Solar Radiation,Sun structure and characteristics

The sun is a hot sphere of gas whose internal temperatures reach over 20 million degrees Kelvin due to nuclear fusion reactions at the sun's core which convert hydrogen to helium.

Sun Radiation outside the earths Atmosphere

Solar radiation in space

The solar irradiance on an object some distance D from the sun is found by dividing the total power emitted from the sun by the surface area over which the sunlight falls.

The total solar radiation emitted by the sun is given by σT4 multiplied by the surface area of the sun (4πR2

sun) ,Rsun is the radius of the sun.

The surface area over which the power from the sun falls will be 4πD2. D is the distance of the object from the sun.

Spectrum of Electromagnetic Radiation

Solar radiation data requirements Solar heating and cooling (below 100 ºC(using flat plate collectors)

Global and diffuse solar radiation

Passive architecture Global solar radiation

Medium/high-temperature industrial process heating(using concentrating collectors)

Direct normal (or beam) radiation

Solar thermal power generation(using concentrating collectors)

Direct normal (or beam) radiation

Photovoltaic applications Global solar radiation

Solar Constant

• DEFINITION• Other details

Spectral distribution

• Diagram to be explained, plus other details on this topic.

Sun earth geometric relationship &

Sun’s trajectories in different seasons

Geometry of the earth–sun–time system

geometric relationship

• The orbit of earth around the sun is elliptical.• Thus, the distance between the earth and the

sun varies from time to time.• It is minimum (147.1 × 106 km) at winter

solstice on 21 December, and the point in the orbit is known as perihelion.

• It is maximum (152.1 × 106 km) at summer solstice on 21 June, and the point in the orbit is known as aphelion.

EQUINOXES and SOLSTICES

SOLSTICESUMMER SOLSTICE: •The first day of the Season of Summer. •On this day (JUNE 21 in the northern hemisphere*) the Sun is farthest north and the length of time between Sunrise and Sunset is the longest of the year.

WINTER SOLSTICE: •The first day of the Season of Winter. •On this day (DECEMBER 22 in the northern hemisphere*) the Sun is farthest south and the length of time between Sunrise and Sunset is the shortest of the year.*In the southern hemisphere, winter and summer solstices are exchanged. Summer: December 22. Winter: June 21.

EQUINOXES and SOLSTICESEQUINOXTwo times of the year when night and day are about the same length. The Sun is crossing the Equator (an imaginary line around the middle of the Earth) and it is an equal distance from the North Pole and the South Pole.SPRING EQUINOX: •The first day of the Season of Spring - and the beginning of a long period of sunlight at the Pole.• In the northern hemisphere: MARCH 20 (the Sun crosses the Equator moving northward). •In the southern hemisphere: SEPTEMBER 22 (the Sun crosses the Equator moving southward).AUTUMN EQUINOX: •The first day of the Season of Autumn - and the beginning of a long period of darkness at the Pole.• In the northern hemisphere: SEPTEMBER 22 (the Sun crosses the Equator moving southward). •In the southern hemisphere: MARCH 20 (the Sun crosses the Equator moving northward).

Sun’s trajectories in different seasons

In Alaska the sun remains visible in the night sky around the time of the summer solstice.

•The polar regions are continually illuminated and there is 24-hour daylight throughout the Arctic Circle. •This is down to a latitude 23 degrees from the pole, matching the angle of Earth's tilt.•While the north enjoys constant daylight the opposite occurs at the South Pole.•When the Northern hemisphere is tilted towards the sun the Southern Hemisphere is tilted away•Anyone in the Antarctic Circle would experience 24-hour darkness.

variation of the extraterrestrialradiation

• The solar radiation at the entrance into the Earth atmosphere is known as extraterrestrial radiation.

• The intensity of extraterrestrial solar radiation is challenge because of the change in distance between the Earth and Sun and because of the Sun activity.

• The value of this radiation during the course of a year changes in the range from 1307 (W/m2) to 1393 (W/m2 )

variation of the extraterrestrialradiation

• This tilt causes the sun to appear higher in the sky in summer, causing more hours of daylight and more intense, direct sunlight, or hotter conditions on the surface of the earth.

• On the other hand, during winter, the sun’s rays hit the earth at a shallow angle. As these rays are more spread out, the amount of energy that hits any given spot gets minimized. Moreover, the long nights and short days prevent the earth from warming up.

changes in the distance

• At the beginning of April and September, the earth is at a mean distance of 149.6 × 106 km from the sun.

• The sun’s visible diameter also changes as the earth moves round its orbit.

• In January, the angle subtended by the diameter of the disc is at its maximum at 32 36 , and in July, it is ′ ′′minimum at 31 32 .′ ′′

• When the earth is at its mean distance from the sun, this angle is about 32′

changesin the radiation flux

• The changes in the distance throughout the year lead to changes in the radiation flux reaching the earth from the sun;

• the radiation flux varies inversely with the square of the distance

estimation

• The variation of the extraterrestrial radiation with the time of the year can be estimated using Equation

• Gon is the extraterrestrial radiation on the nth day of the year (n = 1 for 1 January) and GSC is the solar constant.

SOLAR ENERGY RESOURCES-------Solar angles--------

The Solar angles

• Latitude• Solar declination• Hour angle• Slope • Solar azimuth angle• Surface azimuth angle• Angle of incidence• Zenith angle• Solar altitude angle

LATITUDE

• *One more latitude image*

SOLAR DECLINATION

Solar declination• The angle between the earth–sun line (through their centres) and

the plane through the equator is called the solar declination.• It varies between –23.45º on 21 December and +23.45º on 21 June.• Solar declination can be estimated using the expression.

• where n is the day number during a year (1 January being n = 1).

Hour angle• 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 15º per hour.

(morning negative and afternoon positive)

The different solar angles

SLOPE

• The slope or “tilt” is the angle between the plane of the surface concerned and the horizontal.

• Denoted by β

Solar azimuth angle

• Angular displacement from the south of the projection of the beam radiation on the horizontal plane

Or• Angle between a line due south and the

shadow cast by a vertical rod on Earth.

Surface azimuth angle

Angle made in the horizontal plane between the line due south and the projection of the normal to the surface on the horizontal plane

N

S

Angle of Incidence & Zenith Angle

Angle of Incidence:• Angle between the beam radiation on a

surface and the normal to that surface.

Zenith Angle :• Angle of incidence of the beam radiation on a

horizontal surface.• Special case of incident angle.

SOLAR ALTITUDE ANGLE

• Angle between the horizontal and the line of the sun. Also known as elevation angle

Combined formula• The relationship between incidence angle and other angles : Cos (θ) = Sinϕ(sinδ cosβ + cosδ cosγ cosω sinβ)+ cosϕ (cosδ cosω cosβ –sin

δ cosγ sinβ )+ cosδ sinγ sinω sinβ

Φ=latitude, δ=solar declination , β=slope or tilt, γ=surface azimuth angle , ω=hour angle, θ=

angle of incidence

Numerical

• What is the angle of incidence at noon in Mumbai for a horizontal plane on the 1st November at noon. (Take ϕ=19.1 )

• Here, ω=0 (at noon ω=0), • β=0 as the plane is horizontal

• Thus, the terms that remain are:

Numerical

Cos (θ) = Sinϕ sinδ cosβ + cosϕ cosδ cosω cosβ – eq.1

From data we know that ϕ=19.1 (for mumbai).δ =(-15.36) from following relation

N=305 (as the date is 1st November)

Thus, from eq.1, Angle of incidence (θ)= 34.51

AIR MASSAir mass m is the path length of radiation through the atmosphere.

m=1/cosθ

where θ is the angle from the vertical (zenith angle). When the sun is directly overhead, the Air Mass is 1.

The Air Mass quantifies the reduction in the power of light as it passes through the atmosphere and is absorbed by air and dust.

An easy method to determine the air mass is from the shadow of a vertical pole.

Air mass is the length of the hypotenuse divided by the object height h.

Beam radiationThe part of solar radiation that reaches the earth without any change in direction is called beam radiation. It is also known as direct radiation.

Diffused radiationThe solar radiation received by the earth after its direction gets changed because of scattering in the atmosphere is known as diffuse radiation.

Total solar radiationThe sum of the beam and the diffuse components of solar radiation is called total solar radiation.

IrradianceThe solar irradiance G is the rate at which the radiant energy is incident on a unit area of a surface. It is denoted in terms of W/m2.

Insolation

The incident solar energy radiation (or irradiation) is also termed as insolation. While H is insolation for the day, I is the insolation for a specific time period, usually one hour. H and I are expressed in W-hr/m2/day and W-hr/m2/hr, respectively.