Design and construction of a compact air-cooled absorption machine for solar energy applications

This paper presents a methodical procedure for the design and sizing of a compact, water fired, air- cooled absorption chiller. The proposed compact machine uses Lithium-Bromide/Water as absorbent/refrigerant fluid pair. The machine was designed using a detailed heat transfer analysis for each individual component (i.e. desorber, condenser, absorber and evaporator). The condenser uses conventional fin-tube heat transfer surfaces while the evaporator uses an inner corrugated surface to increase the heat transfer area. The generator has a concentric tube arrangement in which the dilute solution is in the inner section and the heating water flows in the outside section. This arrangement results in a regeneration effect at temperatures close to 75°C which can be easily provided with solar collectors. The absorber and evaporator work together as a single unit. The vapor exiting the evaporator comes into thermal contact with the concentrated LiBr solution that enters the absorber from the top and falls inside the vertical tubes, creating the absorption effect. Moreover, air is cooling the outside surface of the tubes removing the heat released during the absorption process. The evaporator was designed to be a falling film evaporator such that when applying the cooling load, condensed water falling on the evaporator tubes evaporates and rises through the vertical tubes of the absorber. A set of highly efficient fans are used to bring outside air to remove the necessary heat in both the absorber and condenser, respectively. Furthermore, all system components have been constructed and assembled into a working prototype of variable cooling capacity between 10.5 to 17.5 kW having final dimensions equivalent to a volume of 5 m³. The preliminary characterization of the thermal performance of this prototype is presented in the paper with the objective of validating the design methodology.