Modeling and Energetic Assessment of Reactor Technologies for Solar Energy Conversion into Fuels and Chemicals

Kurzfassung

Acknowledging the continued demand for chemical energy carriers and hydrocarbon feedstocks as material inputs for the chemical industry while moving away from fossil hydrocarbons resources in an effort to reduce the resulting carbon dioxide (CO2) emissions, this work is concerned with the study and energetic assessment of solar driven chemical fuel processing technologies via mathematical modeling and simulation. Specifically, three reactor technologies implementing photo-electrochemical, photo-thermal and electrochemical energy conversion are discussed against the background of the FlowPhotoChem research project that developed and demonstrated experimentally these directly or indirectly light driven technologies followed by the integration of the individual reactors into an integrated system for the production of ethylene (C2H4) from CO2 and water (H2O). To aid understanding of these reactor technologies and as prerequisite for numerical simulation, mathematical models on the level of the individual reactors are developed, and associated model parameters are determined via measurement or theoretical calculation. For the photo-electrochemical and electrochemical reactors, zero-dimensional, phenomenological models based on the description via polarization curves are employed. The Chosen phenomenological approach is efficient in terms of modeling effort and computational time while providing sufficient predictive accuracy for the uses within this work considering the limited availability of experimental data. On the other hand, for the photo-thermal reactor, a spatially resolved (three-dimensional), comprehensive multiphysics model was developed, taking into account heat- and multi-component species mass transfer including chemical reactions in porous media. Macroscopic, effective equations and parameters are derived starting from the microscopic or pore level description via the homogenization method. Space discretization of the model equations is performed with the Voronoi box based finite volume method and time discretization with the implicit Euler scheme. The implementation of the discretized equations is realized in the Julia programming language utilizing the package VoronoiFVM.jl. The development of the comprehensive model is motivated by the better availability of experimental data resulting from the close involvement in the development of the reactor and the fact that a phenomenological model with desirable characteristics as for the other two reactors is not expected to exist. Validation of the individual reactor models is performed by comparison with experimental data and metrics for energy conversion efficiency from solar to chemical energy for the respective reactor technologies are derived and evaluated for relevant operating conditions. The models on the individual reactor level are then integrated into a stationary System model which is implemented in the process simulation software Aspen Custom Modeler where the system model closely resembles the physical system constructed in the FlowPhotoChem project. First, the system model is applied to simulate the operating Point exhibiting the highest solar-to-chemical efficiency during experimental testing showing good agreement between simulation and experiment. Second, the parameter space is explored via simulation and improved operating conditions are identified. The work concludes with a summary and an outlook on future research directions taking Advantage of the computational tools developed in this work.