Material Design for Thermochemical Energy Storage with Moving Bed Reactor

Kurzfassung

It is out of the question that effective storage systems for thermal energy have the potential to contribute to a sustainable energy solution: They can help to ensure base load capability despite fluctuating energy sources as well as increase energy efficiency by improved reintegration of industrial waste heat. One way to store thermal energy is by means of reversible gas-solid reactions. Within these thermochemical storage systems, energy is stored by initiating an endothermic reaction. For the storage process the resulting reaction partners are kept separate. To regain the stored energy the reversible exothermic reaction is induced by bringing the partners together again. One very promising thermochemical storage system is based on the reaction of quicklime (CaO) with water vapour to hydrated lime (Ca(OH)2) [1]. This reaction is highly exothermic (reaction enthalpy of 104.4 kJ/mol) and therefore predestined for thermochemical energy storage. Investigations so far also show fast reaction dynamics and good cycling stability of the reaction. All these properties have led to this reaction being used in advanced thermochemical storage systems for thermal energy storage at temperatures between 400 °C and 600 °C [2]. Material characterisation and design of lime for thermochemical storage so far focuses on the microscale properties of the storage materials such as reaction dynamics and temperatures [3,4]. But especially with this low-cost material decoupling of capacity and power is appealing. To enhance the efficiency, moving bed reactors are investigated to separate the storage’s capacity from its power. Such a reactor offers the possibility to lower the cost of the high-capacity storage system’s most cost-intensive part: The reactor itself. New process operations, however, entail new requirements on the material: As many thermochemical storage materials are fine powders, the implementation of this concept is associated with some challenges in the department of material design. As lime powder is not easily moved, efforts made to improve this movability on material basis are the core of this contribution. We present works on modifications of the thermochemical storage material quicklime/hydrated lime as well as the effects these modifications have on important properties such as the materials reactivity and cycling stability. References [1] F. Schaube, L. Koch, A. Wörner, H. Müller-Steinhagen, A thermodynamic and kinetic study of the de- and rehydration of Ca(OH)2 at high H2O partial pressures for thermo-chemical heat storage, Thermochim. Acta. 538 (2012) 9–20. doi:10.1016/j.tca.2012.03.003. [2] M. Schmidt, C. Szczukowski, C. Roßkopf, M. Linder, A. Wörner, Experimental results of a 10 kW high temperature thermochemical storage reactor based on calcium hydroxide, Appl. Therm. Eng. 62 (2014) 553–559. doi:10.1016/j.applthermaleng.2013.09.020. [3] A. Shkatulov, Y. Aristov, Modification of magnesium and calcium hydroxides with salts: An efficient way to advanced materials for storage of middle-temperature heat, Energy. 85 (2015) 667–676. doi:10.1016/j.energy.2015.04.004. [4] J. Yan, C.Y. Zhao, First-principle study of CaO/Ca(OH)2 thermochemical energy storage system by Li or Mg cation doping, Chem. Eng. Sci. (2014). doi:10.1016/j.ces.2014.07.007.