As the decarbonization process of the global power system accelerates, the installed capacity and power proportion of new energy sources such as wind power and photovoltaics continue to increase, and the demand for reliable power support in the power system continues to increase. In recent years, long-term energy storage began to enter the public eye, and quickly triggered a boom in technology research and capital investment., long-term energy storage began to enter the public eye, and quickly triggered a boom in technology research and capital investment.
What is long-term energy storage?
So far, long-term energy storage is still a new thing, and there is no unified definition for long-term energy storage worldwide.
In recent years, long-term energy storage began to enter the public eye, and quickly triggered a boom in technology research and capital investment., the “Issue Brief – Long-duration energy storage” issued by Sandia National Laboratory in the United States believes that long-term energy storage is an energy storage technology with a continuous discharge time of not less than 4 hours. In the last two years, the U.S. Department of Energy will issue a report on supporting long-term energy storage, which defines long-term energy storage as energy storage technologies with a continuous discharge time of not less than 10 hours and a service life of 15-20 years. More scholars have published articles in journals, defining long-term energy storage as energy storage technology that spans from day to season. In order to distinguish it from the 2-hour energy storage system currently being built on a large scale in China, some practitioners also classify energy storage technologies of 4 hours or more as long-term energy storage.
Why develop long-term energy storage?
For this question, we must first start with why energy storage is developed.
A power system without energy storage is relatively unstable. However, at present, the power system mainly based on thermal power still has reliable power support, because the thermal power generated on demand ensures the operation of the power system. However, with the in-depth implementation of the carbon dioxide emission reduction policy in the future, the proportion of highly volatile renewable energy sources (mainly solar and wind energy) will increase to a certain extent, and electricity will be in short supply at night with no wind and no light. In order to improve the reliability of the power system, the development of supporting energy storage is imperative.
So, why develop long-term energy storage?
For example, if the hydroelectric power station regulates the storage capacity of the reservoir for only two hours, then the second half of the night will still be spent in darkness; even if the storage capacity of the reservoir is sufficient for ten hours, we have to pray that the next day will be sunny. Or windy, if the windless cloudy day lasts for two or three days, we have to prepare a larger reservoir, that is, long-term energy storage.
Of course, the future power system will also have other power sources other than wind. Considering the complementary characteristics of power generation, coupled with a long-term energy storage of an appropriate duration, the power system will become a fully regulated hydropower station, which can be worry-free all year round.
Industry experts predict that by 2040, 85-140 billion kWh of long-duration energy storage will be deployed globally.
What long-term energy storage technologies are used in China?
Based on the type of technology, long-term energy storage can be classified as mechanical, electrochemical, thermal storage and chemical. Next, we will introduce several long-term energy storage technologies commonly used in China, namely pumped hydro storage, liquid flow batteries, molten salt thermal storage and compressed air energy storage.
Pumped hydro storage
Pumped storage has two reservoirs, upper and lower. For energy storage, water is pumped to the upper reservoir with electrical energy, which is converted into gravitational potential energy; for power generation, water is released to the lower reservoir to drive the turbine, and gravitational potential energy is converted into electrical energy. Pumped storage power plants have a large construction scale and long construction period, but are able to give more energy to the power system. At present, the individual capacity of pumped storage power plants is 300,000 to 400,000 kilowatts, with a total installed capacity of 300,000 to 3,600,000 kilowatts, and the storage time is generally 4-10 hours.
Liquid flow battery
Liquid flow battery refers to the placement of electrolyte and electric stack separately, and the reversible chemical reaction occurs when the positive and negative electrolyte flows through the electric stack to realize the mutual transformation of electric and chemical energy. The construction cycle of liquid flow battery is short, and by adjusting the capacity of the stack or electrolyte, the power generation and energy storage capacity can be amplified. The current installed capacity of flow battery is 10-100MW, and the energy storage time is generally 2-6 hours.
Molten Salt Thermal Storage
Molten salt thermal storage achieves energy storage by heating molten salt. In the energy release process, high temperature molten salt heat exchange generates high temperature and high pressure steam to drive steam turbines for power generation. Molten salt energy storage is large in scale and has a long service life, and at this stage it is mainly used in CSP power plants. At present, the installed capacity of molten salt storage is 10-100MW, and the storage time is generally 5-15 hours.
Compressed air long-term energy storage
Long-term energy storage of compressed air generally uses salt caverns or containers to store high-pressure air. During the energy storage process, the compressor compresses and stores the air, converting electrical energy into internal air energy; when releasing energy, the high-pressure air drives the turbine to generate electricity, converting internal air energy into electrical energy. Compressed air long-term energy storage power plant has a long construction cycle and high safety, which belongs to mechanical long-term energy storage technology. At present, the installed capacity of compressed air energy storage power station is 10-300 MW, and the energy storage time is generally 4-10 hours.
In addition, salt caverns or containers can be used to store hydrogen to achieve chemical energy storage across weeks or even seasons.
What other strange long-term energy storage technologies exist?
Many cutting-edge technologies of long-term energy storage are still in the early stage of R&D and demonstration, such as iron-air battery energy storage, hot rock energy storage, carbon dioxide battery energy storage, molten aluminum energy storage, etc.
Long-term energy storage of iron-air batteries
An iron-air battery is a type of metal-air battery whose basic principle is based on the reversible oxidation (rusting) of iron. When the iron-air battery is discharged, the oxygen in the air causes the iron to rust; when it is charged, the rust is reduced to iron by the action of an electric current. The only substance emitted by this process is oxygen. Form Energy, the company that invented the technology, claims that an energy storage system using the batteries can last up to 100 hours of discharge at a cost comparable to existing fossil-fuel power plants.
Hot rock long-term energy storage
The storage technology company is working on a technology to store electrical energy in the form of heat in stone, dubbed GridScale. GridScale energy storage consists of one or more sets of steel tanks filled with rubble. Crushed rock is basalt rock crushed to the size of a pea and can withstand repeated heating. Charging and discharging is done through a compressor and turbine system that pumps thermal energy from a tank full of cold stones into a tank full of hot stones. The stones in the cold jar get colder, while the hot jar gets hotter, reaching temperatures of up to 600 degrees Celsius. Energy can be stored in the stone for many days, and when electricity is needed by the grid, the thermal energy is returned from the hot tank to the cold tank through turbines, producing electricity. This is an efficient long-term energy storage solution due to low energy losses, and the number of storage tanks can be selected according to the required storage time and capacity.
Long-term energy storage of carbon dioxide batteries
Italy’s Energy Dome claims its carbon dioxide batteries can provide fast-response, cheap grid-scale energy storage. Its principle is similar to the compressed air long-term energy storage system. The generator compresses carbon dioxide into liquid and stores it, and stores the waste heat generated by the compressed gas; when it needs to support the power grid, the liquid carbon dioxide is evaporated with the previously stored waste heat to drive the second set of turbines. Electricity is generated, and carbon dioxide gas is returned to the air bag.
According to Energy Dome, the full-size carbon dioxide battery has a capacity of up to 25 megawatts and stores 100 to 200 megawatt hours of energy with an optimal charge/discharge cycle of 4 to 24 hours, making it ideal for daily and daytime cycles; its conversion efficiency reaches 75 %, the battery life is expected to be around 25 years, and the levelized cost (LCOS) can be as low as $50-60/MWh in a few years. Energy Dome compared its technology to compressed air energy storage (CAES) and liquid air energy storage (LAES) and concluded that carbon dioxide batteries have 10 to 30 times the energy storage density of CAES, although only two-thirds of that of LAES, But it does not require low temperature and can be stored at ambient temperature.
Long-term energy storage from molten aluminum
Swedish company Azelio’s experimental technology, long-term energy storage from molten aluminum, has been installed in a 580 MW solar complex in Morocco (510 MW CSP and 70 MW PV). The technology uses a phase change material made from recycled aluminum alloy as a thermal storage medium, one way using electricity to heat the recycled aluminum to 600°C through a resistive heater, and another way using a concentrated solar thermal system that melts the aluminum alloy with the heat generated by the system, thus storing the energy as heat. When the energy needs to be released, the alloy cools and re-solidifies, and the released heat is sent to the engine to generate electricity using heat transfer oil. Waste heat at a temperature of 65°C can also be sold to industrial users or to local district heating systems.
Molten Aluminum Long-Term Energy Storage System
According to Azelio, the process has a round-trip efficiency of 90 percent when both electricity and waste heat are used; the technology enables up to 13 hours of electricity storage, can be installed modularly and can provide heat on demand, in hot or cold weather It can be used in all climatic conditions, and the system life is expected to be up to 30 years. At the same time, the scheme can be scaled from 100 kilowatts to 100 megawatts; and the recycled aluminum storage medium can be reused again and again without reducing capacity over time.
All in all, more and more companies are joining the road of long-term energy storage technology research and development and competition. However, the future of long-term energy storage still needs continuous exploration and research.
Knowledeg about long-term energy storage
What is long-term energy storage?
So far, long-term energy storage is still a new thing, and there is no unified definition for long-term energy storage worldwide. In recent years, long-term energy storage began to enter the public eye, and quickly triggered a boom in technology research and capital investment., the “Issue Brief – Long-duration energy storage” issued by Sandia National Laboratory in the United States believes that long-term energy storage is an energy storage technology with a continuous discharge time of not less than 4 hours. In the last two years, the U.S. Department of Energy will issue a report on supporting long-term energy storage, which defines long-term energy storage as energy storage technologies with a continuous discharge time of not less than 10 hours and a service life of 15-20 years. More scholars have published articles in journals, defining long-term energy storage as energy storage technology that spans from day to season. In order to distinguish it from the 2-hour energy storage system currently being built on a large scale in China, some practitioners also classify energy storage technologies of 4 hours or more as long-term energy storage.Why develop long-term energy storage?
For this question, we must first start with why energy storage is developed. A power system without energy storage is relatively unstable. However, at present, the power system mainly based on thermal power still has reliable power support, because the thermal power generated on demand ensures the operation of the power system. However, with the in-depth implementation of the carbon dioxide emission reduction policy in the future, the proportion of highly volatile renewable energy sources (mainly solar and wind energy) will increase to a certain extent, and electricity will be in short supply at night with no wind and no light. In order to improve the reliability of the power system, the development of supporting energy storage is imperative. So, why develop long-term energy storage? For example, if the hydroelectric power station regulates the storage capacity of the reservoir for only two hours, then the second half of the night will still be spent in darkness; even if the storage capacity of the reservoir is sufficient for ten hours, we have to pray that the next day will be sunny. Or windy, if the windless cloudy day lasts for two or three days, we have to prepare a larger reservoir, that is, long-term energy storage. Of course, the future power system will also have other power sources other than wind. Considering the complementary characteristics of power generation, coupled with a long-term energy storage of an appropriate duration, the power system will become a fully regulated hydropower station, which can be worry-free all year round.What long-term energy storage technologies are used in China?
Based on the type of technology, long-term energy storage can be classified as mechanical, electrochemical, thermal storage and chemical. Next, we will introduce several long-term energy storage technologies commonly used in China, namely pumped hydro storage, liquid flow batteries, molten salt thermal storage and compressed air energy storage.Pumped hydro storage
Pumped storage has two reservoirs, upper and lower. For energy storage, water is pumped to the upper reservoir with electrical energy, which is converted into gravitational potential energy; for power generation, water is released to the lower reservoir to drive the turbine, and gravitational potential energy is converted into electrical energy. Pumped storage power plants have a large construction scale and long construction period, but are able to give more energy to the power system. At present, the individual capacity of pumped storage power plants is 300,000 to 400,000 kilowatts, with a total installed capacity of 300,000 to 3,600,000 kilowatts, and the storage time is generally 4-10 hours.Liquid flow battery
Liquid flow battery refers to the placement of electrolyte and electric stack separately, and the reversible chemical reaction occurs when the positive and negative electrolyte flows through the electric stack to realize the mutual transformation of electric and chemical energy. The construction cycle of liquid flow battery is short, and by adjusting the capacity of the stack or electrolyte, the power generation and energy storage capacity can be amplified. The current installed capacity of flow battery is 10-100MW, and the energy storage time is generally 2-6 hours.Molten Salt Thermal Storage
Molten salt thermal storage achieves energy storage by heating molten salt. In the energy release process, high temperature molten salt heat exchange generates high temperature and high pressure steam to drive steam turbines for power generation. Molten salt energy storage is large in scale and has a long service life, and at this stage it is mainly used in CSP power plants. At present, the installed capacity of molten salt storage is 10-100MW, and the storage time is generally 5-15 hours.Compressed air long-term energy storage
Long-term energy storage of compressed air generally uses salt caverns or containers to store high-pressure air. During the energy storage process, the compressor compresses and stores the air, converting electrical energy into internal air energy; when releasing energy, the high-pressure air drives the turbine to generate electricity, converting internal air energy into electrical energy. Compressed air long-term energy storage power plant has a long construction cycle and high safety, which belongs to mechanical long-term energy storage technology. At present, the installed capacity of compressed air energy storage power station is 10-300 MW, and the energy storage time is generally 4-10 hours. In addition, salt caverns or containers can be used to store hydrogen to achieve chemical energy storage across weeks or even seasons.What other strange long-term energy storage technologies exist?
Many cutting-edge technologies of long-term energy storage are still in the early stage of R&D and demonstration, such as iron-air battery energy storage, hot rock energy storage, carbon dioxide battery energy storage, molten aluminum energy storage, etc.Long-term energy storage of iron-air batteries
An iron-air battery is a type of metal-air battery whose basic principle is based on the reversible oxidation (rusting) of iron. When the iron-air battery is discharged, the oxygen in the air causes the iron to rust; when it is charged, the rust is reduced to iron by the action of an electric current. The only substance emitted by this process is oxygen. Form Energy, the company that invented the technology, claims that an energy storage system using the batteries can last up to 100 hours of discharge at a cost comparable to existing fossil-fuel power plants.Hot rock long-term energy storage
The storage technology company is working on a technology to store electrical energy in the form of heat in stone, dubbed GridScale. GridScale energy storage consists of one or more sets of steel tanks filled with rubble. Crushed rock is basalt rock crushed to the size of a pea and can withstand repeated heating. Charging and discharging is done through a compressor and turbine system that pumps thermal energy from a tank full of cold stones into a tank full of hot stones. The stones in the cold jar get colder, while the hot jar gets hotter, reaching temperatures of up to 600 degrees Celsius. Energy can be stored in the stone for many days, and when electricity is needed by the grid, the thermal energy is returned from the hot tank to the cold tank through turbines, producing electricity. This is an efficient long-term energy storage solution due to low energy losses, and the number of storage tanks can be selected according to the required storage time and capacity.Long-term energy storage of carbon dioxide batteries
Italy’s Energy Dome claims its carbon dioxide batteries can provide fast-response, cheap grid-scale energy storage. Its principle is similar to the compressed air long-term energy storage system. The generator compresses carbon dioxide into liquid and stores it, and stores the waste heat generated by the compressed gas; when it needs to support the power grid, the liquid carbon dioxide is evaporated with the previously stored waste heat to drive the second set of turbines. Electricity is generated, and carbon dioxide gas is returned to the air bag. According to Energy Dome, the full-size carbon dioxide battery has a capacity of up to 25 megawatts and stores 100 to 200 megawatt hours of energy with an optimal charge/discharge cycle of 4 to 24 hours, making it ideal for daily and daytime cycles; its conversion efficiency reaches 75 %, the battery life is expected to be around 25 years, and the levelized cost (LCOS) can be as low as $50-60/MWh in a few years. Energy Dome compared its technology to compressed air energy storage (CAES) and liquid air energy storage (LAES) and concluded that carbon dioxide batteries have 10 to 30 times the energy storage density of CAES, although only two-thirds of that of LAES, But it does not require low temperature and can be stored at ambient temperature.Long-term energy storage from molten aluminum
Swedish company Azelio’s experimental technology, long-term energy storage from molten aluminum, has been installed in a 580 MW solar complex in Morocco (510 MW CSP and 70 MW PV). The technology uses a phase change material made from recycled aluminum alloy as a thermal storage medium, one way using electricity to heat the recycled aluminum to 600°C through a resistive heater, and another way using a concentrated solar thermal system that melts the aluminum alloy with the heat generated by the system, thus storing the energy as heat. When the energy needs to be released, the alloy cools and re-solidifies, and the released heat is sent to the engine to generate electricity using heat transfer oil. Waste heat at a temperature of 65°C can also be sold to industrial users or to local district heating systems.