Isothermal compressed air energy storage (CAES) is an emerging technology which attempts to overcome some of the limitations of traditional (diabatic or adiabatic) CAES. Traditional CAES uses turbomachinery to compress air to around 70 bar before storage. In the absence of intercooling the air would heat up to around 900K, making it impossible (or prohibitively expensive) to process and store the gas. Instead the air undergoes successive stages of compression and heat-exchange to achieve a lower final temperature close to ambient. In Advanced-Adiabatic CAES the heat of compression is stored separately and fed back into the compressed gas upon expansion, thereby removing the need to reheat with natural gas – see CAES technical description.
Controlling the pressure-volume (P-V) curve during compression and expansion is the key to efficient CAES. Roughly speaking, the closer the P-V curve resembles an isotherm, the less energy is wasted in the process.
Rather than employing numerous stages to compress, cool, heat and expand the air, isothermal CAES technologies attempt to achieve true isothermal compression and expansion in situ, yielding improved round-trip efficiency and lower capital costs. In principle it also negates the need to store the heat of compression by some secondary means (e.g., oil).
Isothermal CAES is technologically challenging since it requires heat to be removed continuously from the air during the compression cycle and added continuously during expansion to maintain an isothermal process. Heat transfer occurs at a rate proportional to the temperature gradient multiplied by surface area of contact; therefore, to transfer heat at a high rate with a minimal temperature difference one requires a very large surface area of contact.
Although there are currently no commercial Isothermal CAES implementations, several possible solutions have been proposed based upon reciprocating machinery. One method is to spray fine droplets of water inside the piston during compression. The high surface area of the water droplets coupled with the high heat capacity of water relative to air means that the temperature stays approximately constant within the piston – the water is removed and either discarded or stored and the cycle repeats. A similar process occurs during expansion.
The companies developing Isothermal CAES quote a potential round-trip efficiency of 70-80%.
Conclusions and Observations
The technology compresses and expands gas near-isothermally over a wide pressure range, namely from atmospheric pressure (0 psig) to a maximum of about 2,500 psig. This large operating pressure range, along with the isothermal gas expansion (allowing for recovery of heat not achieved with adiabatic expansion), achieves a ~7x reduction in storage cost as compared to classical CAES in vessels.