| Increasing the temperature of the heat source can improve the energy grade and obtain higher thermal efficiency.However,the traditional energy production mode based on heat energy is subject to the upper limit of the thermal cycle parameters and cannot make full use of high-temperature energy.Therefore,the direct use of high-temperature thermal energy to generate electricity,the cascade comprehensive utilization of thermal energy is an important way to achieve efficient use of energy.Thermophotovoltaic is a technology that directly converts radiation into electricity,while at the same time using the excitation of polaritons in metamaterials(changing the emissivity)and the coupling of evanescent modes in near-field radiation(changing the intensity of the photon tunneling induced by the refractive index)to enhance and regulate the spectrum.Therefore,it is of great significance to study the radiation characteristics of thermophotovoltaics from the perspective of spectral modulation and the cascade utilization system of thermal energy with thermophotovoltaics.In this paper,the following route is performed:the theoretical limits of system energy conversion efficiency of thermophotovoltaic→the study of radiation spectral modulation of thermophotovoltaic systems by metamaterial emitters→the study of enhancement of radiation by near-field thermal radiation and the study of dynamic modulation of near-field radiation by graphene→the study of thermophotovoltaic system coupled with near-field radiation and hyperbolic metamaterial→the study of dynamic regulation of near-field thermophotovoltaic performance by graphene and finally,the research studied the thermal energy cascade utilization of thermophotovoltaic—double organic Rankin cycle.To reasonably evaluate the energy conversion performance and reveal the bottleneck restricting the performance of the thermophotovoltaic systems,the study constructs theoretical models of the conversion limit of thermophotovoltaics under different conditions and investigates the effect of temperature and the bandgap on the photoelectric efficiency.Based on the model of the Shockley-Quiser limit(S-Q limit),an ideal thermophotovoltaic system model with non-selective radiation at a temperature of 2000 K(no selective emitter)is established,and the limit efficiency is 31.4%.The theoretical model of an actual thermophotovoltaic system with non-selective radiation at 2000 K(no selective emitter)was studied,and the limit efficiency was13.2%.Further,based on the theoretical model of a single-frequency ideal thermophotovoltaic,the maximum conversion efficiency of the system at 2000 K is 85%.Through the comparison of different models,it can be seen that spectral mismatches constrain the performance of thermophotovoltaics and the spectral modulation of the selective emitter has significantly improved the performance of the thermophotovoltaic systems.The calculation principle of metamaterial emitter is summarized,the metamaterial emitter matching different photovoltaic cells is studied and designed,and the experimental study of metamaterial emitter is carried out.For the In Ga As Sb thermophotovoltaic system,a stacked ring metamaterial with a radiation efficiency of 91.2%at 2000 K was designed to address the disadvantages of existing metamaterials,such as excessive radiation in the lower band,poor polarization independence,and angle independence.To enhance the excitation of magnetic plasma polaritons and improve the selective radiative properties of the emitter,a stacked round table metamaterial emitter(27-layer structure)was designed based on a multilayer pyramidal structure and achieved a spectral efficiency of 95.1%at 2000 K.The stacked round table metamaterial coupled with a three-junction thermophotovoltaic(mid-infrared band)achieved a system energy conversion efficiency of 54.3%at 2000 K.To reduce the difficulty of processing and manufacturing,a stacked round table metamaterial emitter(7-layer)matching In Ga As cell(near-infrared Wavelength)was redesigned and manufactured,and the selective radiation of the stacked round table metamaterial was verified experimentally;The metamaterial achieves a spectral efficiency of 71.32%at 1500 K,increasing the system conversion efficiency to 27.8%at1500 K,which was 2.8%higher than that of existing research.The theoretical model of near-field radiation is introduced,and the improvement of heat flux by near-field radiation is explored by building graphene-plated silicon—silica near-field device,and the near-field heat flux is 1.08-6.78 times that of blackbody radiation.A graphene-coated silicon—silica near-field heat flux modulation device was constructed.The modulation of near-field radiation by graphene Fermi levels under different bias voltages was experimentally explored.The deep mechanism of dynamic modulation of radiation was theoretically analyzed—more surface plasma polaritons in graphene are excited.For the near-field samples at 690±10 nm(emitter at 77°C,receiver at 23°C),the heat flux at a bias voltage of 45 V was enhanced by 58.03±26.57 W/m~2 compared to the zero bias,a relative increase of 9.7±4.5%.The near-field thermophotovoltaic system model based on hyperbolic metamaterials and a multi-junction thermophotovoltaic cell is established,and the hybridized modes,jointly excited by metal coating and hyperbolic metamaterials,modulate and enhance the radiation spectrum,thereby improving the performance of the thermophotovoltaic system.The high k-mode in the hyperbolic surface and the excitation of the broadband singularity in the photon density of states enhance the photon tunneling effect in the near-field thermal radiation,thereby further enhancing the near-field radiation near the bandgap.The results show that the near-field multi-junction system generates 7.8×10~6 W/m~2 of electricity at a system conversion efficiency of 45.8%at 2000K,which is 19.7 times more power than the far-field multi-junction thermophotovoltaic(In As/In Ga As Sb),with a system conversion efficiency of 4.3%higher.The study further improves the performance of multi-junction thermophotovoltaics and guides the design of more efficient near-field multi-junction thermophotovoltaic systems.At the same time,a theoretical model of the near-field thermophotovoltaic system with dynamic modulation of radiation based on hyperbolic metamaterial emitter and graphene-In Sb cell is established(through modulation of the intensity of the photon tunneling),and the improvement of the system by different bias voltages is explored.Theoretical calculations show that the system energy conversion efficiency of the near-field thermophotovoltaic increases from34.6%to 44.9%at 1273 K when the bias voltage modulates the Fermi energy level from 0 e V to0.5 e V.The external bias voltage changes the Fermi energy level of graphene,and modulates the intensity of the surface plasma polaritons of the graphene,thus changing the photon tunneling effect and dynamically regulating the radiation.Based on the techniques of metamaterial and near-field radiation,a thermophotovoltaic-dual organic Rankin cycle thermal energy cascade utilization system is designed.The system uses metamaterials and near-field thermal radiation to modulate and enhance the radiation,improve the conversion efficiency of thermophotovoltaic modules,and recover waste heat by using a two-stage organic Rankine cycle.With the aid of the two-stage organic Rankine cycle module,the system conversion efficiency of the ideal single-frequency thermophotovoltaic of 2000 K is increased from 85%to 87.1%,the output power is increased from 127.4 k W to 130.8 k W;the conversion efficiency of the tandem thermophotovoltaic coupling with the metamaterial is increased from 54%to 61%,the output power is increased from 81.4 k W to 91.8 k W;the conversion efficiency of the near-field tandem thermophotovoltaic system is increased from 59.9%to 66%and the output power is increased from 89.9 k W to 99 k W.The study elucidates the prospect of energy conversion in thermal energy cascade utilization systems based on spectral modulation. |
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