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The Institute of Engineering Thermophysics, Chinese Academy of Sciences, conducts thermodynamic analysis of isothermal compressed air energy storage system with droplet spray

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Technical field: Isothermal compressed air energy storage

Development unit: Chen Haisheng, Institute of Engineering Thermophysics, Chinese Academy of Sciences

Article name: Ziyu Gao, Xinhua Zhang, et al. Thermodynamic analysis of isothermal compressed air energy storage system with droplets injection. Energy, 2023.

Technical breakthrough: Increasing the gas-liquid mass ratio (ML) and reducing the rotation speed can improve the isothermal compression/expansion efficiency, round-trip efficiency and isothermality. When the charging time is 6h, the discharge time is 4h, and the ML is equal to 10, the round-trip efficiency of the single-stage I-CAES system is 83.15%, and the energy density is 1.94 MJ/m3. Under the same conditions, the round-trip efficiency of the two-stage I-CAES system is 82.53%, and the energy density is 39.93 MJ/m3.

Application value: The effects of ML and rotation speed on thermodynamic properties are studied. Considering one-stage and two-stage I-CAES systems, the performance of isothermal compressed air energy storage system is analyzed.

Renewable energy is intermittent and unstable, which may lead to power fluctuations and unstable operation of the grid. Therefore, we need energy storage technology to improve grid stability and reduce the unstable impact of large-scale renewable energy access to the grid. Among the many energy storage technologies, CAES has broad prospects in large-scale and long-term energy storage applications due to its high reliability, economic feasibility and few construction restrictions. The isothermal compressed air energy storage (I-CAES) system achieves near-isothermal compression and expansion processes by controlling the temperature rise of the compression process and the temperature drop of the expansion process, so that the air is always at ambient temperature. Theoretically, the ideal round-trip efficiency is above 90%. According to the different heat transfer methods, I-CAES systems can be divided into direct heat transfer and indirect heat transfer. Droplet spray and liquid piston are one of the most commonly used methods in direct heat transfer technology. Compared with other heat transfer methods, the advantages of adding water droplets are as follows: (1) Water droplets have higher specific heat. (2) The reduction of droplet diameter increases its surface area. (3) Water droplets have a large convective heat transfer coefficient. (4) Water droplets are environmentally friendly. Previous studies have shown that the mass load of the droplets has the greatest impact on the performance of the I-CAES system. However, there are few studies on the change of the droplet mass during the cycle. The comprehensive performance of the system needs to be revealed by multivariate parameter analysis such as isothermal compression/expansion efficiency, energy density and isothermality.

To solve the above problems, researchers from the Institute of Engineering Thermophysics, Chinese Academy of Sciences, conducted a detailed analysis of the multivariate parameters of the compressor/expander, established a thermodynamic model of the I-CAES system with droplet injection method, and gave a calculation formula for the change of droplet mass with crank rotation angle and air mass. The correctness of the simulation model was verified by experimental results. Then, the working process of the isothermal compressor/expander was thermodynamically analyzed. Finally, considering the one-stage and two-stage I-CAES systems, the performance of the I-CAES system was analyzed. The I-CAES system studied is shown in Figure 1. The system is configured using a direct heat transfer method. It mainly consists of an air storage vessel (ASV), a reciprocating piston compressor/expander, a gas-water separator and a motor/generator for storage/power generation. Figure 2 shows the working process of an isothermal reciprocating expander. For a reciprocating compressor/expander, the change in physical quantity relative to the crank angle is often used to represent it. The effects of gas-liquid mass ratio (ML) and rotation speed on thermodynamic properties, including isothermal compression/expansion efficiency, isothermality, round-trip efficiency, and energy density, were studied. The results show that increasing ML and reducing rotation speed can improve isothermal compression/expansion efficiency, round-trip efficiency, and isothermality, among which the effect of ML is more obvious. The round-trip efficiency and energy density can be improved by configuring appropriate ML and rotation speed. At higher ML, the round-trip efficiency and energy density are not significantly improved. The round-trip efficiency of the two-stage I-CAES system is not much different from that of the one-stage I-CAES system, but the two-stage I-CAES system has a higher energy density. When the charging time is 6h, the discharging time is 4h, and the ML is equal to 10, the round-trip efficiency of the single-stage I-CAES system is 83.15% and the energy density is 1.94 MJ/m3. Under the same conditions, the round-trip efficiency of the two-stage I-CAES system is 82.53%, and the energy density is 39.93 MJ/m3, which is 21 times the energy density of the single-stage I-CAES system. (Translated by: Zhou Bingqian, Zhang Xinjing INESA)

Figure 1 (a) Schematic diagram of the first-stage I-CAES system (b) Schematic diagram of the second-stage I-CAES system

Figure 2 (a) Working process of an isothermal reciprocating compressor with water droplets (b) Working process of an isothermal reciprocating expander with water droplets

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Shahrekhod University conducts comprehensive techno-economic evaluation and tri-objective optimization of an innovative integration of compressed air energy storage system and solid oxide fuel cell

Article information

Technical field: compressed air energy storage

Development unit: Shahrekhod University Afrasiab Raisi

Article title: Seyed Meysam Alirahmi, Afrasiab Raisi, et al. Comprehensive techno-economic assessment and tri-objective optimization of an innovative integration of compressed air energy storage system and solid oxide fuel cell. Renewable Energy, 2023.

Technical breakthrough: A new energy storage configuration combining solid oxide fuel cell (SOFC), compressed air energy storage (CAES) and seawater desalination device to generate electricity is proposed. At the TOPSIS point, the system has a round-trip efficiency of 71.03%, a total cost of $34.07/hour, and a pollution rate of 0.184 kg/kWh.

Application value: An innovative energy storage and conversion system is provided, and a comprehensive technical and economic evaluation and optimization are carried out to provide a solution for achieving energy transformation and sustainable development.

Renewable energy is intermittent, and energy storage systems are very necessary for future renewable energy deployment. Compressed air energy storage (CAES) and pumped hydroelectric energy storage (PHES) are the most feasible large-scale application options among the many energy storage systems proposed so far. CAES has advantages such as good efficiency, low cost and long life cycle, and has become a rapidly developing technology. Several studies have combined CASE with gas turbine (GT) systems using combustors, but this will lead to increased carbon dioxide emissions. Integrating fuel cells into GT systems is one of the most reliable ways to reduce emissions. Solid oxide fuel cells (SOFCs) are superior to other types of fuel cells due to their main advantages such as durability, high electrical efficiency, adaptability to fuel use and environmental friendliness. Freshwater production has surpassed electricity production and become one of the most pressing problems for mankind. The most practical solution to the world's water problems and shortages is seawater desalination. Cogeneration technology is a good option that can provide the necessary energy input for the very expensive and energy-intensive seawater desalination process and can reduce costs. The world needs to meet energy and water needs without damaging the environment. This is a major challenge that needs to be solved through knowledge and technology. Therefore, an economic structure that maximizes the use of renewable energy must be established.

To solve the above problems, researchers from Shahrekhod University proposed a system for producing electrical energy and water. As shown in the figure, the system is a new energy storage configuration that combines SOFC, CAES and seawater desalination devices to generate electricity. Compressed air is completed by three compressors running at the same pressure ratio during the charging process, using additional electricity. The gas turbine combustion products preheat the air, and the preheated fuel is mixed with water vapor. Subsequently, the fuel cell uses an electrochemical process to generate electricity from the preheated air and pretreated fuel. Water distillation is heated by waste heat in the gas turbine exhaust. The multi-stage distillation (MED) unit produces fresh water by utilizing the waste heat of compression during air compression and utilizing the waste heat of gas turbine exhaust. The authors mathematically modeled the system and evaluated the system from three aspects: technical, economic, and environmental friendliness using EES software. Then the most accurate model was obtained using a neural network algorithm, which shortened the optimization time. The performance of the system and its influence on the objective function were studied through parameter analysis. Subsequently, the cost rate and CO2 emission index were reduced using the Grey Wolf algorithm while maximizing the exergy efficiency. The results show that the system efficiency decreases and the cost increases with the increase of current density. The optimal point obtained by the TOPSIS decision criterion has an efficiency of 71.03%, a total cost rate of 34.07 USD/h, and a pollution rate of 0.184 kg/kWh. Factors such as the air storage chamber inlet pressure, compressor pressure ratio, and current density have a considerable impact on the system performance. According to the results of the study, the air storage chamber inlet pressure, compressor pressure ratio, current density, and the inlet temperature of the fuel cell should generally be kept as low as possible. (Translated by: Zhou Bingqian, Zhang Xinjing INESA)

Figure 3 Schematic diagram of the proposed CAES-SOFC-MED

Source: International Energy Storage Technology and Alliance