Investigation On The Performance Characteristics Of Graphene-based Thermonic And Thermoradiative Devices

With the increasing of the energy crisis and greenhouse effect,people pay more attentions to the development and utilization of renewable energy.Nowadays,most of the consumed energy flows into the environment in the form of waste heat,which is ubiquitous in vehicle engines,industrial productions,and power electronics.Hence,recycling waste heat can fundamentally improve the utilization rate of energy and alleviate the contradiction among the economic development,environment,and energy.However,traditional solid-state technologies for waste-heat recovery,such as thermoelectric generators and thermophotovoltaic cells,are still limited by low power generation and energy conversion efficiency.With the development of nanoscience and two-dimensional materials,Graphene-based thermionic energy converters and thermoradiative devices have provided new opportunities and approaches for the revolution of the current waste-heat recovery technology.As environmentally sustainable energy systems,these two devices can effectively utilize high-and lowgrade waste heat,and thus achieve higher power output and energy conversion.Additionally,the proposed devices can help to reduce the reliance on fossil fuel,to reduce the pollutants and greenhouse effect.Based on the theoretical framework of finite-time thermodynamics,this thesis studies the Graphene-based thermionic energy converter and thermoradiative system with the non-ideal effect.Not only the effects of non-ideal factors on the system performance are studied,but also the optimum parameter design and operating conditions are given.This thesis is mainly composed of three parts.The first part includes chapters 2-4,which mainly investigate the recovery and utilization of high-grade thermal energy in Graphene-based energy converters and its application in photoelectric energy conversion,and then explore the potential application of three-dimensional Dirac material as the emitter in thermionic energy converters.Based on the thermionic emission theory of Graphene and three-dimensional Dirac materials,a non-ideal thermionic energy converter is constructed.The vacuum thermionic generator with Graphene as the emitter can effectively harvest high-grade heat and possess high power output and energy conversion.Compared with traditional metal materials,Graphene can withstand higher temperatures and emit more electrons,thus exhibiting better performance.On this basis,the graphene-based thermionic solar cell is further proposed.Serving as an emerging concept of photoelectric conversion device,it possesses the advantages of low-cost fabrication,simple structure,and high conversion efficiency.By considering various irreversible losses,the optimal structure parameters and operating conditions of the cell under different solar concentrations are determined.Finally,the thermionic emission from three-dimensional Dirac semimetals is theoretically investigated and its practical application as the cathode of a thermionic energy generator is explored.Based on the linear energy dispersion around Dirac points,the generalized analytical formula of the thermionic emission from three-dimensional Dirac semimetals through the Dirac Hamilton is derived,which is significantly different from the Richardson-Dushman law for metallic materials.The reason for this originates from the fact that electrons behave like massless fermions and follow ultrarelativistic quasiparticle dynamics in three-dimensional Dirac semimetals,deviating from the nonrelativistic electrons with a parabolic energy dispersion in conventional metal materials.Moreover,a theoretical model of the solid-state thermionic energy generator using three-dimensional Dirac semimetals as the cathode is developed and its optimally working regions at different temperatures are obtained.As a research hotspot in electron emission from novel materials,this part is of great theoretical significance for the deep understanding of thermionic emission from Dirac materials,and also provides significant theoretical guidance for the development of promising low-dimensional material thermoelectric devices and its practical applications.The second part contains chapters 5 and 6,which mainly focuses on the thermodynamic optimization of thermoradiative devices and their applications for medium-and low-grade waste-heat recovery.Based on the theory concerning thermal radiation of p-n junction in the semiconductor,non-ideal thermoradiative devices are comprehensively constructed.By introducing a non-ideal factor related to non-radiative recombination,the theoretical model of thermoradiative cells is updated.The influence of non-ideal factors on the system performance of the thermoradiative system is investigated,and the optimal parameter designs and working regions of the updated system are determined.The performance of the thermoradiative system,thermophotovoltaic system,and thermionic system are compared.The results show that the thermoradiative system not only possesses the advantages of simple structure,low-cost production,high power output,and conversion efficiency,but also has more advantages in the utilization of medium-grade thermal energy.A nanoscale InSb thermoradiative system is further designed to explore its potential for the low-grade thermal energy recycle.The optical loss,electrical loss,and heat loss existing in the system as well as the influence of the temperature on the semiconductor parameters are fully considered.Not only are the optimally operating parameters of the system in different temperature regions determined,but the loss mechanism which plays a major role in the system is also clarified.The results show that the system possesses better performance than the traditional solid-state thermoelectric devices in the utilization of low-grade heat energy.In addition,sub-bandgap radiative loss and three kinds of non-radiative recombination losses are the two most important irreversible losses in thermoradiative devices.The proposed InSb thermoradiative device can supply effective theoretical guidance for the optimization design and technical application of the actual device,and thus provide a possible route for the ongoing exploration of emerging solid-state low-grade thermal harvesting devices.The third part includes chapters 7 and 8.Based on the above two parts,a coupling system that hybridizes Graphene-based thermionic devices and thermoradiative cells is proposed and its potential applications in photovoltaics are explored.Such a device can simultaneously utilize both electrons and photons,which enables a significant enhancement on thermal utilization and performance of the system.The results show that the hybrid system possesses the advantages of higher power output and conversion efficiency in an extended temperature range,compared with either the thermionic energy converter or thermoradiative cell.The research content of this part provides a promising route for the optimization design and energy loss mechanism of thermionic-thermoradiative hybrid system,and paves the way for the ongoing development of alternative energy harvesting devices.The research and development of new high-efficient thermoelectric and photoelectric conversion devices and the effective utilization of new energy sources can make up for the shortage of high-quality energy and long-term dependence on coal-based energy structure,alleviate severe ecological and environmental problems,and promote the implementation of national sustainable development strategies.In this thesis,graphene-based thermionic and thermoradiative devices have been studied deeply,and the results obtained are not only of theoretical significance,but also of application prospect,which can promote the development and application of thermionic and thermoradiative devices.