V

Illuminated

Operating point for max. power

Il Conventional Current Direction

Figure 16.20 Nomenclature of solar cells.

cells have been shown to have high end-of-life efficiencies. Solar cells are subject to weathering and radiation damage. Care must be taken in solar arrays to avoid poor interconnection between cells, and increased series resistance due to deterioration of contacts.

Solar cells are arranged in a variety of series and parallel arrangements to give the voltage-current characteristics desired and to assure reliability in case of individual cell failure. Fixed arrays are placed at some optimal slope and usually faced due south in the northern hemisphere. Large arrays are usually placed on a structure allowing tracking of the sun similar to those used for concentrating solar thermal collectors. In some arrays the sunlight is concentrated before it is allowed to impinge on the solar cells. Provision must be made for thermal energy removal since the solar cell typically converts only a small fraction of the incident sunlight into electrical power. Increasing temperature of the cell has a dominant effect on the open circuit voltage, causing the power output and efficiency to decrease. For silicon cells the power output decreases by 0.4 to 0.5 % per degree Kelvin increase.

Provisions must usually be made for converting the direct current generated by the array into the more useful alternating current at suitable frequency and voltage. In many systems where 24 hour/day electricity is needed some type of storage must be provided.

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