Thermodynamics and optimization of chemical and electrochemical energy generators with applications to fuel cells

Aim of project
At present the project is at its beginning (an agreement with the Committee for Scientific Research is pending). The research is performed at the Faculty of Chemical and Process Engineering of Warsaw University of Technology with participation of researchers from the Department of Separation Processes of this Faculty and a researcher from the Energetics Institute in Warsaw (Marcin Błesznowski, M.Sc., Department of Heat Processes, EnI.) The experimental part of the project, done in the fuel cell context, is going to be conducted at the Heat Processes Department (CPC) of the Energetics Institute (EnI). The experimental apparatus is supposed to test electrical properties of fuel cells investigated at EnI.

The theoretical part of the project is conducted at the Department of Separation Processes, Faculty of Chemical and Process Engineering at Warsaw University of Technology (Politechnika Warszawska). Its essentials aim is to extend the optimization approach known in case of thermal power generation systems (worked out by the project participants earlier), such as thermal engines and radiation engines, so to include chemical and electrochemical systems. Effectiveness of such extension was verified by the project participants on a relatively simple model of power generated by a single, simple isomerization reaction, in the present research more complex systems are tested. The difficulties associated with the mentioned extension emerge because practical chemical and electrochemical power generation systems (such as combustion engines and fuel cells) constitute open non-equilibrium systems; this property seriously complicates their thermodynamic description. In case of systems with complex chemical reactors (electrochemical reactors) an additional difficulty, especially for non-stoichiometric mixtures, is caused by complicated system stoichiometry. Finally, a proper approach to the system requires taking into account kinetics of mass diffusion and heat transfer, and suitable linking of these kinetic equations with reaction rate equations in chemically (electrochemically) active systems. All these difficulties have resulted in a rather small number of research projects which propose a sufficiently general and efficient methodological approach to limiting power of chemical (electrochemical) generators; in our opinion, positive, partially successful results are seldom. This substantiates the investigation of theoretical models characterized by possibly large scales of generality in the form of suitable mathematical analyses and computer simulations.

Expected results
One of the expected research results will be determination of fuel cell characteristics (or characteristics of chemical engines) in terms of operation variables, and receiving knowledge-based data on the upper limits of power production in steady and dynamic systems. The data obtained will be analyzed and the conclusions and observation will be available to the scientific community in the form of publications of scientific articles and conference talks, in Poland and abroad. The results of these investigations will lead to thermodynamic limits on production and consumption of power, both mechanical and electrical. In general, the method serves to evaluate enhanced power limits in non-equilibrium processes driven by transfer and rate phenomena. We stress the hierarchical nature of these limits, where so-called endoreversible limits are one step better than classical ones. Solutions will be obtained for chemical systems with linear and nonlinear kinetics. Rate dependent exergies, generalizing classical exergies for systems with finite durations, will be evaluated. Processes bounded by these generalized exergies involve various imperfect phenomena like, e.g., heat conduction or non-ideal compression and expansion. In process modes departing from equilibrium, generalized exergies are larger than in their inversions approaching equilibrium. Corresponding bounds for the mechanical energy yield or consumption are stronger than those defined by classical exergy (enhanced bound). Determined are also efficiencies of fuel cells in terms of operational functions, and properties of optimal dynamical systems which extremize production or consumption of power, for processes with fi nite residence time of reagents and finite number of stages. This will lead to design indices for chemical (electrochemical) energy generators in the form of enhanced power limits, or limits of energy conversion. Some attention will be paid to the effect of pseudochaos, which vanishes with a number of discretizing steps (this will lead to critical assessment of chaos effects in some literature works). Partial results of the project will be used in a future Ph.D. thesis “Thermodynamics and Transport Processes in Fuel Cells” (“Termodynamika i procesy transportowe w ogniwach paliwowych”), prepared by Marcin Błesznowski, M.Sc., supervised by prof. Stanisław Sieniutycz, Ph.D., D.Sc. (FoCPE). The research may lead to our own numerical code, which could be used in future optimization work. Numerous research institutions abroad are a good example showing that sufficiently large scientific information (knowledge-based data) constitutes a suitable starting point to further work and is one of many steps that should be done on the way of improvement of every technology.