1,742 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
Closed-loop approach to thermodynamics
We present the closed loop approach to linear nonequilibrium thermodynamics
considering a generic heat engine dissipatively connected to two temperature
baths. The system is usually quite generally characterized by two parameters:
the output power and the conversion efficiency , to which we add a
third one, the working frequency . We establish that a detailed
understanding of the effects of the dissipative coupling on the energy
conversion process, necessitates the knowledge of only two quantities: the
system's feedback factor and its open-loop gain , the product of
which, , characterizes the interplay between the efficiency, the
output power and the operating rate of the system. By placing thermodynamics
analysis on a higher level of abstraction, the feedback loop approach provides
a versatile and economical, hence a very efficient, tool for the study of
\emph{any} conversion engine operation for which a feedback factor may be
defined
- …