Quantum-dot heat engines with irreversible heat transfer

The ability of the internal states of a working fluid to be in a coherent superposition is one of the basic properties of a quantum heat engine. It was recently predicted that in the regime of small engine action, this ability can enable a quantum heat engine to produce more power than any equivalent classical heat engine. It was also predicted that in the same regime, the presence of such internal coherence causes different types of quantum heat engines to become thermodynamically equivalent. Here, we use an ensemble of nitrogen vacancy centers in diamond for implementing two types of quantum heat engines, and experimentally observe both effects.

When incomplete information about the control parameters is quantified as a prior distribution, a subtle connection emerges between quantum heat engines and their classical analogues. We study the quantum model where the uncertain parameters are the intrinsic energy scales and compare it with the classical models where the intermediate temperature is the uncertain parameter. The prior distribution quantifying the incomplete information has the form π(x)∝1/x in both the quantum and the classical models. The expected efficiency calculated in the near-equilibrium limit approaches the value of one third of Carnot efficiency.

96/06754 Experimental study of turbulent flow in the near wakes of single and tandem prisms

Power optimization of an endoreversible Brayton gas heat engine

Finite time thermodynamic evaluation of irreversible Ericsson and Stirling heat engines

We analyze general models of quantum heat engines operating a cycle of two adiabatic and two isothermal processes. We use the quantum master equation for a system to describe heat transfer current during a thermodynamic process in contact with a heat reservoir, with no use of phenomenological thermal conduction. We apply the endoreversibility description to such engine models working in the linear response regime and derive expressions of the efficiency and the power. By analyzing the entropy production rate along a single cycle, we identify the thermodynamic flux and force that a linear relation connects. From maximizing the power output, we find that such heat engines satisfy the tight-coupling condition and the efficiency at maximum power agrees with the Curzon-Ahlborn efficiency known as the upper bound in the linear response regime.

SYNOPSIS The optimal ecological performance of a universal endoreversible steady flow heat engine cycle consisting of a constant thermal-capacity heating branch, a constant thermal-capacity cooling branch and two adiabatic branches with heat transfer loss is derived by taking an ecological optimisation criterion as the objective, which consists of maximising a function representing the best compromise between the power output and exergy loss rate of the heat engine. Some special examples are discussed. A numerical example is given to show the effects of heat reservoir temperature ratio on the ecological criterion versus the efficiency characteristic of the cycle. The results include the performance characteristics of endoreversible steady flow Diesel, Otto, Atkinson and Brayton cycles. The results can provide some theoretical guidance for the design of practical engines.

Recently, the influences of the Dzyaloshinski-Moriya (DM) interaction on the performances of the basic thermo-dynamical quantities have attracted a lot of attention. A large number of investigations on the quantum coupling systems with DM interaction have been carried out. However, the specific effects of spin-orbit coupling with the performance on the quantum heat engine have not been taken into account in previous studies. DM interaction is a special kind spin-orbit coupling. To enrich the research of the quantum heat engines, the investigation about the effect of DM interaction on its thermodynamic characteristics should be included. In this study, we construct two entangled quantum engines based on spin-1/2 systems with different DM interactions, with the spin exchange constant and magnetic field fixed. The quantum Otto engine and the quantum Stirling engine are discussed in this article. By numerical calculation, we obtain the expressions for several thermodynamic quantities and plot the isoline maps of the variation of the basic thermodynamic quantities such as heat transfer, work with D1 and D2 and their efficiency in the two engines. The results indicate that the DM interaction plays an important role in the thermodynamic quantities for the quantum Otto engine and the quantum Stirling engine. In addition, the positive work condition is discussed with different DM interactions, with the spin exchange constant and magnetic field. Furthermore fixed, it is found that the efficiency of quantum Otto engine cycle is smaller than the Carnot efficiency while the quantum Stirling cycle can exceed the Carnot efficiency by using the regenerator. Finally, the second law of thermodynamics is shown to be valid in the two entangled quantum systems.

Laser and photocell quantum heat engines (QHEs) are powered by thermal light and governed by the laws of quantum thermodynamics. To appreciate the deep connection between quantum mechanics and thermodynamics we need only recall that in 1901 Planck introduced the quantum of action to calculate the entropy of thermal light, and in 1905 Einstein's studies of the entropy of thermal light led him to introduce the photon. Then in 1917, he discovered stimulated emission by using detailed balance arguments. Half a century later, Scovil and Schulz-DuBois applied detailed balance ideas to show that maser photons were produced with Carnot quantum efficiency (see Fig. 1A). Furthermore, Shockley and Quiesser invoked detailed balance to obtain the efficiency of a photocell illuminated by "hot" thermal light (see Fig. 2A). To understand this detailed balance limit, we note that in the QHE, the incident light excites electrons, which can then deliver useful work to a load. However, the efficiency is limited by radiative recombination in which the excited electrons are returned to the ground state. But it has been proven that radiatively induced quantum coherence can break detailed balance and yield lasing without inversion. Here we show that noise-induced coherence enables us to break detailed balance and get more power out of a laser or photocell QHE. Surprisingly, this coherence can be induced by the same noisy (thermal) emission and absorption processes that drive the QHE (see Fig. 3A). Furthermore, this noise-induced coherence can be robust against environmental decoherence.Fig. 1.(A) Schematic of a laser pumped by hot photons at temperature T(h) (energy source, blue) and by cold photons at temperature T(c) (entropy sink, red). The laser emits photons (green) such that at threshold the laser photon energy and pump photon energy is related by Carnot efficiency (4). (B) Schematic of atoms inside the cavity. Lower level b is coupled to the excited states a and β. The laser power is governed by the average number of hot and cold thermal photons, and . (C) Same as B but lower b level is replaced by two states b(1) and b(2), which can double the power when there is coherence between the levels.Fig. 2.(A) Schematic of a photocell consisting of quantum dots sandwiched between p and n doped semiconductors. Open circuit voltage and solar photon energy ℏν(h) are related by the Carnot efficiency factor where T(c) is the ambient and T(h) is the solar temperature. (B) Schematic of a quantum dot solar cell in which state b is coupled to a via, e.g., solar radiation and coupled to the valence band reservoir state β via optical phonons. The electrons in conduction band reservoir state α pass to state β via an external circuit, which contains the load. (C) Same as B but lower level b is replaced by two states b(1) and b(2), and when coherently prepared can double the output power.Fig. 3.(A) Photocell current j = Γρ(αα) (laser photon flux P(l)/ℏ(ν(l))) (in arbitrary units) generated by the photovoltaic cell QHE (laser QHE) of Fig. 1C (Fig. 2C) as a function of maximum work (in electron volts) done by electron (laser photon) E(α) - E(β) + kT(c) log(ρ(αα)/ρ(ββ)) with full (red line), partial (brown line), and no quantum interference (blue line). (B) Power of a photocell of Fig. 2C as a function of voltage for different decoherence rates , 100γ(1c). Upper curve indicates power acquired from the sun.

<div class="htmlview paragraph"><b>T</b>HIS paper discusses first the theoretical relation between thermal efficiency and fuel-air ratio for assumed limited-pressure fuel-air cycles. Data on actual cycles are then presented showing the relation of indicated efficiency to fuel-air ratio. This and other relationships are then used in deriving equations for relating the power output of engines operated at constant speed and throttle setting to ambient conditions.</div> <div class="htmlview paragraph">The fundamental nature of the relation between thermal efficiency and fuel-air ratio is emphasized by showing the variation with fuel-air ratio of computed efficiency of assumed limited-pressure fuel-air cycles. This computed efficiency for a given cycle and compression ratio is determined solely by the thermodynamic properties of the fuel-air medium and is the maximum efficiency theoretically possible for the assumed conditions.</div> <div class="htmlview paragraph">The paper also includes an analysis of published data on compression-ignition engine performance showing the relation between actual indicated efficiency and fuel-air ratio. The data available indicate that this relation is not affected significantly by changes in ambient conditions, provided that the pressure at the intake equals the pressure at the final exhaust. Because this basic relation is independent of ambient conditions, it may be used in predicting the effect of changes in ambient conditions on the power output of compression-ignition engines operated at constant speed and throttle setting. This can be done only if the effect of changes in ambient conditions on fuel-air ratio is known or can be computed. Inasmuch as the quantity of fuel supplied to a compression-ignition engine depends only on the speed and throttle setting and is not affected by changes in ambient conditions, the effect on fuel-air ratio of such changes can be determined when the quantity of air entering the engine at each condition is known. This latter factor is determined by the density of the air and by the volumetric efficiency; both of these factors depend on ambient temperature and pressure. Equations relating all of these factors are presented herein. These equations furnish a rational basis for computing the power output of a compression-ignition engine at a given set of conditions from the power output at some other set of conditions. The precision with which the equations represent test results is shown.</div> <div class="htmlview paragraph">The effect of changes in ambient conditions on the power output of engines in which the fuel-air ratio is constant for operation at constant speed and throttle setting is discussed briefly. This discussion applies to the gasoline or more precisely the spark-ignition engine equipped with a carburetor. It is pointed out that knowledge of the relation between fuel-air ratio and indicated efficiency is not required in formulating equations for computing the effect of ambient conditions on the power output of gasoline engines because fuel-air ratio and, consequently, indicated efficiency are more or less constant, depending on the operating characteristics of the carburetor.</div>

A multistage endoreversible Carnot heat pump (CHP) system with a finite heat sink and generalized convective heat transfer law (HTL) [q ∝ (AT)m] is investigated. For the given initial sink temperature, the minimum power consumption (MPC) is chosen to be optimization objective, the sink temperature is the control variable, and the corresponding continuous Hamilton-Jacobi-Bellman (HJB) equation is established. The detailed expressions of the MPC and the corresponding heat sink temperature for Newtonian HTL (m = 1) are further obtained based on the universal results. While for the cases with other HTLs (m ≠ 1), there exists no analytical solution, so numerical algorithm of dynamic programming (DP) is used, and the difference between the MPC optimization of the multistage endoreversible CHP system and the maximum power output (MPO) of the multistage endoreversible Carnot heat engine (CHE) system is also indicated. The obtained results in this paper could help an engineer in better evaluation of energy limits in practical power-consumption processes.

We present a consistent optimization procedure for the optical measurements in open quantum systems using recently developed incoherent control protocol. Assigning an effective hot bath for the two-entangled-photon pump we recast the transmission of classical probe as a work in a quantum heat engine framework. We demonstrate that maximum work in such a heat engine can exceed that for the classical two-photon and one-photon pumps, while efficiency at maximum power can be attributed to conventional boundaries obtained for the three-level maser heat engine. Our results pave the way for incoherent control and optimization of optical measurements in open quantum systems that involve two-photon processes with quantum light.

Recently, the nanoscale quantum heat engine (QHE) has become one of the most productive areas of research, due to its role as a conduit for excess heat. Regarding the low thermal conductance and suitable structural properties of transition-metal dichalcogenides (TMDs), here, we design a highly efficient spin and valley QHE in a normal-ferromagnetic-normal TMD-based tunnel junction. Spin splitting caused by the exchange field and the huge spin-orbit coupling (SOC) of TMDs leads to a remarkable spin-valley--dependent Seebeck coefficient with a sharp peak at $E\ensuremath{\approx}{E}_{F}$. Application of the gate voltage changes the bands from conduction to valence; thus, a remarkable spin-valley--dependent Seebeck and conductance are observed, leading to a large gate- and exchange-field-tunable spin- and valley-dependent maximum power, which is greatly enhanced compared to other two-dimensional materials such as silicene- and graphene-based QHEs. Additionally, a enormous gate-controllable figure of merit and, consequently, a high efficiency at maximum power are achieved in this device, which confirms the advantage of using TMD-based junctions for highly efficient QHEs. These findings may open the door to the design of spin-valley caloritronics devices based on TMDs.

Numerical investigation of forced convection heat transfer and flow irreversibility in a novel heatsink with helical microchannels working with biologically synthesized water-silver nano-fluid

Quantum heat engine with ideal gas has been well studied, yet the role of interaction was seldom explored. We construct a quantum Otto heat engine with N repulsive Bosonic particles in a 1D hard wall box. With the advantage of exact solution using Bethe Ansatz, we obtain not only the exact numerical result of efficiency in all interacting strength c, but also analytical results for strong interaction. We find the efficiency \eta recovers to the one of non-interacting case $\eta_{\mathrm{non}}=1-(L_{1}/L_{2})^{2}$ for strong interaction with asympotic behavior $\eta\sim\eta_{\mathrm{non}}-4(N-1)L_{1}\left(L_{2}-L_{1}\right)/(cL_{2}^{3})$. Here, $L_{1}$ and $L_{2}$ are two trap sizes during the cycle. Such recovery reflects the duality between 1D strongly repulsive Bosons and free Fermion. We observe and explain the appearance of a minimum efficiency at a particular interacting strength c, and study its dependence on the temperature.