Heat and Mass Transfer Characteristics of a Zeolite 13x/cacl2 Composite Adsorbent in Adsorption Cooling Systems

The performance of the adsorption cooling system using the zeolite 13X/CaCl2 composite adsorbent was studied using a numerical simulation. The novel zeolite 13X/CaCl2 composite adsorbent with superior adsorption properties was developed in previous studies [11]. It has high equilibrium water uptake of 0.404 g/g between 25C and 100C 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 75C. 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 75C to 100C. 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. INTRODUCTION Along with rapid development of modern society, energy demand rises in wide range of sectors from our daily usages to industrial applications. High energy consumption has become a worldwide problem. According to U.S. Department of Energy, total worldwide primary energy consumption was 520 EJ in 2008 while it was 400 EJ in 1998 and 360 EJ in 1988 [1]. It is predictable that the demand on energy will not be reduced and will increase continuously along with the rapid development of the cities all over the world. In many places, air-conditioning is a daily necessity but it also contributes significantly to electricity consumption 183 billion kWh, or 16% of electricity consumption by U.S. households, for example [2]. In 1997, 72.5% of homes in the U.S. were equipped with air conditioning, and the figure has been increasing [3]. Air conditioning systems play an even more important role in regions with higher average ambient temperatures and humidity. For example, it contributes up to 40 – 50% of the total building electricity consumption in Asian metropolitan cities such as Hong Kong [4]. To ease the problems of energy shortage, adsorption cooling systems (ACS)s can be a good alternative. ACS offers a number of distinct advantages, as ACS is an environmentallyfriendly thermal system where low grade thermal energy, e.g. solar energy or waste heat from industrial processes, boilers in hospitals and hotels, commercial kitchens, etc., can be used as the input energy source. The working principle and shortages of ACSs have been described in many literatures, see for example [5 – 10]. Major challenges facing the commercialization of ACS include low thermal conductivity and low uptake capacity of the currently available adsorbents, which lead to low specific cooling capacity (SCP) and low coefficient of performance (COP), which in turn lead to bulky and inefficient ACS.