Theoretical investigation of performances of zeolite Y and SAPO-34 coatings for adsorption heat pump applications

Abstract

SAPO-34 and Y are two of the most commonly investigated zeolites for adsorption heat pump/cooling applications, due to their favorable adsorption properties. Some observations that mass transfer may be a limiting factor for SAPO-34 and faujasite (X/Y) coatings deserve further inquiry to understand better the performance limits of these materials. In this study, a theoretical investigation was made to predict the performances of zeolite Y and SAPO-34 coatings for an adsorption cooling system operated by waste heat, under various conditions. A mathematical model developed and tested previously for coatings of A and X type zeolites on various substrates was used for this purpose. SAPO-34 generally provided relatively high maximum cooling power, owing to its high water sorption capacity coupled with the relatively low regeneration temperature. However, mass transfer resistances became quite significant at relatively high coating thicknesses, originating from the rather slow water diffusion in this zeolite. Utilizing a relatively low desorption temperature of 100 °C, instead of 150 °C, favored the relative performance of SAPO-34 coatings. The strong temperature dependence of water diffusion in zeolite Y reduced the performance of this material. The enhancement of the adsorption temperature from 25 °C to 60 °C resulted in improved performances for NaY coatings. When enhanced diffusivity values were used in the calculations, to represent coatings with more open texture, the cooling power increased notably for both zeolites.