Performance Characteristics And Optimal Analysis Of Quantum Dot Heat Engines And Refrigerators

Due to its important application in development of miniaturization andmicro-scale energy converter,as well as the advantages compare with the traditionalenergy conversion, such as high reliability, lack of moving parts, scalability, absenceof emissions, etc., nanothermoelectric devices have attract considerable interests ofmany researchers both at home and abroad. The thermodynamic performance ofnanothermoelectric devices, such as superlattice, nanowires, nanotubes, molecularjunctions and quantum dots, have been investigated widely. On the other hand, basedon the finite time thermodynamic, optimize performance of thermodynamics devicein finite time thermodynamics also attracted more and more attention. Since then,much explicit application for different nanothermoelectric devices models have beenreported, such as stochastic heat engines, Feynman ratchet and pawl, molecularmotors and quantum dot thermoelectric heat engines. So it has important theoreticaland realistic significance to study the thermodynamic performance characteristics andoptimization of the thermoelectric devices under the principle and method of finitetime thermodynamics.In this thesis, the thermodynamic performance characteristics and optimalanalysis of a quantum dot thermoelectric engines and refrigerators are studied. Thequantum dot nanothermoelectric devices are consisting of quantum dot embeddedbetween two electron reservoirs at different temperatures and chemical potentials.Electronic can transport through tunneling between quantum dot and two reservoirsunder temperature and chemical potential gradients. In the limit of weak contactcoupling (kBT) and the sequential tunneling approximation, co-tunnelingprocesses and coherence may be neglected. The evolution of the occupationprobability of the system state is described by the stochastic master equation. In thesteady-state case, based on the master equation the expressions for the charge currentand the heat current of system are derived. Moreover, according to the second law ofthermodynamics, the thermodynamic performance parameters of quantum dotsthermoelectric devices are defined (such as efficiency and coefficient of performance), and the performance characteristics and optimal analysis are further discussed. Thecontent of the thesis includes:In chapter1, we first introduce the overview of the study background of thethesis, and then mainly introduce the theory of the master equation and thethermodynamics and finite time thermodynamics.In chapter2, the optimal performance of a single-level quantum dot refrigeratorwith Maxwell demon feedback is investigated. The main focus of this chapter is tostudy the influence of Maxwell demon feedback acting on the coefficient ofperformance at maximum of the quantum dot refrigerator, and compared it withthe optimized coefficient of performance of classical Carnot-type refrigerator.In chapter3, we investigate the efficiency at maximum power of a quantum dotheat engine in external magnetic field. Once external magnetic field is applied to theengine, the two spin states associated with a given orbital level are no longerdegenerate and split into two energy levels, the level spacing is determined by themagnetic field strength. Based on the master equation, the expressions for the heatcurrent of system are derived. The efficiency at maximum power of heat engine indifferent external magnetic field is obtained by the numerical calculation, andcompared with the classical CA (Curzon-Ahlborn) efficiency.The performance characteristics and optimization of an interacting quantum dotthermoelectric refrigerator are investigated in chapter4. The main focus of thischapter is to study the influence of the Coulomb interaction on the performancecharacteristics and the optimal performance of the refrigerator. Electron tunnelingenergy threshold will increase because the coulomb interaction. The characteristiccurves between the cooling power and the coefficient of performance at differentCoulomb interaction are plotted through numerical calculation. The variations of theoptimal performance parameters at maximum coefficient of performance andmaximum cooling power with the Coulomb interaction and temperature ratio areplotted.In chapter5, we establish the model of a two-level quantum dot heat engine andrefrigerator, and the influence of the strength of variations in electron-electroninteraction on the optimal performance parameters is analyzed in detail. We plot the characteristic curves of heat engine at different strength of variation inelectron-electron interaction. The variations of the optimal performance parameters atmaximum power and maximum efficiency with the strength of variations are plotted.The focus is to study the variation of the efficiency at maximum power withtemperature ratio, and compare with CA efficiency. For the refrigerator, thecharacteristic curves and optimal performance at maximum coefficient ofperformance and maximum cooling power with strength of variation are investigated.In chapter6, we study the thermodynamic performance of a nanosizedphotoelectric refrigerator consisting of three coupled single-level quantum dotsembedded between two reservoirs at different temperatures. Based on the masterequation, we derive expressions for the coefficient of performance and cooling powerof the refrigerator and analysis the performance characteristic. The optimalperformance parameters under conditions of maximum cooling power and maximumcoefficient of performance are discussed in detail. The influence of the nonradiativeprocesses, energy level difference and the temperature ratio on the optimalperformance is analyzed. The general conclusions are obtained.In chapter7, we give the mainly summarize the previous work and furtherresearch direction.