Combined thermochemical energy storage and heat transformation for industrial waste heat recovery

Abstract

To quantify the value of thermal energy as an “energy currency”, it needs more than solely the energy’s amount given in Joules: the temperature level at which, for example, excess heat is available from industrial processes determines whether this thermal energy can directly be re-integrated as process heat or is emitted to the ambient as waste heat. A further limitation is the temporal coupling of heat supply and heat demand. The latter can be resolved by using thermal energy storage systems, which, however, further “downgrade” the thermal energy in terms of its temperature level, and hence, lead to exergy losses. In this thesis, a thermochemical energy storage system was developed based on the reversible chemical reaction of strontium bromide anhydrate to strontium bromide monohydrate. The phase transition from the anhydrous to the monohydrous phase allows for storage operation in the temperature range from 150°C to 300°C, which is particularly interesting for industrial applications. The temperature downgrade between charging and discharging can be compensated by means of the so-called "thermochemical heat transformation": if the gas pressure is raised between charging and discharging, the stored thermal energy is released at a higher temperature compared to its transfer to the storage. Thermo-dynamically, this system corresponds to the coupling of an energy storage with a thermally driven heat pump. For the thermochemical reaction system SrBr2/H2O, the pressure-dependent re-action temperatures of the hydration and dehydration reaction were experimentally investigated, and an empirical description of the reaction rate was derived from thermogravimetric measurements. The experimental proof-of-concept was performed with an effective thermal upgrade from 180°C (charging temperature at 1 kPa steam pressure) to 280°C (discharging temperature at 560 kPa steam pressure), using a reactor concept scalable for large industrial applications. The operating characteristics of the storage module were experimentally and numerically studied to quantitatively explain the performance-dominating processes in the storage reactor. By means of the experimentally validated simulation study, the limitation of the storage module’s thermal performance by heat transfer was proven. Subsequently, a thermal sensitivity study was executed, which shows that at the moment of maximum thermal power, the major contribution is attributed to the interface between the porous bulk and the heat exchanger wall - and not, as it is often assumed for other packed-bed storage geometries, within the porous medium. In addition to the proof-of-concept, the present study provides the necessary fundamentals for detailed potential analyses of various industrial thermal energy storage and heat transformation applications and the optimization of the storage integration.