure sealed facility with a typical storage pressure of 7.5 MPa . During peak electricity demand, the compressed air is released to drive the gas turbine. The world's first commercial CAES power plant was the Huntdorf plant built in Germany in 1978, with an installed capacity of 290 MW and a conversion efficiency of 77%. To date, it has been started over 7,000 times, primarily used for hot standby and load smoothing. Compared to pumped storage power plants, CAES power plants offer greater site flexibility, eliminating the need for surface reservoirs and easily meeting terrain requirements. Currently, compressed air energy storage power plants are widely used in some developed countries.
Flywheel energy storage ( FW ) achieves charging and discharging through the interconversion of mechanical and electrical energy. It uses a high-speed rotating flywheel core as the medium for storing mechanical energy, and utilizes an electric motor/generator and an energy conversion control system to control energy input and output. Flywheel energy storage has very high requirements for the raw materials and technology used in its manufacture, and its rapid development only began in the 1990s, with applications in uninterruptible power supplies (UPS/EPS), power grid peak shaving, and frequency control. Research in this area in China is still in its early stages.
Physical energy storage, such as pumped hydro storage and compressed air storage, has advantages such as large scale, long cycle life, and low operating costs. However, it requires specific geographical conditions and sites, resulting in significant limitations in construction. Furthermore, it incurs high initial investment costs, making it unsuitable for small-capacity off-grid power generation systems. From a developmental and practical perspective, chemical energy storage has a broader application prospect than physical energy storage.
2. Chemical energy storage—Lithium-ion battery energy storage is currently the most feasible technological route.
Lead-acid batteries are the oldest and most mature chemical energy storage method, with a history of over 100 years. They are widely used in automotive starting power supplies, electric bicycle or motorcycle power supplies, backup power supplies, and lighting power supplies. The electrodes of lead-acid batteries are mainly made of lead and its oxides, and the electrolyte is a sulfuric acid solution. During charging, the positive electrode is mainly composed of lead dioxide, and the negative electrode is mainly composed of lead; during discharging, both the positive and negative electrodes are mainly composed of lead sulfate. Lead-acid batteries have good reliability, readily available raw materials, and are inexpensive . However, their optimal charging current is around 0.1C, and the charging current cannot exceed 0.3C . The discharging current is generally required to be between 0.05C and 3C, making it difficult to meet the large-scale energy storage requirements that simultaneously balance power and capacity. Furthermore, lead-acid batteries cannot be deeply charged or discharged; 100% discharge conditions have a significant impact on battery life (the cycle life of a battery under full charge and discharge conditions is less than 300 cycles). Also, water decomposes into hydrogen and oxygen gases at the end of charging, requiring frequent addition of acid and water, resulting in heavy maintenance work. Therefore, they are not suitable for application in the smart grid field.
Currently, the main chemical power sources that can be applied to the field of smart grids are sodium-sulfur batteries, flow batteries, and lithium-ion batteries.
Sodium-sulfur (NaS) batteries were first invented and published by Ford Motor Company in 1967. They use metallic sodium as the negative electrode, sulfur as the positive electrode, and a ceramic tube as the electrolyte membrane. At a certain operating temperature, sodium ions permeate through the electrolyte membrane and undergo a reversible reaction with sulfur, releasing and storing energy (see Figure 2). NaS batteries have high specific energy (theoretically up to 760Wh/kg), can be charged and discharged at high currents, and have a long service life (10-15 years), making them one of the most economical and practical energy storage methods currently available. Their main applications are in power plant load leveling, UPS emergency power supplies, and instantaneous compensation power supplies. Japan is currently the leading country in NaS battery technology, with an annual production exceeding 100MW as of 2007. In 2008, the Futamata Wind Power Plant in Japan introduced 17 NaS battery systems from NGK, with a storage capacity of 34MW. This successfully suppressed power fluctuations in the 51MW wind turbine, enabling planned power output and laying the foundation for grid-connected wind power generation. In 2009, the Shanghai Institute of Ceramics in my country successfully developed key technologies for 100kW-level sodium-sulfur single-cell batteries, becoming the second country in the world, after Japan, to master the core technology of large-capacity sodium-sulfur single-cell batteries. The developed sodium-sulfur battery is shown in Figure 3. However, sodium-sulfur batteries require high temperatures of 350°C to melt sulfur and sodium, necessitating additional heating equipment to maintain the temperature. Furthermore, overcharging is dangerous, thus presenting shortcomings in terms of safety and maintenance-free operation.
Research on vanadium redox flow batteries began in 1984 with the Skyllas-kazacos research group at the University of New South Wales, Australia. It is a redox regenerative fuel cell energy storage system based on vanadium metal, and its working principle is illustrated in Figure 4. The flow battery uses a proton exchange membrane as the separator of the battery pack. The electrolyte solution flows parallel across the electrode surface and undergoes an electrochemical reaction. Current is collected and conducted through dual electrode plates, converting the chemical energy stored in the solution into electrical energy. The rated power and rated capacity of the flow battery system are independent. The power depends on the battery stack, while the capacity depends on the electrolyte. Battery capacity can be increased by increasing the amount of electrolyte or increasing the electrolyte concentration, and "instant recharging" can be achieved by replacing the electrolyte. Flow batteries have an theoretically unlimited shelf life, long storage life, no self-discharge, and can be discharged to 100% depth without damage. These characteristics make flow batteries one of the preferred energy storage technologies. Currently, flow energy storage technology has been demonstrated in developed countries such as the United States, Germany, Japan, and the United Kingdom, while my country is still in the research and development stage. The challenge of vanadium redox flow batteries lies in the fact that the total vanadium ion concentration used is usually below 2 mol/L, resulting in a specific energy of only 25-35 Wh/kg. The electrolyte storage tank is large and difficult to manage. Moreover, the pentavalent vanadium in the positive electrode liquid is prone to precipitating vanadium pentoxide when left to stand or at a temperature above 45°C, which affects the battery's lifespan.
In comparison, lithium-ion battery energy storage is currently the most feasible technological route for energy storage product development. Lithium-ion batteries have advantages such as high energy density, low self-discharge, no memory effect, wide operating temperature range, rapid charging and discharging, long lifespan, and no environmental pollution, earning them the title of "green batteries." Table 1 compares lead-acid batteries, sodium-sulfur batteries, flow batteries, and lithium-ion batteries with lithium titanate as the negative electrode. It can be seen that lead-acid batteries have a shorter lifespan, sodium-sulfur batteries suffer from high operating temperatures, flow batteries have lower energy density, while lithium-ion batteries with lithium titanate as the negative electrode exhibit comprehensive performance advantages. Figure 5 is a schematic diagram of the working principle of a lithium-ion battery with lithium titanate as the negative electrode.
Because lithium titanate is a zero-strain material, it avoids structural damage caused by the expansion and contraction of electrode materials, thus significantly improving the lifespan of lithium-ion power batteries. Furthermore, due to its high operating potential, lithium titanate is unlikely to form lithium dendrites on the negative electrode even during overcharging, greatly enhancing the safety of lithium-ion power batteries. These improvements make the application of lithium-ion power batteries in energy storage possible, and lithium-ion power battery energy storage technology using lithium titanate as the negative electrode is currently a hot topic of development both domestically and internationally. In 2008, Altairnano in the United States developed a 1MW lithium titanate energy storage battery system, which, after trial operation, showed that it could output 250kWh of energy with an energy conversion efficiency greater than 90%. In 2010, Toshiba announced at its annual management policy meeting that it would develop a super lithium-ion battery (SCiB) for energy storage using lithium titanate as the negative electrode material. Based on the successful commercialization of high-power SCiB lithium titanate batteries, Toshiba's SCiB energy storage batteries are expected to enter the market soon. After five years of technological development, CITIC Guoan Mengguli Power Technology Co., Ltd. in China developed a 35Ah battery for energy storage applications in 2010.
The battery has a cycle life of nearly 8,000 cycles, can be charged and discharged at a 5C rate, and has excellent safety performance. The company is currently working with partners to develop a megawatt-level energy storage system, and the product is expected to be available on the market in 2011.
Besides lithium-ion batteries with lithium titanate as the negative electrode, which can be used in energy storage, the application of lithium iron phosphate (LFP) cathode materials has significantly improved the lifespan and safety of traditional carbon-based LFP batteries, making them suitable for energy storage as well. In 2010, Sony launched a 1.2 kWh LFP energy storage battery module with a maximum output power of 2.5 kW. However, LFP batteries currently suffer from significant consistency issues. Even if a single cell can achieve a lifespan of over 2000 cycles, the lifespan of assembled batteries is greatly reduced. Furthermore, the core patents for LFP materials are held by several large international companies, posing patent disputes for LFP battery production. Therefore, using lithium titanate batteries for energy storage is currently the most feasible technological approach.
3. Other energy storage technologies
Superconducting electromagnetic energy storage converts electrical energy into magnetic energy and stores it in the magnetic field of a superconducting coil. Charging and discharging of the energy storage device are achieved through electromagnetic inversion. Because the coil has no resistance in the superconducting state, energy loss in superconducting energy storage is extremely low. However, since superconductivity requires the coil to be at extremely low temperatures, and low temperatures consume a large amount of energy and are difficult to miniaturize, this technology is currently in the research and development stage.
Phase change energy storage utilizes the phase change of certain substances at specific temperatures to absorb or release energy, such as ice for cooling and water for heating. It can be applied to fields such as central air conditioning and is an emerging energy storage technology.
II. Market Prospects for Energy Storage Technology—Lithium-ion Batteries Will Become the Ideal Choice
According to data from the Wind Energy Professional Committee of the China Renewable Energy Society, China's (excluding Taiwan) cumulative installed wind power capacity reached 25,805.3 MW in 2009. Based on calculations by Guodian Corporation, the energy storage industry in China has a potential market of approximately 5,161 to 7,742 MW. By 2020, both wind and solar power installed capacities in China will reach tens of millions of kilowatts, and the energy storage battery market will reach 70 billion RMB. Energy storage products will become the most worthwhile investment area and the most capital-intensive market sector in the future.
Lithium-ion batteries are one of the most significant achievements in high-tech research over the past decade, representing the most advanced level of chemical power source development. Due to the significant advantages of this new system, including high specific energy, long cycle life, and environmental friendliness, it has become the main power source for various advanced portable electronic products, possessing an absolute advantage in mobile applications. Currently, the global annual demand for lithium-ion batteries has reached 1.3 billion units, with annual sales of $27 billion, undoubtedly making it one of the leading players in the rechargeable battery market. With the research and development of new lithium-ion battery materials, innovation in battery manufacturing technology, and the participation of numerous research institutions and enterprises, the performance of lithium-ion batteries is continuously improving, battery costs are decreasing, and battery safety is greatly enhanced. Lithium-ion batteries are gradually demonstrating their application advantages in the electric vehicle field. Fuji Keizai of Japan believes that lithium-ion batteries will begin to gradually replace nickel-metal hydride batteries starting in 2011, and the mainstream technology route of lithium-ion batteries is undeniable. With the development and application of new materials such as nano-lithium titanate and nano-lithium iron phosphate, lithium-ion batteries will become an ideal choice for a series of major high-tech applications such as clean transportation and photovoltaic energy storage. At present, the State Grid Corporation of China is actively carrying out a pilot project of a 10MW-level lithium-ion battery energy storage system, which will trigger a new round of investment in related manufacturing equipment and plants. At the same time, many new entrants to the lithium-ion power battery and material industry will intensify the technological competition in related fields, and large-capacity lithium-ion battery energy storage power stations will gradually emerge on this basis.
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