Introduction: The application of renewable energy is an inevitable trend in the world today, and the use of liquefied air energy storage technology can store renewable energy. Liquefied air energy storage technology can not only be applied to the storage of renewable energy, but also can be used to solve the problem of peak-to-valley differences in the power grid. This article elaborates on the current development of liquefied air energy storage technology, analyzes various parameters of liquefied air energy storage technology, and compares it with other energy storage technologies, which is conducive for engineers and technicians to understand liquefied air more intuitively. Energy storage technology.
introduction
Today's people's living environment is seriously polluted, such as the greenhouse effect, ozone layer depletion and acid rain, and the root of all this is the burning of fossil fuels. In order to reduce the environmental pollution caused by the burning of fossil fuels, a new, safe, sustainable, and environmentally friendly energy supply system is needed to regenerate the energy supply system. At present, renewable energy accounts for 5% of the world's electricity capacity and 3.4% of global electricity generation. This does not include water power generation (approximately 15% of global electricity generation). The British government has set a goal to increase the electricity generated from renewable energy from the current 4.6% to 20% in 2020 and reach 80% by 2050. The European Union recently proposed a higher goal, and the use of renewable energy should reach 30% to 40% growth by 2020. In the near future, renewable energy such as wind, solar, ocean energy, biomass, and geothermal resources will account for most of the entire generation capacity.
Although the use of renewable energy is an effective method to solve environmental pollution problems, there are still some problems in the use of renewable energy. Renewable energy sources, especially wind energy and solar energy, are intermittent energy sources. The generated energy is not a sustained energy and does not meet people’s actual demand for energy. At the same time, some renewable energy sources such as ocean energy and wind energy are also limited by geographical environment. Therefore, a suitable energy storage system is required to store the energy generated by these renewable energy sources.
At present, there are many methods for the storage of renewable energy. Most energy storage technologies are very difficult to store these intermittent renewable energy sources, and there are restrictions on geographical and environmental conditions. However, liquefied air energy storage (LAES) not only can effectively store intermittent renewable energy, but also is not limited by geographical environment and is easy to manage and transport. The following is a detailed overview of liquefied air energy storage technology.
1 Liquefied air energy storage technology
In general, the liquefied air energy storage system (LAES) includes three processes: liquefaction process, energy storage process, and power recovery process, as shown in FIG.
(1) Liquefaction process. The surplus electric energy of the electric grid drives the liquefied air device at the night, so that the air in the environment is clean and then compressed, and then it is passed into the heat exchanger to exchange heat with the cool air returned by the gas-liquid separator and the cold air in the cold storage device. The cooled cold air passes through the expander and the throttle valve in turn, and is lowered in temperature and pressure. A part of the cooled air is condensed into a liquid, and part of the air is still a gas, and finally is separated in a gas-liquid separator. Cold air from the upper port of the gas-liquid separator is returned to the heat exchanger to cool the air compressed by the compressor.
(2) Energy storage process. The liquid air separated by the gas-liquid separator flows from the lower port of the gas-liquid separator to the liquefied air storage tank, and most of the electric energy consumed in the liquefaction process is converted into the cold energy of the liquid air.
(3) Power recovery process. The liquid air in the cryogenic storage tank is led out, pressurized by the cryogenic pump and sent to the gasification heat exchanger for heat absorption and vaporization. The vaporized air is passed into the heat exchanger and heated further to raise the temperature and increase the pressure. The high-pressure gas from the heat exchanger passes through the turbine to do work, and the turbine is connected to the generator to drive the generator to rotate and generate electricity. The high-temperature air coming out of the turbine is cooled in turn through a heat exchanger and a gasification heat exchanger, and then it flows into the cold storage device and exchanges heat with the air compressed by the compressor in the heat exchanger. Because the boiling point of the liquid air is relatively low, the heat supplied to the low-temperature air in the heat exchanger during the power recovery process may be the heat from the liquefaction process or the heat of the external environment.
Liquefied air energy storage media are readily available air, and the entire energy storage process does not require fossil fuels as a supplement and is completely “greenâ€. When the temperature of the air becomes liquid by cooling to about -196[deg.]C using a liquefaction apparatus, typically 700L of ambient air can become about 1L of liquid air. Liquid air as a storage medium has a high energy storage density, and the effective energy per unit volume can reach 660 MJ.
The liquefied air energy storage cycle is essentially a combination of a Linde cycle (liquefaction process) and a Rankine cycle (electric recovery process), but the liquefaction process differs from the classical Linde cycle because the cold air coming out of the expander is Used to cool the air at the inlet of the expander. Some of the effective energy lost in the Rankine cycle is also used to cool the air at the inlet of the expander. Therefore, the effective energy input in the Linde cycle comes from the Rankine cycle and the compressor, respectively. Due to the influence of the external environment, there will be some energy loss in the storage process of the liquefied air energy storage system. Real work will be limited by the real cycle efficiency. Excluding some of the irreversible losses in the internal cycle, the effective energy at the Linde cycle exit exists as liquid air. In the Rankine cycle, the output power of the turbine not only comes from the input energy of the liquefaction process, but also from the input heat of the external environment. However, in this process, the loss of effective energy is also accompanied, but the loss of effective energy is greater than the input heat. The effective energy in the exchanger is large, so it is necessary to make full use of the effective energy in the input heat exchanger to improve the efficiency of the cycle. The loss of effective energy in the Rankine cycle is determined by the pressure at the inlet of the turbine, and higher pressure will result in less effective energy loss. Under conditions of high pressure and high relative saturation temperature, the loss of effective energy will be relatively low. From the liquefied air energy storage cycle, even if it is adiabatic expansion, the work required by the liquefaction process is higher than the work done by the expansion of liquid air, which requires the combination of the liquefaction process and the expansion process, using heat recovery between the two processes To improve overall efficiency.
2 Development of Liquefied Air Energy Storage Technology
The development of liquefied air energy storage technology can be traced back to 1977. Smith proposed the use of adiabatic compression and expansion devices and reported a 72% energy recovery efficiency. But to achieve this efficiency, you need an energy storage device that can withstand temperatures between -200 and 800°C and pressures up to 10,000 Pa. Ameel et al. analyzed the liquefaction process in conjunction with the Rankine cycle and Linde cycle, and reported that the efficiency of the liquefaction process was 43%. The cycle proposed here differs from the previous study in two aspects. First, in order to overcome the difficulties of manufacturing large pressure vessels, the energy storage device needs to be operated at low pressure. Second, the liquefier employs a Claude cycle, wherein the cooling process includes an isentropic expansion process in one or more expanders and an equal expansion process in the throttle. The Claude cycle is the most commonly used method for large-scale liquefaction, and is more effective than the Linde cycle.
In recent years, Japan has also actively carried out research on liquefied air energy storage technologies, such as Mitsubishi and Hitachi, but due to its low system efficiency, it does not have much practical value.
Since 2007, the Institute of Engineering Thermophysics has jointly developed a liquid air energy storage system with British Gaozhan Company and the University of Leeds, UK. At present, the first liquefied air energy storage prototype (rated power 500 kW, storage capacity of about 2 MWh) of Highview Power Storage Co. of the United Kingdom using this technology has been demonstrated and operated in London, UK. Since 2011, Highview's LAES technology has been used by the Southern Energy Corporation of Scotland (SSE) in its 350kW/2.5MWh liquefied air energy storage system for its 80MW biomass heat and power plant. In late 2012, Highview PowerStorage built a 3,500 kW commercial system in Scotland and built an 8000 to 10000 kW storage power station in early 2014. In February 2014, Viridor selected Highview to design and build a demonstration demonstration plant for liquefied air storage of 5MW/15MWh commercial demonstrations with the aid of 8 million pounds from the Department of Energy and Climate Change (DECC). The liquefied air energy storage plant is built in the Viridor landfill gas power plant. In the spring of 2015, the UK's Highview Power Storage Corporation demonstrated for the first time the use of LAES technology in a commercial scale. The LAES facility will be powered by GE's turbine generators [20].
3 Technical and economic analysis of liquefied air energy storage system
The economic analysis of liquefied air energy storage technology is a comprehensive evaluation of technical indicators such as technology maturity, cycle efficiency, energy density and other economic indicators of energy storage technology. The following analysis of these impact indicators is carried out.
As shown in Table 1, the storage capacity of liquefied air energy storage technology can reach 10 to 200 MW, which is equivalent to half of the large compressed air energy storage capacity. The specific energy of liquefied air energy storage technology is 214Wh/kg, which is equivalent to four times that of large-scale compressed air energy storage technology. The duration of energy storage in liquefied air energy storage technology can reach more than 12 hours and the service life is 25 years, which is relatively high. The efficiency of liquefied air energy storage is 55% to 90%, and its efficiency value is closely related to whether the entire system energy can be fully utilized. In order to improve the efficiency of the liquefied air energy storage system, it is necessary to select a suitable liquefied air energy storage device to minimize unnecessary energy loss during the operation of the device. The waste heat generated during the liquefaction process can be used to heat the liquid air during the power recovery process so that the energy can be fully utilized and the efficiency of the entire cycle can be improved. The heat used to heat the liquid air for the liquefaction process can also be the heat in the environment and the waste heat generated in the industry. In the same way, it can also be used to precool the gaseous air during the energy storage process by applying the cooling capacity generated by the gasification of the liquid air, which can also improve the efficiency of the liquefied air energy storage system. The liquefied air energy storage system is now widely used in the United Kingdom and is a relatively mature energy storage technology.
As shown in Table 2, under the same conditions, the comparison of three kinds of energy storage technologies: liquefied air energy storage, compressed air energy storage, and pumped energy storage, the liquefied air energy storage technology has the largest energy storage density. Therefore, in an energy storage system that requires the same storage capacity, the storage container required for the liquefied air energy storage system is relatively small among the three, and the liquefied air energy storage system is less affected by the restrictions of geographical and environmental conditions. The area is very wide. From Table 2, it can also be seen that for the compressed air energy storage system, the energy storage density increases with the increase of the storage pressure and becomes a nearly linear relationship.
Cost is one of the most important indicators of technical economy. The cost of energy storage systems mainly includes initial investment costs and operation and maintenance costs. Table 3 lists the cost of various energy storage technologies in kilowatts. The cost of energy storage for sodium-sulfur batteries is 600-2800 $/kW, which is a relatively expensive technology; the cost of pumped water storage is 600-2000. $/kW, lower unit cost; liquefied air energy storage costs 400 ~ 800 $/kW, its cost is equivalent to one-third of the cost of sodium-sulfur battery and half of the pumped storage cost; liquefied air energy storage unit costs the same Large-scale compressed air energy storage is equivalent, but it is half of the small compressed air energy storage. With the maturity of the technology and the simplification of the equipment, there will be some room for decline.
According to the LUX study, as shown in Figure 2, the potential energy storage of the global power grid by 2017 is expected to reach US$113.5 billion with a capacity of 185 GWh (52 GW). As a new, large-scale, long-lasting energy storage system with a low energy storage cost, a liquefied air energy storage system can be deployed where it is needed to meet a wide range of market and application needs.
4 Summary
The liquefied air energy storage system can store intermittent renewable energy and excess electricity in the nighttime power grid. Its storage capacity is relatively large, up to 200MW, but storage devices are relatively small and are less affected by geographical constraints, so they are suitable Most areas promote use. As a new type of energy storage technology, liquefied air energy storage technology has a long history of development, its technical level is relatively mature, and liquefied air energy storage system is pollution-free, good for the environment, relatively low cost of energy storage, economic benefits High, it will play an important role in the future of low-carbon energy occupying major markets, and has good prospects for development.
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