page_title: Compressed-air energy storage
section_title: Nearly isothermal
hierarchical_section_title: Compressed-air energy storage / Types of systems / Nearly isothermal
caption_attribution_description: English: Schematic views of a near isothermal compressor and expander. Left view with piston fully retracted right view with piston fully inserted.
context_page_description: Compressed-air energy storage is a way to store energy generated at one time for use at another time using compressed air. At utility scale, energy generated during periods of low energy demand can be released to meet higher-demand periods. The first utility scale CAES project was built in Huntorf and is still operational. While the Huntorf CAES plant was initially developed as a load balancer for fossil fuel generated electricity in mind, the global shift towards renewable yet highly intermittent energy sources has led to a renewed interest in CAES systems. Thus, CAES systems can have a vital impact in making sure that renewable energy can satisfy fluctuating electricity demands
  . One ongoing challenge in large scale CAES 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 CAES architectures lies in the thermal engineering. On the other hand, small-scale systems have long been used as propulsion of mine locomotives.
context_section_description: A number of methods of nearly isothermal compression are being developed. Fluid Mechanics has a system with a heat absorbing and releasing structure (HARS) attached to a reciprocating piston. Light Sail inject a water spray into a reciprocating cylinder. SustainX use an air-water foam mix inside a semi-custom, 120 rpm compressor/expander. All these systems ensure that the air is compressed with high thermal diffusivity compared to the speed of compression. Typically these compressors can run at speeds up to 1000 rpm. To ensure high thermal diffusivity, the average distance a gas molecule is from a heat-absorbing surface is about 0.5 mm. These nearly isothermal compressors can also be used as nearly isothermal expanders and are being developed to improve the round-trip efficiency of CAES.
caption: Schematic views of a nearly isothermal compressor and expander. Left view with piston fully retracted, right view with piston fully inserted.
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