SrBr2/H2O as reaction system for thermochemical heat transformation

Kurzfassung

In chemical industries, waste heat usually occurs at low temperature levels, whereas process heat is mostly needed at higher temperatures [1]. To bridge this temperature gap, heat pumps are commonly used absorbing thermal energy at low temperatures and releasing it at a higher temperature level. This process is driven by external energy such as electrical energy. However, there is no heat pump available yet on industrial scale that offers an output temperature of more than 140 °C, which is required for many applications [2]. This is why thermochemical heat transformation based on gas-solid reactions has come into the focus of interest [3]. 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 temperature of the exothermic reaction can be adjusted. It is therefore possible to perform the endothermic reaction at lower temperatures than the exothermic reaction. This process is comparable to conventional heat pumps, since it leads to a temperature lift between energy input and energy output. Another positive aspect is the possibility to store thermal energy which extends the range of application, e.g. to batch processes. In order to apply these reactions to thermochemical energy storage systems, the following requirements have to be met: chemical reversibility of the reaction, high reaction enthalpy, small reaction hysteresis and fast reaction kinetics, amongst others. Based on a screening of more than 300 different binary salts, the reversible reaction of SrBr2 anhydrate to its monohydrate has been chosen for further analysis as reaction material for thermochemical heat transformation [4, in preparation]. Using this single step reaction, an energy storage density of 170 kWh/m3 [5] (calculation based on anhydrate and a 50 % powder bed porosity) and heat transformation at high temperature levels (150 – 300 °C) is possible. The oral contribution will outline the thermodynamic principle of thermally driven heat transformation and its main difference to conventional heat pumps. Additionally, the potential of the working pair SrBr2/H2O will be discussed based on experimental data from thermogravimetric analysis at different partial vapor pressures. It will also include first results of the thermodynamic and kinetic analysis of the reaction system. References: [1] BRUECKNER, S.; LIU, S.; MIRO, L.; RADSPIELER, M.; CABEZA, L.F.; LAEVEMANN, E. Industrial waste heat recovery technologies: An economic analysis of heat transformation technologies. Applied Energy, 2015, Volume 151,157-167. [2] BLESL, M.; WOLF, S.; FAHL, U. Large scale application of heat pumps. 7th EHPA European Heat Pump Forum, Berlin, Germany, May 2014. [3] 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. [4] RICHTER, M. et. al. A systematic screening of salt hydrates as materials for a thermochemical heat transformer. In preparation. [5] WAGMAN, D.D.; EVANS, W.H.; PARKER, V.B.; SCHUMM, R H.; HALOW, I.; BAILEY, S.M.; CHURNEY, K.L.; NUTTALL, R.L. The NBS tables of chemical thermodynamic properties. Selected values for inorganic and C1 and C2 organic substances in SI units. Journal of Physical and Chemical Reference Data, 1982, Volume 11, Supplement No. 2.