Salt hydrates for thermochemical storage in a thermal battery

Publikation: Bücher und AnthologienDissertationsschriftenForschung

Authors

Thermochemical heat storage is a highly promising technology, because of the possibility of theoretical unlimited storage time of heat energy without losses. Additionally, the power you can get from, is adjustable, and decoupled from the charging power. Potentially, the thermochemical heat storage has a high energy storage density. Therefore, this technology can be important for a more efficient energy infrastructure. However, reactions with an usable heat exchanges show some challenges, justifying why there is not yet a thermochemical heat storage system on the market place – contrary to heat storage systems based on adsorption materials. The Dissertation was authored during the research project “Thermal Battery” of the Leuphana University Lüneburg, which had the task to construct an economically operating heat storage battery for the use in combination with a micro-combined heat and power plant (micro-CHP). What is required for such a heat storage system are the energy storage density and the efficiency of the used heat storage material, which are important. The project-based approach of the research group and the related parallel research, planning and construction (of the nonchemical battery elements) reasoned a lack of knowledge of the application parameters at the start of the work on the dissertation. This was compiled by the research group throughout the entire term of the project. Thus, a very broad research with parallel lab-scale and TGA/DSC experiments was performed. Accompanying the project three screening series on materials to evaluate their fitness for thermochemical heat storage were done. Firstly, the reaction enthalpy depending on the reaction temperature was investigated and as a result the reaction system of the MgCl2 • 2 H2O and a mixture of MgCl2 • 4 H2O and MgCl2 • 5 H2O was suggested. Secondly, the chemical reversibility and environmental friendly behavior of the material were analyzed. The result was the second suggestion: CaCl2 • 2 H2O and also again a mixture of CaCl2 • 4 H2O and CaCl2 • 5 H2O. [2] Third, the general material properties such as toxicology and explosion risk, and reaction properties of 125 salt hydrates were analyzed depending on one possible thermal battery scenario. Based on theoretical calculations and TGA/DSC – experiments three salt hydrates were found. For a scenario of a low temperature heat storage application with charging at a temperature of 105 °C and discharging at 60 °C SrBr2 • 6 H2O, MgSO4 • 7 H2O and LaCl3 • 7 H2O were suggested. [3] A very important point is the cycle stability. Therefore different thermogravimetrical experiments were performed: 1.) at stabile water vapor pressure during charging (dehydration) and discharging (hydration) [4,5], 2.) charging below dry nitrogen and discharging below humidified nitrogen [6], 3. charging and discharging at stabile water vapor pressure and additional “stress cycles” (extended hydration periods). [5] The salt hydrates of MgCl2, CaCl2 and MgSO4 were tested with this methods. An important result was, that the cycle stability of the different hydrates of each salt were different. A discharging to the hexahydrate caused dissolving in captured water (MgCl2) or melting (CaCl2), and as a result, the rapidly worsening of the cycle stability. [6,7]. An increased cycle stability were obtained by controlled hydration to tetra/ penta hydrate of the salts. Surprisely, the highest cycle stability was obtained by targeted overhydration called “disturbed cycles”. The salt hydrates of MgSO4 showed no cycle stability. The main reason for this was the low kinetic during the hydration and this could not be lifted by greatly extension of the hydration period. Finally, mixtures were created based on the mentioned salt hydrates and were also tested on cycle stability. The mixture of MgCl2 and CaCl2 showed a great cycle stability during mentioned cycle terms 2 and 3. The mixture of MgCl2 and MgSO4 had only during the cycle term 2 and the mixture of CaCl2 and MgSO4 only during the cycle term 3 good cycle stability. The mixture of MgCl2 and CaCl2 were also measured with a powder XRD after the cycle stability test. This investigations strengthened the suspicion of the synthesis of tachyhydrite (CaMg2Cl6 • 12 H2O) during the cycle measurements. The measurement of the reaction enthalpy, estimation of the energy density and the investigation of the cycle stability of the mentioned salt hydrates in this thesis enables the use of this materials for different heat storage applications.
Titel in ÜbersetzungSalzhydrate für die thermochemische Speicherung in einer Wärmebatterie
OriginalspracheEnglisch
ErscheinungsortLüneburg
Anzahl der Seiten127
PublikationsstatusErschienen - 09.02.2016

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