128 research outputs found
Continuity and boundary conditions in thermodynamics: From Carnot's efficiency to efficiencies at maximum power
[...] By the beginning of the 20th century, the principles of thermodynamics
were summarized into the so-called four laws, which were, as it turns out,
definitive negative answers to the doomed quests for perpetual motion machines.
As a matter of fact, one result of Sadi Carnot's work was precisely that the
heat-to-work conversion process is fundamentally limited; as such, it is
considered as a first version of the second law of thermodynamics. Although it
was derived from Carnot's unrealistic model, the upper bound on the
thermodynamic conversion efficiency, known as the Carnot efficiency, became a
paradigm as the next target after the failure of the perpetual motion ideal. In
the 1950's, Jacques Yvon published a conference paper containing the necessary
ingredients for a new class of models, and even a formula, not so different
from that of Carnot's efficiency, which later would become the new efficiency
reference. Yvon's first analysis [...] went fairly unnoticed for twenty years,
until Frank Curzon and Boye Ahlborn published their pedagogical paper about the
effect of finite heat transfer on output power limitation and their derivation
of the efficiency at maximum power, now known as the Curzon-Ahlborn (CA)
efficiency. The notion of finite rate explicitly introduced time in
thermodynamics, and its significance cannot be overlooked as shown by the
wealth of works devoted to what is now known as finite-time thermodynamics
since the end of the 1970's. [...] The object of the article is thus to cover
some of the milestones of thermodynamics, and show through the illustrative
case of thermoelectric generators, our model heat engine, that the shift from
Carnot's efficiency to efficiencies at maximum power explains itself naturally
as one considers continuity and boundary conditions carefully [...]
On the efficiency at maximum cooling power
The efficiency at maximum power (EMP) of heat engines operating as generators
is one corner stone of finite-time thermodynamics, the Curzon-Ahlborn
efficiency being considered as a universal upper bound. Yet, no
valid counterpart to has been derived for the efficiency at
maximum cooling power (EMCP) for heat engines operating as refrigerators. In
this Letter we analyse the reasons of the failure to obtain such a bound and we
demonstrate that, despite the introduction of several optimisation criteria,
the maximum cooling power condition should be considered as the genuine
equivalent of maximum power condition in the finite-time thermodynamics frame.
We then propose and discuss an analytic expression for the EMCP in the specific
case of exoreversible refrigerators
Thermodynamics of Thermoelectric Phenomena and Applications
Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value
Circuits of thermodynamic devices in stationary non-equilibrium
We introduce a non-linear theory of thermodynamic circuits in non-equilibrium
stationary states. The non-equilibrium conductance matrix of a composite device
is obtained from the ones of its sub-devices. This generalizes to thermodynamic
devices the concept of equivalent impedance defined in electronics. An abstract
example and the serial connection of two thermoelectric generators (TEG) with
constant thermoelectric coefficients are considered. Interestingly, a
current-dependent electrical resistance emerges from this connection.Comment: 5 pages, 3 figure
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
Macroeconomic Dynamics in a finite world: the Thermodynamic Potential Approach
This paper presents a conceptual model describing the medium and long-term
co-evolution of natural and socio-economic subsystems of Earth. An economy is
viewed as an out-of-equilibrium dissipative structure that can only be
maintained with a flow of energy and matter. The distinctive approach
emphasized here consists in capturing the economic impact of natural ecosystems
being depleted and destroyed by human activities via a pinch of thermodynamic
potentials. This viewpoint allows: (i) the full-blown integration of a limited
quantity of primary resources into a non-linear macrodynamics that is
stock-flow consistent both in terms of matter-energy as well as economic
transactions; (ii) the inclusion of natural and forced recycling; (iii) the
inclusion of a friction term which reflects the impossibility of producing
goods and services in high metabolising intensity without exuding energy and
matter wastes; (iv) the computation of the anthropically produced entropy as a
function of intensity and friction. Analysis and numerical computations confirm
the role played by intensity and friction as key factors for sustainability.
Our approach is flexible enough to allow for various economic models to be
embedded into our thermodynamic framework.Comment: 30 pages, 10 figure
Fégréac – Berges de la Vilaine
La problématique du franchissement de la Vilaine entre Rieux (Morbihan) et Fégréac (Loire-Atlantique) de l’époque gallo-romaine à la fin du Moyen Âge a été développée selon trois axes : dépouillement des archives, prospection sur les berges du fleuve et prospection subaquatique. Les conclusions des auteurs du xixe s., notamment L. Maître, qui voient un gué à l’époque gallo-romaine et un pont à partir de la fin du xiiie s., ne sont désormais retenues que comme hypothèses de travail. En effet, ..
- …