Regeneration of Liquid Desiccants in Evacuated Tube Solar Collectors

Hubler, Darrell J.

Graduate Thesis Or Dissertation

Regeneration of Liquid Desiccants in Evacuated Tube Solar Collectors Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/h702q677r

Abstract

Liquid desiccants can be regenerated by heating the solution to evaporate water from the solution. While conventional methods rely on the vapor pressure difference between water in the solution and that in moist air, it has been proposed to directly boil the desiccant in evacuated tube solar collectors. Regeneration by boiling makes the evacuated tube regenerator less susceptible to performance decreases due to humid weather. The low thermal loss through the walls of the evacuated tube also allows regeneration in colder climates. An experimental study was conducted to characterize the opportunity to directly regenerate liquid desiccants in evacuated tube solar collectors. Measurements were performed on a single tilted evacuated tube during outdoor tests using solutions of calcium chloride and water at nominal mass fractions of 10%, 30%, and 50%. The amount of water evaporated from a desiccant solution was determined by measuring both the change in mass of a solution and the mass of water evaporated during regeneration. The test apparatus was further instrumented to provide incident solar radiation on the tube and transient solution temperature measurements inside the collector tube. Test results indicated that the desiccant could be regenerated without significant concentration stratification in the tube.A model of the collector tube was developed, based on well-established efficiency relationships for solar collectors that relate useful energy production to the incident solar radiation and the thermal losses. Using model parameters identified by the experimental data, the model was demonstrated to effectively predict desiccant regeneration rates. The results suggest that regeneration performance can be predicted using simple models with conventional collector efficiency curves.

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