Efficiency at Maximum Power Output of a Quantum-Mechanical Brayton Cycle

The performance in finite time of a quantum-mechanical Brayton engine cycle is discussed, without intro- duction of temperature. The engine model consists of two quantum isoenergetic and two quantum isobaric processes, and works with a single particle in a harmonic trap. Directly employing the finite-time thermodynamics, the efficiency at maximum power output is determined. Extending the harmonic trap to a power-law trap, we find that the efficiency at max/mum power is independent of any parameter involved in the model, but depends on the confinement of the trapping potential.     

Universal Expression of Efficiency at Maximum Power:A Quantum-Mechanical Brayton Engine Working with a Single Particle Confined in a Power-Law Trap

We propose a quantum-mechanical Brayton engine model that works between two superposed states,employing a single particle confined in an arbitrary power-law trap as the working substance. Applying the superposition principle,we obtain the explicit expressions of the power and efficiency,and find that the efficiency at maximum power is bounded from above by the function: η+= θ/(θ+1),with θ being a potential-dependent exponent.     

Bounds of Efficiency at Maximum Power for Normal-, Sub- and Super-Dissipative Carnot-Like Heat Engines

The Carnot-like heat engines are classified into three types (normal-, sub- and, super-dissipative) according to relations between the minimum irreversible entropy production in the "isothermal" processes and the time for completing those processes. The efficiencies at maximum power of normal-, sub- and super-dissipative Carnot-like heat engines are proved to be bounded between η C /2 and η C /(2 η C ), η C /2 and η C , 0 and η C /(2 η C ), respectively. These bounds are also shared by linear, sub- and super-linear irreversible Carnot-like engines [Tu and Wang, Europhys. Lett. 98 (2012) 40001] although the dissipative engines and the irreversible ones are inequivalent to each other.     

A New Step in the Optimization of the Chambadal Model of the Carnot Engine

This paper presents a new step in the optimization of the Chambadal model of the Carnot engine. It allows a sequential optimization of a model with internal irreversibilities. The optimization is performed successively with respect to various objectives (e.g., energy, efficiency, or power when introducing the duration of the cycle). New complementary results are reported, generalizing those recently published in the literature. In addition, the new concept of entropy production action is proposed. This concept induces new optimums concerning energy and power in the presence of internal irreversibilities inversely proportional to the cycle or transformation durations. This promising approach is related to applications but also to fundamental aspects.     

Consistency of optimizing finite-time Carnot engines with the low-dissipation model in the two-level atomic heat engine

The efficiency at the maximum power (EMP) for finite-time Carnot engines established with the low-dissipation model, relies significantly on the assumption of the inverse proportion scaling of the irreversible entropy generation ΔS^(ir) on the operation time τ, i.e. ΔS^(ir) ∝ 1/τ. The optimal operation time of the finite-time isothermal process for EMP has to be within the valid regime of the inverse proportion scaling. Yet, such consistency was not tested due to the unknown coefficient of the 1/τ-scaling. In this paper, we reveal that the optimization of the finite-time two-level atomic Carnot engines with the low-dissipation model is consistent only in the regime of η_C < 2(1 − δ)/(1 + δ), where η_C is the Carnot efficiency, and δ is the compression ratio in energy level difference of the heat engine cycle. In the large-η_C regime, the operation time for EMP obtained with the low-dissipation model is not within the valid regime of the 1/τ-scaling, and the exact EMP of the engine is found to surpass the well-known bound η₊ = η_C/(2 − η_C).