Application of zeolitic materials prepared from fly ash to water vapor adsorption for solar cooling

The water vapor adsorption properties of raw and hydrothermally treated fly ashes with NaOH and their application prospect as evaporative coolers of roof surfaces were studied. Initially, samples were characterized through techniques like elemental analysis, X-ray diffraction, thermogravimetry, reflectance measurements and water vapor adsorption isotherms. Moreover, the water adsorption properties and the associated temperature variations were determined in a specific wind tunnel with controllable environmental conditions. The adsorption isotherms for fly ash were of type III indicating hydrophobic material with low water vapor adsorption. The hydrothermal treatment in an alkaline solution transformed the fly ash in hydrophilic material of type IV. Moreover, the treated samples were capable of lowering their surface temperatures due to water evaporation and the release of the latent heat. The maximum difference of temperature increase under simulated solar irradiation was observed between the treated fly ash and the concrete with values of 5.0, 5.4 and 7.5 °C for the surface, middle and bottom position, respectively. The results indicate that the zeolitic materials prepared from the fly ash samples have a significant potential for solar cooling applications.

► Water vapor isotherms of lignite ash are of type III with low adsorption capacity.
► Mild fly ash hydrothermal treatment produces a hydrophilic zeolitic material.
► The zeolitic material exhibits a type IV water vapor adsorption isotherm.
► Under irradiation, its surface temperature is reduced due to water desorption.
► Potential application of the fly ash zeolitic material for solar cooling.