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Geometries for enhancing the output from PV cells
Högskolan Dalarna, Akademin Industri och samhälle, Miljöteknik.
1997 (engelsk)Rapport (Annet vitenskapelig)
Abstract [en]

In a world of limited resources and increasing demand for energy sources, especially in developing countries, PV cells are of great interest. Despite fairly large efforts in certain countries, among them Sweden, in R&D of new technology, the cells are still expensive regarding cost per watt. Obviously there should be interest also in other solutions than increasing cell efficiency for enhancing the output from the PV systems. Some solutions could be different geometries of both the PV cells and the PV cells combined with concentrating equipment such as cones and cornets, lenses, or V-trough mirror concentrators. Geometries studied in this thesis are: square comets, linear Fresnel lenses, V-trough/Fresnel lens combinations, concave and convex cylindrical surfaces, and PV panels mounted an an adjustable rack. Square cornets with flat mirrors have even light distribution at the exit aperture, which is an advantage over circular cornets and CPC cones which have uneven light distribution and cause hot spots at the exit aperture. Measurements and calculations on silver coated and aluminum coated cornets indicated that arrangements with comets having a PV cell at the exit aperture are technically worthwhile. The concentration depends on the geometry of the comet and the reflector material of the surface. The low cost, compared to PV cells, of polished and coated 0.7 mm aluminum makes it probable that the use of cornet concentrators would reduce the cost per produced kWh. Factors to take into account are the higher labor and material costs for manufacturing modules with cornets. Direct sunlight can be concentrated by lenses. Since large ordinary lenses are too bulky, flat linear or circular Fresnellenses are the only practical lenses for solar energy applications. The Fresnel lens is however less perfect than ordinary lenses since not all incident rays hit facets. The linear Czech Fresnel lens is made of glass and mass-produced in an inexpensive and technically simple method, but the surface is not optically perfect due to a slight sagging of the rolled grooved surface during the hardening of the glass. The ray tracing results have therefore been supplemented with measurements. Both rays incident on the flat surface and on the grooved surface were studied, as well as normal incidence and various combinations of meridional and sagittal angles of incidence. The measurements show that focallength varies from 35 cm to 60 cm for focal band widths about 5 cm, depending on incidence angles and whether the flat or grooved surface faces the sun. This Fresnellens could be combined with a PV panel consisting of solar cells connected in series in one row, which is placed in the focal band. The Czech Fresnellens could also be combined with a V-trough mirror concentrator and with one row of PV cells or one long, thin-film PV cell at the bottom of the trough. A test bench has been constructed, and six different geometries have been studied: For all of these geometries four cases have been studied: troughs without the lens, with the flat surface of the lens and the grooved surface facing the sun, and the lens only with the mirrors covered. The concentration was measured by comparing the short circuit current from a PV cell at the bottom of the trough. The measurements indicated that the acceptance angle is larger when the grooved surface faces the sun than when the flat surface faces the sun, but the concentration is slightly larger when the flat surface faces the sun. For the investigated geometries the light concentration varies from 1.9 to 3.2. The reflection losses from a flat surface are large for high incidence angles. At high latitudes a substantial part of the sunlight hits the surface of a stationary PV panel at high incidence angles. In order to reduce the reflection losses one solution could be to produce thin-film PV panels with concave or convex surfaces. Calculations have been carried out for six different refractive indices of the surface and six different ratios between the depth of the curved surface and the width of the surface. For all these combinations the conditions for both one surface and two surfaces (panes) have been calculated. The results show that for transparent concave panes the overall reflection losses could be reduced by one to two percent units compared to a flat surface. The last idea investigated was the improvement of PV panel output by changing the panel azimuth thrice daily. Calculations were carried out for three different latitudes at locations in the north, the middle and the south of Sweden in order to estimate the most suitable azimuths and tilt angles for the three azimuth changes. The calculations show that the insolation could be increased by 38 %,30 %, and 28 %, respectively, as compared with a stationary panel. Measurements were carried out in Borlänge, the middle location. The voltage and current from two panels, one fixed at the best tilt and one turned thrice daily, were measured every sixth minute and stored using a data logger. For four days in July with normal weather the gain was 30 % compared to the stationary panel, and for seven days in August with sunny weather the gain was 39 %. If a simple and inexpensive adjustable mount could be manufactured, this idea would be very interesting in order to enhance the PV output for a small panel of a few modules to be used, for instance, at a summer cottage.

sted, utgiver, år, opplag, sider
Högskolan Dalarna , 1997.
Serie
Centrum för solenergiforskning, Högskolan Dalarna, ISSN 1401-7555 ; DU-SERC--62--SE
Identifikatorer
URN: urn:nbn:se:du-844OAI: oai:dalea.du.se:844DiVA, id: diva2:522735
Tilgjengelig fra: 2005-01-12 Laget: 2005-01-12 Sist oppdatert: 2012-04-24bibliografisk kontrollert

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