**To Understand how Solar Energy Varies we need to understand the various terms and parameters Involved. Here are a few Terms and Parameters Related to Solar Energy. Actually there are a large number of parameters but these are just few important of them.
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**Solar Constant:**** **The solar constant G_{SC} is the energy received from the sun, per unit time, on a unit area of surface perpendicular to the direction of propagation of the radiation at the mean earth-sun distance. There were several measurements of solar radiation, but the value 1367W/m², adopted by the world radiation center, is now commonly used.

**ZENITH****:** the point directly overhead is known as zenith

**Solar Time:**

*S.T. =LST+ET+4(l _{st} + l_{local})*

LST: Local Standard Time

ET = Equation Of time

l_{st }= Standard Time Meridian

l_{local }= Local Longitude

Both lst, l_{local} are taken positive from GM.

ET (in minutes)=9.87sin2B-7.23cosB-1.5sinB

Where B=(n-81)360/364

Where n=day of year

**Air Mass****:**** **It is the path length of radiation through the atmosphere, considering the vertical path at sea level as unity. At sea level, AM=1, when the sun is at the zenith and AM=2 for a zenith angle θz of 60°.

AM=1/cos θz

For zenith angles θz between 0° and 70° at sea level.

**Beam Radiation****:** The part of solar radiation that reaches the earth’s without any change in the direction is called beam radiation.

**Diffuse Radiation:** The solar radiation received by the earth after its direction gets changed because of scattering in the atmosphere is known as diffused radiation.

**Total Solar Radiation (Global Radiation):**** **The** **sum of the beam and diffuse solar radiation is called total solar radiation.

**Irradiance****:** The solar irradiance G is the rate at which the radiant energy is incident on a unit area of a surface. Denoted in terms of W/m², it is used for beam, diffuse or spectral radiation with suitable subscripts.

**Insolation:**** **The incident solar energy radiation (or irradiation) is also termed as insolation. While H is the insolation for the day, I is the insolation for a specific period, usually one hour. H and I are expressed in W-hr/m²/day or W-hr/m²/hr. When the values are measured on an hourly basis, I is numerically equal to G (Solar Irradiance). H and I can represent beam, diffuse or total radiation be it on any orientation.

**Solar Radiation at different surfaces**

The geometric relationship between a plane of any particular orientation relative to earth at any time( whether plane is fixed or moving relative to earth) and the incoming beam radiation, that is, the position of the sun relative to that plane, can be described in terms of several angles.

1) **Latitude (****φ):** The angle made by the radial line joining the location to the center of the earth, with projection of the line on the equatorial plane. By convention latitude is measured as positive for northern hemisphere. Varies from -90° to + 90°

2) **Solar Declination (δ):** Since the earth’s axis is inclined at an angle of 23.45° to the axis of its orbit around the sun, this tilt causes variation in the available solar radiation at any location. The angle between the earth sun-line and the plane through the equator is called solar declination. It varies from -23.45° on 21^{st} December and 23.45° on 21^{st} June. Declinations towards the north of equator are positives whereas those to south are negatives.

δ =23.45sin [(360/365)(284+n)] where n is the day number

3) **Slope (β):** Angle between plane of surface concerned and horizontal. Varies from 0 to 180°.

4) **Surface Azimuthal Angle (γ) :**Angle made in horizontal plane between the line due south and the projection of the normal to the surface on the horizontal plane. Due south is taken as 0, East of south positive and west of south negative. Varies from -180° to 180°.

5) **Hour Angle (ω):**It is the angular displacement of the sun, east or west of the local meridian, due to rotation of the earth on its axis at 15° per hour, morning negative and afternoon positive.

6) **Angle of Incidence (θ):**It is the angle between the beam radiation on a surface and a normal to that surface.

7) **Zenith Angle(θz**):It is the angle between the vertical and the line to the angle of incidence of the beam radiation on a horizontal surface.

8) **Solar Azimuthal Angle(α _{s}):**It is the angle between the horizontal and line of sun, that is, compliment of zenith angle.

9) **Solar Azimuthal Angle(γ _{s}):** It is the angular displacement of beam radiation on the horizontal plane. Displacements east of south are negative and west of south are positive.

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