Can you play with air like this? Underwater constant pressure compressed air energy storage is here!

Produced by: Science Popularization China

Author: Liu Changchun (Associate Researcher, Institute of Engineering Thermophysics, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to unveil the mystery of scientific work, the China Science Popularization Frontier Science Project launched a series of articles called "Me and My Research", inviting scientists to write articles themselves, share their scientific research experiences, and create a scientific world. Let us follow the explorers at the forefront of science and technology and embark on a journey full of passion, challenges, and surprises.

Offshore renewable energy generation, especially wind power, has entered a period of large-scale development. According to reports from the International Renewable Energy Agency (IEA) and the China Wind Energy Association (CWEA), the global offshore wind power installed capacity will increase by 7.3GW in 2023, exceeding 50GW in total. Among them, China's cumulative offshore wind power installed capacity will reach 37.7GW, ranking first in the world.

Since renewable energy is volatile, random and unpredictable, it is difficult to meet the stable energy demand of residential users. Energy storage can achieve smooth output of renewable energy, guarantee user energy demand, and establish a flexible link between the power generation side and the user side by storing surplus electricity when there is a surplus of electricity and releasing the stored energy to supplement the power gap when there is a shortage of electricity.

With the large-scale development of offshore renewable energy, the demand for offshore energy storage has increased dramatically. How to develop economical, applicable and reliable offshore energy storage technology is the first problem that energy storage practitioners need to solve.

Compressed air energy storage technology

Compressed air energy storage technology is a physical energy storage technology developed based on gas turbine technology. The system principle is shown in the figure below. It has the characteristics of large energy storage scale, long discharge time, low construction and operation costs, and long life.

When storing energy, excess or non-peak electrical energy is used to drive the electric motor to rotate, converting electrical energy into mechanical energy. The motor drives the compressor (a machine that raises low-pressure gas to high-pressure gas) to compress the air from a low-pressure state to a high-pressure state, and the high-pressure air is stored in a gas storage device (salt caverns, artificial chambers or gas tanks), ultimately converting electrical energy into air thermal energy and pressure energy.

When releasing energy, high-pressure air is released from the gas storage device and enters the combustion chamber to burn with the fuel, or is heated by other hot fluids in the heat exchanger. The high-temperature and high-pressure gas drives the turbine (a machine that converts the energy in the fluid medium into mechanical work) to rotate, and the turbine drives the generator to generate electricity, ultimately converting the internal energy of the air into electrical energy.

Schematic diagram of compressed air energy storage technology

(Image source: Institute of Engineering Thermophysics, Chinese Academy of Sciences)

Since 2004, the Institute of Engineering Thermophysics of the Chinese Academy of Sciences has been conducting research on advanced compressed air energy storage technology that does not require the combustion of fuel. It has completed the transformation of advanced compressed air energy storage technology from kW level to 300MW level, and successfully promoted advanced compressed air energy storage technology from theoretical research to commercial application stage.

Research and development history of compressed air energy storage technology at the Institute of Engineering Thermophysics

(Image source: Institute of Engineering Thermophysics, Chinese Academy of Sciences)

A different approach

Whether it is traditional compressed air energy storage or advanced compressed air energy storage that has entered the early stage of commercialization, both use gas storage devices with constant volume, which belongs to constant volume compressed air energy storage. However, the current constant volume compressed air energy storage technology is difficult to meet the urgent demand for energy storage technology in the development of offshore renewable energy. It faces three key bottlenecks:

First, in the special geographical environment of the coast, there are no well-sealed underground salt caverns, it is impossible to build underground artificial gas storage chambers, and the ground space is insufficient to accommodate large-scale gas storage tanks, so it is difficult to find a suitable large-scale gas storage site;

Second, constant volume gas storage is used. During the energy storage and release process, the internal pressure and temperature of the gas storage device are constantly changing. In order to make the turbine output power relatively stable, the intake pressure needs to be adjusted through the throttle valve, which results in large energy loss and the efficiency needs to be further improved.

Third, due to the internal pressure changes and regulation requirements of the gas storage device, the equipment needs to constantly change its operating state to adapt to the pressure and regulation requirements inside the gas storage reservoir. The efficiency drops sharply under frequently changing working conditions, and there is a lack of relevant theoretical support for integrated energy storage systems on the renewable energy side.

In response to the above bottlenecks, researchers from the Energy Storage R&D Center of the Institute of Engineering Thermophysics, Chinese Academy of Sciences, are preparing to take a different approach - developing underwater constant-pressure compressed air energy storage technology.

We know that the water pressure at a specific underwater location corresponds to the water depth one by one. As long as the water depth remains unchanged, the water pressure will remain unchanged. Therefore, by finding a way to transfer the water pressure to the air inside the air storage device, constant-pressure air storage and constant-pressure air release can be achieved.

Researchers have therefore developed two types of underwater constant pressure gas storage devices: closed flexible gas storage devices and open rigid gas storage devices.

The outer wall of the flexible gas storage device is in contact with water, and the water pressure is transmitted to the air inside the device through the flexible gas storage device. Changes in the amount of gas inside the gas storage device will only affect the actual size of the space inside the gas storage device and will not cause pressure changes.

The open rigid gas storage device has a hole at the bottom, which is in direct contact with water. During the inflation and deflation process, water enters or is discharged from the gas storage device through the hole. Similarly, changes in the amount of gas inside the gas storage device will not cause changes in pressure.

Both of these gas storage devices can achieve complete release of the air inside the device while keeping the exhaust pressure unchanged, fully utilize the gas storage space, and have a high energy storage density.

Since the pressure in the gas storage reservoir remains unchanged during the energy storage/release process, the working pressure of the compressor and turbine can also be optimized according to the design pressure of the gas storage reservoir, and always operates near the design point, with small system energy loss and high operating efficiency.

Through comparative studies, it was found that the constant pressure system is 3%-6% more efficient than the constant volume system, and the greater the gas storage pressure, the more obvious the energy storage density advantage of the constant pressure system. The energy storage density of the adiabatic constant pressure system can reach 3 times or more than that of the constant volume system.

Continuously optimize offshore and land applications

The current compressed air energy storage technology is limited by coastal land resource conditions, while the underwater constant pressure compressed air energy storage technology can utilize the vast underwater seabed and the underwater constant temperature and pressure environment as a gas storage site. The gas storage scale is not restricted, providing efficient and low-cost energy storage technology support for the large-scale development of offshore renewable energy.

Through the co-construction and coordinated planning of underwater constant-pressure compressed air energy storage and offshore renewable energy, it is possible to achieve a smooth output of unstable and uncontrollable renewable energy, providing coastal users with a stable and reliable supply of green electricity.

In addition to being used in offshore renewable energy development, this technology can also be used to upgrade existing compressed air energy storage power stations. By adding a ground water pool to the existing compressed air energy storage power station and laying a direct pipeline to the bottom of the gas storage device to achieve constant pressure operation, the system's rated efficiency is expected to increase by 3%-6%, avoiding constant volume gas storage that causes the system to deviate from the design operating conditions, reducing the difficulty of power station operation and maintenance, and greatly increasing the power station's operating life.

In recent years, we have conducted research on constant-pressure compressed air energy storage technology from three levels: optimized design, optimized operation, and experimental verification.

In terms of optimization design: an analysis method suitable for underwater constant-pressure compressed air energy storage was established, the source of energy loss was determined, the theory of synergistic and efficient storage of pressure energy and thermal energy was revealed, and an optimization method for minimizing energy loss was further established;

In terms of optimized operation: through a method combining theoretical analysis with experimental verification, the key parameter adjustment characteristics of constant-pressure compressed air energy storage are revealed, and a multi-parameter joint variable operating condition control strategy is proposed, which greatly broadens the range of efficient operation.

In terms of experimental verification: In order to break through the limitations of underwater experimental sites and costs, we proposed a constant pressure compressed air energy storage experimental technology based on a deep-water simulation device, using high-pressure water and high-pressure gas to simulate the external deep-water environment of the flexible airbag, and built a megawatt-level constant pressure compressed air energy storage system experimental platform. The design gas storage pressure is equivalent to a water depth of about 700 meters. We have completed the system performance experiment and test. After a third-party test with CNAS qualifications, the system efficiency has reached the international leading level, 6.7% higher than the constant volume system of the same scale.

At the same time, we also carried out experimental verification of the coupling control of the energy storage system and renewable energy. The results showed that the system has good load following performance, the experimental power following error does not exceed ±5%, and the efficiency is maintained at more than 90% of the rated efficiency, verifying the feasibility of the constant voltage system as energy storage on the power generation side.

Schematic diagram of constant pressure compressed air energy storage test platform

(Image source: Institute of Engineering Thermophysics, Chinese Academy of Sciences)

Megawatt-level constant pressure compressed air energy storage experimental platform

(Image source: Institute of Engineering Thermophysics, Chinese Academy of Sciences)

Conclusion

In the future, we will further conduct in-depth research on the key components of underwater constant-pressure compressed air energy storage, break through the ability of key equipment to operate stably for a long time in special environments such as high salt fog and high humidity in coastal areas, overcome the problem of compressed air dissolution in water in open underwater constant-pressure compressed air energy storage, and the anchoring problem of flexible air storage devices in closed underwater constant-pressure compressed air energy storage systems, and carry out underwater compressed air energy storage technology engineering demonstrations.

I believe that in the near future, underwater constant-pressure compressed air energy storage technology will gradually mature and enter the industrialization stage, which will safeguard the development of offshore renewable energy and inject new vitality into the realization of the "dual carbon" goals.

References:

[1] Changchun Liu, Xu Su, Zhao Yin, Yong Sheng, Xuezhi Zhou, Yujie Xu, Xudong Wang, Haisheng Chen. Experimental study on the feasibility of isobaric compressed air energy storage as wind power side energy storage. Applied Energy. 2024;364:123129.

[2] Changchun Liu, Zhao Yin, Xu Su, Xuehui Zhang, Zhitao Zuo, Yong Sheng, Xuezhi Zhou, Xudong Wang, Yujie Xu, Haisheng Chen. Megawatt Isobaric Compressed Air Energy Storage: an Experimental Study on the Discharge Process. Energy Proceedings; 2024;47.

Note: The author of this article, Liu Changchun, is an associate researcher at the Institute of Engineering Thermophysics, Chinese Academy of Sciences. He has long been engaged in research on key technologies such as compressed air energy storage and distributed energy supply, multi-energy coupling mechanisms and control methods, as well as distributed energy and storage planning.