Research On Thermophotovoltaic System Based On Photonic Crystal Selective Emitter

In deep space mission,conditional Solar cells can not provide sufficient energy for spacecraft.The heat released from the nuclear fuel is converted to electricity by nuclear batteries,which has a high power density and is not influenced by environment,such as low or high temperature.Various types of nuclear batteries is investigated,including thermal-electric generators（RTG）,alkali-metal thermal-to-electric converter（AMTEC）,the Advanced Stirling engines（ASRG）,and the radioisotope thermophotovoltaic systems（RTPV）.Comparing to other competitors,the advantages of RTPV system are the high efficiency,absence of moving portion and consisting of only solid-state parts and reliability.The process of radioisotope thermophotovoltaic（RTPV）concept,which is similar to the conventional solar cells,directly converts the infrared photons from a high temperature surface of emitter into electricity by low bandgap thermophotovoltaic（TPV）elements.In this work,we focus on the design and selection of system parts,and optimize the system to reach a maximum efficiency.The major work is shown as follows:1.Selection of PV cells.We compare the advantages and disadvantages of varies of PV cells,and select the GaSb cells for our thermal-electric elements according to the bandgap and electric performance.In addition,considering to the difficulty of being obtained,Si cells is selected for the candidate.Then,we write a Python program based on the model and governing equation of PV cells.The calculating results are verified to that of literature,proving the correctness of the program.2.Design of photonic crystal emitter.Based on the bandgap of GaSb an Si cell and the equation of cut-off wavelength,the width of microcavities is estimated,and the characteristic parameters,such as width,period and depth,are tentatively designed.Then,based on the theory of diffraction,we qualitatively analyze the direction of optimization and further optimize the width,period and depth.Finally,tungsten 2D photonic crystal selective emitters are designed to match the bandgap of GaSb and Si cells.3.Selection of Multi-layer materials.In order to reduce the radiative heat loss of RTPV generator,proper multi-layer materials are selected.Zr O₂powder and Mo foil are selected for the multi-layer insulation,insuring that there is no outgassing to cause contaminants on the surface of Ph C emitter and cells.Moreover,multi-layer insulation can shield the heat radiation and reduce the heat loss.4.Employment of support materials.Support materials are mostly based on superalloys and ceramics according to their high strength and creep resistances at a high temperature.A Max phase material,Ta₂Al C,and Inconel are employed for support rod.The support is design to hollow rod to reduce conductive loss,at the same time,it has enough mechanical strength.5.Optimization of system.By varying the area of emitter,when GaSb cell is used,the optimal area of emitter is 88.36cm²with a operating temperature of 1343K.The system efficiency is reached to 12.3%.When Si cell is employed,the area is 29.12cm²with a corresponding operating temperature of 1655K,and the maximum efficiency is1.4%.6.Summarize the problems in RTPV cells.