The world's existing energy storage systems fall into five main categories

When it comes to energy storage, it is easy to think of batteries, but the existing battery technology is difficult to meet the requirements of grid-level energy storage. In fact, the market for energy storage is huge, with $122 billion expected to be spent on global energy storage projects between 2011 and 2021, according to market research firm PikeResearch. In the large-scale energy storage system, the most widely used pumped storage and compressed air energy storage and other traditional energy storage methods are also undergoing continuous improvement and innovation. Today, caixinenergy recommends an article that analyzes current global energy storage technologies and their impact on the power grid.

The existing energy storage systems are mainly divided into five categories: mechanical energy storage, electrical energy storage, electrochemical energy storage, thermal energy storage, and chemical energy storage. At present, pumped storage accounts for the highest proportion in the world, with its total installed capacity reaching 127GW, accounting for 99% of the total energy storage capacity, followed by compressed air energy storage with 440MW, and sodium sulfur battery with 316MW.

Existing energy storage systems around the world

1. Mechanical energy storage

Mechanical energy storage mainly includes pumped storage, compressed air storage, and flywheel storage.

Pumped storage: (1) the excess power grid when trough using electricity as the liquid water from the lower reservoir energy media to draw the terrain of the reservoir, high terrain at peak load of power grid in a reservoir water back into the reservoir under the driving turbine generators, efficiency is about 75% commonly, commonly known as into 4 out of 3, have daily regulation ability, used to load and set aside.

Disadvantages: difficult site selection, and its dependence on topography; Large investment cycle, high loss, including pumping storage loss + line loss; At the present stage, it is also restricted by China's electricity price policy. Last year, more than 80% of China's electricity storage and pumping were basking in the sun. Last August, the national development and reform commission issued a policy on electricity storage and pumping.

(2) the compressed air energy storage (CAES) : compressed air energy storage is the use of electric power system load is low battery, the motor drives the air compressor, air pressure into the airtight large underground cavern gas storage room, when the system is lack of electricity, the compressed air through the heat exchanger is mixed with oil or natural gas combustion, import the gas turbine power generation. Foreign research is more, technology is mature, our country begins a bit late, it seems that academician lu qiang is more to this respect research, what cold electricity couplet produces and so on.

Compressed air storage also has a peak-regulating function, suitable for large-scale wind farms, because the mechanical work generated by wind energy can directly drive the compressor rotation, reducing the intermediate conversion into electricity, thus improving efficiency.

Cons: one big drawback is low efficiency. The reason is that the temperature of the air rises when it is compressed and decreases as it expands. In compressed air some of the energy is lost as heat and must be reheated before it can expand. Natural gas is usually used as a heat source to heat air, which leads to a decrease in energy storage efficiency. It is also conceivable that large gas storage facilities, certain geological conditions, and dependence on the burning of fossil fuels are needed.

(3) flywheel energy storage: it USES a high-speed rotating flywheel to store energy in the form of kinetic energy. When energy is needed, the flywheel slows down and releases the stored energy. Flywheel energy storage technologies are basically available in China (but the gap with foreign countries is more than 10 years). The difficulty lies in the development of new products with different functions according to different USES. Therefore, flywheel energy storage power supply is a high-tech product, but its original innovation is not enough, which makes it difficult to obtain the support of national scientific research funds.

Disadvantages: low energy density, high self-discharge rate, such as stop charging, energy will be exhausted within a few to dozens of hours. Only suitable for some market segments, such as high-quality uninterruptible power supply.

2. Electrical energy storage

(1) supercapacitor energy storage: a double-layer structure composed of activated carbon porous electrodes and electrolytes is used to obtain an extremely large electrical capacity. Unlike batteries which use chemical reactions, the charging and discharging process of supercapacitors is always a physical process. Short charging time, long service life, good temperature characteristics, energy saving and environmental protection. There is nothing too complicated about ultracapacitors, that is, capacitor charging, and the rest is the problem of materials. At present, the research direction is whether the area is small and the capacitance is larger. The development of supercapacitors is still very fast. At present, the new type of supercapacitors based on graphene materials are very popular.

ElonMusk, Tesla's chief executive, said as early as 2011 that batteries in conventional electric cars were obsolete and that new cars powered by supercapacitors would replace them in the future.

Cons: compared to batteries, its energy density results in relatively low energy storage under the same weight, which directly leads to poor battery life and depends on the creation of new materials, such as graphene.

(2) superconducting energy storage (SMES) : devices that store electrical energy using the zero-resistance property of superconductors. Superconducting energy storage system generally includes superconducting coil, cryogenic system, power regulation system and monitoring system. The development of superconducting material technology is the priority among priorities of superconducting energy storage technology. Superconducting materials can be roughly divided into low-temperature superconducting materials, high-temperature superconducting materials and room-temperature superconducting materials.

Cons: the high cost of superconducting energy storage (materials and cryogenic refrigeration systems) limits its use. Limited by reliability and economy, commercial application is still far away.

3. Electrochemical energy storage

(1) lead-acid battery: it is a battery whose electrode is mainly made of lead and its oxides and whose electrolyte is the sulfuric acid solution. At present, it is widely used in the world, with a cycle life of about 1000 times, an efficiency of 80-90%, and a high-cost performance. It is often used in power system emergency power supply or standby power supply.

Disadvantages: if the depth, fast power discharge, available capacity will decline. It is characterized by low energy density and short life. Lead-acid batteries have improved their cycle life this year by adding super-active carbon materials to their negative plates.

(2) lithium ion battery: it is a kind of battery with lithium metal or lithium alloy as a negative electrode material and non-aqueous electrolyte solution. Mainly used in portable mobile devices, its efficiency can reach more than 95%, the discharge time can reach several hours, the number of cycles can reach 5000 or more, the response is fast, is the battery of the highest energy practical battery, currently the most used. In recent years, technology is being upgraded, and there are many applications of anode and cathode materials.

There are three main types of power lithium batteries on the market: lithium cobalt acid batteries, lithium manganese acid batteries and lithium iron phosphate batteries. The former has a high energy density but is less safe. On the contrary, domestic electric cars such as BYD currently mostly use lithium iron phosphate batteries. But it seems that foreigners are playing three lithium battery and lithium iron phosphate battery?

Lithium-sulfur battery is also very popular, is the sulfur element as a positive pole, lithium metal as a negative pole of a battery, its theoretical than energy density can reach 2600wh/kg, the actual energy density can reach 450wh/kg. But how to improve the cycle life and safety of the battery is also a big problem.

Disadvantages: high price (4 yuan /wh), overcharge leading to heating, combustion and other safety problems, need to be charged for protection.

(3) sodium sulfur battery: it is a secondary battery with metal sodium as the negative electrode, sulfur as the positive electrode and ceramic tube as the electrolyte membrane. Cycle can reach 4500 times, discharge time 6-7 hours, cycle round trip efficiency 75%, high energy density, fast response time. At present, there are more than 200 such energy storage power stations in Japan, Germany, France and the United States, which are mainly used for load leveling, peak shifting and power quality improvement.

Cons: easy to burn at high temperatures due to liquid sodium. And in case the grid runs out of power, diesel generators will be needed to keep temperatures high or to help keep batteries cool.

(4) liquid-flow battery: a kind of high-performance battery that USES positive and negative electrolytes to separate and circulate separately. The power of the battery is not related to energy. The energy stored depends on the size of the storage tank, so it can be stored for hours to days, with a capacity of up to MW. This battery has many systems, such as iron chromium system, zinc bromine system, sodium polysulfide bromine system and all vanadium system, vanadium battery is the most popular.

Disadvantages: the battery is too large; The battery requires too high ambient temperature; High prices (probably a short-term phenomenon); Systems are complex (pumps and pipelines are not as simple as non-liquid flow batteries such as lithium).

Battery energy storage has more or less environmental problems.

4. Heat storage

Thermal energy storage: in a thermal energy storage system, heat energy is stored in a medium in an insulated vessel and is converted back to electrical energy when needed or can be used directly without being converted back to electrical energy. Heat storage can be divided into sensible heat storage and latent heat storage. Heat storage can store a lot of heat, so it can be used to generate electricity from renewable sources.

Shortcomings: thermal energy storage to a variety of high - temperature chemical thermal working medium, more limited use occasions.

5. Chemical energy storage

Chemical class energy storage: the use of hydrogen or synthesis gas as a secondary energy carrier, used excess hydrogen production, can be directly use hydrogen as an energy carrier, can also be its reaction with carbon dioxide to synthesis gas (methane), hydrogen and synthesis gas besides can be used to generate power, use other ways such as transport and so on. Germany is keen to promote the technology and has demonstration projects in operation.

Disadvantages: low full-cycle efficiency, hydrogen production efficiency of only 40%, synthetic natural gas efficiency of less than 35%.

To quote from the previous summary:

PHS- pumped storage; CAES- compressed air; Lead-Acid battery; NiCd: NiCd battery; NaS: sodium sulfur batteries; ZEBRA: nickel chloride batteries; Li-ion: lithium batteries; Fuelcell: Fuelcell; Metal-air: Metal air battery; VRB: liquid flow battery; ZnbBr: liquid flow battery; PSB: liquid flow battery; SolarFuel: solar energy fuel cell; SMES: superconducting energy storage; Flywheel: Flywheel; Capacitor/Super capacitor: Capacitor/ supercapacitor; Al-tes: water/ice heat storage/cold system; CES: low temperature energy storage system; Ht-tes: heat storage system.

In general, the current research and development mainly focuses on ultracapacitors and batteries (lithium battery, liquid flow battery). Breakthroughs in materials are key.

What will a reliable energy storage grid look like?

1. Support the realization of energy Internet and smart power grid

Energy storage is important equipment to realize bidirectional energy interaction in smart grid. Without energy storage, a complete smart grid is out of the question.

2. Use energy storage technology to face the test of new energy

The main purpose is to stabilize and stabilize the output power of intermittent renewable energy generation, such as wind power and solar energy, and improve the grid's ability to accept intermittent renewable energy.

3. Reduce peak and valley differences and improve equipment utilization

Power grid enterprises can obtain more peak load benefits while the peak load regulation and supply pressure are alleviated.

4. Improve the safety and reliability of power grid and power quality

Provide emergency power supply; Reduce the loss caused by various transient power quality problems.

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