SOLAR RADIATION & SOLAR RADIATION & ITS MEASUREMENTITS MEASUREMENT

CHAPTER 2CHAPTER 2

BY BY

DR.A.M.SURENDRA KUMARDR.A.M.SURENDRA KUMAR

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OUTLINEOUTLINE Properties of Radiations:Properties of Radiations: Solar constant Solar constant Solar Radiation at the Earths surfaceSolar Radiation at the Earths surface Sun at ZenithSun at Zenith Solar Radiation GeometrySolar Radiation Geometry

Altitude angleAltitude angle Zenith AngleZenith Angle SSolar Azimuth angleolar Azimuth angle Local Solar timeLocal Solar time

Solar Radiation measurementSolar Radiation measurement PyranometerPyranometer PyrheliometerPyrheliometer

Estimation of Average solar RadiationEstimation of Average solar Radiation Solar Radiation on Tilted surfaceSolar Radiation on Tilted surface

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INTRODUCTIONINTRODUCTION

Solar energy - Received in the form of RadiationSolar energy - Received in the form of Radiation

Converted Directly /indirectly - Heat & electricity.Converted Directly /indirectly - Heat & electricity.

DrawbacksDrawbacks::

1. Intermittent & variable manner in which arrives at the earths 1. Intermittent & variable manner in which arrives at the earths

surfacesurface

2. Large area required to collect the energy 2. Large area required to collect the energy

Radiated as electromagnetic wavesRadiated as electromagnetic waves

Electromagnetic waves- Wave length range from 0.2 to 4.0 micrometerElectromagnetic waves- Wave length range from 0.2 to 4.0 micrometer

Consist of 1. Ultraviolet radiation – 8% Consist of 1. Ultraviolet radiation – 8%

2. Visible light - 46%2. Visible light - 46%

3. Infrared Radiation - 46%3. Infrared Radiation - 46%

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SOLAR CONSTANTSOLAR CONSTANT

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SOLAR CONSTANT [SOLAR CONSTANT [IIscsc]] The rate at which solar energy The rate at which solar energy

arrives at the top of the atmosphere.arrives at the top of the atmosphere.

This is the amount of energy received in unit time on a This is the amount of energy received in unit time on a

unit area perpendicular to the suns direction at the mean unit area perpendicular to the suns direction at the mean

distance of the earth from the sun.distance of the earth from the sun.

According to NASA , According to NASA , IIsc sc = 1353 W/m= 1353 W/m22

= 116.5langleys/hr. = 116.5langleys/hr.

= 1165 = 1165

kcal/mkcal/m22/hr/hr

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The distance between the earth & the Sun varies a The distance between the earth & the Sun varies a little through the year.little through the year.

The earth is closest to the sun in the summer and The earth is closest to the sun in the summer and farthest away in the winter.farthest away in the winter.

This variation in the distance produces a nearly This variation in the distance produces a nearly sinusoidal variation in the sinusoidal variation in the intensityintensity of solar radiation, of solar radiation, I I that reaches the earth.that reaches the earth.

This can be approximated by the equation This can be approximated by the equation

where where nn is the day of the year is the day of the year

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365

n x 360cos 0.033 1

scI

I

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The air mass AM = 1 when the sun is at the The air mass AM = 1 when the sun is at the zenith i.e. directly over headzenith i.e. directly over head

( Sun at Zenith : Position of the sun directly ( Sun at Zenith : Position of the sun directly over head)over head)

AM = 2 when zenith angle is 60AM = 2 when zenith angle is 600 0

AM = sec AM = sec ΘΘ when AM > 3 when AM > 3

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ZENITH ANGLE ZENITH ANGLE ΘΘZZ

It is the complementary angle of sun’s altitude angle. It is a It is the complementary angle of sun’s altitude angle. It is a vertical angle between the sun’s rays and a line vertical angle between the sun’s rays and a line perpendicular to the horizontal plane through the point. i.e. perpendicular to the horizontal plane through the point. i.e. the angle between the beam from the sun and the vertical.the angle between the beam from the sun and the vertical.

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2

Z

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(Altitude angle)

Azimuth angle(Azimuth angle(γγss))

It is the horizontal angle measured from north It is the horizontal angle measured from north to the horizontal projection of the sun’s raysto the horizontal projection of the sun’s rays

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LOCAL SOLAR TIMELOCAL SOLAR TIME

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The Local Solar Time (LST) is based on the definition

of noon at a given location as being the instant when the

Sun is at its highest position (greatest altitude above the

horizon) in the sky.

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The Equation of Time (EoT) is an

empirical equation that factors in the

eccentricity of the Earth’s elliptical orbit and

the tilt of the Earth’s axis.

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LST = ST LST = ST 4( ST LONGITUDE – L O C) + EOT 4( ST LONGITUDE – L O C) + EOT

Where Where

LST = Local solar TimeLST = Local solar Time

ST = Standard TimeST = Standard Time

LOC = Longitude of Location LOC = Longitude of Location

EOT = Equation of Time correctionEOT = Equation of Time correction

The negative sign is applicable for the eastern The negative sign is applicable for the eastern

HemisphereHemisphere

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HOUR ANGLE (HOUR ANGLE ()) The hour angle The hour angle is the angle through which the is the angle through which the

earth must turn to bring the meridian of a point earth must turn to bring the meridian of a point

directly in line with the sun’s rays.directly in line with the sun’s rays.

equivalent to 15equivalent to 1500 per hour per hour

It is measured from noon based on the LSTIt is measured from noon based on the LST

It is positive in the morning and negative in the It is positive in the morning and negative in the

afternoon.afternoon.

= 15( 12 – LST)= 15( 12 – LST)

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DAY LENGTHDAY LENGTH Since 150 of the hour angle are equivalent to 1 Since 150 of the hour angle are equivalent to 1

hour, hour, The Day length tThe Day length tdd ( in hours) ( in hours)

ttdd = (2 = (2 )/15)/15

= =

where where is the latitude of location is the latitude of location

is the declination angleis the declination angle

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tantancos15

2 1

AVERAGE SOLAR RADIATIONAVERAGE SOLAR RADIATION According to Angstrom, the monthly average According to Angstrom, the monthly average

horizontal solar radiation is given by horizontal solar radiation is given by

Where a & b are constants depending upon Where a & b are constants depending upon the locationthe location

n is the average daily hrs. of bright sunshinen is the average daily hrs. of bright sunshine N = tN = td d is the max. daily hrs of bright sun is the max. daily hrs of bright sun

shine for the same periodshine for the same period

N

nb H 0 aH av

N= tN= tdd = daily length = daily length

= =

tantancos15

2 1

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