HASELiNNOVATION
Personal Website of Yousef Haseli
Entropy in Endoreversible Engines
The objective of this study was to to investigate the thermal efficiency and power production of typical models of endoreversible heat engines at the regime of minimum entropy generation rate. The study considers the Curzon-Ahlborn engine, the Novikov’s engine, and the Carnot vapor cycle. The operational regimes at maximum thermal efficiency, maximum power output and minimum entropy production rate are compared for each of these engines. The results reveal that in an endoreversible heat engine, a reduction in entropy production corresponds to an increase in thermal efficiency. The three criteria of minimum entropy production, the maximum thermal efficiency, and the maximum power may become equivalent at the condition of fixed heat input.
Entropy in Irreversible Engines
A thermodynamic analysis is presented by means of mathematical formulation to examine the performance of the most common types of heat engines including Otto, Diesel, and Brayton cycles, at the regime of minimum entropy generation. All engines are subject to internal and external irreversibilities. It is shown that minimum entropy production criterion neither correlates with maximum thermal efficiency design nor with maximum work output criterion. The results demonstrate that the production of entropy is not necessarily equivalent to the energy losses taking place in real devices.
Entropy & Chemical Equilibrium
A literature survey reveals significant inaccuracy of the prediction of equilibrium models. A thermodynamic analysis is presented to show that the equilibrium calculations rest on a critical assumption of reversible heat exchange between a reactive system and its surrounding. Indeed, a correct application of the energy conservation and entropy balance equation leads to a modified Gibbs function. Minimization of the modified Gibbs function happens to be identical to maximization of the total entropy generation. The actual chemical equilibrium is shown through a methane steam reforming, as an illustrative example, to be correctly predicted by kinetic modeling. The state of chemical equilibrium does not necessarily correspond to maximum entropy generation. Once a chemical equilibrium has been established, both the total entropy generation and the modified Gibbs function remain unaltered and independent of time.
Entropy & Fuel Cells
We apply the conservation of energy and entropy balance equations to derive expressions for the maximum work of hydrogen-oxygen, hydrogen-air and methane-air fuel cells. We show that the theoretical efficiency of a fuel cell may exceed that of a Carnot engine operating between the same low and high temperatures. Contrary to past studies in that the efficiency of an ideal hydrogen fuel cell is shown to decline with temperature, the maximum efficiency is observed to first decrease with reactants temperature, then remains unaltered and finally rises. The lowest value of the maximum efficiency is found to be 79.3%, 75.7%, and 82.1% for hydrogen-oxygen, hydrogen-air and methane-air fuel cells, respectively. By increasing the stoichiometric coefficient of air, the efficiencies of both hydrogen-air and methane-air fuel cells monotonically increase and they approach the 100% limit at a stoichiometric coefficient of 7.2 and 9.8, respectively. It is shown that a Carnot engine whose heat is supplied by an isothermal combustor proposed in some past studies is not a correct means for comparison of the ideal performance of fuel cells and heat engines.