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Salt mixtures in porous ceramics for thermochemical storage of low temperature heat

Salzmischungen in poröser Keramik für die thermochemische Speicherung von Niedrigtemperatur-Wärme
 
: Jänchen, J.; Brettschneider, T.; Lauenroth, S.; Fischer, G.; Hellwig, U.

International Solar Energy Society -ISES-:
30th ISES Biennial Solar World Congress 2011. Vol.6 : Kassel, Germany, 28 August - 2 September 2011
Red Hook, NY: Curran, 2012
ISBN: 978-1-618-39364-7
S.4858-4862
Solar World Congress (SWC) <30, 2011, Kassel>
Englisch
Konferenzbeitrag
Fraunhofer IKTS ()

Abstract
Long term storage of solar heat or transformation of solar heat into could demands often a thermochemical process with materials having a low charging temperature to meet the requirements of simple inexpensive solar collectors. A number of storage materials are available such as silica gel or so called composites (Levitskij et al. 1996, Jänchen et al., 2004, 2005) hosting a hygroscopic salt hydrate. Those salt hydrates, showing a high potential for thermochemical heat storage, may have disadvantages like slow hydration/dehydration kinetics (magnesium sulfate as an example) or a too low value of the deliquescence relative humidity (DRH) typically for halides. The latter leads to liquefaction of the salts at rather low relative humidity already and may result in leakage problems. We have the refore investigated a new approach of utilization of salt hydrates in porous hosts by using a mixture of salts (Posern and Kaps, 2010) to prevent the disadvantages of the single salts.Porous ceramics could be prepared with mean pore diameters varying between 0.3 and 13 µm with corresponding pore volumes of 0.2 to 0.6 cm 3/g. About % of this pore volume of the support can be filled by a hygroscopic salt mixture MgCl 2/MgSO 4 using a saturated solution for the impregnation procedure. The isotherms at 293-313 K show a stepwise curse due to formation of individual hydrates with different strength upon rising humidity. The part of the hydration/dehydration isotherms above 50% relative humidity is reversible whereas the piece below this humidity shows a hysteresis due to the DRH value of the sal t mixture of about 50%. Most probably kinetic effects cause the hysteresis at RH < DRH were solidification of the salt mixtures occurs. The hydrothermal/thermal stability of the composite salt mixture/ceramics in high relative humidity is limited to temperatures up to 150 °C. First tests of a honeycomb structure in an open storage system confirm to some extent suitability of the chosen combination mullite/salt mixture for low temperature utilization of heat. The useable storage capacity, however, remains low so far.

: http://publica.fraunhofer.de/dokumente/N-218697.html