Theoretical Analysis Of Irreversible Thermodynamic Cycles And Study On Practical Thermodynamic Systems For Low Grade Energy Utilization

Nowadays, most of the energy we utilize orginates from the combustion of the fossil fuels, which has induced severe environmental problems. The adverse exhaust such as sulphur oxides, particles (PM 2.5) has caused severe catastrophe for human beings and the nature. Furthermore, refrigerators are one of the most energy-consumption apparatus in our life. Approximately 15% of all energy consumed is used by cooling systems. The optimization and improvement of the thermodynamical devices (heat engines and refrigerators) and utilization of the renewable energy and low grade waste heat can efficiently relieve such issues.On the theorectical research of the thermodynamical cycles, systematical analysis of the thermal efficiency and coefficient of performance (COP) for general heat engines and refrigerators with non-isothermal processes and internal disspations are conducted under the ? criteria, trade-off criteria and the ecological cirteria, respectively. Under these criteria, the analytical bounds of the thermal efficiency and COP have been proposed. In addition, a performance optimization for minimally nonlinear heat engines and refrigerators is conducted under the ? criteria and ecological criteria. The analytical bounds of the thermal efficiency and coefficient of performance have also been studied. The relation of the minimally nonlinear models and the low dissipation models has been further investigated. In order to study the macro/micro heat engine cycles, the performance of micro two-level heat engines with prior information have been analysed under the maximum power output. Results reveal that the model proposed in this paper can describe any specified models with concrete prior probability distribution such as two-level quantum heat devices and Brownian heat devices with prior information. In addition, the performance of Feynman's ratchet refrigerator with heat leak has been studied, under the maximum COP, maximum cooling rate and maximum ? figure of merits. The traditional performance region between maximum cooling rate and maximum COP can be divided into two more specific ones (the region between maximum cooling rate and maximum ?,an the region between maximum?an maximum COP), which represent two different operating demands. Furthermore, the performance of a quantum Otto refrigerator coupled to a squeezed cold reservoir has been evaluated using the X figure of merit. Results reveal that the COP under the maximum x figure of merit is of the Curzon-Ahlborn (CA) style that cannot surpass the actual Carnot COP, and is consistent with the second law of thermodynamics.As to the actual thermodynamical cycles, the internal and external exergy efficiencies are adopted to analyze the impact of working fluids on the performance of the organic Rankine cycle (ORC). Results show that working fluids with lower critical temperature lead to a higher optimal evaporation temperature, which results in higher overall exergy efficiency. In addiotn, innovative thermodyamcal cycles can also offer alternative methods to recovery low grade waste heat. The performance of a thermally regenerative electrochemical cycle (TREC) and a regenerative Ericsson pyroelectric cycle (REPC) for harvesting waste heat has been investigated based on finite time analysis. Parameters have been systematically investigated to evaluate the performance of the TREC and REPC systems. Furthermore, in order to achieve an optimum solution for the conflicting objectives, an optimization analysis of a continuous TREC based on NSGA-II algorithms was conducted with maximum power output and exergy efficiency as the objective functions simultaneously. Results reveal that multi-objective optimization could coordinate well both the power output and the exergy efficiency of the TREC system, and may serve as a more promising guide for operating and designing TREC systems.As to the research on the complex systems for energy untilization, for electricity generation, a hybrid system consisting of a compound parabolic collector system, a solid oxide electrolyzer system and a proton exchange membrane fuel cell (PEMFC) system is proposed to harvest solar energy. And an analysis of a solar-powered electrochemical refrigeration system consisting of a photovoltaic (PV) system and a thermally regenerative electrochemical refrigerator (TRER) was also conducted. These two solar energy harvesting systems may be applicable in remote and rural areas and the space station etc..In practice, cascade or staged utilization of thermal energy could be more efficient than a single cycle system. In this thesis, a dual loop thermally regenerative electrochemical cycle (DLTREC) system consisting of two hot electrochemical cells and a cold one is proposed for harvesting waste heat in a more efficient manner. For the prescribed heat source inlet temperature of 393.15 K, the maximum power output of the DLTREC system was 50.11% larger than that of the conventional TREC system and the electrical efficiency improved by 13.31%. Meanwhile, a new hybrid system consisting of a PEMFC subsystem and TREC subsystem is proposed to convert the waste heat produced by the PEMFC system into electricity. The power output of the hybrid system is 6.85%-20.59% larger than that of the PEMFC subsystem. And the total electrical efficiency is improved by 4.56%-13.81%. In addition, this thesis also reports on the study of a cascade utilization system based on ORC and TREC subsystems. With 423.15 K as the inlet temperature of the heat source and R141b selected as the working fluid, the optimal power output of the cascade system is found to be 62.3% larger than that of a sole ORC system, and 5.2% larger than that of a sole TREC system. The exergy efficiency of the cascade system is 14.7% higher than that of a sole ORC system, and 7.3% higher than that of a sole TREC system.