Performance Analysis On The Nonlinear Didoe And Quantum Dot Engines

Heat engines and refrigerators are both such devices that are carried out by various working substances transferring heat. Generally, the traditional heat engines and refrigerators use the classical substances as the heat transfer media in the macroscopic size. Recently, with the further theory research and the development of science and technology, the power generation and refrigeration technologies are studied gradually from the macroscopic size to the microscopic scope. Especially, electron engines and refrigerators using the electrons as a kind of working substance have greatly evoked people's interest.Two kinds of the engine models are mainly studied in the thesis. One is a Brownian heat engine constituted by the nonlinear diode system. In two reservoirs with different temperatures, the electron scattering resulting from ion oscillation corresponds to the collision of liquid molecule with Brownian particles. Based on the theory of the thermal fluctuations, by solving Fokker-Planck equation, the performance characteristics of the engine are analyzed. The other is an electronic engine model constituted by a quantum dot system. Based on the theory of electron transport through a quantum system, by solving quantum master equation, the performance characteristics of a single electron tunnelling through the quantum dot are studied. The content of the thesis is arranged as follow:In chapter 2, the engine model consisting of two nonlinear diodes switched in the opposite directions, paralleling to a capacitor, and located in two heat reservoirs with different temperatures is studied. The electron scattering resulting from ion oscillation in the conductor corresponds to the collision of liquid molecule with Brownian particles and the motion of electrons in the nonlinear diodes is similar to that of Brownian particles in the environment with non-uniform friction efficiency. Based on the theory of the thermal fluctuations, the Fokker-Planck equation is obtained. When an ideal current is applied, the system acts as a heat engine and does work on the environment. After taking into account the heat leak between the two reservoirs, the performance characteristic curves are plotted and the characteristic relations between the optimum performance parameters and the ratio of the temperatures are shown by numerical simulation. The influence of the main parameters, including the nonlinearity of the diodes, the loss of heat leak and the ratio of temperatures of two reservoirs, on the performance characteristics of the engine is analyzed. Lastly, the performance characteristics of the ideal diodes engine are discussed.In chapter 3, the effect of an applied magnetic field on the performance characteristics of a quantum dot engine is studied. The quantum dot engine consists of a single energy level quantum dot embedded between two reservoirs at different temperatures and chemical potentials. The spin-degenerated electrons with resonant energy transfer heat flux or heat through the quantum dot between two reservoirs under the temperature and chemical potential gradients. Once a magnetic field is applied to the system, the spin degeneracy is broken and the energy of the electron is cut into two energy levels. The whole tunnelling process of the electron through the quantum dot is described by quantum master equation. The effect of magnet strength on the performance characteristics of the quantum dot engine is analyzed by numerical simulation.In chapter 4, the performance characteristics of a two-energy level refrigerator are studied. Two leads with different temperatures and chemical potentials act as two electron heat reservoirs. The electrons transfer heat between two reservoirs by tunnelling through these two quantum dots under temperature and chemical potential gradients. The whole tunnelling process of the electron through the two levels is described by quantum master equation. The steady expressions of the probability current and heat per unit time extracted from two reservoirs are derived analytically. The cooling rate and the coefficient of performance (COP) are obtained. The influence of the energy space of two levels on the performances and optimal performances of the refrigerator are analyzed in detail by numerical simulation.