Finding optimal conductive additive content to enhance the performance of coated sorption beds: An experimental study

Abstract

Adding natural graphite flakes to sorbents of sorption cooling systems can significantly enhance the overall thermal diffusivity, while reducing the active material and increasing mass transfer resistance. To find an optimum compromise between these counteracting trends, the sorption performance of CaCl2-silica gel composite sorbents with 0–20 wt% graphite flakes content are tested using a custom-built gravimetric large pressure jump (G-LPJ) test bed. It is observed that in the early stages of sorption, i.e. sorption time less than 20 min, graphite flakes additives increase the specific cooling power (SCP) (from 365 to 604 W/kg for 5 min sorption time, a 65% increase) and coefficient of performance (COP) (from 0.46 to 0.54 for 5 min sorption time, a 17% increase) due to the enhanced sorbent thermal diffusivity (from 0.23 to 1.38 mm2/s, a 500% increase). Over time, as the sorbent approaches equilibrium, the performance enhancement deteriorates by increasing graphite flakes content since there is less active material in the composite. Furthermore, adding 20 wt% graphite flakes to the composite sorbent has led to a 67% increase in SCP0.8 (SCP when the sorbent reaches 80% of equilibrium).