Modeling and simulation of closed low-pressure zeolite adsorbers for thermal energy storage

Abstract

Thermal energy storages are essential for the efficient implementation of renewable energy sources. Therefore, thermal energy storages based on closed adsorption are studied in literature. Against this background, closed low-pressure adsorbers with zeolite 13X adsorbent of varying configuration (powder, granules, honeycomb) are modeled, simulated and compared in this work. The focus is on the discharging process of the adsorber and special emphasis is put on the accurate incorporation of the rarefaction effects, resulting from low pressure. The simulation results reveal similar behavior for honeycomb and granules adsorbents (temperature wave), while a qualitatively different behavior is observed for the powder adsorbent case. The different behavior for the powder adsorbent case results from a modification of the vapor supply. With respect to modeling, it is found that equilibrium assumptions can not be applied in general. Only for the case of powder adsorbent it is valid to neglect the intra-particle mass transfer resistance and assume instant adsorption equilibrium. Furthermore, rarefaction effects are found to be relevant only for small channel and particle diameters dc/p⩽1mm. Regarding the application, the discharging performance in terms of a defined discharging degree is strongly influenced by the channel and particle diameter. An optimum channel and particle diameter respectively for the honeycomb and granules adsorbent is determined with an maximum discharging degree of 80%. The optimum is a result of limitations by inter- and intra-particle mass transfer resistances. Finally, the discharging degree also strongly depends on the discharging conditions and decreases with increasing discharging temperature and increasing volume flow of the heat transfer fluid.