7.2 Compressed air energy storage systems

Compressed air energy storage (CAES) is a mechanical energy storage method that uses electricity to compress air, storing it for later use. This section explores three types of CAES systems: adiabatic, diabatic, and isothermal, each with unique characteristics and efficiency levels.

The CAES process involves compression, storage, and expansion stages, with various infrastructure requirements. Understanding these systems is crucial for grasping their role in grid-scale energy storage and integration with renewable energy sources.

Compressed Air Energy Storage (CAES) Types

Adiabatic CAES

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• Stores compressed air without heat exchange with the environment
• Captures and stores heat from compression separately using thermal energy storage (molten salt, ceramic bricks)
• During expansion, stored heat is used to reheat the air, increasing efficiency (up to 70%)
• Avoids the need for fuel combustion during the expansion process (natural gas)
• Requires advanced insulation and heat storage materials (firebrick, molten salt)

Diabatic CAES

• Compressed air is stored without the heat generated during compression
• Heat is dissipated to the environment during compression and not captured
• During expansion, air is reheated using an external heat source (natural gas combustion)
• Lower efficiency compared to adiabatic CAES (40-50%) due to heat loss and fuel consumption
• Simpler system design and lower capital costs compared to adiabatic CAES

Isothermal CAES

• Aims to maintain a constant temperature during compression and expansion processes
• Heat is continuously removed during compression and added back during expansion
• Approaches a thermodynamically reversible process, potentially achieving high efficiencies (up to 80%)
• Requires advanced heat exchange systems to maintain near-isothermal conditions (spray cooling, porous media)
• Currently in the research and development stage with pilot projects underway (SustainX, LightSail Energy)

CAES Process

Compression Stage

• Air is compressed using electrically-driven compressors during off-peak hours
• Compression raises the air temperature and pressure (up to 70°C and 100 bar)
• Compressed air is cooled and stored in underground caverns or above-ground tanks
• Intercooling between compression stages improves efficiency by reducing compressor work

Expansion Stage

• Stored compressed air is released and expanded through turbines to generate electricity during peak demand
• In diabatic CAES, air is preheated using external heat sources (natural gas combustion) before expansion
• In adiabatic CAES, stored heat from compression is used to reheat the air
• Expansion drives the turbine generator, converting kinetic energy into electrical energy

System Performance

• Round-trip efficiency is the ratio of electrical energy output to input, typically 40-50% for diabatic CAES and up to 70% for adiabatic CAES
• Power-to-heat ratio compares the electrical power output to the thermal energy input from fuel combustion (diabatic CAES)
• Higher power-to-heat ratios indicate better utilization of stored energy and lower fuel consumption
• Isothermal CAES aims for high round-trip efficiencies (up to 80%) by minimizing temperature changes during compression and expansion

CAES Infrastructure

Compressed Air Storage

• Large-scale CAES typically uses underground caverns for air storage (salt caverns, hard rock caverns, aquifers)
• Caverns provide high storage capacities (hundreds of MWh) and air tightness for long-term storage
• Above-ground storage options include steel tanks and pipelines, suitable for smaller-scale applications
• Storage pressure ranges from 50 to 100 bar, depending on the depth and type of storage reservoir

Grid Integration

• CAES plants are connected to the electrical grid through high-voltage transmission lines
• Acts as a grid-scale energy storage system, providing multiple services (peak shaving, load shifting, frequency regulation)
• Complements intermittent renewable energy sources (wind, solar) by storing excess energy and dispatching it when needed
• Existing CAES plants have power capacities ranging from 50 to 300 MW and storage durations of 4 to 24 hours (Huntorf, Germany; McIntosh, USA)