Crystallization as a limit to develop solar air-cooled LiBr–H2O absorption systems using low-grade heat

In the present work the use of low-temperature solar heat is studied to produce cooling at 5°C, using a double-stage LiBr–H₂O air-cooled absorption cycle. A solar plant, consisting of flat plate collectors feeding the generators of the absorption machine, has been modeled. Operating conditions of the double-stage absorption machine, integrated in the solar plant without crystallization problems for condensation temperatures up to 53°C, are obtained. Results show that about 80°C of generation temperature are required in the absorption machine when condensation temperature reach 50°C, obtaining a COP equal to 0.38 in the theoretical cycle.A comparative study respect to single-stage absorption cycles is performed. Efficiency gain of the double-stage solar absorption system, over the single-stage one, will increase with higher condensation temperatures and lower solar radiation values. Single-stage cycles cannot operate for condensation temperatures higher than 40°C using heat from flat plate collectors. For higher condensation temperatures (45°C) the generation temperatures required (105°C) are very high and crystallization occurs. Condensation temperatures able to use in double-stage cycles may be increased until 53°C using heat from flat plate collectors without reaching crystallization.