1. SOLAR ENERGY IRRADIATION AND MEASUREMENTS
Sudarshan B S
Assistant Professor
Dept. of EEE
RVCE, Bangalore

2. Basics of Solar Radiation
Solar radiation is the radiant energy emitted by the Sun in the form of
electromagnetic waves.
The sun emits vast amount of radiant energy.
The earth intercepts only a fraction of it.
It is essential to drive directly or indirectly all biological and physical
processes on the Earth.
The earth is the only planet in the solar system, which receives an optimum amount of solar radiation that makes life sustainable on it.
Solar spectrum resembles to that of a black body at approximately
5800 K.
98% of the total emitted energy lies in the spectrum ranges from
0.25 μm to 3.00 μm. About half of the radiation is in the visible short-
wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum.

3. Basics of Solar Radiation Cont.
Solar radiation having wavelength less than μm (called
ultraviolet) is absorbed by ozone layer in stratosphere. The ultraviolet radiation not absorbed by the atmosphere is responsible for the change of color in skin pigments.
The solar radiation, that traverses the atmosphere further, is subjected to scattering, reflection and absorption by air molecules, aerosols, gases and clouds.
The radiation budget represents the balance between incoming energy from the Sun and outgoing thermal (longwave) and reflected (shortwave) energy from the Earth. Globally, the budget is balanced. Otherwise the temperature would rise constantly. Locally, the budget is not balanced. Tropical areas get more than they release, while higher latitudes of the winter hemisphere release more than they receive.

4. Breakdown of incoming solar energy

5. DEPLETION OF SOLAR RADIATION BY ATMOSPHERE
The earth is surrounded by an atmosphere containing various gases, dust and other suspended particles, water vapour and clouds of various types. The solar radiation during its passage in the atmosphere gets partly absorbed, scattered and reflected in different wavelength bands selectively.
Radiation gets absorbed in water vapor, Ozone, CO2 , O2 in
certain wavelengths.
Radiation gets scattered by molecules of different gases and small dust particles known as Rayleigh scattering where the intensity is inversely proportional to the fourth power of wavelength of light (l  1/4).
If the size of the particles are larger than the wavelength of light then Mie Scattering will takes place.
There will be a reflection of radiation due to clouds, particles of larger size and other material in the atmosphere.
Considerable amount of solar radiation also gets absorbed by clouds which are of several types.

6. Factors Affecting Solar Radiation
Location (space and time)
Clouds (droplet and ice)
Total precipitable water
Aerosols and dust
Surface Albedo
Total Solar Irradiance (Solar Constant)
Ozone
Mixed gases (CO2, N2….)

7. DEPLETION OF SOLAR RADIATION BY ATMOSPHERE (cont.)
Some radiation gets reflected back in the atmosphere due
to reflection from the ground, from the clouds, and scattering. This fraction of radiation reflected back is called albedo of the ground and on an average the albedo is 0.2.
The solar radiation which reaches on the earth surface unattenuated (after scattering, reflection and absorption) is called direct radiation or beam radiation.
The radiation which gets reflected, absorbed or scattered is not completely lost in the atmosphere and comes back on the surface of the earth in the short wavelength region and called sky or diffuse solar radiation.
The sum of the diffuse and direct radiation on the surface of the earth is called global or total solar radiation.

8. SPECTRAL DISTRIBUTION OF EXTRATERRESTRIAL RADIATION
In addition to the total energy in the solar spectrum (i.e. the solar constant), it is useful to know the spectral distribution of the extraterrestrial solar radiation, that is, the solar radiation that would be received in the absence of the atmosphere.
A standard spectral irradiance curve based on high altitude and space measurements is shown here which is found to be similar to the 5777K blackbody spectrum.
From this figure following observations are made:
The peak solar intensity is w/m2 at a wavelength of 0.48 m.
The solar spectrum varies from 0.2 – 3.0 m,
The energy in various spectral ranges is as follows:
Ultravoilet
Visible
Infrared
Wavelength Energy (W/m2) Percent
0.2 – 0.38m) 88 6
(0.38 – 0.78 m)
656 48
(0.78 – 3.0 m)
623 46

9. Terrestrial solar spectrum
The atmosphere absorbs extraterrestrial radiation at certain wavelengths, resulting in an altered spectral distribution for terrestrial radiation.

10. Types of Solar Radiation Irradiance (G, W/m2) Direct / Beam (DNI)•
Irradiance
Insolation
(G,
(H,
W/m2)
J/m2) •
Direct / Beam (DNI)
Diffuse irradiance (DHI)
Global / Total solar irradiance
(GHI) = DNI.
Cosθ + DHI
Ground Reflected radiation (Albedo)

11. Radiation Terminology
Irradiance
(W/m2):
Amount
of radiant
energy
incident on a
surface per unit area per unit time.
Direct solar irradiance: Solar irradiance on a surface held perpendicular to sun rays and diffuse sky radiation obstructed.
Diffuse solar irradiance: Solar irradiance on a horizontal surface due to sky radiation only.
Global solar irradiance: Solar irradiance on a horizontal surface due to both direct sun rays and diffuse sky radiation.
Reflected solar irradiance: Upward radiant exitance in the short wave range.
Net terrestrial radiation: Upward radiant exitance minus downward irradiance in long wave range through a horizontal surface near earth surface
Net total irradiance: Downward irradiance minus Upward radiant exitance in entire spectrum

12. DEFINITION OF SOLAR CONSTANT
The amount of the suns energy that reaches the
earth (before entering the atmosphere)
The average value of irradiance per year is called the solar constant (Ion)and is equivalent to , 1367 or 1373 W/m2 depending on who you believe
1353 (1.5%) from Thekaekara (1976) – derived
measurements at very high atmosphere and used by NASA
from
1367 (1%) Adopted by the World Radiation Centre
1373 (1-2%) from Frohlich (1978) - derived from satellite data

13. Radiation measuring Instruments
Pyranometer
Pyrheliometer
Pyrgeometer
Net Radiometer
Sunshine Recorder
First Factor
Second Factor
Third Factor
Sensitivity
Stability
Accuracy

14. INSTRUMENTS USED GLOBAL SOLAR RADIATION:
Direct + diffuse radiation on horizontal surface - PYRANOMETER
DIFFUSE SOLAR RADIATION:
Short wave radiation from entire hemispherical sky - PYRANOMETER WITH SHADING RING / SHADING DISK
DIRECT RADIATION
Direct radiation from sun - PYRHELIOMETER
REFLECTED SOLAR RADIATION
Short wave radiation reflected from ground - PYRANOMETER FACING DOWNWARDS
LONGWAVE RADIATION
Emitted from ground (upward direction)
Atmospheric radiation (Downward direction) - PYRGEOMETER & NET PYRADIOMETER

15. Measuring Solar Radiation Thermocouples/ Thermopile
Measurement devices for solar radiation employ thermocouples, which use the thermoelectric effect : Thermocouples contain two dissimilar metal conductors in contact, which produce a voltage when heated

16. A Typical Pyranometer
Pyranometers are used to measure global and diffuse solar radiation (from the halfspace). The thermopile is composed of several thermocouples, connected in series. The output is a voltage proportional to the temperature difference between the black surface of the sensor element and the housing as reference. Two quartz domes and a ventilation system (shall) minimize external influences

17. Eppley Precision Pyranometer
A pyranometer measures total global solar irradiance from the whole sky.

18. Specifications of Eppley Precision Pyranometer
Sensitivity: approx. 9 µV/Wm-2.
Impedance: approx. 650 Ohms.
Temperature Dependence: ±1% over ambient temperature
range -
20 to +40°C
Linearity: ±0.5% from 0 to 2800 Wm-2.
Response time: 1 second (1/e signal).
Cosine:
±1% from normalization 0-70° zenith angle;
±3% 70-80° zenith angle.
Calibration: integrating hemisphere.
Orientation: Performance is not affected by orientation or tilt.

19. A PYRANOMETER SHOULD HAVE THE FOLLOWING CHARACTERSTICS
It should not be wavelength-selective
Absence of zero drift
Calibration factor must be independent of the intensity
Response time should be as small as possible
Calibration Factor must be independent of time
Temperature response should be minimum
The calibration factor must be independent of temperature
Cosine and azimuthal response or spatial variation in the sensitivity of the detector should be minimum
Sensitivity should be as large as possible

20. Classification of pyrheliometers
STANDARD PYRHELIOMETERS Absolute cavity radiometer
Angstrom electrical compensation pyrheliometer Abbot silver – disk pyrheliometer
FIRST – CLASS PYRHELIOMETER Michelson bimetallic pyrheliometer
Linke – Feussner iron – clad pyrheliometer
New eppley pyrheliometer (temperature compensated)
Yanishevsky thermoelectric pyrheliometer
SECOND CLASS PYRHELIOMETERS Moll – Gorczynski pyrheliometer
Old Eppley pyrheliometer (not temperature compensated)
The smithsonian water – flow pyrheliometer was omitted from the list of standard instrument, but it has been one of the primary standard of the United States.

21. Instrument characteristics
Sensitivity Impedance
Receiver
9 microvolts per watt meter-2 approx.
650 ohms approx.
Circular 1 cm-2, coated with Parsons’ black optical lacquer
Temperature dependence
 1 per cent over ambient temperature range -20 to +40C (temperature compensation of sensitivity can be supplied over other ranges at additional charge
Linearity response time cosine
0.5 per cent from 0 to 2800 watts m-2 1 second (i/e signal)
1 percent from normalization 0-70 zenith angle
3 percent 70-80 zenith angle
Orientation mechanical vibration calibration
No effect on instrument performance tested upto 20g’s without damage integrating hemisphere (approx. 700 watts
/ meter arbient temperature +25C): calibration reference
Eppley primary standards
Readout
Reproducing the World Radiation Reference

22. DETECTORS FOR RADIATION MEASUREMENT
CALORIMETRIC SENSORS
The radiant energy is incident on a high conductivity metal
coated with a nonselective black paint of high absorptance.
THERMOMECHANICAL SENSORS
The radiant flux is measured through bendings of a bimetallic strip.
THERMOELECTRIC SENSORS
Consists of two dissimilar metallic wires with their ends connected.
PHOTOELECTRIC SENSORS
Photovoltaic instruments are most numerous in the field of solar radiation measurement. A photovoltaic device is made of a semiconducting material such as silicon.

23. General characteristics of sensors for radiant energy measurements
Effect used
Wave length (m)
Sensitivity
Linearity
Selectivity
Calorimetric Thermoelectric Photoelectric Photographic Visual
All 5 2
1.2
0.4 – 0.75
Low Good High High high
V. Good Good Poor Bad Bad
Absent Absent High High High

24. Absolute cavity Radiometer from Eppley

25. Measurement of Direct radiation at normal incidence

26. NORMAL INCIDENCE PYRHELIOMETER

27. Normal Incidence Pyrheliometer from Eppley Laboratory, USA
( SPECIFICATIONS)
Sensitivity: approx. 8 µV/Wm-2.
Impedance: approx. 200 Ohms.
Temperature Dependence: ± 1% over ambient temperature range -20 to +40°C.
Linearity: ±0.5% from 0 to 1400 Wm-2.
Response time: 1 second (1/e signal).
Calibration reference : Eppley primary
standard group of pyrheliometers.

28. Measurement of global and diffuse solar radiation on horizontal surface

29. Pyranometer with shading disk
Diffuse solar irradiance can be measured by adding a shadowing device to a pyranometer, which blocks the direct component of total irradiance.

30. Hand held pyranometers
Handheld pyranometers use less precise sensors than precision pyranometers but are more suitable for field measurements.

31. Ground measurements vs. satellite derived data
Satellite data
Advantages
Advantages
high accuracy (sensor dependent)
high time resolution
spatial resolution
long-term data (more than 20 years)
effectively no failures
no soiling
no ground site necessary
low costs
Disadvantages
high costs for installation & O&M
soiling of the sensors
sometimes sensor failure
no possibility to gain data of past
Disadvantages
lower time resolution
low accuracy without ground measurement
High quality ground measurement data is the basic ingredient for solar
radiation mapping

32. SRRA Network MNRE through its autonomous institute, National
Institute of wind energy (NIWE) at Chennai, has established solar radiation resource assessment (SRRA) stations (in phase-1 and phase-
2) in different parts of country which fall in the zones expected to receive good solar radiation. It is also proposed that the project will be extended to other parts of the country and monitored through NISE.

33. Parameters being Measured1
Global Horizontal Irradiance (GHI): The total amount of solar radiation per unit area that is intercepted by a flat, horizontal surface. 2
Direct Normal Irradiance (DNI): The amount of direct beam solar radiation per unit area that is intercepted by a flat surface that is at all times pointed in the direction of the sun. 3
Diffuse Horizontal Irradiance (DHI): The amount of diffuse solar radiation per unit area that is intercepted by a flat, horizontal surface that is not subject to any shade or shadow and does not arrive on a direct path from the sun. 4
Humidity 8
Barometric Pressure 5
Wind Speed 9
Rainfall 6
Wind Direction 10
Aerosol Optical Depth (Not Required but recommended to include at few sites) 7
Temperature
Frequency of Measurement:
1 minute interval
Duration of Measurement:
10 years, in order to establish TMYs

34. A TYPICAL SOLAR RADIATION RESOURSE ASSESSMENT STATION

35. Sl.No
State/UT
Phase I
Phase II
Total 1
Andhra Pradesh 6 3 9 2
Bihar -
Chhattisgarh 4
Gujarat 11 13 5
Haryana
Himachal Pradesh 7
Jammu & Kashmir 8
Jharkhand
Karnataka 10
Kerala
Madhya Pradesh 12
Maharashtra
Orissa 14
Punjab 15
Rajasthan 16
Tamil Nadu 17
Uttar Pradesh
Uttarakhand 18
West Bengal 19
North-East 20
Union Territories 51 60
111

36. Solar Resource Assessment
Information of solar radiation as available on the ground surface
•Quantity
•Spatial distribution
•Temporal characteristics

37. E-mail : hpgarg01@rediffmail.com
INDIA SOLAR RESOURCE SS
ByProf. (Dr.) H.P. Garg Sr.Consultant,
National Institute of Solar Energy (NISE) Gwal Pahari,Gurgaon Phone No. :