Compressors driven by thermal solar energy: entropy generated, exergy destroyed and exergetic efficiency

Abstract

This work is devoted to the study of the entropy generated, the exergy destroyed and the exergetic efficiency of lithium-bromide absorption thermal compressors of single and double effect, driven by the heat supplied by a field of solar thermal collectors. Two different applications have been considered and compared: air-cooled and water-cooled units. Water-cooled compressors work with temperatures and pressures lower than air-cooled compressors considering, in both cases, the same suction temperature, equal to 5°C. While the absorption temperature in water-cooled compressors can reach 40°C, in air-cooled systems it can vary between 30°C and more than 50°C. Under these conditions, the discharge temperature (boiling temperature within the desorber) of a single effect air-cooled unit lies between 65 and 110°C, the maximum discharge pressure being around 0.12 bar. The discharge temperatures (boiling temperature within the high pressure desorber) of the double effect air-cooled thermal compressor lies between 110°C for a final absorption temperature of 30°C, and 180°C for a final absorption temperature of 50°C. Discharge pressures can reach values of 0.3 and 1.5 bar, respectively. The lithium-bromide air-cooled thermal compressors of double-effect can be viable with absorption temperatures around 50°C, when the temperature difference between the lithium-bromide solution and the outside air is about 8°C. The double effect thermal compressor generates less entropy and destroys less exergy than the single effect unit, leading to a higher exergetic efficiency. In both cases, the compression process of the cooling fluid occurs with entropy reduction.