F. Angulo-Brown

Compression ratio of an optimized air standard Otto-cycle model

We propose an irreversible simplified model for the air standard Otto thermal cycle. This model takes into account the finite-time evolution of the cycles compression and power strokes and it considers global losses lumped in a friction like term. The proposed model permits the maximization of quantities such as the power output and the efficiency in terms of the compression ratio r. The optimum r values obtained compare well with standard r values for real Otto engines. Our model leads to loop-shaped power-versus-efficiency curves as is common to almost all real heat engines.

A non-endoreversible Otto cycle model: improving power output and efficiency

We propose a finite-time thermodynamics model for an Otto thermal cycle. Our model considers global losses in a simplified way lumped into a friction-like term, and takes into account the departure from an endoreversible regime through a parameter (R) arising from the Clausius inequality. Our numerical results suggest that the cycles power output and efficiency are very sensitive to that parameter. We find that R is the ratio of the constant-volume heat capacities of the reactants and products in the combustion reaction occurring inside the working fluid. Our results have implications in the search for new fuels for internal combustion engines.

Local stability analysis of an endoreversible Curzon-Ahborn-Novikov engine working in a maximum-power-like regime

A local stability analysis of an endoreversible Curzon–Ahborn–Novikov (CAN) engine, working in a maximum-power-like regime, is presented. The CAN engine in the present work consists of a Carnot engine that exchanges heat with the heat reservoirs T1 and T2 (T1 >T 2) through a couple of thermal conductors, both having the same conductance (α). In addition, the working fluid has the same heat capacity (C) in the two isothermal branches of the cycle. From the local stability analysis we conclude that the CAN engine is stable for every value of α, C and τ = T2/T1; that after a perturbation the system state exponentially decays to the steady state with either of two different relaxation times; that both relaxation times are proportional to C/α; and that only one of them depends on τ , being a monotonically decreasing function of τ . Finally, when comparing with the system steady-state energetic properties, we find that as τ increases, the system stability is improved, while the system power and efficiency decrease; this suggests a compromise between the stability and energetic properties, driven by τ .

A general property of non-endoreversible thermal cycles

In this work it is shown that a general property of endoreversible Curzon-Ahlborn-Novikov (CAN) cycles previously demonstrated can be extended for non-endoreversible CAN-cycles. This general property is based on the fact that at the so-called maximum ecological regime the efficiency is the average of the Carnot and the maximum-power efficiencies, and that in such a regime the power output is 75% of the maximum power of the CAN-cycle and the entropy produced is only 25% of that produced in the maximum power point. This property is independent of the heat transfer law.

Dynamic robustness and thermodynamic optimization in a non-endoreversible Curzon-Ahlborn engine

Abstract In this work we analyze the stability of a non-endoreversible Curzon–Ahlborn engine, taking into account the engines implicit time delays. When comparing the systems dynamic stability with its thermodynamic properties (efficiency and power output), we find that the temperature ratio τ = T 2/T 1 (T 1 > T 2 being the temperatures of the external heat reservoirs) represents a trade-off between stability and energetic properties. This result is in agreement with previous studies of the endoreversible Curzon–Ahlborn engine. The only dierence is that, in the non-endoreversible case, τ can only increase up to R (with R < 1, a parameter measuring the degree of internal irreversibilities), while in the endoreversible case it can grow up to one. Finally, we demonstrate that the total time delay does not destabilize the system steady-state, regardless of its length, and thus it does not seem to play a role in the dynamic-thermodynamic property trade-off.

Thermodynamic optimality in some biochemical reactions

SummaryIn this short communication we discuss the possibility that anaerobic glycolisis and (aerobic) respiration, both for adenosine triphosphate (ATP) production, be chemical reactions that follow different thermodynamic-optimization criteria. The former reaction maximizing power output and the latter maximizing a function that represents an advantageous compromise between high power output and low entropy production. Our approach is by means of finite-time thermodynamics (FTT).

On Some Nonendoreversible Engine Models with Nonlinear Heat Transfer Laws

In this work, we analyze a nonendoreversible thermal engine model with a nonlinear heat transfer law between the heat reservoirs and the working fluid under two optimization criteria: the maximum power regime and the so-called ecological criterion. We find that this nonendoreversible model has a similar behaviour to that shown by De Vos (Am. J. Phys. 53, 570 (1985)) for endoreversible models with two thermal conductances with only one superior conductance and with only one inferior conductance, respectively. The model is compared with two sets of real power plants, the first one containing power plants of old design (before 1960s) and the second one being formed by modern nuclear power plants. Our results suggest that the first group was designed under conditions which, are reminiscent of a maximum power regime and the second one under an ecological-like criterion. We also study some general properties of nonendoreversible thermal engine models.

A variational approach to ecological-type optimization criteria for finite-time thermal engine models

In this paper we apply variational calculus procedures for the optimization of a Curzon-Ahlborn thermal cycle under the so-called modified ecological criterion. Our result for the optimum efficiency is the same that Velasco et al (2000 J. Phys. D 33 355) previously obtained by means of the method of the saving functions. Besides, we show that both the saving functions and the modified ecological criteria are equivalent.

First-order irreversible thermodynamic approach to a simple energy converter

Several authors have shown that dissipative thermal cycle models based on finite-time thermodynamics exhibit loop-shaped curves of power output versus efficiency, such as it occurs with actual dissipative thermal engines. Within the context of first-order irreversible thermodynamics (FOIT), in this work we show that for an energy converter consisting of two coupled fluxes it is also possible to find loop-shaped curves of both power output and the so-called ecological function versus efficiency. In a previous work Stucki [J. W. Stucki, Eur. J. Biochem. 109, 269 (1980)] used a FOIT approach to describe the modes of thermodynamic performance of oxidative phosphorylation involved in adenosine triphosphate (ATP) synthesis within mithochondrias. In that work the author did not use the mentioned loop-shaped curves and he proposed that oxidative phosphorylation operates in a steady state at both minimum entropy production and maximum efficiency simultaneously, by means of a conductance matching condition between extreme states of zero and infinite conductances, respectively. In the present work we show that all Stuckis results about the oxidative phosphorylation energetics can be obtained without the so-called conductance matching condition. On the other hand, we also show that the minimum entropy production state implies both null power output and efficiency and therefore this state is not fulfilled by the oxidative phosphorylation performance. Our results suggest that actual efficiency values of oxidative phosphorylation performance are better described by a mode of operation consisting of the simultaneous maximization of both the so-called ecological function and the efficiency.

Connection between maximum-work and maximum-power thermal cycles.

A new connection between maximum-power Curzon-Ahlborn thermal cycles and maximum-work reversible cycles is proposed. This linkage is built through a mapping between the exponents of a class of heat transfer laws and the exponents of a family of heat capacities depending on temperature. This connection leads to the recovery of known results and to a wide and interesting set of results for a class of thermal cycles. Among other results it was found that it is possible to use analytically closed expressions for maximum-work efficiencies to calculate good approaches to maximum-power efficiencies. Behind the proposed connection is an interpretation of endoreversibility hypothesis. Additionally, we suggest that certain reversible maximum-work cycles depending on working substance can be used as reversible landmarks for FTT maximum-power cycles, which also depend on working substance properties.