Compressed air energy storage systems use compressed air as a medium for storing energy during off-peak times and generating energy during peak hours. The CAES system utilizes electricity during off-peak times to drive the air compressor for air compression. Also, the electricity is used to store the compressed air in underground caverns.

CAES systems are regarded as hybrid systems because electricity from fossil fuels is required to recover the stored energy. Two main types of CAES systems include adiabatic and diabatic CAES systems.

Diabatic compressed air energy storage (D-CAES)

The diabatic compressed air energy storage system is the oldest CAES concept. It is the simplest type of compressed air energy storage plant. Almost all the existing CAES plants are adiabatic. In D-CAES systems, the motor that drives the compressor is driven by electricity.

After every compression stage, the temperature of the air is reduced to near-ambient temperature. The resulting waste heat from air compression is discharged into the environment via intercoolers. This decreases the required power consumption by increasing the pressure ratios of the compressor.

The compressed air is stored in underground caverns at a low temperature to reduce the thermal stress in the cavern wall. The heat is neither given off nor absorbed. This happens at thermal equilibrium between the two systems.

For electricity to be generated, compressed air is released and heated. Heating is needed to enhance the efficiency of the system. The air is used to drive a turbine via heating by natural gas combustion. To produce electricity, the turbine drives a generator.

Huntorf commercial CAES plant in Germany utilizes a 60-megawatt compressor power and generates a 321-megawatt output power. The plant operates a diabatic CAES architectural design where the air compressed at 5-7 MPa is stored in a 310,000 cubic meters volume and 600 m depth mined salt cavern.

Another commercial diabatic CAES plant is the McIntosh CAES plant in the United States. The plant has a 50-megawatt compressor and generates 110-megawatt of power. It preheats the inlet of the gas turbine with high-temperature exhaust air from the gas turbine via a heat exchanger. The McIntosh CAES plant stores the compressed air in mined salt caverns.

Adiabatic compressed air energy storage (A-CAES)

The A-CAES was developed to enhance the efficiency of D-CAES systems. In A-CAES, the heat generated from the compressor is stored and used to warm he air entering the turbines. This eliminates the need to combust natural gas and the accompanying environmental and efficiency impacts.

A-CAES exists in two types: A-CAES with thermal energy storage (TES) and A-CAES without thermal energy storage. In A-CAES without TES, the air is not cooled after the compression stages and the storage of the air is at higher temperatures. Thus, the air does not need heating before its expansion. A-CAES without TES reduces thermal energy losses. The disadvantages of this system are the inability to compress the air to high pressures because of the high temperature. This reduces the potential of the A-CAES system without TES to store energy. Also, more expensive storage vessels are required to store compressed air at very high temperatures.

In A-CAES with TES, the heat is withdrawn from the air after every compression stage. However, unlike D-CAES, the heat is stored in an appropriate storage medium like packed bed heat storage. The stored thermal energy is used to heat the compressed air before the air is sent to the turbine. This eliminates the heat energy requirement from an additional thermal energy source. The cost of the TES element is one of the significant challenges of A-CAES systems.

D-CAES vs. A-CAES

A-CAES and D-CAES systems can be compared with respect to the number of expansion and compression stages, expansion and compression ratios, and the effect of ambient temperature. The efficiencies of the expanders and compressors in the A-CAES and D-CAES systems are also compared.

For both adiabatic CAES and diabatic CAES systems, the consumed work per unit mass of compressor air increases with an increase in ambient temperature. Also, the work generation per unit mass per expander air increases with ambient temperature increase. As the ambient temperature increases, the roundtrip efficiency decreases.

In D-CAES systems, increasing the expansion and compression ratios would decrease the work generation per unit mass of the expander air and increase the work consumption per unit mass of the compressor air. As the expansion and compression ratios increase, the roundtrip efficiency decreases.

Conversely, in AA-CAES, increasing the expansion and compression ratios would increase the work generation per unit mass of the expander air and the work consumption per unit mass of the compressor air. As the expansion and compression ratios increase, the roundtrip efficiency increases.

The work generation per unit mass of expander air and work consumption per unit of compressor air increase with an increase in the number of compression stages in D-CAES systems. On the other hand, the work generation per unit mass of expander air and work consumption per unit mass of compressor air decrease with an increase in the number of compression stages in A-CAES systems.

Increasing the number of expansion and compression stages in D-CAES systems would increase the roundtrip efficiency. However, a negative increase would be observed in an A-CAES system.

Future directions

To get the best out of the air discharged from the A-CAES system and improve system performance, several future research directions are proposed. Research is needed to improve the thermal energy efficiency and roundtrip efficiency of A-CAES systems. A modified A-CAES system that can simultaneously provide efficient cooling power, thermal energy, and mechanical energy should be developed.

Developing fully optimized operation, size and structure for the entire CAES plant can enhance efficiency. However, such a plant requires entire system simulations using sophisticated software tools. To successfully achieve increased deployment of CAES systems on a commercial scale, the storage of heat and cold energies in large timescales and quantities need to be studied.