Study Of Energy Transition Process In The Plane Cell Type Micro-thermophotovoltaic System

With the powerful impulse of micro-machining technology, persistent breakthroughs have been obtained on the development of micro-machinery and electromechanical products. However, no remarkable progress has been obtained in its power supply part, which makes the complete system too big and heavy or can not woking for long time, so this problem has become the bottleneck of MEMS development. In recent years, due to the advantages of high energy density and long working time as so as small volume, several micro power generators which based on combustion are expected to well solve this problem. Their energy densities are supposed to exceed 100kW/kg, so have aroused public concern around the world and are becoming one of the focus in the scientific research competition.As one kind of typical power generating machine, micro-thermophotovoltaic system utilize thermal energy to heat the outer surface of combustor which coming from combustion of hydrocarbon fuels in the micro-combustor, and electricity can be generated when the sufficiently higher-energy photon reaches PV cells. Compared to other power generating machines, its greatest advantages are including no moving components and relatively easy to manufacture and assemble.By modifing the structure of previous cylindrical combustor system, a novel modular micro-thermophotovoltaic system with plane cell structure is proposed in this paper. Systematic research have been carried out through numerical analysis, and some achievements with scientific significance and values have been acquired:(1) Each links of energy conversion is analysed for the new plane cell type micro-thermophotovoltaic system, which including flow, heat transfer and combustion coupling process, radiation process of combustor's outer wall and photoelectric transformation process. Then, on the premise of non-uniformity of wall temperature distribution, a three-dimensional energy transition computational model is constructed through combining the softwares of Fluent and Matlab which describes the whole transition process of chemical energy to electricity, and the accuracy of model has been verified by preliminary experiment results. (2) The system adopting round nozzle combustor and without filter is taken as study object, by changing distance between radiation wall and PV cell, cell types, working temperature of radiation surface and cell, a variety of preliminary calculations about system performance are maken. According to the analysis of calculation results, some basic conditions for system operation are obtained which can provide reasonable references for the further optimal design of plane cell system.(3) Considering the significance of radiation wall temperature distribution to the system working performance, a great amount of simulations about combustion in the plane cell microchannel are done in the following study. Firstly, measures of improving microscale combustion are presented such as replacing the round nozzle with elongated rectangular nozzle, approximately decreasing width of combustion channel. By further analyzing the effect factors in the combustion process, suitable operating parameters for plane cell combustor such as mixture flux 1500mL/min and channel height 0.6mm are gained. Then, three optimized combustor structures namely porous media in the combustion channel, catalysis on inside walls, convex blocks for steady flow at the inlet and another new plane cell micro-combustor with preheating channels are designed. Combustion characteristics and wall temperature distribution of these optimized structures are analyzed, and the active effects of improving the system working performance are contrasted. Among these four structures, an overall efficiency 1.12% can be realized by using inlet convex blocks for steady flow at mixture flux 1500mL/min, which is 33.9% higher than the original combustor with rectangular nozzle.(4) To further increase the energy conversion efficiency, a one-dimension Si/SiO2 photonic crystal filter is set in the system to ensure recycle of unavailable radiation energy. The basic structure design and corresponding optical characteristics are obtained through optical thin film theory and transmission matrix method, and a improved design is done according to the narrower first reflection band of basic structure. Calculation results show that the using of filter can effectively lower the burden of cell cooling, while at the same time boost the temperature of radiation wall so as to raise system output performance. System output power density and total efficiency reaches 5.46W and 2.6% when using the improved structure filter at mixture flux 1500mL/min, which is 5.7% higher than the result of using basic structure filter.(5) Micro power system is not merely minification of their conventional scale prototype, and each device should has its dimension limits, but few scholar have undertaken related study on the whole device's miniaturization limitations. In the end of this paper, the minimum dimension of one energy transition cell in the plane cell micro thermophotovoltaic system is determined by considering parts strength and manufacture difficulty and combining related reference, micro combustion experiment and calculation analysis of PV cell cooling, the whole size of energy transition cell is 10mm X 8mm X 2.5mm.The power density is up to 17.55W/cm3 when the mixture flux is 1500mL/min, which fully demonstrates the advantages of plane cell micro-thermophotovoltaic system as the power sources of MEMS.This work provides a practicable research thought to rationally develop and correctly evaluate of working performance for micro-thermophotovoltaic system. However, the methods and conclusions presented in this paper can also be applied to other power systems.