We study a class of cyclic Brownian heat engines in the framework of finite-time thermodynamics. For infinitely long cycle times, the engine works at the Carnot efficiency limit producing, however, zero power. For the efficiency at maximum power, we find a universal expression, different from the endoreversible Curzon-Ahlborn efficiency. Our results are illustrated with a simple one-dimensional engine working in and with a time-dependent harmonic potential.

Efficiency at maximum power: An analytically solvable model for stochastic heat engines

Artificially designed hyperbolic metamaterial (HMM) possesses extraordinary electromagnetic features different from those of naturally existing materials. In particular, the dispersion relation of waves existing inside the HMM is hyperbolic rather than elliptical; thus, waves that are evanescent in isotropic media become propagating in the HMM. This characteristic of HMMs opens a novel way to spectrally control the near-field thermal radiation in which evanescent waves in the vacuum gap play a critical role. In this paper, we theoretically investigate the performance of a near-field thermophotovoltaic (TPV) energy conversion system in which a W/SiO2-multilayer-based HMM serves as the emitter at 1000 K and InAs works as the TPV cell at 300 K. By carefully designing the thickness of constituent materials of the HMM emitter, the electric power of the near-field TPV devices can be increased by about 6 times at 100-nm vacuum gap as compared to the case of the plain W emitter. Alternatively, in regards to the electric power generation, HMM emitter at experimentally achievable 100-nm vacuum gap performs equivalently to the plain W emitter at 18-nm vacuum gap. We show that the enhancement mechanism of the HMM emitter is due to the coupled surface plasmon modes at multiple metal-dielectric interfaces inside the HMM emitter. With the minority carrier transport model, the optimal p-n junction depth of the TPV cell has also been determined at various vacuum gaps.

Hyperbolic metamaterial-based near-field thermophotovoltaic system for hundreds of nanometer vacuum gap = 하이퍼볼릭 메타물질 기반의 근접장 열광전지 장치

The energy selective electron device works among electron reservoirs with different temperatures and chemical potentials. Electrons obey the Fermi-Dirac distribution, and with the help of resonant filters, a part of electrons with specific energy levels can tunnel among reservoirs and provide current to an external circuit. Herein, an irreversible three-terminal energy selective electron generator model is proposed. Using statistical mechanics and finite-time-thermodynamics, analytical expressions of power and efficiency are derived, and the optimal performance of the device is investigated. Results show that the central energy level difference of filters, the chemical potential difference of low-temperature reservoirs, the interval of mean-central-energy-level of filters and the mean-chemical-potential of low-temperature reservoirs can be optimized to maximize power and efficiency. On the basis of power and efficiency analyses, performance characteristics under different objective functions, including efficient power and ecological function, are discussed and the corresponding optimal performance regions are obtained. The relationship between the entropy generation rate and the efficiency is investigated, and it is shown that the minimum-entropy-generation-state does not coincide with the maximum-efficiency-state.

Performance optimization of three-terminal energy selective electron generators

The authors propose a general mechanism of bath spectral filtering to boost the performance of a quantum heat manager.

Minimal quantum heat manager boosted by bath spectral filtering

Strongly anisotropic media, where the principal components of the dielectric tensor have opposite signs, are called hyperbolic. Such materials exhibit unique nanophotonic properties enabled by the highly directional propagation of slow-light modes localized at deeply sub-diffractional length scales. While artificial hyperbolic metamaterials have been demonstrated, they suffer from high plasmonic losses and require complex nanofabrication, which in turn induces size-dependent limitations on optical confinement. The low-loss, mid-infrared, natural hyperbolic material hexagonal boron nitride is an attractive alternative. Here we report on three-dimensionally confined 'hyperbolic polaritons' in boron nitride nanocones that support four series (up to the seventh order) modes in two spectral bands. The resonant modes obey the predicted aspect ratio dependence and exhibit high-quality factors (Q up to 283) in the strong confinement regime (up to λ/86). These observations assert hexagonal boron nitride as a promising platform for studying novel regimes of light-matter interactions and nanophotonic device engineering.

Sub-diffractional, volume-confined polaritons in a natural hyperbolic material: hexagonal boron nitride

This work establishes a quantum heat engine considering qubits as the external heat transfer mediums. The authors find that the efficiency and the power output of the quantum heat engine are closely dependent on the microproperties of qubits, and consequently relate the irreversible heat transfer with quantum effects. The equivalent conditions between the quantum heat engines and the classical endoreversible Carnot heat engines are revealed

Quantum-dot heat engines with irreversible heat transfer

We study the performance of a thermal management device with small scales by considering a strong coupling between quantum qubits. A small change of the thermal current at the base will cause a great change to the thermal current at the emitter and collector, reaching its promise for large thermal amplification. The competition between the quantum coherence and the incoherence induces a significant variation in the amplification factor and consequently relates the thermal controls with quantum effects. The results obtained here will provide a feasible scheme for the realization of quantum thermal management devices.

Quantum thermal management devices based on strong coupling qubits.

A new scheme of near-field thermophotovoltaic devices is proposed by introducing the intermediate-band absorber. The two-step excitation via the intermediate band provides a large photogenerated current density and maintains a high voltage output for the thermal-electric conversion. Energy dissipation processes in devices are analyzed by using the detailed balance model. Results show that the powers and efficiencies of thermophotovoltaic devices with intermediate bands in appropriate positions are clearly larger than those of the conventional counterparts. Based on the optical properties observed in experiments, three types of absorber materials are compared, which indicates that InAs with low energy bandgap is more suitable as a high-temperature material for photon absorptions than GaSb and Si. Comparing the performance of our model with experiments, we show that the InAs based thermophotovoltaic device allows the enhancement of efficiency over a range of gap sizes. The proposed model may open a new field in the application of thermophotovoltaic devices.

Intermediate-band-assisted near-field thermophotovoltaic devices with InAs, GaSb, and Si based absorbers

https://iopscience.iop.org/article/10.1088/1361-6463/ab601e/pdf

Wei Shen

Papers

Quantum-dot heat engines with irreversible heat transfer

Quantum thermal management devices based on strong coupling qubits.

Intermediate-band-assisted near-field thermophotovoltaic devices with InAs, GaSb, and Si based absorbers

Thermodynamic coupling rule for quantum thermoelectric devices

The entropy production rate of double quantum-dot system with Coulomb coupling