Fundamentals and applications of cryogen as a thermal energy carrier: A critical assessment

This paper reviews and assesses the current status of the use of cryogen as an energy carrier, with a focus on the thermodynamic aspects and cryogenic energy extraction. Cryogen as an energy carrier is different from normal heat storage media in that the energy storage in a cryogen occurs through decreasing its internal energy while increasing its exergy. It is shown that cryogens have a higher energy density than other commonly used thermal energy storage media, and cryogen can be efficient working media for recovering low grade heat due to their low critical temperatures. If there are high grade heat sources, a combination of the direct expansion with a Brayton cycle is shown to be the most efficient method to extract the cryogenic exergy for most cryogens. This, however, is not true for hydrogen as its latent heat accounts for only a small portion of the released cold and a simple Brayton cycle is more suitable for the exergy recovery. If there is only ambient and/or a low grade heat source, a combination of direct expansion and a Rankine cycle is more attractive due to its low power consumption in the compression process, and this appears to be more promising when carbon dioxide capture is considered.