Compressed-air energy storage
Compressed-air-energy storage is a way to store energy for later use using compressed air. At a utility scale, energy generated during periods of low demand can be released during peak load periods.
The first utility-scale CAES project was in the Huntorf power plant in Elsfleth, Germany, and is still operational as of 2024. The Huntorf plant was initially developed as a load balancer for fossil-fuel-generated electricity, but the global shift towards renewable energy renewed interest in CAES systems, to help highly intermittent energy sources like photovoltaics and wind satisfy fluctuating electricity demands.
One ongoing challenge in large-scale design is the management of thermal energy, since the compression of air leads to an unwanted temperature increase that not only reduces operational efficiency but can also lead to damage. The main difference between various architectures lies in thermal engineering. On the other hand, small-scale systems have long been used for propulsion of mine locomotives. Contrasted with traditional batteries, compressed-air systems can store energy for longer periods of time and have less upkeep.
Types
Compression of air creates heat; the air is warmer after compression. Expansion removes heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, then the efficiency of the storage improves considerably. There are several ways in which a CAES system can deal with heat. Air storage can be adiabatic, diabatic, isothermal, or near-isothermal.Adiabatic
Adiabatic storage continues to store the heat energy produced by compression and returns it to the air as it is expanded to generate power. This is a subject of an ongoing study, with no utility-scale plants as of 2015. The theoretical efficiency of adiabatic storage approaches 100% with perfect insulation, but in practice, round trip efficiency is expected to be 70%. Heat can be stored in a solid such as concrete or stone, or in a fluid such as hot oil or molten salt solutions. Storing the heat in hot water may yield an efficiency around 65%.Packed beds have been proposed as thermal storage units for adiabatic systems. A study numerically simulated an adiabatic compressed air energy storage system using packed bed thermal energy storage. The efficiency of the simulated system under continuous operation was calculated to be between 70.5% and 71%.
Advancements in adiabatic CAES involve the development of high-efficiency thermal energy storage systems that capture and reuse the heat generated during compression. This innovation has led to system efficiencies exceeding 70%, significantly higher than traditional Diabatic systems.
Diabatic
Diabatic storage dissipates much of the heat of compression with intercoolers into the atmosphere as waste, essentially wasting the energy used to perform the work of compression. Upon removal from storage, the temperature of this compressed air is the one indicator of the amount of stored energy that remains in this air. Consequently, if the air temperature is too low for the energy recovery process, then the air must be substantially re-heated prior to expansion in the turbine to power a generator. This reheating can be accomplished with a natural-gas-fired burner for utility-grade storage or with a heated metal mass. As recovery is often most needed when renewable sources are quiescent, the fuel must be burned to make up for the wasted heat. This degrades the efficiency of the storage-recovery cycle. While this approach is relatively simple, the burning of fuel adds to the cost of the recovered electrical energy and compromises the ecological benefits associated with most renewable energy sources. Nevertheless, this is thus far the only system that has been implemented commercially.The McIntosh, Alabama, CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value of gas for each MJ of energy output, corresponding to an energy recovery efficiency of about 27%. A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 1.85 MJ of gas per MJ generated, a 54% thermal efficiency.
To improve the efficiency of Diabatic CAES systems, modern designs incorporate heat recovery units that capture waste heat during compression, thereby reducing energy losses and enhancing overall performance.
Isothermal
Isothermal compression and expansion approaches attempt to maintain operating temperature by constant heat exchange to the environment. In a reciprocating compressor, this can be achieved by using a finned piston and low cycle speeds. Current challenges in effective heat exchangers mean that they are only practical for low power levels. The theoretical efficiency of isothermal energy storage approaches 100% for perfect heat transfer to the environment. In practice, neither of these perfect thermodynamic cycles is obtainable, as some heat losses are unavoidable, leading to a near-isothermal process. Recent developments in isothermal CAES focus on advanced thermal management techniques and materials that maintain constant air temperatures during compression and expansion, minimizing energy losses and improving system efficiency.Near-isothermal
Near-isothermal compression is a process in which a gas is compressed in very close proximity to a large incompressible thermal mass such as a heat-absorbing and -releasing structure or a water spray. A HARS is usually made up of a series of parallel fins. As the gas is compressed, the heat of compression is rapidly transferred to the thermal mass, so the gas temperature is stabilized. An external cooling circuit is then used to maintain the temperature of the thermal mass. The isothermal efficiency is a measure of where the process lies between an adiabatic and isothermal process. If the efficiency is 0%, then it is totally adiabatic; with an efficiency of 100%, it is totally isothermal. Typically with a near-isothermal process, an isothermal efficiency of 90–95% can be expected.Hybrid CAES systems
Hybrid Compressed Air Energy Storage systems integrate renewable energy sources, such as wind or solar power, with traditional CAES technology. This integration allows for the storage of excess renewable energy generated during periods of low demand, which can be released during peak demand to enhance grid stability and reduce reliance on fossil fuels. For instance, the Apex CAES Plant in Texas combines wind energy with CAES to provide a consistent energy output, addressing the intermittency of renewable energy sources.Other
One implementation of isothermal CAES uses high-, medium-, and low-pressure pistons in series. Each stage is followed by an airblast venturi pump that draws ambient air over an air-to-air heat exchanger between each expansion stage. Early compressed-air torpedo designs used a similar approach, substituting seawater for air. The venturi warms the exhaust of the preceding stage and admits this preheated air to the following stage. This approach was widely adopted in various compressed-air vehicles such as H. K. Porter, Inc.'s mining locomotives and trams. Here, the heat of compression is effectively stored in the atmosphere and returned later on.Compressors and expanders
Compression can be done with electrically-powered turbo-compressors and expansion with turbo-expanders or air engines driving electrical generators to produce electricity.Storage
Air storage vessels vary in the thermodynamic conditions of the storage and on the technology used:- Constant volume storage
- Constant pressure storage
Constant-volume storage
The storage vessel is often a cavern created by solution mining or by using an abandoned mine; use of porous and permeable rock formations, such as those in which reservoirs of natural gas are found, has also been studied.
In some cases, an above-ground pipeline was tested as a storage system, giving some good results. Obviously, the cost of the system is higher, but it can be placed wherever the designer chooses, whereas an underground system needs some particular geologic formations.
Constant-pressure storage
In this case, the storage vessel is kept at constant pressure, while the gas is contained in a variable-volume vessel. Many types of storage vessels have been proposed, generally relying on liquid displacement to achieve isobaric operation. In such cases, the storage vessel is positioned hundreds of meters below ground level, and the hydrostatic pressure of the water column above the storage vessel maintains the pressure at the desired level.This configuration allows:
- Improvement of the energy density of the storage system because all the air contained can be used.
- Removal of the requirement of throttling prior to the expansion.
- Avoidance of mixing of heat at different temperatures in the Thermal Energy Storage system, which leads to irreversibility.
- Improvement of the efficiency of the turbomachinery, which will work under constant-inlet conditions.
- Use of various geographic locations for the positioning of the CAES plant.
A different approach consists of burying a large bag buried under several meters of sand instead of water.
Plants operate on a peak-shaving daily cycle, charging at night and discharging during the day. Heating the compressed air using natural gas or geothermal heat to increase the amount of energy being extracted has been studied by the Pacific Northwest National Laboratory.
Compressed-air energy storage can also be employed on a smaller scale, such as exploited by air cars and air-driven locomotives, and can use high-strength air-storage tanks. In order to retain the energy stored in compressed air, this tank should be thermally isolated from the environment; otherwise, the energy stored will escape in the form of heat, because compressing air raises its temperature.