The Institute of Engineering Thermophysics, Chinese Academy of Sciences, conducts thermodynamic analysis research on droplet spray isothermal compressed air energy storage systems

Article Information

Technical field: Isothermal compressed air energy storage

Developer: Chen Haisheng, Institute of Engineering Thermophysics, Chinese Academy of Sciences

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

Technological breakthrough: Increasing the gas-liquid mass ratio (ML) and reducing the rotational speed can enhance the isothermal compression/expansion efficiency, round-trip efficiency and isothermal performance. When the charging time is 6 hours, the discharging time is 4 hours, 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 influence of ML and rotational speed on thermodynamic performance was studied. Considering the first-stage and two-stage I-CAES systems, the performance of the isothermal compressed air energy storage system was analyzed.

Renewable energy is intermittent and unstable, which may lead to power fluctuations and unstable operation of the power grid. Therefore, we need energy storage technology to enhance the stability of the power grid and reduce the unstable impact of large-scale renewable energy integration into the grid. Among various energy storage technologies, CAES, with its advantages of high reliability, economic feasibility, and few construction restrictions, has broad prospects in large-scale and long-term energy storage applications. Isothermal compressed air energy storage (I-CAES) systems achieve nearly isothermal compression and expansion processes by controlling the temperature rise during the compression process and the temperature drop during the expansion process, thereby keeping the air at the ambient temperature all the time. Theoretically, the ideal round-trip efficiency is above 90%. According to the different heat transfer methods, the I-CAES system can be divided into two types: direct heat transfer and indirect heat transfer. Droplet spraying and liquid piston are among 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 a higher specific heat. (2) The reduction of droplet diameter increases its surface

Accumulation. (3) Water droplets have a relatively large convective heat transfer coefficient. (4) Water droplets are environmentally friendly. Previous studies have shown that the mass loading of droplets has the greatest impact on the performance of the I-CAES system. However, there are few studies on the change of droplet mass during circulation. The comprehensive performance of the system needs to be revealed through the analysis of multiple parameters such as isothermal compression/expansion efficiency, energy density and isothermal property.

To solve the above problems, researchers from the Institute of Engineering Thermophysics, Chinese Academy of Sciences, conducted a detailed analysis of the multivariable parameters of the compressor/expansion machine, established the thermodynamic model of the droplet injection I-CAES system, and provided the calculation formula for the droplet mass varying with the crank rotation Angle and air quality. The experimental results verified the correctness of the simulation model. Then, a thermodynamic analysis was conducted on the working process of the isothermal compressor/expander. Finally, considering the first-level and two-level I-CAES systems, the performance of the I-CAES system was analyzed. The studied I-CAES system is shown in Figure 1, and this system is configured using the direct heat transfer method. It is mainly composed of a gas storage container (ASV), a reciprocating piston compressor/expander, a gas-water separator and an electric motor/generator, and is used for storage/power generation. Figure 2 shows the working process of an isothermal reciprocating expander. For reciprocating compressors/expanders, the change in physical quantities relative to the crank Angle is often used to represent it. The influences of gas-liquid mass ratio (ML) and rotational speed on thermodynamic properties were studied, including isothermal compression/expansion efficiency, isothermal property, round-trip efficiency and energy density. The results show that increasing ML and reducing the rotational speed can improve the isothermal compression/expansion efficiency, round-trip efficiency and isothermal property, among which the influence of ML is more obvious. By configuring appropriate ML and rotational speed, the round-trip efficiency and energy density can be enhanced. At higher ML, the round-trip efficiency and energy density did not improve significantly. The round-trip efficiency of the two-stage I-CAES system is not much different from that of the first-stage I-CAES system, but the two-stage I-CAES system has a higher energy density. When the charging time is 6 hours, the discharging time is 4 hours, 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 39.93MJ/m3, which is 21 times that of the single-stage I-CAES system. (Compiled by Zhou Bingqian and Zhang Xinjing, INESA)

Figure 1(a) Schematic diagram of the primary I-CAES system (b) Schematic diagram of the secondary 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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The University of Salekod is conducting a comprehensive economic evaluation and three-objective optimization research on the integration technology of compressed air energy storage systems and solid oxide fuel cells

Article Information

Technical field: Compressed air energy storage

Developer: Afrasiab Raisi, University of Shahrekod

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.

Technological breakthrough: A new type of energy storage configuration combining solid oxide fuel cells (SOFC), compressed air energy storage (CAES), and seawater desalination devices for power generation has been proposed. At the TOPSIS point, the round-trip efficiency of this system is 71.03%, the total cost is $34.07 per hour, and the pollution rate is 0.184 kg/kWh.

Application value: It provides an innovative energy storage and conversion system and conducts a comprehensive technical and economic assessment and optimization of it, offering a solution for achieving energy transition and sustainable development.

Renewable energy has intermittent characteristics, and energy storage systems are very necessary for the future deployment of renewable energy. Compressed air energy storage (CAES) and Pumped hydropower energy storage (PHES) are the most feasible large-scale application options among the numerous 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 combustion chambers, but this has led to an increase in carbon dioxide emissions. Integrating fuel cells into GT systems is one of the most reliable ways to reduce emissions. Solid oxide fuel cells (SOFC) outperform other types of fuel cells due to their main advantages such as durability, high electrical efficiency, adaptability to fuel usage, and environmental friendliness. Freshwater production has surpassed electricity production and become one of the most urgent problems for humanity. The most practical solution to water resource problems and shortages in the world is seawater desalination. Combined heat and power technology is a good choice that can provide the necessary energy input for the very expensive and energy-intensive seawater desalination process and can also reduce costs. The world needs to meet the demands for energy and water without damaging the environment. This is a major challenge that needs to be addressed through knowledge and technology. Therefore, an economic structure that maximizes the utilization of renewable energy must be established.

To address the above issues, researchers from Shahrekod University have proposed a system for generating electricity and water, as shown in the figure. This system is a new type of energy storage configuration that combines SOFC, CAES and seawater desalination devices for power generation. Compressed air is produced by three compressors operating at the same pressure ratio during the charging process, using additional electricity. The combustion products of the gas turbine preheat the air. The preheated fuel is mixed with steam. Subsequently, the fuel cell generates electricity from the preheated air and the pre-treated fuel using an electrochemical process. Water distillation is heated by the waste heat in the exhaust gas of the gas turbine. The multi-stage distillation (MED) unit produces fresh water by utilizing the waste heat from compression and the waste heat from gas turbine exhaust during the compression of air. The author conducted mathematical modeling of the system and evaluated the system from three aspects: technology, economy and environmental friendliness by using the EES software. Then the most accurate model is obtained by using the neural network algorithm, which shortens the optimization time. The performance of the system and its influence on the objective function are studied through parameter analysis. Subsequently, the grey Wolf algorithm was used to reduce the cost rate and CO2 emission index, while maximizing exergy efficiency. The results show that as the current density increases, the system efficiency decreases and the cost rises. The optimal advantages obtained through the TOPSIS decision-making criterion have an efficiency of 71.03%, a total cost rate of $34.07 per hour, and a pollution rate of 0.184 kilograms per kilowatt-hour. Factors such as the inlet pressure of the gas storage chamber, the pressure ratio of the compressor and the current density have a considerable impact on the system performance. According to the research results, the inlet pressure of the gas storage chamber, the pressure ratio of the compressor, the current density and the inlet temperature of the fuel cell should usually be maintained at the lowest possible level. (Compiled by Zhou Bingqian and Zhang Xinjing, INESA)

Figure 3 Schematic diagram of CAES-SOFC-MED proposed

Source: International Energy Storage Technology and Alliance