CaO/Ca(OH)2 for Moving Bed Thermochemical Energy Storage

Kurzfassung

To realise a sustainable and cost-effective energy solution, the development of efficient energy storage technologies is needed. Especially the storage of thermal energy contains a so far largely untouched potential for such technologies. The least researched of the thermal energy storage types is thermochemical storage, where heat is stored as reaction enthalpy of reversible chemical reactions. Major advantages of thermochemical compared to sensible and latent thermal energy storage are higher storage densities and the possibility of long-term storage. Suitable thermochemical storage materials are generally characterised by high reaction enthalpies, fast dynamics and cycling stability. For a successful transition from material characterisation to storage solution, however, sufficient heat and mass transfer through the reaction bed and within the particles is also vital. A highly promising reaction system for applications between 400 °C and 600 °C (e.g. thermal solar power or industrial processes) has been found in the hydration of quicklime (CaO). Well known from applications in the building industry the reaction CaO(s) + H2O(g) ⇌ Ca(OH)2(s) + HR is currently being investigated with regard to thermal energy storage. Principle suitability was proven as reversibility, reaction dynamics and cyclability were studied and demonstrated by means of thermogravimetric analysis [1] as well as lab- [2] and pilot-scale reactors [3]. Research so far has focused on fixed reactions beds. In order to improve the storage’s efficiency, a moving bed reactor is proposed. However, due to the very fine particle size, the material is a Geldart C powder and therefore difficult to move. The aim of this work is therefore a modification of the raw material in order to enable the movement of the storage material without negatively affecting the material properties. In this contribution, the effects of the investigated modifications on the movability are discussed. Based on important criteria for thermochemical heat storage such as cycling stability and reactivity as well as impacts on the heat and mass transport properties of the bulk, the proposed modifications are evaluated. 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] F. Schaube, A. Kohzer, J. Schütz, A. Wörner, H. Müller-Steinhagen, De- and rehydration of Ca(OH)2 in a reactor with direct heat transfer for thermo-chemical heat storage. Part A: Experimental results, Chem. Eng. Res. Des. 91 (2013) 856–864. doi:10.1016/j.cherd.2012.09.020. [3] 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.