Brief Discussion on Principles and Development Routes of Compressed Air Energy Storage

Energy storage technologies mainly address three core issues: facilitating the consumption of renewable energy, improving grid performance, and building the Energy Internet. At present, 

diverse energy storage solutions are available, which can be broadly classified into physical energy storage and chemical energy storage. Among them, 

pumped hydro storage, battery energy storage and compressed air energy storage are viable for large-scale deployment.

Pumped hydro storage is constrained by geographical and climatic conditions, requiring terrain with sufficient elevation difference. In regions with extremely low winter temperatures in northern areas, 

water may freeze. Although battery energy storage enjoys robust development momentum, it faces prominent challenges concerning safety, service life and environmental protection, which will pose severe constraints in the future.

Technical Routes of Compressed Air Energy Storage
Compressed Air Energy Storage (CAES) refers to a technology that stores energy via compressed air.

Working principle: During off-peak electricity demand periods, electric power drives compressors to pressurize air into high-pressure air stored in underground caverns or pressure vessels, 

converting electric energy into air internal energy for storage. During peak demand periods, high-pressure air is released from the air storage chamber, 

heated by fuel combustion inside the combustion chamber, and drives turbines together with generators to produce electricity.

A complete CAES system consists of five key components: compressors, coolers, pressure vessels, recuperators and turbines.
Compressor: Compresses air and converts electric energy into internal energy of air. Air pressure can reach 70–100 bar, and temperature up to 1000°C.
Cooler: Heat exchange equipment for cooling air prior to storage in pressure vessels, preventing pressure reduction of air inside vessels or caverns.
Pressure Vessel: Stores cooled air. For cavern storage, geological formations must feature high pressure resistance and good tightness.
Recuperator: Heat exchanger or combustion chamber that raises air temperature to roughly 1000°C to sustain stable long-term turbine operation and improve turbine efficiency.
Turbine: Air expands and depressurizes inside the turbine, converting internal energy into kinetic energy.
Generator: Mostly synchronous generators, converting kinetic energy into electric energy.
CAES technologies fall into three main categories: diabatic, adiabatic and isothermal systems.
Diabatic CAES: Compression heat is removed by cooling water, and turbine inlet air is preheated by external heat sources.
Adiabatic CAES: Compression heat is captured and stored by thermal storage media to preheat turbine inlet air; no external heat source is required for the system.
Isothermal CAES: An ideal technical approach adopting water mist injection to realize near-isothermal compression and expansion. This technology is still immature at present.
Two major technical routes exist:

The first route is conventional compressed air energy storage. Its advantage lies in large power generation capacity, while disadvantages include low energy storage density, 

massive air storage space demand, and drastic pressure fluctuation inside air storage chambers.

The second route is Liquid Air Energy Storage (LAES). It liquefies air to achieve higher energy storage density. Free from geographical restrictions, it enables low-pressure cryogenic storage of liquid air with stable pressure.

The working principle of CAES is straightforward. During the charging phase, intermittent renewable energy or off-peak grid electricity drives compressors to compress air; 

electric energy is stored as high-pressure air in storage chambers, 

and compression heat is recovered simultaneously. During the discharging phase, high-pressure air is stabilized via throttle valves, heated through various preheating methods, 

and high-temperature & high-pressure gas drives air turbines to rotate and generate electricity for grid connection.

Advantages of Compressed Air Energy Storage
As large-scale energy storage technology whose capacity can rival pumped hydro storage, CAES is gaining growing popularity. Its merits include:
Fast startup time (<15 minutes)
High energy density and power density
Black start capability
Low daily operation cost
Abundant underground storage space on the earth’s surface
Long service life and low equipment loss
Low self-discharge rate of stored compressed air

Adiabatic CAES achieves high system efficiency (70%–75%). It requires no fossil fuel to heat compressed air and can genuinely realize carbon neutrality. 

CAES supports grid auxiliary services including secondary and tertiary frequency regulation, 

voltage regulation, peak load shifting, load balancing, spinning reserve and black start, boasting broad application prospects. The technology features strong regional adaptability, 

with huge development potential in the Three-North Region of China. It can also be deployed for offshore wind energy storage (utilizing salt caverns in the North Sea).

Nevertheless, CAES is restrained by multiple limiting factors:
High capital investment and long payback period (payback period > 25 years)
High self-discharge rate of thermal storage units for adiabatic systems; low efficiency (<55%) for non-adiabatic systems
Constructed systems require specific geological conditions (pressure-sealed caverns); salt cavern construction entails high costs
Limited practical operational experience; only two older non-adiabatic CAES plants are in commercial operation globally
Intensified competition from distributed storage systems; CAES lacks competitiveness against other technologies for small-capacity energy storage scenarios

After decades of development, one of the biggest bottlenecks for CAES is air storage technology. Large-scale power stations connected to power grids demand enormous compressed air storage volume. 

Underground rock caverns represent the primary storage method at present. Although rock caverns offer large storage capacity, gas pressure continuously declines during air discharge. 

Throttle valves are required to stabilize pressure. Some scholars have proposed constant-pressure storage schemes, yet such structures are complex and remain unapplied commercially.

Moreover, conventional air compression systems deliver efficiency merely between 40% and 55%, lower than the 80% efficiency of pumped hydro storage. 

Thermodynamically, a large portion of energy is converted into heat during air compression without effective utilization, which constitutes the key cause of low efficiency. To improve system efficiency, 

heat generated during compression can be stored by thermal accumulators and reused to preheat compressed air during power generation for heat recovery. 

This optimized technology is known as Adiabatic Compressed Air Energy Storage (AA-CAES). No demonstration projects of AA-CAES have been put into commercial operation yet. 

Its biggest challenge lies in extending the heat retention duration of thermal storage equipment and developing economically viable system designs.

To tackle air storage challenges, numerous researchers have proposed cryogenic Liquid Air Energy Storage (LAES). Essentially, LAES integrates the Linde liquefaction cycle and Rankine power cycle.

Working principle: In the charging stage, intermittent renewable energy or off-peak grid electricity drives compressors to compress air. High-pressure air is pre-cooled by regenerators and throttled for liquefaction. 

Electric energy is stored in the form of low-pressure cryogenic liquid air, while compression heat is recovered. In the discharging stage, liquid air is pressurized by cryogenic pumps,

releases cold energy and vaporizes inside regenerators, and is heated through preheating processes. High-temperature high-pressure gas drives air turbines to generate grid-connected power. 

The land occupation for LAES air storage is only 1/15 of conventional CAES, significantly improving operational flexibility.

The world’s first LAES demonstration project was jointly developed by the University of Birmingham and Highview Power in the UK, with an installed capacity of 350kW / 2.5MWh. 

The two parties are currently constructing 5MW / 15MWh demonstration units and planning a 250MWh system in the UK. In China, a 100kW / 100kWh LAES system has been built in Langfang Industrial Park.

In the future, air energy storage technologies may evolve into energy base stations that absorb curtailed wind power, photovoltaic power, 

off-peak electricity and medium & low-temperature waste heat. Such stations can flexibly supply electricity to the grid or provide cooling, heating and gas according to demand. 

Building on previous R&D experience with a 500kW CAES system and a 100kW LAES system, the Chinese Academy of Sciences is constructing a 10MW / 100MWh LAES system and planning a hundred-megawatt-class LAES facility to realize combined cooling, 

heating and power (CCHP) supply.

CAES Project Cases

As early as the 1940s, foreign researchers put forward the CAES concept. Limited by low energy storage demand at that time, the technology failed to develop rapidly. 

In the 1960s, driven by the expansion of large-scale nuclear and thermal power plants, grids required peak shaving and valley filling, prompting many countries to launch relevant R&D.

In 1978, the world’s first 290MW CAES power station was built in Huntorf, Germany. In 1991, the second global commercial CAES station with 110MW capacity was commissioned in McIntosh, USA.

In 2003, Alstom of France proposed Advanced Adiabatic Compressed Air Energy Storage (AA-CAES) to eliminate fossil fuel consumption, realizing green electric energy storage by capturing high-temperature compression heat. 

In 2010, ESPC (USA) proposed a combined cycle integrating gas turbines and CAES.

The Huntorf Plant in Germany entered commercial operation in 1978 and remains operational up to now. It is the largest CAES facility in the world. 

The compressor unit consumes 60MW, and the power output during discharge reaches 290MW with a maximum rated continuous output duration of 2 hours. 

The system stores compressed air in abandoned mines 600 meters underground with a total cavern volume of 3.1×10⁵ m³, 

and the maximum compressed air pressure reaches 10 MPa. The unit can perform continuous air charging for 8 hours and continuous power generation for 2 hours. 

Between 1979 and 1991, the plant was synchronized to the grid more than 5,000 times with an average startup reliability of 97.6%. Adopting natural gas supplementary combustion, 

its overall operational efficiency stands at approximately 42%, and the net efficiency excluding supplementary fuel consumption is 19%.

The McIntosh CAES Power Station in Alabama, USA, was commissioned in 1991 as the second commercial CAES plant worldwide. Its compression unit power is 50MW, 

and generation capacity is 110MW. The air storage cavern is located 450 meters underground with a total volume of 5.6×10⁵ m³ and operating pressure of 7.5MPa. 

It supports 41 consecutive hours of air compression and 26 hours of power generation; the unit takes roughly 9 minutes from startup to full load. 

The plant is remotely and automatically controlled by the energy control center of Alabama Power Company. Similar to the Huntorf plant, it adopts natural gas supplementary combustion, 

achieving an operational efficiency of about 54%, and net efficiency of 20% after deducting fuel energy consumption.