Spectral Cascade Regulation Mechanism And Performance Study Of Solar Photovoltaic-Thermal System

Solar energy utilization technology is an effective way to speed up the adjustment of energy structure.Solar photovoltaic power generation technology is considered to be an ideal solar energy utilization technology.Although the power generation efficiency of photovoltaic cells has been significantly improved,due to the inherent semiconductor band gap characteristics,photovoltaic cells have a large energy loss and relatively low conversion efficiency.The conversion efficiency of photovoltaic cells depends on the frequency of incident light.Photons with energy lower than the bandgap of semiconductor materials（photothermal band）will be converted into thermal energy by photovoltaic cells,increasing the temperature of photovoltaic cells,resulting in a decrease in the energy conversion efficiency of photovoltaic cells.When the operating temperature of photovoltaic cells increases by 1 K,the power generation efficiency will be relatively reduced by 0.4%-0.5%.In addition,photons with energy higher than the band gap of semiconductor materials（photovoltaic band）cannot be fully utilized by photovoltaic cells,due to the interface reflection,parasitic absorption,and unsaturated absorption,the optical loss reaches to 25%.Therefore,the low utilization efficiency of the full spectrum of solar energy is an important challenge in solar energy utilization.Aiming at the problem of low utilization efficiency of full-spectrum solar energy in photovoltaic cells,new photovoltaic cells are usually designed from the perspective of material and structure design to improve photoelectric conversion efficiency,or from the perspective of convective heat transfer to reduce the temperature of photovoltaic cells.However,the above method ignores the grade of solar energy,and it is difficult to use photons of different grades at the same time,which restricts the utilization of full-spectrum solar energy.This dissertation is based on the different characteristics of photovoltaic bands and solar thermal bands in the solar full spectrum,aiming to regulating spectral characteristics of photovoltaic bands through micro-nanostructure,and reveal the enhanced light absorption mechanism of micro-nanostructured photovoltaic cells;establishes a method for micro-nano structures to regulate photovoltaic and photothermal dual-band spectral characteristics,and solves the problem of high absorption rate of photovoltaic cells in the photothermal band;integrates micro-nano structures,solar photovoltaic devices and solar thermal devices,develop full-spectrum solar energy technology according to energy grades,carry out experimental research on spectral splitting full-spectrum solar energy utilization systems,and improve the efficiency of full-spectrum solar energy utilization.The research contents are:In the photovoltaic band,the surface periodic micro-nano structure is designed through wave optics theory to strengthen the photon absorption of photovoltaic cells in the photovoltaic band and solve the problem of large optical loss of photovoltaic cells in the photovoltaic band.The influence of the surface periodic micro-nano structure on the optical and electrical properties of photovoltaic cells was analyzed,the geometric parameters of the surface periodic micro-nano structure were optimized by using the particle swarm optimization algorithm,and the enhanced photon absorption mechanism of the surface periodic nanosphere structure was clarified.The results show that the surface periodic nanosphere structure has a micro-lens concentrating effect on short-wavelength light and a long-range scattering effect on long-wavelength light,which effectively enhances the photon absorption of photovoltaic cells at 400-1000 nm.Afterwards,the 200-nm-thick photovoltaic cell produced the same current as a conventional 300-nm-thick photovoltaic cell.Surface micro-nano structure on the photovoltaic cell is proposed,designed and optimized to solve the problem of large optical loss in the photovoltaic band and high absorption rate in the photothermal band.The influence of the structural parameters of the micro-nano structure laying on the surface of the multi-layer nano-dielectric film on the optical and electrical properties of photovoltaic cells was explored,and the optical performance of the photovoltaic cell with the micro-nano structure on the surface of the multi-layer nano-dielectric film was analyzed.The results show that the micro-nano structure on the surface of the optimized multi-layer nano-dielectric film is a double-layer SiO₂/TiO₂ film structure with a thickness of 95 nm and 57 nm,which effectively enhances the absorption rate of the photovoltaic cell in the photovoltaic band,and simultaneously enhances the reflectivity in the photothermal band.The reflectivity of the photothermal band increased from 23%to 78%,the temperature of the photovoltaic cell was reduced by7°C,and the photoelectric conversion efficiency of the photovoltaic cell was increased by 0.6%.SiO₂/TiO₂ nano-interference films is designed and prepared,established a method for controlling the spectral characteristics of photovoltaic bands and photothermal bands based on SiO₂/TiO₂ nano-interference films according to energy grades,and constructed SiO₂/TiO₂ nano-interference films spectral splitting full-spectrum solar energy photovoltaic-thermal system,developed SiO₂/TiO₂ nano-interference film spectral splitting full-spectrum solar energy utilization technology with outdoor dual-axis tracking with concentrating function,and carried out outdoor experimental test research of full-spectrum solar energy utilization system.The results show that the SiO₂/TiO₂ nano-interference film can regulate the full spectrum of solar energy according to the energy grade,realize the reflection of the photovoltaic band and the transmission of the photothermal band,curb the absorption of the photothermal band by the photovoltaic cell,reduce the temperature of the photovoltaic cell by 11 K,improve photoelectric conversion efficiency by 1.3%,and the full-spectrum energy conversion efficiency and exergy efficiency of solar energy are increased by 5.8%and 1.5%,respectively.Developed and established a method based on single nanofluids and mixed nanofluids to regulate the spectral characteristics of photovoltaic bands and photothermal bands according to energy grades,analyzed the influence of nanoparticle shape,concentration and optical thickness on the selective transmission and absorption characteristics of nanofluids,and constructed a nanofluid spectral splitting full-spectrum solar energy photovoltaic-thermal system based on dual-axis sun tracking.The results show that the spectral characteristics of solar energy can be adjusted according to energy grades through single nanofluids and mixed nanofluids,and the photovoltaic band transmission and photothermal band absorption characteristics can be realized,and nanofluids can directly absorb and convert solar light in the photothermal band into useful thermal energy.The experimental test shows that the full-spectrum energy conversion efficiency of solar energy can reach 24.07%,and the filtering efficiency of nanofluid based on various nanoparticles can reach 35%.A full-spectrum solar energy utilization technology based on the interface structure is proposed,and the photovoltaic cell itself can use the photovoltaic band and the photothermal band according to the energy grade.A spectral splitting full-spectrum solar photovoltaic-thermal system with semi-transparent photovoltaic cells with interface structure was established,and the influence of the bionic moth-eye structure on the surface of semi-transparent photovoltaic cells and the serrated groove bottom structure on the spectral characteristics were analyzed.The results show that semi-transparent photovoltaic cells with interfacial structure can regulate the energy grade of solar energy,realize the multifunctional compatibility of photovoltaic cells with high absorption in photovoltaic bands,high transmission in photothermal bands,and high concentration of light.The average absorption factor and average transmission factor are increased by 8%and 14%,and the full-spectrum solar energy utilization efficiency of the spectral splitting full-spectrum solar photovoltaic/concentrating solar thermal system reaches 77%.