Technology

AE-CAES couples the flow of heat and work in a novel fashion, which can be thought of as an open, Rankine-cycle-like heat engine / pump that uses air as its working fluid and adsorption as its phase change

It is well known that certain kinds of nano-porous materials, most notably zeolites, are able to adsorb substantial quantities of nitrogen and oxygen. The problem is that most of the air comes off at low pressures, so the resulting energy density is too low to be useful even for stationary applications.

It is also known, however, that the amount of air such materials adsorb depends very strongly on temperature: whereas the density of gaseous air is proportional to the inverse absolute temperature, the amount of air adsorbed by a given volume of a zeolite depends exponentially on the inverse temperature. In effect, we have a new equation of state for air!

This allows us to use a new thermodynamic cycle for CAES, in which the pressure is kept approximately constant while the temperature is varied, as opposed to “isothermal” CAES where the temperature is kept approximately constant while the pressure is varied (see figure on right). With presently available zeolites at 10 bar, a temperature swing between –40°C and 100°C is able to store nearly 10 times more air would be stored in the same empty volume at 10 bar.

Because the heat capacities of zeolites are similar to those of sand or rock, it follows this system will store substantial amounts of cold along with the mechanical work that the compressed air can perform as it desorbs. Cold and work are both forms of energy!

The AE-CAES storage cycle combines the pressure swing used in conventional CAES with a temperature swing. In the latter, the temperature of the adsorbent is controlled so that it adsorbs or desorbs air at a constant rate, thereby keeping the pressure over it approximately constant.