6 where r b r b on the representative day r d r d

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? + ? 𝑑 ? 𝑔 ? ? + ? ? - - - 6 where , r ̅ b = r b on the representative day r ̅ d = r d = ( 1+cosβ ) / 2 r ̅ r = r r = ρ ( 1- cos β ) / 2 The daily radiation falling on the surface is of interest in many applications. The ratio of daily radiation falling on such surface ( H T ) to the daily global radiation on a horizontal surface ( H g ), given by Liu and Jordan is ? 𝑇 ? 𝑔 = [1 − ? 𝑑 ? 𝑔 ] ? ? + ? 𝑑 ? 𝑔 ? ? + ? ? - - - 7 For a south facing surface, γ = 0 ? ? = 𝜔 ?? ?𝑖? 𝛿 ?𝑖?(𝜑−𝛽)+??? 𝛿?𝑖? 𝜔 ?? ???(𝜑−𝛽) 𝜔 ? ?𝑖? 𝜑 ?𝑖?𝛿+??? 𝜑 ??? 𝛿 ?𝑖?𝜔 ? - - - 8 R d = r d = (1+ cos β ) / 2 - - - 9 R r = r r = ρ (1- cos β ) / 2 - - - 10 In Equation 8, ω st and ω s are the sunrise and sunset angles expressed in radians for the tilted surface and a horizontal surface respectively.
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7 Equation 7 can also be used for calculating the monthly average daily radiation falling on a tilted surface if the values required are calculated for the representative day of the month. It is then used in the modified form as ? 𝑇 ? 𝑔 = [1 − ? 𝑑 ? 𝑔 ] ? ? + ? 𝑑 ? 𝑔 ? ? + ? ? - - - 11 where , b = R b on the representative day d = R d = ( 1+cosβ ) / 2 r = R r = ρ ( 1- cos β ) / 2 3.6 Measurement of Solar Radiation The instruments used for solar radiation measurement are discussed below. Solar radiation flux is usually measured with the help of a pyranometer or a pyrheliometer. 3.6.1 Pyranometer A pyranometer is an instrument which measures either global or diffuse radiation falling on a horizontal surface over a hemispherical field of view. A sketch of one type of pyranometer as installed for the measurement of global radiation is shown in Fig. 3.3. Basically the pyranometer consists of a 'black' surface which heats up when exposed to solar radiation. Its temperature increases until the rate of heat gain by solar radiation equals the rate of heat loss by convection, conduction and reradiation. The hot junctions of the thermopile are attached to the black surface, while the cold junctions are located under a guard plate so that they do not receive the radiation directly. As a result, an emf is generated. This emf which is usually in the range of 0 to 10 mV can be read, recorded or integrated over a period of time and is a measure of the global radiation. Fig. 2.4: Pictorial view and sketch of Pyranometer for measuring global radiation The pyranometer shown in fig 2.4 is used commonly in India it has its hot junctions arranged in the form of a circular disc of diameter 25 mm and is coated with a special black lacquer having a very high absorptivity in the solar wave length region. Two concentric hemispheres, 30 and 50 mm in diameter respectively made of optical glass having excellent transmission
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8 characteristics are used to protect the disc surface from the weather. An accuracy of about ±2 percent can be obtained with the Instrument. 3.6.2 Shading ring Pyranometer The Pyranometer can also be used for measuring diffused radiation.
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