HEAT AND MASS TRANSFER CHARACTERISTICS OF A ZEOLITE 13X/CACL_2 COMPOSITE ADSORBENT IN ADSORPTION COOLING SYSTEMS

摘要

The performance of the adsorption cooling system using the zeolite 13X/CaCl_2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl_2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25°C and 100°C under 870Pa. The system specific cooling power (SCP) and coefficient of performance (COP) were successfully predicted for different operation parameters. The simulated COP with the composite adsorbent is 0.76, which is 81% higher than a system using pure zeolite 13X under desorption temperature of 75°C. The SCP is also increased by 34% to 18.4 W/kg. The actual COP can be up to 0.56 compared to 0.2 for zeolite 13X-water systems, an increase of 180%. It is predicted that an adsorption cooling system using the composite adsorbent could be powered by a low grade thermal energy source, like solar energy or waste heat, using the temperature range of 75°C to 100°C. The performance of the adsorber with different design parameters was also studied in the present numerical simulation. Adsorbents with smaller porosity can have higher thermal conductivity and may result in better system performance. The zeolite bed thickness should be limited to 10mm to reduce the thermal response time of the adsorber. Addition of high thermal conductivity materials, for example carbon nanotube, can also improve the performance of the adsorber. Multi-adsorber tube connected in parallel can be employed to provide large heat transfer surface and maintain a large SCP and COP. The desorption temperature also showed a large effect on the system performance.