Researchers from North China Electric Power University have looked into methods for improving the efficiency of compressed air energy storage (CAES) systems, which are used to store excess energy from solar and wind power plants. They focused on the isothermal storage technology and the coordinated operation of two-stage liquid pistons.
The CAES technology is based on the principle of compressing air during periods of excess electricity and later expanding it to generate energy during high-demand hours. However, traditional CAES systems are considered inefficient, as the air gets very hot during compression and the resulting heat gets lost. Subsequently, there is no heat source during the reverse expansion process, and energy efficiency decreases. To solve this problem, the researchers have proposed the isothermal compressed air energy storage (ICAES) technology, in which the air temperature is maintained at a nearly constant level. This is achieved via liquid pistons, in which air is compressed not by a mechanical piston but by a moving liquid (such as water) with a high heat capacity. The liquid is injected into the working chamber, effectively removing heat and ensuring uniform compression and energy loss reduction.
Work under high pressure remains a key technical difficulty, as it entails large volumes of liquid and complicates the operation of the control system. In an attempt to overcome these limitations, the Chinese scientists have proposed using two sequentially operating stages of liquid pistons. Unlike classic two-stage schemes, where the second stage is launched only after the first stage has completed its operation, the proposed system allows both stages to operate simultaneously, in a single flow. This ensures continuous and coordinated air compression.
In addition, the proposed system includes a constant-pressure tank in which compressed air is stored under a pressure of up to 10 MPa. The pressure is regulated by water: when air is fed inside, water gets displaced, and when air is released, it comes back. This design makes it possible to stabilize the storage process and return some energy through the movement of liquid. In other words, the system consists of low- and high-pressure liquid pistons through which the air passes two compression stages before being fed into the storage. In the generation mode, the process is reversed: the air expands and activates the same mechanisms, which now operate as generators.
The mathematical model created by the researchers showed that, for the given parameters of accumulation (200 kW·h) and storage pressure (up to 10 MPa), the overall efficiency of the system stands at about 68%, which is significantly higher than that of traditional CAES units, whose efficiency rarely exceeds 50%. A key advantage of the proposed technology is high energy storage density: thanks to stage compression, it was possible to reduce the required volume of the tank by more than six times. The system also has good scalability prospects: one could add additional stages to further increase pressure and energy capacity.
The researchers note that the disadvantages of the new technology include energy consumption associated with constant pressure in the tank: part of the energy is spent on moving the liquid, which somewhat reduces overall efficiency. Nevertheless, the proposed solution demonstrates considerable potential for use in hybrid energy systems and could become a promising alternative to large-sized batteries.
