Solar Energy: SOLAR CONSTANT, SPECTRAL DISTRIBUTION, TERRESTRIAL SOLAR RADIATION

Solar Energy

1. Renewable & Non Conventional Energy (PEC-EEE-501B) Credit: 3 Lecture-1 By Prof. Pratik De Sarkar

2. SOLAR CONSTANT Solar rays subtend an angle of only 32 minutes on earth Energy flux received from the sun before entering the earth’s atmosphere is a constant quantity The solar constant, I sc , is the energy from the sun received on a unit area perpendicular to the solar rays at the mean distance from the sun outside the atmosphere The standard value of the solar constant is 1367 W/m 2

3. SPECTRAL DISTRIBUTION The curve rises sharply with the wavelength and reaches the maximum value of 2074 W/m 2 / µ m at a wavelength of 0.48 µ m. It then decreases asymptotically to zero, showing that 99% of the sun’s radiation is obtained up to a wavelength of 4 µ m.

4. SPECTRAL DISTRIBUTION The distance between the sun and the earth varies due to the elliptical motion of the earth. The extraterrestrial flux also varies, which can be calculated (on any day) by the equation where n is the day of the year counted from the first day of January.

5. TERRESTRIAL SOLAR RADIATION Beam radiation ( I b ): Solar radiation received on the earth’s surface without change in direction, is called beam or direct radiation . Diffuse radiation ( I d ): The radiation received on a terrestrial surface (scattered by aerosols and dust) from all parts of the sky dome, is known as diffuse radiation . Total radiation ( I T ): The sum of beam and diffuse radiations ( I b + I d ) is referred to as total radiation. When measured at a location on the earth’s surface, it is called solar insolation at the place. When measured on a horizontal surface, it is called global radiation ( I g ). Sun at zenith: It is the position of the sun directly overhead.

6. TERRESTRIAL SOLAR RADIATION Air mass (AM): It is the ratio of the path length of beam radiation through the atmosphere, to the path length if the sun were at zenith. At sea level AM = 1, when the sun is at zenith or directly overhead; AM = 2 when the angle subtended by zenith and line of sight of the sun is 60°; AM = 0 just above the earth’s atmosphere. At zenith angle θ z , the air mass is calculated as Irradiance (W/m 2 ): The rate of incident energy per unit area of a surface is termed irradiance . Albedo: The earth reflects back nearly 30% of the total solar radiant energy to the space by reflection from clouds, by scattering and by reflection at the earth’s surface. This is called the albedo of the earth’s atmosphere system.

7. Renewable Energy Sources and Emerging Technologies, D.P. KOTHARI, PHI, Second Edition, 2011 Chapter-3