Abstract
Perfect (reversible) cyclic heat engines operate at Carnot efficiency. Perfect reversible) nonheat engines and noncyclic heat engines operate at unit (100%) efficiency. But a usually necessary, although not always sufficient, requirement to achieve reversibility is that an engine must operate infinitely slowly, i.e., quasi-statically. And infinitely slow operation, which implies infinitesimally small power output, is obviously impractical. Most real heat engines operate, if not at maximum power output, then at least closer to maximum power output than to maximum efficiency. Endoreversible heat engines delivering maximum power output operate at Curzon-Ahlborn efficiency. Irrespective of efficiency, engines' work outputs are in almost all cases totally frictionally dissipated as heat immediately (e.g., an automobile operating at constant speed) or on short time scales. But if a heat engine's work output must be frictionally dissipated, it is best to dissipate it not into the cold reservoir but at the highest practicable temperature. We dub this as high-temperature recharge (HTR). This is not always practicable. But if it is practicable, it can yield improved heat-engine performance. We discuss improvements of the Carnot and Curzon-Ahlborn efficiencies achievable via HTR, and show consistency with the First and Second Laws of Thermodynamics. We reply to criticisms of HTR.
Highlights
Introduction, overview, and general considerationsPerfect cyclic heat engines operate at Carnot efficiency [1–7]
A simple example of a noncyclic heat engine is the one-time expansion of a gas pushing a piston
By the First and Second Laws of Thermodynamics, for a standard reversible heat engine operating at Carnot efficiency, the heat input QH from its hot reservoir, the work output W, the waste heat QC exhausted to its cold reservoir, the efficiency εCarnot,std, the entropy change ΔSH,Carnot,std of its hot reservoir, the entropy change ΔSC,Carnot,std of its cold reservoir, and the total entropy change
Summary
Perfect (reversible) cyclic heat engines operate at Carnot efficiency [1–7]. Perfect (reversible) nonheat engines and noncyclic (necessarily one-time, singleuse) heat engines operate at unit (100%) efficiency. [7] considers absorption refrigeration, wherein the entire energy output is into an intermediate-temperature (most typically ambient-temperature) reservoir, and for which HTR is even more strongly never practicable.]} They are not practicable for cyclic heat engines in cases wherein a cyclic heat engine’s work output is not frictionally dissipated immediately or on short time scales [16, 17], for example, as gravitational potential energy sequestered for a long time interval in the construction of a building.
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