226 research outputs found
The absoption refrigerator as a thermal transformer
The absorption refrigerator can be considered a thermal transformer, i.e. a
device that is analogous to the electric transformer. The analogy is based on a
correspondence between the extensive quantities entropy and electric charge and
that of the intensive variables temperature and electric potential
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
Driven Spin Systems as Quantum Thermodynamic Machines: Fundamental Limits
We show that coupled two level systems like qubits studied in quantum
information can be used as a thermodynamic machine. At least three qubits or
spins are necessary and arranged in a chain. The system is interfaced between
two split baths and the working spin in the middle is externally driven. The
machine performs Carnot-type cycles and is able to work as heat pump or engine
depending on the temperature difference of the baths and the energy
differences in the spin system . It can be shown that the efficiency
is a function of and .Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
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 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
Irreversible Performance of a Quantum Harmonic Heat Engine
The unavoidable irreversible losses of power in a heat engine are found to be
of quantum origin. Following thermodynamic tradition a model quantum heat
engine operating by the Otto cycle is analyzed. The working medium of the model
is composed of an ensemble of harmonic oscillators. A link is established
between the quantum observables and thermodynamical variables based on the
concept of canonical invariance. These quantum variables are sufficient to
determine the state of the system and with it all thermodynamical variables.
Conditions for optimal work, power and entropy production show that maximum
power is a compromise between the quasistatic limit of adiabatic following on
the compression and expansion branches and a sudden limit of very short time
allocation to these branches. At high temperatures and quasistatic operating
conditions the efficiency at maximum power coincides with the endoreversible
result. The optimal compression ratio varies from the square root of the
temperature ratio in the quasistatic limit where their reversibility is
dominated by heat conductance to the temperature ratio to the power of 1/4 in
the sudden limit when the irreversibility is dominated by friction. When the
engine deviates from adiabatic conditions the performance is subject to
friction. The origin of this friction can be traced to the noncommutability of
the kinetic and potential energy of the working medium.Comment: 25 pages, 7 figures. Revision added explicit heat-transfer expression
and extended the discussion on the quantum origin of frictio
Carnot cycle for an oscillator
Carnot established in 1824 that the efficiency of cyclic engines operating
between a hot bath at absolute temperature and a bath at a lower
temperature cannot exceed . We show that linear
oscillators alternately in contact with hot and cold baths obey this principle
in the quantum as well as in the classical regime. The expression of the work
performed is derived from a simple prescription. Reversible and non-reversible
cycles are illustrated. The paper begins with historical considerations and is
essentially self-contained.Comment: 19 pages, 3 figures, sumitted to European Journal of Physics Changed
content: Fluctuations are considere
Optimal performance of endoreversible quantum refrigerators
The derivation of general performance benchmarks is important in the design of highly optimized heat engines and refrigerators. To obtain them, one may model phenomenologically the leading sources of irreversibility ending up with results that are model independent, but limited in scope. Alternatively, one can take a simple physical system realizing a thermodynamic cycle and assess its optimal operation from a complete microscopic description. We follow this approach in order to derive the coefficient of performance at maximum cooling rate for any endoreversible quantum refrigerator. At striking variance with the universality of the optimal efficiency of heat engines, we find that the cooling performance at maximum power is crucially determined by the details of the specific system-bath interaction mechanism. A closed analytical benchmark is found for endoreversible refrigerators weakly coupled to unstructured bosonic heat baths: an ubiquitous case study in quantum thermodynamics
The falling chain of Hopkins, Tait, Steele and Cayley
A uniform, flexible and frictionless chain falling link by link from a heap
by the edge of a table falls with an acceleration if the motion is
nonconservative, but if the motion is conservative, being the
acceleration due to gravity. Unable to construct such a falling chain, we use
instead higher-dimensional versions of it. A home camcorder is used to measure
the fall of a three-dimensional version called an -slider. After
frictional effects are corrected for, its vertical falling acceleration is
found to be . This result agrees with the theoretical
value of for an ideal energy-conserving -slider.Comment: 17 pages, 5 figure
Brownian Carnot engine
The Carnot cycle imposes a fundamental upper limit to the efficiency of a
macroscopic motor operating between two thermal baths. However, this bound
needs to be reinterpreted at microscopic scales, where molecular bio-motors and
some artificial micro-engines operate. As described by stochastic
thermodynamics, energy transfers in microscopic systems are random and thermal
fluctuations induce transient decreases of entropy, allowing for possible
violations of the Carnot limit. Despite its potential relevance for the
development of a thermodynamics of small systems, an experimental study of
microscopic Carnot engines is still lacking. Here we report on an experimental
realization of a Carnot engine with a single optically trapped Brownian
particle as working substance. We present an exhaustive study of the energetics
of the engine and analyze the fluctuations of the finite-time efficiency,
showing that the Carnot bound can be surpassed for a small number of
non-equilibrium cycles. As its macroscopic counterpart, the energetics of our
Carnot device exhibits basic properties that one would expect to observe in any
microscopic energy transducer operating with baths at different temperatures.
Our results characterize the sources of irreversibility in the engine and the
statistical properties of the efficiency -an insight that could inspire novel
strategies in the design of efficient nano-motors.Comment: 7 pages, 7 figure
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