Water–Lithium Bromide Absorption Chillers for Solar Cooling

Solar thermal resources can be effectively utilized to meet the refrigeration and air-conditioning demands for both household and industrial purposes. Considerable fraction of total available electricity is consumed by the conventional vapor compression refrigeration systems (VCRS) during the summer season in countries with tropical climate. The leakage of VCRS refrigerants in the atmosphere has also been identified as one of the major contributors toward ozone layer depletion and hence global warming. The utilization of solar thermal energy for obtaining refrigeration and air-conditioning is the key to address these issues concerning high electricity demand as well as the environmental pollution. Solar thermal energy, being one of the leading resources of green energy, can reduce the carbon footprint considerably, when used for sorption cooling process. The advantages of using sorption cooling systems powered by solar thermal energy over VCRS are twofold when we consider environmental issues. Sorption-based solar thermal cooling reduces the electricity demand for cooling to a large extent, which in turn reduces usage of fossil fuels to produce this electricity, and thus leads to low-carbon footprint. Also, the refrigerants used for sorption cooling are less prone to cause ozone layer depletion. Although sorption-based refrigeration systems driven by solar thermal energy are mature technologies, wide acceptability of such cooling system is yet to be achieved. Two major limitations of sorption-based solar thermal cooling are relatively low coefficient of performance (COP), and large volume requirement. Other than these two limitations, the intermittent nature of solar thermal resource and heat exchanger and control mechanism design complicacies also pose considerable challenge. Sorption cooling technology can be broadly classified base on absorption and adsorption processes. Absorption is a volumetric phenomenon where a substance of one state gets absorbed in another substance in a different state with or without having chemical reaction, such as liquid being absorbed by solid or gas being absorbed by liquid. On the other hand, adsorption is a surface phenomenon due to physical bonding forces such as Van der walls forces between a solid surface and adjacent fluid or due to chemical bonding between the two. The discussion in this chapter is attributed to Water–Lithium Bromide-based absorption cooling systems. The discussion emphasizes on fundamental concepts of absorption refrigeration cycle, starting with simplest intermittent vapor absorption refrigeration system and gradually elaborating toward the operating principles of commercially used chillers at the end. Cycle analysis of most commonly used single-effect absorption chillers is discussed in a detailed manner along with the background knowledge of how to determine pertinent thermodynamic properties at the inlet and outlet of individual components. Finally, methods and design criteria that can improve the system performance are discussed.