Current and power output of a solar panel is very sensitive to the intensity of light falling on it. Current and power output are proportional to the light intensity. Fig. 1 shows this effect on a typical photovoltaic panel.
A solar array that gives 1000 W peak output at full light intensity will be able to give only about 800 W peak output when the light intensity falls by about 30%. (Peak output of a panel is available at the knee point of the V-I characteristics.)
Operating temperature also affects the current output and the peak power output of a panel as shown in fig. 1.
Shading:
A solar panel is made up of a number of solar cells interconnected in series-parallel combinations to achieve the rated voltage and power output. This aspect has a strong bearing on the manner in which solar panels are installed. If even one of the cells in a panel gets shaded by the shadow of a branch of a tree (or even the droppings of a bird) it may pull down the output of the panel by as much as 50%. Extreme care needs to be taken in positioning solar panels, especially when used for urban home installations.
You can ensure getting maximum power output from a solar panel by mounting it so as to catch light at maximum intensity. This is, however, easier said than done. Position of sun in the sky changes during a day; it also changes with the season. Moreover, the location of the installation on earth also influences the positioning of the solar panel.
A solar array (assembly of solar panels) should be tilted at an angle approximately equal to the latitude of the site, and facing within 15° due south (when situated in the northern hemisphere, as in the US) in order to be able to capture maximum solar radiation over a year. The sun is higher in the sky during the summer and lower in the winter. Therefore for optimum performance in winter the solar array can be tilted 15° more than the latitude angle, and for optimum summer performance, 15° less than the latitude angle.
Smaller installations of solar panels such as for homes are generally on the roofs of the homes. Naturally a south facing roof would be ideal. Flush mounted panels on roof would obviously be cheaper if the tilt of the roof is correct. Otherwise generally the solar array is mounted in a rack which has provision for changing the tilt, at least in two fixed positions to permit near optimum performance.
Large installations, where economics permits the added cost, generally pole- or ground-mounted arrays are another choice. Facility for adjustment of mounting racks enables you to set the angle of your solar panels as the season changes, keeping them aimed more directly at the sun. The solar electric system’s annual energy production increases by a few percent by providing adjustable tilt angle. The tilt angle of pole- or ground-mounted solar arrays can be changed more quickly and safely as compared to the roof mounted ones.
Solar cell modules are expensive. For large output, obviously a larger assembly of modules will be necessary with the attendant higher cost. A concentrator may be useful and possibly, more economical in overall sense, in such situations. Figure 2 shows typical basic construction of a solar concentrator that helps in reducing the cost while providing an efficient solar electricity generator.
It uses plastic Fresnel lenses mounted suitably above solar cells in housing. We all know the lens that is used as a magnifying glass. It can also be used to concentrate light from the sun to a sharp focus where it can even ignite material because of the concentrated solar power. The Fresnel lens is a rather special type of lens which is very light and can concentrate light from over a fairly large area to a small area much more efficiently than a conventional lens can.
Fresnel lens directs the rays from sun to the solar cell mounted underneath. The light beam goes through a further secondary concentrator before it falls on the solar cell. Thus a concentrator in effect brings more solar radiation to the PV modules increasing their output at a reasonably low cost.
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