| Thermophotovotaic (TPV) is a kind of technology that can directly converts the infrared radiation of high temperature heat source into electrical energy by semiconductor PN junction. TPV systems were reported with potential high output energy density and theoretical efficiency. Besides, due to the diversity of heat sources, portability, no moving parts, little pollution and cogeneration, TPV technology has potential applications in the fields of industry, commerce, military and aerospace etc.At present, one of the biggest challenges for TPV technology is that system thermoelectric conversion efficiency is low. In the case that the research of TPV cells could not get a breakthrough in a short time, high efficient spectrum control methods are very important to the improvement of TPV system efficiency. Selective emitter is one of the most important spectrum control technologies. In this thesis, selective emitters are studied from materials and structures.First, in order to solve the problem that the high temperature stability of rare-earth-type selective emitter is poor, an Er2O3coating-type selective emitter for thermo-photovotaic application was prepared by plasma spray technology. The test results show that plasma spray technology could be used to prepare Er2O3coatings selective emitter with good stability at1400℃. Base on the measurements of the high temperature normal spectral emissivity and the spectral hemispherical emissivity of the samples at room temperature, the influence of the coating thickness was discussed, and the selective emission performance of the sample was evaluated using radiative efficiency as the criterion. The results demonstrate that the emission of substrate could not be neglected unless the coating thickness is larger than the penetration depth, which is around100μm. The selective emission peak of Er2O3coating occurs at1550nm, matched well with GaSb cells. However, the radiative efficiency is no larger than that of SiC emitter, because the non-convertible emission of1.725-5μm accounts for a large proportion of the total radiation power, especially at high temperature. Effective suppression of this band emission is essential to improve the radiation efficiency of the emitter.In order to solve the narrowband emission and high emission at unconvertible wavelength which exist in Erbium oxide selective emitter, selective emitter which doped nickel oxide in the high reflective host material (magnesium oxide) was prepared based on the character of Ni2+that high emission can be realized in visible and near infrared band by energy level transition. The spectral emissivity was measured and the influence of the NiO content to emissivity of the samples was analyzed. The results showed that the selective emission peak increased with the content of NiO, while the emissivity of non-convertible wavelength was also increased. Because the emission peak was still narrow, Er2O3was doped into the sample to expand the emission peak in near infrared band. This was confirmed by the experimental results. The emission peak caused by Ni2+decreased with the increase of the content of Er2O3. The simulation results of TPV system illustrated that the improvement on effective power density and cell efficiency was little because of the influence of the emission of2.68～4.00μm, and spectral efficiency and system efficiency significantly decreased. Using radiative efficiency and TPV system efficiency as the criterion, the selective emission performance of the sample whose proportion is MgO:NiO:Er2O3=63:27:10is the best. At the same temperature, while this sample was used as selective emitter, TPV system efficiency can be theoretically increased by43.87%, compared with SiC emitter; at the same input power density (10W/cm2), TPV system efficiency and electric output power density can be5to6times that of SiC system.Compared with material-type selective emitter, structure-type selective emitter realized selective emission by plasma polarization, microcavity effect and photonic bandgap effect, the designability is much better. Based on the high melting point of vanadium dioxide (VO2) and the character that VO2is metal phase at high temperature, we calculated and analyzed spectral emissivities of2D cylindrical and rectangular cavity photonic crystals by Finite Difference Time Domain method (FDTD). The impacts of geometric dimension on spectral emissivity of2D photonic crystals were discussed combined with resonant cavity theory. The results showed that the emissivity enhancement is dominated by the microcavity effect. For radiation with wavelengths shorter than the fundamental cavity resonant wavelength (cutoff wavelength), the enhancement in emissivity is achieved by coupling with resonant electromagnetic modes of the cavity. The position of cutoff wavelength is dominated by the physical dimension, especially by the sectional dimension. The average emissivity with wavelength shorter than cutoff wavelength increased with the depth of cavity. The impacts of periodicity on cutoff wavelength is small. Diffractive modes decrease the emissivity below the cut-off, as higher-order resonances may couple with multiple channels corresponding to diffraction, thus increasing total reflectivity. No matter the microcavity section is a circular or rectangular,2D photonic crystal selective emitter matched with GaSb cells can be designed by selected the appropriate size. The emissivity of emitter at the wavelength below bandgap of GaSb cells can reach0.95, while the emissivity at non-convertible wavelength is below0.30. |
PDF Full Text Request