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- water adsorption (3)
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Faujasite zeolites with the highest possible Al-content and different cations as well as a dealuminated zeolite Y have been chosen to study the impact on the adsorption behavior in view of differed adsorptive agents for heat pumps or thermochemical storages. Our results show advantages and disadvantages for water, methanol or ethanol adsorption due to the structure of the anion skeleton, kind of cations and size of the adsorbat molecule.
The water adsorption properties of modified porous sorbents for solar thermal energy storage and heat transformation have been investigated by thermogravimetry (TG) differential thermogravimetry (DTG), microcalorimetry, measurements of water adsorption isotherms, and storage tests. A chabazite type SAPO, a dealuminated faujasite type zeolite, and a mesostructured aluminosilicate, have been synthesized and compared with common zeolites X, Y and silica gel. It has been found that optimized lattice composition and pore architecture contribute to well adapt hydrophilic properties and a beneficial steep isotherm.
The influence of the dealumination degree of NaY with respect to the water adsorption properties was investigated by infrared spectroscopy, thermogravimetry and isotherm measurements. The modification of the samples is a result of a steaming process at different temperatures in dependence of time. It was found that dealumination controls the hydrophilic behavior of NaY and contributes to defined low desorption temperatures. Similar to microporous SAPO's steamed Y-zeolites can be used for low temperature applications of heat transformation and thermal adsorption storage.
We quantitatively studied the hydration and dehydration behaviour of the three astrobiological model lichens Xanthoria elegans, Buellia frigida and Circinaria gyrosa by thermoanalysis and gravimetric isotherm measurements under close-to-Martian environmental conditions in terms of low temperature and low pressure. Additionally, the impact of UVC exposure on the isolated symbionts of B. frigida and X. elegans was studied by thermoanalysis and mass spectrometry as well as by gravimetric isotherm measurements. The thermal analysis revealed whewellite as a component of C. gyrosa which was not found in B. frigida and X. elegans. Neither the water retention nor the thermal behaviour of symbionts changed when irradiated with UVC under dry conditions. On the other hand, UVC irradiation of the wet mycobiont of B. frigida had a distinct impact on the hydration/dehydration ability which was not observed for the mycobiont of X. elegans. Possibly the melanin of B. frigida's mycobiont, that is not present in X. elegans, or a specifically damaged acetamido group of the chitin of B. frigida may be the sources of additional UVC-induced sorption sites for water associated with the UVC exposure.
Motivated by findings of new mineral related water sources for organisms under extremely dry conditions on Earth we studied in an interdisciplinary approach the water sorption behaviour of halite, soil component and terrestrial Nostoc commune biofilm under Mars relevant environmental conditions. Physicochemical methods served for the determination of water sorption equilibrium data and survival of heterotrophic bacteria in biofilm samples with different water contents was assured by recultivation. Deliquescence of halite provides liquid water at temperatures <273 K and may serve as water source on Mars during the morning stabilized by the CO2 atmosphere for a few hours. The protecting biofilm of N. commune is rather hygroscopic and tends to store water at lower humidity values. Survival tests showed that a large proportion of the Alphaproteobacteria dominated microbiota associated to N. commune is very desiccation tolerant and water uptake from saturated NaCl solutions (either by direct uptake of brine or adsorption of humidity) did not enhance recultivability in long-time desiccated samples. Still, a minor part can grow under highly saline conditions. However, the salinity level, although unfavourable for the host organism, might be for parts of the heterotrophic microbiota no serious hindrance for growing in salty Mars-like environments.
Increasing interest in the application of the thermochemical storage or adsorption heat pumps for utilization of industrial waste heat and solar heat stimulated several experimental and theoretical studies on short and long term heat storage (Tchernev, 2001, Mittelbach et al., 2000, Hauer et al.,1999) as well as heat driven heat pumps (Meunier,, 1986, Dieng, and Wang, 2001). However, less attention was paid to the properties of the porous materials such as common zeolites, silica gels or aluminosilicates which are mainly designed for catalysis and adsorption technologies but are not optimized for thermochemical heat storage applications. In recent years some new storage materials were suggested and characterized such as the so called composite adsorbens (Levitskij et al., 1996, Jänchen et al., 2000) which have considerable high storage densities, a low charging temperature but their temperature lift is limited. Very recently (Jänchen, et al., 2002) we suggested the introduction of the AlPO4’s for heat storage and heat transformation purposes. AlPO4’s are zeolites-like microporous materials, which are mildly hydrophilic and may have the potential to close the gap between silica gels and zeolites in respect to the optimization of the adsorption strength of the water and the ability to be desorbed at mild conditions. Those microporous aluminophosphates can be modified by incorporation of some silicon into the lattice to tailor their adsorption properties with respect to the water adsorption/desorption properties. The aim of this paper is to characterize the state of the art of the optimization of those new thermochemical storage materials, based on recent results, as an starting point for a continuation of the this project.