Exergetic analysis of a double stage LiBr-H2O thermal compressor cooled by air/water and driven by low grade heat

In the present paper, an exergetic analysis of a double stage thermal compressor using the lithium bromide-water solution is performed. The double stage system considered allows obtaining evaporation temperatures equal to 5 degreesC using solar heat coming from flat plate collectors and other low grade thermal sources. In this study, ambient air and water are alternatively used as cooling fluids without crystallization problems up to condensation-absorption temperatures equal to 50 degreesC. The results obtained give the entropy generated, the exergy destroyed and the exergetic efficiency of the double stage thermal compressor as a function of the absorption temperature. The conclusions obtained show that the irreversibilities generated by the double stage thermal compressor will tend to increase with the absorption temperature up to 45 degreesC. The maximum value corresponds to 1.35 kJ kg(-1) K-1. The entropy generated and the exergy destroyed by the air cooled system are higher than those by the water cooled one. The difference between the values increases when the absorption temperature increases. For an absorption temperature, equal to 50 degreesC, the air cooled mode generates 14% more entropy and destroys 149/6 more exergy than the water cooled one. Also, the results are compared with those of previous studies for single and double effect air cooled and water cooled thermal compressors. The conclusions show that the double stage system has about 22% less exergetic efficiency than the single effect one and 32% less exergetic efficiency than the double effect one. (C) 2004 Elsevier Ltd. All rights reserved.