Thermochemical energy storage and heat transformation based on SrBr2: generic reactor concept for validation experiments

Kurzfassung

Since energy efficiency of chemical processes becomes more and more important, recovery of thermal waste heat offers an increasing potential for industrial applications. In general, re-integrating waste heat into a chemical process not solely depends on the simultaneous presence of availability and demand. It is also limited by the temperature level of the heat flows, as waste heat flows usually occur at lower temperatures than the actual required process heat. A heat pump could principally be used to close this temperature gap. However, there is no heat pump commercially available yet that offers output temperatures of more than 140 °C [1]. Therefore, thermochemical energy storage based on gas-solid reactions has come into the focus of interest [2]. Such reactions can generally be described by the following reaction equation: A(s) + B(g) <=> AB(s) + ΔRH. By varying the partial pressure of the gaseous reaction partner B, the required reaction temperature can be adjusted. Thereby, it is possible to perform the endothermic reaction at lower temperatures than the exothermic reaction, and hence achieve a temperature lift between energy input and energy output. Additionally, gas-solid reactions can also be used for storing thermal energy with high storage densities, which makes them very attractive candidates for waste heat recovery. In this work, SrBr2/H2O has been chosen as a working pair of materials. The reversible reaction of SrBr2 monohydrate to the hydrate SrBr2 x 6 H2O has been applied for thermochemical energy storage for domestic use below 80 °C [3, 4]. However, by using a different reaction step, namely a lower degree of hydration, energy storage as well as heat transformation at temperatures relevant for industrial waste heat recovery (150 – 300 °C) seems thermodynamically possible. In order to investigate the application potential of this reaction, it was analyzed considering technically relevant boundary conditions. In the oral presentation, a comparison of experimental thermodynamic and kinetic data at two mass scales will be discussed: on the one hand, 15 mg SrBr2 monohydrate were tested using thermogravimetric analysis. On the other hand, 1 kg of the solid was analyzed in a lab-scale reactor which was mainly designed to obtain experimental data, e.g. for model validations. Due to its generic geometry, it allows to test the effects of various process parameters, such as pressure variations or different gas in- and outlet conditions, on the performance of the reactive bulk. This consequently leads to a deeper understanding of material requirements for the applications mentioned above, since thermodynamic and kinetic limitations of the reactive material can be properly distinguished from macroscopic effects, e.g. the effects of heat and mass transfer within its bulk. References: [1] REISSNER, F.; GROMOLL, B.; SCHAEFER, J.; DANOV, V.; KARL, J. Experimental performance evaluation of new safe and environmentally friendly working fluids for high temperature heat pumps. European Heat Pump Summit, Nürnberg, Germany, October 2013. [2] YU, Y.Q.; ZHANG, P.; WU, J.Y.; WANG, R.Z. Energy upgrading by solid-gas reaction heat transformer: A critical review. Renewable and Sustainable Energy Reviews, 2008, Volume 12, 1302-1324. [3] LELE, A.F.; KUZNIK, F.; OPEL, O.; RUCK, W.K.L. Performance analysis of a thermochemical based heat storage as an addition to cogeneration systems. Energy Conversion and Management, 2015, Volume 106, 1327–1344. [4] MICHEL, B.; MAZET, N.; NEVEU, P. Experimental investigation of an innovative thermochemical process operating with a hydrate salt and moist air for thermal storage of solar energy: Global performance. Applied Energy, 2014, Volume 129, 177-186.