Solar cooling systems: Mass transfer studies for a liquid desiccant dehumidifier

Abstract

Liquid desiccant cooling systems have the advantage over conventional compression systems of being able to operate with largely solar thermal energy sources, and of efficiently handling the latent load. The solar energy is used to regenerate the liquid desiccant by removing the water absorbed from air in the dehumidifier. A packed-bed liquiddesiccant (LiBr) dehumidification unit has been operated with varying air conditions and liquid streams and with three levels of packing (0, 28 and 40 cm). Number of transfer units (NTU) values of 2–2.5 were obtained in the best performing configuration; the corresponding height of transfer unit (HTU) values were 0.25–0.31 m. Overall, gas-side mass transfer coefficients calculated for the dehumidifier are made up of contributions from the packed bed and spray sections of the tower. With full packing and a higher solution flow rate, the overall Kya was 151.3 g mol/sm3 contact-volume log mean concentration driving force. Spray-only contacting at the higher solution flow rate gave a Kya of 15.7 g mol/sm3 contact-volume log mean concentration driving force. The individual mass transfer coefficients for the two sections have been separated; to the authors' knowledge, this is the first time the separate contributions of spray and packing have been quantified in a composite dehumidifier tower. Spray contributions were found to contribute from 10 to 70% of the mass transfer occurring in the dehumidifier, the higher percentages being found for a very inefficient deep bed and low liquid flow conditions.