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Y. Apertet

Publications and source records attributed to Y. Apertet.

At least 19 recordsLinked to original sources
arXiv2018

Comment on "Carnot efficiency at divergent power output" (and additional discussion)

In a recent Letter [EPL, 118 (2017) 40003], Polettini and Esposito claimed that it is theoretically possible for a thermodynamic machine to achieve Carnot efficiency at divergent power output through the use of infinitely-fast processes. It appears however that this assertion is misleading as it is not supported by their derivations as demonstrated below. In this Comment, we first show that there is a confusion regarding the notion of optimal efficiency. We then analyze the quantum dot engine described in Ref. [EPL, 118 (2017) 40003] and demonstrate that Carnot efficiency is recovered only for vanishing output power. Moreover, a discussion on the use of infinite thermodynamical forces to reach Carnot efficiency is also presented in the appendix.

cond-mat.stat-mech↗
arXiv2018

Comment on "Optical Imaging of Light-Induced Thermopower in Semiconductor" [Phys. Rev. Applied 5, 024005 (2016)]

In a recent article [Phys. Rev. Applied 5, 024005 (2016)], Gibelli and coworkers proposed a method to determine the thermopower, i.e. the Seebeck coefficient, using photoluminescence measurements. The photoluminescence spectra are used to obtain the local gradients of both the electrochemical potential difference between electron and holes and the temperature of the electron-hole plasma. However, the definition of the thermopower given in that article seems erroneous due to a confusion between the different physical quantities needed to derive this parameter.

cond-mat.mtrl-sci↗
arXiv2017

Small-signal model for frequency analysis of thermoelectric systems

We show how small-signal analysis, a standard method in electrical engineering, may be applied to thermoelectric device performance measurement by extending a dc model to the dynamical regime. We thus provide a physical ground to \textit{ad-hoc} models used to interpret impedance spectroscopy of thermoelectric elements from an electrical circuit equivalent for thermoelectric systems in the frequency domain. We particularly stress the importance of the finite thermal impedance of the thermal contacts between the thermoelectric system and the thermal reservoirs in the derivation of such models. The expression for the characteristic angular frequency of the thermoelectric system we obtain is a generalization of the expressions derived in previous studies. In particular, it allows to envisage impedance spectroscopy measurements beyond the restrictive case of adiabatic boundary conditions often difficult to achieve experimentally, and hence \emph{in-situ} characterization of thermoelectric generators.

cond-mat.mes-hall↗
arXiv2016

Causality in thermoelectric systems: Insights from block diagrams

While Carnot's model engines demonstrate ideal performances regarding conversion efficiency, they cannot be actually used as energy converters since they are non causal systems. Such an unphysical behavior indeed restrains the working conditions to a single point where, in the case of a refrigerator (generator), the cooling power (output power) vanishes. Focusing on the example of a thermoelectric refrigerator, we study the impact of different dissipation sources on the causality of such systems. Basing our analysis on the block diagram description of this system, we discuss particularly the fact that heat conduction cannot ensure causality.

cond-mat.stat-mech↗
arXiv2016

A note on the electrochemical nature of the thermoelectric power

While thermoelectric transport theory is well established and widely applied, there remains some degree of confusion on the proper thermodynamic definition of the Seebeck coefficient (or thermoelectric power) which is a measure of the strength of the mutual interaction between electric charge transport and heat transport. Indeed, as one considers a thermoelectric system, it is not always clear whether the Seebeck coefficient is to be related to the gradient of the system's chemical potential or to the gradient of its electrochemical potential. This pedagogical article aims to shed light on this confusion and clarify the thermodynamic definition of the thermoelectric coupling. First, we recall how the Seebeck coefficient is experimentally determined. We then turn to the analysis of the relationship between the thermoelectric power and the relevant potentials in the thermoelectric system: As the definitions of the chemical and electrochemical potentials are clarified, we show that, with a proper consideration of each potential, one may derive the Seebeck coefficient of a non-degenerate semiconductor without the need to introduce a contact potential as seen sometimes in the literature. Furthermore, we demonstrate that the phenomenological expression of the electrical current resulting from thermoelectric effects may be directly obtained from the drift-diffusion equation.

cond-mat.mtrl-sci↗
arXiv2016

On the fundamental aspect of the first Kelvin's relation in thermoelectricity

Kelvin's relations may be considered as cornerstones in the theory of thermoelectricity. Indeed, they gather together the three thermoelectric effects, associated respectively with Seebeck, Peltier and Thomson, to get a unique and consistent description of thermoelectric phenomena. However, their physical status in literature are quite different. On the one hand, the second Kelvin's relation is associated with the microscopic reversibility, considered as a fundamental thermodynamical property. On the other hand, the first Kelvin's relation is traditionally introduced only as a convenient mathematical relation between Seebeck and Thomson coefficients. In the present article, we stress that, contrary to common believes, this relation may demonstrates deeper insights than a bold mathematical expression between thermoelectric coefficients. It actually reflects the coexistence of two different mechanisms taking place inside thermoelectric systems: Energy conversion and reorganization of heat flow when Seebeck coefficient varies.

cond-mat.mtrl-sci↗
arXiv2015

Green thermoelectrics: Observation and analysis of plant thermoelectric response

Plants are sensitive to thermal and electrical effects; yet the coupling of both, known as thermoelectricity, and its quantitative measurement in vegetal systems never were reported. We recorded the thermoelectric response of bean sprouts under various thermal conditions and stress. The obtained experimental data unambiguously demonstrate that a temperature difference between the roots and the leaves of a bean sprout induces a thermoelectric voltage between these two points. Basing our analysis of the data on the force-flux formalism of linear response theory, we found that the strength of the vegetal equivalent to the thermoelectric coupling is one order of magnitude larger than that in the best thermoelectric materials. Experimental data also show the importance of the thermal stress variation rate in the plant's electrophysiological response. Therefore, thermoelectric effects are sufficiently important to partake in the complex and intertwined processes of energy and matter transport within plants.

physics.bio-ph↗
arXiv2015

Equivalent parameters for series thermoelectrics

We study the physical processes at work at the interface of two thermoelectric generators (TEGs) thermally and electrically connected in series. We show and explain how these processes impact on the system's performance: the derivation of the equivalent electrical series resistance yields a term whose physical meaning is thoroughly discussed. We demonstrate that this term must exist as a consequence of thermal continuity at the interface, since it is related to the variation of the junction temperature between the two TEGs associated in series as the electrical current varies. We then derive an expression for the equivalent series figure of merit. Finally we highlight the strong thermal/electrical symmetry between the parallel and series configurations and we compare our derivation with recent published results for the parallel configuration.

cond-mat.mtrl-sci↗
arXiv2014

Influence of thermal environment on optimal working conditions of thermoelectric generators

Optimization analyses of thermoelectric generators operation is of importance both for practical applications and theoretical considerations. Depending on the desired goal, two different strategies are possible to achieve high performance: through optimization one may seek either power output maximization or conversion efficiency maximization. Recent literature reveals the persistent flawed notion that these two optimal working conditions may be achieved simultaneously. In this article, we lift all source of confusion by correctly posing the problem and solving it. We assume and discuss two possibilities for the environment of the generator to govern its operation: constant incoming heat flux, and constant temperature difference between the heat reservoirs. We demonstrate that, while power and efficiency are maximized simultaneously if the first assumption is considered, this is not possible with the second assumption. This latter corresponds to the seminal analyses of Ioffe who put forth and stressed the importance of the thermoelectric figure of merit $ZT$. We also provide a simple procedure to determine the different optimal design parameters of a thermoelectric generator connected to heat reservoirs through thermal contacts with a finite and fixed thermal conductance.

cond-mat.other↗
arXiv2014

Comment on "Effective thermal conductivity in thermoelectric materials"

In a recent article, Baranowski et al. [J. Appl. Phys. 113, 204904 (2013)] proposed a model that allegedly facilitates optimization of thermoelectric generators operation as these latter are in contact with hot and cold temperature baths through finite conductance heat exchangers. In this Comment, we argue that the results and analyses presented by these authors are misleading since their model is incomplete and rests on an inappropriate assumption derived from thermoelectric compatibility theory.

cond-mat.other↗
arXiv2014

Revisiting Feynman's ratchet with thermoelectric transport theory

We show how the formalism used for thermoelectric transport may be adapted to Smoluchowski's seminal thought experiment also known as Feynman's ratchet and pawl system. Our analysis rests on the notion of useful flux, which for a thermoelectric system is the electrical current and for Feynman's ratchet is the effective jump frequency. Our approach yields original insight into the derivation and analysis of the system's properties. In particular we define an entropy per tooth in analogy with the entropy per carrier or Seebeck coefficient, and we derive the analogue to Kelvin's second relation for Feynman's ratchet. Owing to the formal similarity between the heat fluxes balance equations for a thermoelectric generator (TEG) and those for Feynman's ratchet, we introduce a distribution parameter that quantifies the amount of heat that flows through the cold and hot sides of both heat engines. While it is well established that $γ$ = 1/2 for a TEG, it is equal to 1 for Feynman's ratchet. This implies that no heat may be rejected in the cold reservoir for the latter case. Further, the analysis of the efficiency at maximum power shows that the so-called Feynman efficiency corresponds to that of an exoreversible engine, with $γ$ = 1. Then, turning to the nonlinear regime, we generalize the approach based on the convection picture and introduce two different types of resistance to distinguish the dynamical behavior of the considered system from its ability to dissipate energy. We finally put forth the strong similarity between the original Feynman ratchet and a mesoscopic thermoelectric generator with a single conducting channel.

cond-mat.stat-mech↗
arXiv2014

Comment on "Heat dissipation in atomic-scale junctions"

We discuss the recent results of Lee et al. [Nature 498, 209 (2013)] using the notion of thermoelectric convection. In particular, we highlight the fact that this contribution to the thermal flux is not a dissipative process.

cond-mat.mes-hall↗
arXiv2013

A linear nonequilibrium thermodynamics approach to optimization of thermoelectric devices

Improvement of thermoelectric systems in terms of performance and range of applications relies on progress in materials science and optimization of device operation. In this chapter, we focuse on optimization by taking into account the interaction of the system with its environment. For this purpose, we consider the illustrative case of a thermoelectric generator coupled to two temperature baths via heat exchangers characterized by a thermal resistance, and we analyze its working conditions. Our main message is that both electrical and thermal impedance matching conditions must be met for optimal device performance. Our analysis is fundamentally based on linear nonequilibrium thermodynamics using the force-flux formalism. An outlook on mesoscopic systems is also given.

physics.class-ph↗
arXiv2012

Internal convection in thermoelectric generator models

Coupling between heat and electrical currents is at the heart of thermoelectric processes. From a thermal viewpoint this may be seen as an additional thermal flux linked to the appearance of electrical current in a given thermoelectric system. Since this additional flux is associated to the global displacement of charge carriers in the system, it can be qualified as convective in opposition to the conductive part associated with both phonons transport and heat transport by electrons under open circuit condition, as, e.g., in the Wiedemann-Franz relation. In this article we demonstrate that considering the convective part of the thermal flux allows both new insight into the thermoelectric energy conversion and the derivation of the maximum power condition for generators with realistic thermal coupling.

cond-mat.mtrl-sci↗
arXiv2012

Irreversibilities and efficiency at maximum power of heat engines: illustrative case of a thermoelectric generator

Thermoelectric generators are particularly suitable to investigate the irreversible processes which govern the coupled transport of matter and heat in solid-state systems. We study the efficiency at maximum power in the strong coupling regime, where the thermal flux is proportionnal to the electrical current inside the generator. We demonstrate that depending on the source of irreversibility we obtain either the Curzon-Ahlborn efficiency for external dissipation or a universal efficiency at maximum power for internal dissipation. A continuous change between these two extremes is evidenced. Effects of dissymetry of thermal contact conductance are also investigated.

cond-mat.stat-mech↗
arXiv2012

Efficiency at maximum power of thermally coupled heat engines

We study the efficiency at maximum power of two coupled heat engines, using thermoelectric generators (TEGs) as engines. Assuming that the heat and electric charge fluxes in the TEGs are strongly coupled, we simulate numerically the dependence of the behavior of the global system on the electrical load resistance of each generator in order to obtain the working condition that permits maximization of the output power. It turns out that this condition is not unique. We derive a simple analytic expression giving the relation between the electrical load resistance of each generator permitting output power maximization. We then focuse on the efficiency at maximum power (EMP) of the whole system to demonstrate that the Curzon-Ahlborn efficiency may not always be recovered: the EMP varies with the specific working conditions of each generator but remains in the range predicted by irreversible thermodynamics theory. We finally discuss our results in light of non-ideal Carnot engine behavior.

cond-mat.stat-mech↗
arXiv2012

Thermoelectric internal current loops inside inhomogeneous systems

Considering a system composed of two different thermoelectric modules electrically and thermally connected in parallel, we demonstrate that the inhomogeneities of the thermoelectric properties of the materials may cause the appearance of an electrical current, which develops inside the system. We show that this current increases the effective thermal conductance of the whole system. We also discuss the significance of a recent finding concerning a reported new electrothermal effect in inhomogeneous bipolar semiconductors, in light of our results.

cond-mat.mtrl-sci↗