Compressed-air energy storage technology involves storing energy as pressurized air, with the system’s capacity depending on the size of the storage vessel, the pressure at which the air is held, and the temperature. Different types of CAES exist, including diabatic, adiabatic, isothermal, supercritical, micro, and underwater.
Isothermal CAES is one of the major CAES systems, along with adiabatic and diabatic, while supercritical CAES is still in the experimental and research stages but has much more potential. This article discusses isothermal and supercritical CAES systems in detail.

Isothermal CAES

Isothermal CAES is an advanced technology that seeks to address some of the problems associated with conventional (adiabatic or diabatic) CAES. The conventional CAES system compresses air up to 70 bars before storing it. However, without intercooling, the temperature of the air rises to an impractical 900 K. In contrast, isothermal CAES uses a succession of compression and heat-exchange stages to reduce the temperature to near ambient. Furthermore, the heat of compression is stored separately and reintroduced to the compressed gas during expansion to negate the need for reheating with natural gas.
The isothermal CAES system functions by managing the pressure-volume (P-V) relationship during compression and expansion, which is a critical aspect of an efficient CAES system. Rather than using multiple stages to compress, cool, heat, and expand the air, isothermal CAES technologies try to achieve true isothermal in-situ expansion and compression, leading to better round-trip efficiency and cheaper capital expenses. In theory, it also eliminates the need to store the heat of compression through other means, such as oil.
The isothermal Compressed Air Energy Storage process is highly demanding from a technological standpoint since it necessitates constant air cooling during the compression cycle and heating during expansion to keep the process isothermal. Heat transmission depends on the temperature difference multiplied by the surface area of contact, meaning a colossal surface area of contact is required to transfer heat with a minimal temperature difference effectively.

Advantages of Isothermal Compressed Air Energy Storage (CAES) Systems

Many isothermal CAES setups have been proposed in the past. One such proposition is to inject a mist of water droplets into the piston chamber during its compression. This is beneficial due to the large surface area of the water droplets and its comparatively high heat capacity compared to air, which helps to maintain a consistent temperature in the piston. During the expansion phase, we actively mirror this process, either disposing of or storing the water for future use.
Isothermal CAES developers cite a potential efficiency of between 70-80%. This technology compresses and expands gas at temperatures near the same level across a large pressure spectrum from atmospheric pressure (0 bar) up to a maximum of nearly 172 bar. This large operating pressure range, combined with the isothermal gas expansion (which permits the retrieval of heat not possible with adiabatic expansion), leads to an approximate 7x decrease in storage cost compared to traditional CAES in tanks.

Supercritical CAES

The supercritical CAES system is advantageous because it combines the benefits of liquid air energy storage (LAES), such as high energy and power densities, and those of adiabatic CAES, such as its long storage time, efficiency, environmentally friendly nature, and high-power rating. The setup of this system comprises of a generator, a motor, a cryogenic storage tank, a cryopump, a valve or liquid expander, a heat exchanger/ heat or cold storage system, a multi-stage compressor with intercoolers, and a multi-stage expander with reheaters.
When storing energy, the compressor compresses atmospheric air to a supercritical state, capturing and storing the heat generated in the heat storage/heat exchanger. The cold storage/heat exchanger employs retained cool energy to cool the supercritical air and transform it into a liquid form, while a valve or liquid expander reduces the liquid air to atmospheric pressure. A cryogenic storage tank stores the liquid air, and the cold storage/heat exchanger receives the gaseous air to release cold energy.
During the energy release process, the system transports liquid air to the cold storage/heat exchanger and raises its temperature to atmospheric levels using a cryopump, thereby increasing it to supercritical pressure. The high-pressure air from the cold storage/heat exchanger actively flows to the heat storage/heat exchanger, where it absorbs compression heat (or industrial waste heat), and then undergoes expansion in the expander to generate work.

Features of Compressed Air Energy Storage Systems

During the process, the cold storage/heat exchanger recovers and preserves the frigid energy from the liquid air. The multi-stage compressor with intercoolers in an SC-CAES system not only diminishes the energy needed for compression but also reclaims the heat created from the compression. The multi-stage expander with reheaters can use the recovered compression heat to augment the power output.
When a suitable cycling working medium is available, the heat storage/heat exchanger and the cold storage/heat exchanger can utilize packed bed thermal energy storage and double-tank thermal energy storage. The valve and the liquid expander actively reduce the high-pressure liquid air to atmospheric pressure. The air expansion process in a valve is categorized as an isenthalpic process, while in a liquid expander, it is categorized as an isentropic process.

As a result, opting for a liquid expander can boost the liquefaction rate. However, the two-phase gas-liquid flow in the liquid expander is intricate, and the liquid expander is pricier than the valve. The system utilizes a cryopump to actively raise the pressure of liquid air from the cryogenic storage tank. The cryogenic storage tank stores liquid air, and to sustain a consistent pressure inside the cryogenic storage tank, it is necessary to promptly discharge the redundant gas air and pipe it to the cold storage/heat exchanger. Research shows that the round-trip efficiency of the supercritical CAES system is about 64.4%, with an energy density of 6.43 – 105 kJ m3.

Future research directions

As researchers, we need to actively conduct further studies to ensure that we realize the potential of both isothermal and supercritical CAES systems in the near future. We can achieve this by actively engaging in ongoing research and development efforts. Future research should focus on improving the round-trip efficiencies of these systems. Achieving this requires actively optimizing the operation, structure, and size of the CAES plant. Another avenue for future research is integrating the CAES systems with renewable energy and other energy storage systems. This integration can enhance the performance of each element of CAES systems.