Study On Efficiency Of Finite Power Engine

In this thesis,we study the finite-time performance of(irreversible)heat engines which produce finite power and are thus more realistic models(compared with the quasi-static engines of vanishing power).For various cyclic engines with their working systems ranging from micro-scale to macro-scale level,we analyze their thermodynamics and finite-power performance,and determine the limited bounds and possible universality of the machine efficiency under maximal power.The first chapter introduces the research area and theoretical tools for analyzing the performance of a heat engine in finite time,outlining rationale for the present study.In second chapter,we study the minimally nonlinear irreversible heat engines in which the time-reversal symmetry for the systems may be broken.The expressions for the power and the efficiency are derived,in which the effects of the nonlinear terms due to dissipations are included.We show that,as within the linear responses,the minimally nonlinear irreversible heat engines can enable attainment of Carnot efficiency at positive power.We also find that the Curzon-Ahlborn limit imposed on the efficiency at maximum power can be overcome if the time-reversal symmetry is broken.The Chapter three addresses the finite-power performance of quantum heat engines working in the linear response regime where the temperature gradient is small.The engine cycles with working substances of ideal harmonic systems consist of two heat transfer and two adiabatic processes,such as the Carnot cycle,Otto cycle,and Brayton cycle.By analyzing the optimal protocol under maximum power we derive the explicitly analytic expression for the irreversible entropy production,which becomes the low dissipation form in the long duration limit.Assuming the engine to be endoreversible,we derive the universal expression for the efficiency at maximum power,which agrees well with that obtained from the phenomenological heat transfer laws holding in the classical thermodynamics.Through appropriate identification of the thermodynamic fluxes and forces that a linear relation connects,we find that the quantum engines under consideration are tightly coupled,and the universality of efficiency at maximum power is confirmed at the linear order in the temperature gradient.In the fourth chapter we briefly summarize the content of the entire thesis and point out the limitations of the present work,and we also raise several questions that deserve to be studied in future.