Influence of Cycle Temperatures on the Thermochemical Heat Storage Densities in the Systems Water/Microporous and Water/
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Influence of Cycle Temperatures on the Thermochemical Heat Storage Densities in the Systems Water/Microporous and Water/Mesoporous Adsorbents H. STACH ZeoSys GmbH, Volmerstr. 13, 12489 Berlin J. MUGELE TU-Berlin, Institut f¨ur Haustechnik ¨ J. JANCHEN AND E. WEILER ZeoSys GmbH, Volmerstr. 13, 12489 Berlin
Abstract. The adsorption equilibrium of water on microporous adsorbents (zeolites of NaA-, NaY- and NaXtype as well as their ion exchanged forms) and on mesoporous adsorbents (different silica gels and composite material i.e. silica gel + salt hydrate) has been studied experimentally and theoretically. Using the Dubinin theory of pore filling the characteristic curves of the adsorption systems and other relevant dependences such as isotherms, isobars, isosteres and the curve of the differential heat of adsorption were calculated. For all systems investigated the adsorption were calculated. Aads and the desorption potential Ades of the closed heat storage system were estimated. These values define the working range of the adsorption/desorption cycle and allow to calculate the specific heat storage density h sp . On the basis of h sp the different adsorbents were compared in order to select the optimal porous storage material for a given application. The presented experimental and theoretical investigations show that the adsorption systems water-zeolite and water-composites are promising working pairs for thermochemical heat storage processes for hot tap water supply and space heating of single family dwellings. The advantage of the water-composite system is the low desorption temperature (solar energy) the main shortcoming the low temperature lift. The advantage of the water zeolite system is the high temperature lift, the shortcoming are the relative high desorption temperatures. Keywords: thermochemical heat storage, water adsorption equilibrium, thermo-dynamical data, heat storage density, zeolites, composite adsorbents Introduction Microporous and some mesoporous adsorbents are of increasing interest as storage materials mostly in combination with water as working fluid (Stach et al., 2001; Stach and J¨anchen, 2000; J¨anchen et al., 2004; Mugele, 2004) for application as heat transformers in adsorption heat pumps and thermochemical heat storage. These materials may be used in storages for the application of low temperature heat (e.g. solar energy) but as well of industrial waste heat. In most cases zeolites
are the preferred microporous materials. They show a high energy density (if properly dehydrated) and as well a high temperature lift. Mesoporous adsorbents, in many cases they are represented by silica gels and aluminosilicates, posses a medium energy density and a limited temperature lift. Comparing both groups of adsorbents with analogous chemical composition but different pore systems it can be stated that the advantage of the mesoporous adsorbents are the low charging temperatures making it a preferred material for solar application.
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