28 research outputs found
Thermodynamic Bounds on Efficiency for Systems with Broken Time-reversal Symmetry
We show that for systems with broken time-reversal symmetry the maximum
efficiency and the efficiency at maximum power are both determined by two
parameters: a "figure of merit" and an asymmetry parameter. In contrast to the
time-symmetric case, the figure of merit is bounded from above; nevertheless
the Carnot efficiency can be reached at lower and lower values of the figure of
merit and far from the so-called strong coupling condition as the asymmetry
parameter increases. Moreover, the Curzon-Ahlborn limit for efficiency at
maximum power can be overcome within linear response. Finally, always within
linear response, it is allowed to have simultaneously Carnot efficiency and
non-zero power.Comment: Final version, 4 pages, 3 figure
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.Comment: 11 pages, 7 figure
Efficiency of a thermodynamic motor at maximum power
Several recent theories address the efficiency of a macroscopic thermodynamic
motor at maximum power and question the so-called "Curzon-Ahlborn (CA)
efficiency." Considering the entropy exchanges and productions in an n-sources
motor, we study the maximization of its power and show that the controversies
are partly due to some imprecision in the maximization variables. When power is
maximized with respect to the system temperatures, these temperatures are
proportional to the square root of the corresponding source temperatures, which
leads to the CA formula for a bi-thermal motor. On the other hand, when power
is maximized with respect to the transitions durations, the Carnot efficiency
of a bi-thermal motor admits the CA efficiency as a lower bound, which is
attained if the duration of the adiabatic transitions can be neglected.
Additionally, we compute the energetic efficiency, or "sustainable efficiency,"
which can be defined for n sources, and we show that it has no other universal
upper bound than 1, but that in certain situations, favorable for power
production, it does not exceed 1/2
Efficiency at maximum power of low dissipation Carnot engines
We study the efficiency at maximum power, , of engines performing
finite-time Carnot cycles between a hot and a cold reservoir at temperatures
and , respectively. For engines reaching Carnot efficiency
in the reversible limit (long cycle time, zero dissipation),
we find in the limit of low dissipation that is bounded from above by
and from below by . These bounds are reached when
the ratio of the dissipation during the cold and hot isothermal phases tend
respectively to zero or infinity. For symmetric dissipation (ratio one) the
Curzon-Ahlborn efficiency is recovered.Comment: 4 pages, 1 figure, 1 tabl
Coefficient of performance at maximum figure of merit and its bounds for low-dissipation Carnot-like refrigerators
The figure of merit for refrigerators performing finite-time Carnot-like
cycles between two reservoirs at temperature and () is
optimized. It is found that the coefficient of performance at maximum figure of
merit is bounded between 0 and for the
low-dissipation refrigerators, where is the
Carnot coefficient of performance for reversible refrigerators. These bounds
can be reached for extremely asymmetric low-dissipation cases when the ratio
between the dissipation constants of the processes in contact with the cold and
hot reservoirs approaches to zero or infinity, respectively. The observed
coefficients of performance for real refrigerators are located in the region
between the lower and upper bounds, which is in good agreement with our
theoretical estimation.Comment: 5 journal pages, 3 figure
Bounds of Efficiency at Maximum Power for Normal-, Sub- and Super-Dissipative Carnot-Like Heat Engines
The Carnot-like heat engines are classified into three types (normal-, sub-
and super-dissipative) according to relations between the minimum irreversible
entropy production in the "isothermal" processes and the time for completing
those processes. The efficiencies at maximum power of normal-, sub- and
super-dissipative Carnot-like heat engines are proved to be bounded between
and , and , 0 and
, respectively. These bounds are also shared by linear, sub-
and super-linear irreversible Carnot-like engines [Tu and Wang, Europhys. Lett.
98, 40001 (2012)] although the dissipative engines and the irreversible ones
are inequivalent to each other.Comment: 1 figur
Bounds of efficiency at maximum power for linear, superlinear and sublinear irreversible Carnot-like heat engines
The efficiency at maximum power (EMP) of irreversible Carnot-like heat
engines is investigated based on the weak endoreversible assumption and the
phenomenologically irreversible thermodynamics. It is found that the weak
endoreversible assumption can reduce to the conventional one for the heat
engines working at maximum power. Carnot-like heat engines are classified into
three types (linear, superlinear, and sublinear) according to different
characteristics of constitutive relations between the heat transfer rate and
the thermodynamic force. The EMPs of Carnot-like heat engines are proved to be
bounded between and for the linear type, 0 and
for the superlinear type, and and for
the sublinear type, respectively, where is the Carnot efficiency.Comment: 6 journal pages, 1 figure, EPL (in press
From thermal rectifiers to thermoelectric devices
We discuss thermal rectification and thermoelectric energy conversion from
the perspective of nonequilibrium statistical mechanics and dynamical systems
theory. After preliminary considerations on the dynamical foundations of the
phenomenological Fourier law in classical and quantum mechanics, we illustrate
ways to control the phononic heat flow and design thermal diodes. Finally, we
consider the coupled transport of heat and charge and discuss several general
mechanisms for optimizing the figure of merit of thermoelectric efficiency.Comment: 42 pages, 22 figures, review paper, to appear in the Springer Lecture
Notes in Physics volume "Thermal transport in low dimensions: from
statistical physics to nanoscale heat transfer" (S. Lepri ed.