Optimization of a PVD Deposition System for the Realization of Dichroic Filters used in CPV spectral Separation System for the Energy Production

Raniero, Walter (2015) Optimization of a PVD Deposition System for the Realization of Dichroic Filters used in CPV spectral Separation System for the Energy Production. PhD thesis, University of Trento.

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Abstract

Photovoltaic technology in the field of renewable energy has reached a high commercial interest over the past decade. The traditional silicon photovoltaic systems that is currently the most widespread, mainly due to government subsidies, have a low energy production. The wide use of material and the low efficiency of the silicon modules required the research and development of photovoltaic systems more efficient. The most promising technology is the photovoltaic concentration that increases the efficiency of the modules by reducing the area of the PV cell. The concentration photovoltaic has had considerable technological progress related to the development of multi-junction PV cells with high efficiency. Another approach is the technology of photovoltaic concentration with the spectral separation, so using the interference filters the solar spectrum is splitted into different optical bands. In this research was designed and built a CPV prototype system with spectral separation. The interference filters such as anti-reflection and dichroic mirror are made up of silicon dioxide and titanium dioxide. These oxides have been realized by means of physical vapor deposition reactive magnetron sputtering technique. The PVD technique allows to deposit thin films with a homogeneous process reproducible and reliable. In the first part of the work, the characterization of individual layers of oxide materials have allowed to extrapolate the optical constants. This is necessary for the design of the optical multilayer. The characterization has nvolved various analyzes such as atomic force microscopy (AFM) to determine the thickness and the roughness, compositional analysis Rutherforf backscattering spectrometry (RBS), and optical analysis UV-Vis-NIR. These analyzes were necesary to calibrate the deposition system in order to subsequently to realize the multilayer optics. The as deposited optical multilayers not confirm the optical design, and it was necessary to carry out an annealing at 350°C. In the second part of the work, there were also micro structural characterizations for evaluating the phase variation of the titanium dioxide with the annealing treatment. The Fourier transform infrared (FT-IR) analysis has checked the absorption peak of the Ti-O-Ti of the crystalline phase. In addition, X-ray diffraction (XRD) analysis verified the phase variation of titanium dioxide from purely amorphous phase with a slight presence of rutile to the anatase phase. Through the optical analysis it was possible to extrapolate the new optical constants corresponding to the phase of anatase. In the third part of the work, the ray tracing design of optical splitting of the CPV prototype was carry out. The CPV system is designed by coupling a concentration Fresnel a dichroic mirror. The focus of the radiation on the PV cell, is simulated by two ideal detector. The optical optimization as function of the f-number of the lens has allowed to define the layout for the prototyping phase. A further optimization is to insert a secondary optics element (SOE) of homogenization. The secondary optics will also limits the optical losses due to a misalignment of the CPV prototype. In the last part of this thesis is devoted to the preparation and the characterization of the CPV prototype. Were performed measures of solar radiation, which combined with the characteristic I-V-P curves of the solar cells have enable to evaluate the efficiency of the prototype system. The efficiency of the spectral separation system was compared with concentration multi-junction PV cells. Daily measurement were performed to compare the spectral separation technology than to the multi-junction technology. The results show that the separation system maintains a more constant performance during the day. Finally, thermal measurements were conducted on the component of the CPV prototype separation system. The experimental results allows to guarantee that the spectral separation is also a selective filter of temperature. This allows the solar cells to maximize the photovoltaic conversion and to reduce the overheating.

Item Type:Doctoral Thesis (PhD)
Doctoral School:Materials Science and Engineering
PhD Cycle:28
Subjects:Area 02 - Scienze fisiche > FIS/03 FISICA DELLA MATERIA
Repository Staff approval on:14 Dec 2015 14:19

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