Solar Radiation, Electromagnetic Spectrum Solar Constant, Measurement of Diffuse, Global and Direct Solar Radiation

Solar Radiation:

Solar radiation is Energy coming from the Sun in ultra-violet, visible, and IR electromagnetic radiation.

• Annual solar radiation on a horizontal surface at the equator is over 2000kWh/m²

•  In Northern Europe, this falls to about 1000kWh/m² _{(per annum)}

•   The tilt between the Sun and the land reduces the intensity of the mid-day sun

 

Fig: Solar Radiation Spectrum

Ultraviolet	0.20 - 0.39µ
Visible	0.39 - 0.78µ
Near-Infrared	0.78 - 4.00µ
Infrared	4.00 - 100.00µ

•  The Sun produces light with a distribution similar to what would be expected from a 5525 K (5250 °C) blackbody, which is approximately the Sun's surface temperature

Radiation interacts with matter in several ways:

–                Absorption

–                Transmission

–                Scattering

–                Reflection

Factors affecting spectral distribution and intensity of solar radiation

Fig : Effect of  Atmosphere; PV Education

                    a)              The path followed by the Sunlight

                    b)             Composition of atmosphere

–                Water content

–                Ozone content

–                Depth of cloud cover

–                Reflection from the ground (Albedo)

–                Turbidity coefficient: a measure of dust and other forms of scattering

Effect of the atmosphere on Sunlight

When Sunlight enters the Earth's atmosphere, some portion of incident solar radiation gets absorbed, some radiation gets scattered, and some pass through and reach the Earth's surface.

Different composition of the atmosphere absorbs a considerable amount of solar radiation at a specific wavelength. E.g., in the infrared and visible range, water vapor and carbon dioxide absorb the Sunlight, while in the UV region, the Ozone absorbs the Sunlight.

Electromagnetic spectrum

·       The PV process converts solar radiation into useful electrical energy. Therefore, it can be considered that the fuel for the generation of solar electricity is the energy received from the Sun in the form of radiation.

·       The full range of electromagnetic radiation is known as the electromagnetic spectrum.

·       Only part of the spectrum that human eyes can detect is visible light (ROYGBIV), Wavelength (Violet 400nm; Red 700 nm)

·       All other radiations have wavelengths that are too long or too short for our eyes to detect.

Insolation

        ·  Insolation is a measure of solar radiation energy received on a given surface area in a given time.

        ·  It is commonly expressed as average irradiance in watts per square meter (W/m2) per day.

Solar constant:

The solar constant I_sc is the rate at which Energy is received from the Sun on a unit area perpendicular to the rays of the Sun outside the atmosphere at the mean distance from the Earth. A standard value of the solar constant is 1367 W/m².

Variation of extraterrestrial radiation

The value on any day can be calculated from the equation

                                I’_sc = I_sc [1+ 0.033 *cos( 360*n / 365)]

Where n is the day of the year.

           I_sc = Standard value of solar constant is 1367 W/m²

Solar time

Local Solar Time (LST): Twelve-noon local solar time (LST) is defined as when the Sun is highest in the sky. Local time (LT) usually varies from LST because of the eccentricity of the Earth's orbit and human adjustments such as time zones and daylight saving.

Local Time (LT): Local meantime is a form of solar time that corrects the variations of apparent local time, forming a uniform time scale at a specific longitude. Its uniformity depends only on the accuracy of the clocks used to measure it.

Local Standard Time Meridian (LSTM): The Local Standard Time Meridian (LSTM) is a reference meridian used for a particular time zone.

Equation of time

The Equation of Time is the difference between your local apparent solar time, when the Sun crosses your meridian, and local mean solar time, which is the average rate of solar time used in clocks. Because the Earth travels at varying speeds as it traverses its elliptical orbit, the difference can be as much as 16 minutes. 

- Kathleen Hill Zeller 

The solar energy varies because of the relative motion of the sun.

ET= 9.87 sin(2D) -7.53cos(D) -1.5 (D)

D =360 * (n-81)/365

n=n^th day of the year

Measurements of diffuse, global, and direct solar radiation

 

fig: Direct, Diffused and total Solar Radiation; basic-photovoltaic-principles-and-methods

Direct normal solar radiation

Direct beam radiation is the part of Sunlight that comes directly from the Sun. This would exclude diffuse radiation, such as that which would through on a cloudy day. It's an Indication of the clearness of the sky.

Diffuse sky radiation

Diffused radiation is solar radiation reaching the Earth's surface after being scattered from the direct solar beam by molecules or suspended particles in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the color of the sky.

Total light removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the Sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the Earth as diffuse sky radiation.

Global Horizontal Radiation

Total solar radiation, the sum of direct, diffuse, and ground-reflected radiation, is global horizontal radiation. However, because ground reflected radiation is usually insignificant compared to direct and diffuse, global radiation is said to be the sum of direct and diffuse radiation only for all practical purposes.

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