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Wireless+Networking+in+the+Developing+World_Part12 - 7...

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7 Solar Power This chapter provides an introduction to the components of a standalone photovoltaic system . The word standalone refers to the fact that the system works without any connection to an established power grid. In this chapter, we will present the basic concepts of the generation and storage of photovol- taic solar energy. We will also provide a method for designing a functional solar system with limited access to information and resources. This chapter only discusses the use of solar energy for the direct production of electricity ( photovoltaic solar energy ). Solar energy can also be used to heat fl uids ( thermal solar energy ) which can then be used as a heat source or to turn a turbine to generate electricity. Thermal solar energy systems are beyond the scope of this chapter. Solar energy A photovoltaic system is based on the ability of certain materials to convert the radiant energy of the sun into electrical energy. The total amount of solar energy that lights a given area is known as irradiance ( G ) and it is measured in watts per square meter ( W/m 2 ). The instantaneous values are normally averaged over a period of time, so it is common to talk about total irradiance per hour, day or month. Of course, the precise amount of radiation that arrives at the surface of the Earth cannot be predicted with high precision, due to natural weather varia- tions. Therefore it is necessary to work with statistical data based on the "so- lar history" of a particular place. This data is gathered by a weather station over a long period and is available from a number of sources, as tables or 211
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databases. In most cases, it can be dif fi cult to fi nd detailed information about a speci fi c area, and you will need to work with approximate values. A few organizations have produced maps that include average values of daily global irradiation for different regions. These values are known as peak sun hours or PSH s. You can use the PSH value for your region to simplify your calculations. One unit of "peak sun" corresponds to a radiation of 1000 Watts per square meter. If we fi nd that certain area has 4 PSH in the worst of the months, it means that in that month we should not expect a daily irradiation bigger than 4000 W/m 2 (day). The peak sun hours are an easy way to repre- sent the worst case average of irradiation per day. Low resolution PSH maps are available from a number of online sources, such as http://www.solar4power.com/solar-power-global-maps.html . For more de- tailed information, consult a local solar energy vendor or weather station. What about wind power? It is possible to use a wind generator in place of solar panels when an autonomous system is being designed for installation on a hill or mountain. To be effective, the average wind speed over the year should be at least 3 to 4 meter per second, and the wind generator should be 6 meters higher than other objects within a distance of 100 meters. A location far away from the coast usually lacks suf fi cient wind energy to support a wind powered system.
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