SolarEnergyResource - 7 3/8 x 9 1/4 Technical Energy...

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7 3/8 x 9 1/4 Technical / Energy Systems Engineering / Vanek / 0071495932 / Chapter 9 CHAPTER 9 The Solar Resource 9-1 Overview This chapter explains how light from the sun becomes available to do useful work on earth, as a basis for looking at how that sunlight can be turned into electrical or thermal energy. Topics covered here include the measurement of energy available from the sun, the effect of the atmosphere on light transmission, and the impact of angular geometry between sun, earth, and the surface of a solar device, instantaneous versus integrated values, and variations of real data from averaged values. 9-2 Introduction Solar power and solar energy resources on earth are enormous, nonpolluting, and virtually inexhaustible. Moreover, solar energy is the driving mechanism behind other renewable energy sources such as wind, hydropower, biomass, and animal power. Until the past two centuries and the exploitation of coal, oil, and natural gas, civilization grew and developed almost entirely based on solar energy in its various manifestations. Solar intensity is more than 1 continuous kW/m 2 outside the earth’s atmosphere, while on the surface of the earth the average daily interception is nearly 4 kWh/m 2 . The solar energy intercepted by earth in less than 1 month is the equivalent of all the energy originally stored in the conventional energy resources of coal, petroleum, and natural gas on the planet. This chapter reviews techniques for measuring and understanding the availability of the solar resource, as background for understanding the devices explained in Chaps. 10 and 11. 9-2-1 Availability of Energy from the Sun and Geographic Availability The intensity of energy arriving from the sun in space just outside the earth’s atmosphere is approximately 1367 W/m 2 , called the solar constant . Although it is termed a “constant,” it varies over time. Solar flares and sun spots change the value slightly. The distance from the sun to the earth is a greater factor, varying the value during the year. The daily solar constant, I 0 , can be calculated from the following equation where N is the day number of the year (e.g., January 1 is day 1; December 31 is day 365). N is also called the “Julian date,” from the Julian calendar. A Julian date calendar is provided, for convenience, as an appendix to this chapter. I 0 = 1367(1 + 0.034 cos(2 p N /365 )) (9-1) 225 Vanek_ch09-p225-248.indd 225 Vanek_ch09-p225-248.indd 225 3/28/08 8:38:06 PM 3/28/08 8:38:06 PM
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226 C h a p t e r N i n e 7 3/8 x 9 1/4 Technical / Energy Systems Engineering / Vanek / 0071495932 / Chapter 9 Average insolation , or solar energy reaching a given location on earth, will be lower than the amount available outside the atmosphere due to absorption and diffraction of sunlight in the atmosphere, changing weather, loss of sunlight at night, and so on.
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