Showing papers on "Thermal reservoir published in 2015"

Three-terminal energy harvester with coupled quantum dots

Abstract: Rectification of thermal fluctuations in mesoscopic conductors is the key idea behind recent attempts to build nanoscale thermoelectric energy harvesters to convert heat into useful electric power. So far, most concepts have made use of the Seebeck effect in a two-terminal geometry, where heat and charge are both carried by the same particles. Here, we experimentally demonstrate the working principle of a new kind of energy harvester, proposed recently, using two capacitively coupled quantum dots. We show that, due to the novel three-terminal design of our device, which spatially separates the heat reservoir from the conductor circuit, the directions of charge and heat flow become decoupled. This enables us to manipulate the direction of the generated charge current by means of external gate voltages while leaving the direction of heat flow unaffected. Our results pave the way for a new generation of multi-terminal nanoscale heat engines.

Occupation of topological Floquet bands in open systems

Abstract: Floquet topological insulators are noninteracting quantum systems that, when driven by a time-periodic field, are described by effective Hamiltonians whose bands carry nontrivial topological invariants A longstanding question concerns the possibility of selectively populating one of these effective bands, thereby maximizing the system's resemblance to a static topological insulator We study such Floquet systems coupled to a zero-temperature thermal reservoir that provides dissipation We find that the resulting electronic steady states are generically characterized by a finite density of excitations above the effective ground state, even when the driving has a small amplitude and/or large frequency We discuss the role of reservoir engineering in mitigating this problem

Functional integral approach to time-dependent heat exchange in open quantum systems: general method and applications

Abstract: We establish the path integral approach for the time-dependent heat exchange of an externally driven quantum system coupled to a thermal reservoir. We derive the relevant influence functional and present an exact formal expression for the moment generating functional which carries all statistical properties of the heat exchange process for general linear dissipation. The method is applied to the time-dependent average heat transfer in the dissipative two-state system (TSS). We show that the heat can be written as a convolution integral which involves the population and coherence correlation functions of the TSS and additional correlations due to a polarization of the reservoir. The corresponding expression can be solved in the weak-damping limit both for white noise and for quantum mechanical coloured noise. The implications of pure quantum effects are discussed. Altogether a complete description of the dynamics of the average heat transfer ranging from the classical regime down to zero temperature is achieved.

Review on Heat Transfer Mechanisms and Characteristics in Encapsulated PCMs

Abstract: Latent heat storage (LHS) is a particularly promising technique compared with the conventional sensible heat storage (SHS) as it provides a high-energy storage density with a small volume. However, there are difficulties in practical engineering applications of LHS due to the heat releasing/absorbing, which involves phase transition and moving boundary problems and the unacceptable low thermal conductivity of the phase-change material (PCM). Furthermore, the encapsulation would affect the heat transfer characteristics of PCM significantly, depending on the parameters of various encapsulations and boundary conditions. Hence, this review analyzes heat transfer mechanisms during the phase-change process and numerical analysis for heat transfer in macroencapsulated PCMs according to the shape of containment. The effective heat capacity method and the enthalpy method, two of the most widely used numerical approaches for phase-change problems, are presented in detail. Besides numerical models for different PCM ...

New Entropy Formula with Fluctuating Reservoir

Abstract: Finite heat reservoir capacity, C , and temperature fluctuation, Δ T / T , lead to modifications of the well known canonical exponential weight factor. Requiring that the corrections least depend on the one-particle energy, ω , we derive a deformed entropy, K ( S ) . The resultingformula contains the Boltzmann–Gibbs, Renyi, and Tsallis formulas as particular cases. For extreme large fluctuations, in the limit C Δ T 2 / T 2 → ∞ , a new parameter-free entropy–probability relation is gained. The corresponding canonical energy distribution is nearly Boltzmannian for high probability, but for low probability approaches the cumulative Gompertz distribution. The latter is met in several phenomena, like earthquakes, demography, tumor growth models, extreme value probability, etc.

Laser-induced cooling of broadband heat reservoirs

Abstract: We explore, theoretically and experimentally, a method for cooling a broadband heat reservoir, via its laser-assisted collisions with two-level atoms followed by their fluorescence. This method is shown to be advantageous compared to existing laser-cooling methods in terms of its cooling efficiency, the lowest attainable temperature for broadband baths, and its versatility: it can cool down any heat reservoir, provided the laser is red detuned from the atomic resonance. It is applicable to cooling down both dense gaseous and condensed media.

Wellbore–reservoir coupled simulation to study thermal and fluid processes in a CO2-based geothermal system: identifying favorable and unfavorable conditions in comparison with water

Abstract: Using CO2 as a heat transmission fluid to extract geothermal energy is currently considered as a way to achieve CO2 resource utilization and geological sequestration. As a novel heat transmission fluid, the thermophysical properties of CO2 are quite different from those of water. CO2 has many advantages, such as larger mobility and buoyancy resulted from the lower density and viscosity. This will reduce the consumption of pressure driving the circulation, and save the energy of external equipment. The cycle even can be achieved by siphon phenomenon under a negative circulating pressure difference. However, there are still some disadvantages for CO2 as the heat transmission fluid, such as small heat capacity, leading to a less heat at the same mass flow rate. At the same time, because of the lager expansion and compression coefficient for CO2, changes in temperature and pressure may cause a more complex flow and thermodynamic processes. The lager compressibility makes it possible to get high temperature at the bottom of the injection well, whereas the lager expansion coefficient makes the temperature drop rapidly along the production well. Therefore, how to scientifically control the production pressure to guarantee sufficient high temperatures at the head of production well and, thereby, improve the efficiency of heat extraction are the key issues needed to be further addressed. The geological and geothermal conditions correspond to the central depression of the Songliao Basin located in the Northest of China. This depression has a high geothermal gradient and heat flow. In this article, a classic idealized “five-spot” reservoir model coupled with wellbores is used for simulations and analyses. The objectives of the present work are: (1) to investigate the fluid flow and thermal processes of supercritical CO2 along the wellbore and in the reservoir, (2) to understand the heat-extracting mechanism, (3) to identify advantages and disadvantages of using CO2 as the heat transmission fluid, and (4) to provide a theoretical basis for the selection of heat transmission fluid.

Thermal gating of charge currents with Coulomb coupled quantum dots

Abstract: We have observed thermal gating, i.e. electrostatic gating induced by hot electrons. The effect occurs in a device consisting of two capacitively coupled quantum dots. The double dot system is coupled to a hot electron reservoir on one side (QD1), while the conductance of the second dot (QD2) is monitored. When a bias across QD2 is applied we observe a current which is strongly dependent on the temperature of the heat reservoir. This current can be either enhanced or suppressed, depending on the relative energetic alignment of the QD levels. Thus, the system can be used to control a charge current by hot electrons.

Floquet systems coupled to particle reservoirs

Abstract: Open quantum systems, when driven by a periodic field, can relax to effective statistical ensembles that resemble their equilibrium counterparts. We consider a class of problems in which a periodically driven quantum system is allowed to exchange both energy and particles with a thermal reservoir. We demonstrate that, even for noninteracting systems, effective equilibration to the grand canonical ensemble requires both fine tuning the system-bath coupling and selecting a sufficiently simple driving protocol. We study a tractable subclass of these problems in which the long-time steady state of the system can be determined analytically, and demonstrate that the system effectively thermalizes with fine tuning, but does not thermalize for general values of the system-bath couplings. When the driven system does not thermalize, it supports a tunable persistent current in the steady state without external bias. We compute this current analytically for two examples of interest: (1) a driven double quantum dot, where the current is interpreted as a dc electrical current, and (2) driven Dirac fermions in graphene, where it is interpreted as a valley current.

Multiscale dynamics of open three-level quantum systems with two quasi-degenerate levels

Abstract: We consider a three-level quantum system interacting with a bosonic thermal reservoir. Two energy levels of the system are nearly degenerate but well separated from the third one. The system-reservoir interaction constant is larger than the energy difference of the degenerate levels, but it is smaller than the separation between the latter and the remaining level. We show that the quasi-degeneracy of energy levels leads to the existence of a manifold of quasi-stationary states, and the dynamics exhibits two characteristic time scales. On the first, shorter one, initial states approach the quasi-stationary manifold. Then, on the much longer second time scale, the final unique equilibrium is reached.

Effects of Heat Generations on the Thermal Response of Channels Partially Filled with Non-Darcian Porous Materials

Abstract: The forced convective thermal response of channels partially filled with porous materials is analytically studied. It is assumed that internal heat generations exist within both the fluid and solid phases. Effects of internal heat generations within the both phases on the thermal response of the channel resulting in the heat flux bifurcation phenomenon are discussed for the first time. No study previously analyzed the heat flux bifurcation in a non-Darcian porous medium with heat-generating porous materials. To obtain the most general thermo-hydraulic behavior, the Darcy–Brinkman equation of motion and the two-energy model (local thermal non-equilibrium) along with two practical thermal boundary conditions (models A and B) are used. Consequently, two possible thermal responses are obtained for each phase. Results show that insertion of a porous material inside a heat-convecting fluid leads to have a more uniform temperature distribution which translates to a lower thermal resistance and an enhanced heat transfer. Furthermore, it is seen that increasing the porous material thickness increases the ratio of heat transferred by the porous medium compared to that convected by the clear fluid flow. In addition, the internal heat generations drastically change the temperature distribution. Finally, it is shown that the internal heat generations can inverse the heat flux direction at the porous–fluid interface (the heat flux bifurcation phenomenon). Criteria for the heat flux bifurcation for a partially porous-filled channel are presented under the Darcy’s law of motion.

Thermal energy storage: the role of the heat pipe in performance enhancement

Abstract: Heat pipes and thermosyphons—devices of high effective thermal conductivity—have been studied for many years for enhancing the performance of solid, liquid and phase change material (PCM) heat stores. However, as the applications of heat storage widen, from micro-electronics thermal control to concentrated solar heat storage and vehicle thermal management, and even for chemical reactor isothermalization, the challenges facing heat storage increasingly are moving from those associated with the ‘standard’ diurnal storage, in itself a problem for low thermal conductivity materials, to response times measured in a few hours or even minutes. While high thermal conductivity metals such as foams can be impregnated with a PCM, for example, to increase local conductivity, the rapid heat input and removal necessitates a more radical approach—heat pipes, possibly with feedback control, with innovative PCM interfaces. This paper reviews the use of heat pipes in conventional and rapid response PCM and liquid or cold storage applications and introduces some novel concepts that might overcome current limitations.

Quantum interferometric measurements of temperature

Abstract: We provide a detailed description of the quantum interferometric thermometer, which is a device that estimates the temperature of a sample from the measurements of the optical phase We rigorously analyze the operation of such a device by studying the interaction of the optical probe system prepared in a single-mode Gaussian state with a heated sample modeled as a dissipative thermal reservoir We find that this approach to thermometry is capable of measuring the temperature of a sample in the nanokelvin regime Furthermore, we compare the fundamental precision of quantum interferometric thermometers with the theoretical precision offered by the classical idealized pyrometers, which infer the temperature from a measurement of the total thermal radiation emitted by the sample We find that the interferometric thermometer provides a superior performance in temperature sensing even when compared with this idealized pyrometer We predict that interferometric thermometers will prove useful for ultraprecise temperature sensing and stabilization of quantum optical experiments based on the nonlinear crystals and atomic vapors

Thermal Gating of Charge Currents with Coulomb Coupled Quantum Dots

Abstract: We have observed thermal gating, i.e. electrostatic gating induced by hot electrons. The effect occurs in a device consisting of two capacitively coupled quantum dots. The double dot system is coupled to a hot electron reservoir on one side (QD1), whilst the conductance of the second dot (QD2) is monitored. When a bias across QD2 is applied we observe a current which is strongly dependent on the temperature of the heat reservoir. This current can be either enhanced or suppressed, depending on the relative energetic alignment of the QD levels. Thus, the system can be used to control a charge current by hot electrons.

Loudspeaker protection circuitry and methods

Abstract: This application relates to methods and apparatus for thermal protection of a loudspeaker ( 102 ). A controller ( 106 ) is configured to generate a control signal (G mod ) for modulating the gain of a signal processing chain ( 101 ) that drives the loudspeaker. The controller is configured such that the control signal is generated as a function of each of: an indication of voice coil temperature of the loudspeaker (T VC ); an indication of power dissipation in the voice coil of the loudspeaker (P VC ); and an indication of a reservoir temperature (T res ) of a thermal reservoir for heat flow from the voice coil. The reservoir temperature may be an ambient temperature of the environment or a temperature of a component of the loudspeaker or apparatus that acts as a heat sink.