Reaction of Calcium Chloride and Magnesium Chloride and their Mixed Salts with Ethanol for Thermal Energy Storage

AbstractThe use of thermochemical energy storage systems increasingly gains interest in order to meet the energy targets of the European renewable energy directive. In this study the suitability of calcium chloride, magnesium chloride and mixed salt ethanolates as heat storage materials for practical implementation was determined by investigating specific thermodynamic properties and estimating the materials’ lifetime at various operating conditions. It was proven that the reaction of the before mentioned metal salts with ethanol depends on the applied ethanol vapour pressure. The ethanol sorption increased in the following order: MgCl2 < CaCl2 < 2CaCl2*MgCl2. The enthalpies followed the same sequence. Over-stoichiometric ethanol uptake, in particular for CaCl2 and 2CaCl2*MgCl2 with increasing C2H5OH vapour pressure, was observed. However, the reaction systems CaCl2-C2H5OH and 2CaCl2*MgCl2 -C2H5OH showed the best sorption properties and cycle stability and thus have a great potential for low-grade thermal energy storage as well as cold storage due to their low reaction temperatures in comparison with salt-water-systems. In general, physically mixing of single salts from the same family with different chemical properties leads to superior thermal behaviour with higher heat storage capacities and material stabilities

Thermal synthesis of a thermochemical heat storage with heat exchanger optimization

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Thermal conductivity measurement of thermochemical storage materials

International audienceThermal properties related to heat and mass transfer are crucial when designing thermochemical heat storage systems. Therefore, enhancing this phenomenon lies in the thermal conductivity of the used material. The effective thermal conductivity of salt hydrates and host matrices is measured using two different methods by differential scanning calorimeter from 100 to 200 °C and radial flow apparatus called guarded hot cartridge from 20 to 70 °C, where the method effect is less than 12%. On this latter, the results as function of temperature was modelled and theoretical correlation of effective thermal conductivity of the material bed presented. Four inorganic salts often used in thermochemical energy storage (CaCl2, MgCl2, SrBr2 and MgSO4) and host matrices (activated carbon, expanded natural graphite and silica gel) were used as samples and the results on both systems for only salts give a thermal conductivity in the range of 0.3–1.3 W m−1 K−1 with measurement uncertainty less than 14%. These obtained data are satisfactory with literature values. Regarding the results, the need of composite design is mandatory to achieve great thermal performances in thermal storage systems, especially in closed systems. The presented results can be used for the evaluation and the improvement of heat and mass transfer in thermochemical and sorption heat storage systems

Sorption and thermal characterization of composite materials based on chlorides for thermal energy storage

International audienceThermochemical heat storage is a promising technology towards efficient use of renewable energy resources. Materials based on salts and their hydrates have a high potential for a good energy storage density and the benefit of long-term storage ability. However, the process has not yet been successfully implemented due to limitations in mass and heat transfer. This paper investigates how to improve the less desirable properties of CaCl2 and its hydrates such as low melting points, agglomeration, low cycle stability and low sorption rates. The optimization of CaCl2 properties was achieved by mixing with KCl and impregnation in carrier materials to obtain a composite material. The tests show at first that, with the admixtures of KCl, water uptake during hydration is 2 times higher than that of CaCl2. Water release during dehydration is 1.3 times higher than that of CaCl2. Secondly, the use of compacted expanded natural graphite (ENG) or activated carbon foam (ACF) increases the cycle stability, thermal conductivity and the water sorption performance. Due to their hydrophobic nature those matrices have no influence on the reaction scheme, thus the total amount of water molecules sorbed by the salt-in-matrix is close to the value of CaCl2. The degree of impregnation varies from 31 to 90 wt% depending on the host matrix and the impregnating medium used. The water vapour uptake is up to 0.61 g g−1 and the water released ranges from 0.12 to 0.72 g g−1. The thermal conductivity of CaCl2-in-matrixis is 3 times higher than that of sole CaCl2