Page - 96 - in Clean Water Using Solar and Wind - Outside the Power Grid

96 Clean Water Using Solar and Wind: Outside the Power Grid water operations is illustrated in 8.5. Economic aspects of solar PV operations are further reviewed in Chapter 12. 8.1 UTILISING THE SUN The solar radiation that reaches the outer atmospheric layers of the Earth has a particular spectral profile. Its intensity is highest between the wavelengths 0.3 and 4 μm. About 7% of the extra-terrestrial spectrum energy intensity is at ultraviolet wavelengths, 47% in the visible range and 46% infrared. 8.1.1 Irradiance Irradiance is the measure of solar power at a certain geographical location and is measured in W/m2 (see also 3.5). A significant amount of solar radiation is attenuated as it travels through the atmosphere. This attenuation is due to: • Absorption of solar radiation by different particles in the atmosphere, • Backward scattering and reflection of solar radiation by air particles, water vapour, dust etc. On a clear day some 80% of the radiation reaches the ground and is absorbed. Around 5% is scattered and reflected into space. 10–15% can be absorbed by particles in the atmosphere and typically 2% is reflected from the ground. On a cloudy day 30–60% of the radiation can be reflected and 5–20% absorbed by the clouds. There is a diffuse radiation from the clouds, which reaches the ground, so the total irradiation at ground level may be 0–50% of the radiance. A solar irradiance of 1,000 W/m2 is known in the PV community as one equivalent sun. 8.1.2 Global horizontal irradiance A solar cell will absorb a combination of several parts: • Direct radiation comes as a direct beam from the sun to the solar cell; • Ground-reflected radiation is the part of the radiation that is first reflected on the ground and then reaches the solar cell. Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user