Micro-structure Based Spectrum Modulation And Its Applications In Solar Thermophotovoltaics

Solar thermophotovoltaics(STPV)is an efficient energy conversion technology,which absorbs the whole solar spectrum and emits narrow band infrared radiation to the PV cells.The technology can break the efficiency limit for solar cells.The solar thermophotovoltaics technology can solve the mismatch problem of solar spectrum and bandgap,so that an efficiency of 85.4%can be achieved.In this thesis,we study the basic principles,structural designs and fabrications of all the critical components:spectrum selective solar absorber,optical filter and hybrid absorber-emitter for solar thermophotovoltaics.In addition,we build up a solar thermophotovoltaic system.The cut-off width of selective absorbers refers to the wavelength width of the transition from high absorption to low absorption.The photothermal conversion efficiency of the selective absorber decreases with the increase of the cut-off width.We report a semiconductor nanowire enabled selective solar absorber with not only high absorption selectivity but also sharp absorption cut-off.Due to the intrinsic spectral selectivity of semiconductors,our absorber has an extremely sharp absorption cut-off(transition region?200 nm),the sharpest selective absorber demonstrated so far.Such absorber can efficiently reduce the radiation loss in system,especially for STPV with low solar concentrations.For the PV cell in the STPV system,a narrow band optical filter can reduce the bandwidth of emission,so that a high system efficiency can be achieved.However,the emission spectrum of most reported emitters are very wide,more than 1000 nm,which greatly reduces the photoelectric conversion efficiency of the emission spectrum.We design and fabricate a SiO2/Si3N4 multilayer filter for narrow band application on the PV cell(cold side of the system),which shows a 130 nm passband at 2?m.Although the absorption and emission spectra of the system have been optimized,there will still be a lot of radiation losses for an open structure of STPV.To solve this problem,a bifunctional hybrid absorber-emitter(HAE)for efficient STPV by sequentially depositing HfO2/Mo/HfO2 films on a metallic surface is demonstrated here.By fine tuning coherent perfect absorption of the ultrathin Mo film and destructive interference of interfacial reflections across the top HfO2 layer,the HAE exhibits a measured emissivity of 0.97 at 1.8 ?m as well as an average absorptivity of?80%over 400-1000 nm,respectively,both of which are thermally stable up to 1373 K.It is theoretically shown that the HAE-based cage-type STPV can enable an efficiency improvement of 15%compared to double-side planar systems.On the basis of the above,we summarized two basic requirements of STPV,the maintenance of operating temperature and eficient radiation transfer.We have established a STPV test system with good temperature control performance.And we have achieved an overall efficiency of 5.04%,with input power of 16 W and a temperature of 1241 K for absorber and emitter.