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Biaxial model for the incidence angle dependence of the optical efficiency of photovoltaic and solar thermal systems with two-dimensional reflectors
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Technology, Department of Engineering Sciences.
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Manuscript (Other academic)
URN: urn:nbn:se:uu:diva-91327OAI: oai:DiVA.org:uu-91327DiVA: diva2:164020
Available from: 2004-02-05 Created: 2004-02-05 Last updated: 2010-01-13Bibliographically approved
In thesis
1. Optical Efficiency of Low-Concentrating Solar Energy Systems with Parabolic Reflectors
Open this publication in new window or tab >>Optical Efficiency of Low-Concentrating Solar Energy Systems with Parabolic Reflectors
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solar electricity is a promising energy technology for the future, and by using reflectors for concentrating solar radiation onto photovoltaic cells, the cost per produced kWh can be significantly reduced. The optical efficiency of a concentrating system determines the fraction of the incident energy that is transferred to the cells and depends on the optical properties of the system components. In this thesis, low-concentrating photovoltaic and photovoltaic-thermal systems with two-dimensional parabolic reflectors were studied and optimised, and a new biaxial model for the incidence angle dependence of the optical efficiency was proposed.

Concentration of light generally results in high cell temperatures, and the uneven irradiance distribution on cells with parabolic reflectors leads to high local currents and temperatures, which reduce fill-factor and voltage. Cooling the cells by means of water increases the voltage and makes it possible to utilize the thermal energy. The performance of a 4X concentrating photovoltaic-thermal system was evaluated. If operated at 50°C, this system would produce 250 kWhelectrical and 800 kWhthermal per m2 cell area and year. Optical performance can be increased by 20% by using better reflectors and anti-reflectance glazing.

Low-concentrating photovoltaic systems for façade-integration were studied and optimised for maximum annual electricity production. The optimisation was based on measured short-circuit currents versus solar altitude. Measurements were performed outdoors and in a solar simulator. It was found that the use of 3X parabolic reflectors increases the annual electricity production by more than 40%. High solar reflectance is crucial to system performance but by using a low-angle scattering reflector, the fill-factor and power are increased due to a more even irradiance on the modules.

Long-term system performance depends on the durability of the components. The optical properties and degradation of reflector materials were assessed using spectrophotometry, angular resolved scatterometry, Fresnel modelling, optical microscopy, and surface profilometry before and after ageing. The degradation of reflectors was found to be strongly dependent on material composition and environmental conditions. Back surface mirrors, all-metal reflectors, and polymer-metal laminates degraded in different ways, and therefore accelerated ageing must be tailored for testing of different types of reflector materials. However, new types of reflector laminates showed a potential for increasing the cost-effectiveness of low-concentrating solar energy systems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 160 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 934
Engineering physics, Photovoltaic cells, Photovoltaic-thermal systems, Parabolic reflectors, Optical properties, Reflector materials, Optical efficiency, Accelerated ageing, Outdoor ageing, Building-integrated photovoltaics, Teknisk fysik
National Category
Other Engineering and Technologies
urn:nbn:se:uu:diva-3988 (URN)91-554-5867-X (ISBN)
Public defence
2004-02-27, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1A, Uppsala, 09:30
Available from: 2004-02-05 Created: 2004-02-05 Last updated: 2011-02-02Bibliographically approved

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