Development of zeolite and hygroscopic salt composite adsorbents for use in adsorption cooling system

The current challenge on cooling system all over the world was reviewed. The adsorption cooling system is believed to be able to solve the problems of energy shortage and environmental pollution. Composite adsorbents synthesized from zeolite 13X and CaCl2 were investigated for applications in solar adsorption cooling systems. The effect of Ca-ion-exchange on the adsorption properties of zeolite 13X was studied by different characterization techniques, including XRF, TGA, BET, BJH and XRD. Sodium ions in the zeolite structure were replaced by calcium ions by ion exchange. It was found that the Ca-ion-exchange process decreased the specific surface areas of the Ca-ion-exchanged zeolites while the total pore volumes were increased. The optimized Ca-ion-exchange condition existed when soaking zeolite 13X in 46wt% CaCl2 solution for 24 hours. The increase in the total pore volume is good for further impregnating the zeolite with CaCl2. A large difference in equilibrium water uptake, 0.4g/g, between 25°C and 75°C under 870Pa was recorded for the 13X/CaCl2 composite adsorbent impregnated in 40wt% CaCl2 solution. This was 419% of that of zeolite 13X under the same condition. The 13X/CaCl2 composite adsorbent showed a high potential in replacing vapor compression chillers in producing chilled water for central air-conditioning systems. The performance of the adsorber using the zeolite 13X/ water pair was also studied in numerical simulation. It was suggested that the adsorbate gas diffusion can be omitted in the future simulation. The zeolite bed thickness shall be limited to 10mm to reduce the bad effect caused by the poor thermal conductivity of the zeolite. Muti-adsorber tube connected in parallel can be employed to provide enough cooling power and at the same time maintain a large SCP and COP. The desorption temperature should be about 200°C for maximizing the COP