47 research outputs found
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.Comment: Modified version compared to the manuscript submitted to EP
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
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.Comment: Chapter 14 in "Thermoelectric Nanomaterials", Editors Kunihito
Koumoto and Takao Mori, Springer Series in Materials Science Volume 182
(2013
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.Comment: 8 pages, 3 figure