What innovative thermal storage techniques are being developed for CAES

Innovative thermal storage techniques for Compressed Air Energy Storage (CAES) are focused on improving energy efficiency by capturing and reusing the heat generated during air compression, thus avoiding fuel use during air expansion and increasing the overall system efficiency.

Key Innovative Thermal Storage Techniques in CAES

1. Adiabatic CAES with Thermal Energy Storage
Adiabatic CAES (A-CAES) systems compress air and store the heat generated during compression in a dedicated thermal storage medium instead of letting it dissipate. This stored thermal energy is then used to reheat the air during expansion, enabling electricity generation without additional fuel. The thermal storage can be in the form of sensible heat stored in solids or liquids, or phase change materials, depending on the temperature range and containment considerations. Siemens Energy highlights the use of such thermal storage integrated with pressure storage in caverns, where both stored heat and compressed air are recuperated for electricity generation.

2. Sensible Heat Storage in Liquids or Solids
One common approach is to use materials that absorb and release heat through temperature changes (sensible heat). Materials like molten salts, ceramics, or concrete can be used as thermal storage media. The selection of the medium depends on factors such as the operating temperature range, heat exchange fluids used, energy loss rates, and containment strategies.

3. Phase Change Material (PCM) Storage
PCMs store thermal energy during phase transitions (such as melting or solidification), offering high energy density and stable temperature regulation. This approach can improve thermal energy retention and efficiency of heat transfer between compression and expansion stages.

4. Near-Isothermal and Isothermal Compression Techniques
– Near-Isothermal Compression: This involves compressing air in close contact with a high thermal mass—often a finned heat absorbing and releasing structure (HARS)—to rapidly transfer the heat of compression to the thermal mass, stabilizing temperature. An external cooling circuit maintains the thermal mass temperature. Near-isothermal processes have efficiencies around 90-95%, approaching the ideal of constant temperature compression and expansion, thereby reducing thermal losses and improving round-trip efficiency.
– Isothermal Compression: Attempts to maintain constant air temperature throughout compression and expansion by continuous heat exchange, using advanced heat exchangers such as finned pistons and low cycle speeds. Although practically challenging at high power levels, advanced materials and heat exchanger designs aim to approach near-100% efficiency in thermal management.

5. Low-Cost and Alternative High-Temperature Thermal Storage Materials and Designs
Research and development continue into novel, cost-effective thermal storage solutions including materials that can tolerate high temperatures for extended cycles with minimal degradation, and designs that reduce thermal losses during storage. These innovations target scaling CAES economically while maintaining high efficiency.

In summary, current innovative thermal storage techniques for CAES emphasize adiabatic cycles with integrated sensible or phase change heat storage, advances in near-isothermal and isothermal compression-expansion processes, and development of cost-effective high-temperature thermal materials. These approaches collectively aim to improve CAES round-trip efficiency, reduce fuel dependence, and lower costs for grid-scale energy storage.