The document provides an overview of solar resource evaluation methodology. It discusses the need to evaluate solar radiation for energy system studies and compares classical evaluation using measurements to evaluation from satellite images. The procedure involves determining what inputs are needed like location and technology information before choosing a methodology. Measurement-based or satellite-based approaches are outlined. Key aspects of solar radiation like daily and seasonal cycles are summarized. Methods for estimating components like beam and diffuse radiation are presented. The document also reviews instrumentation and databases for measurements.

1. Luis Martín Pomares
IrSOLaV
Calle Santiago Grisolia nº2, 28760 Tres Cantos (Madrid)
luis.martin@irsolav.com
www.irsolav.com / www.solarexplorer.info

2. Introduction
 Solar resources evaluation is a necessary first step for the
study of any energy system.
 The objective is the determination of the solar radiation
collected in a specific site, for its use in a specific solar
technology.
 As inputs, it is necessary to have information related to the
source and to the technology.
 The methodologies can be classified as: classical evaluation
(from measurements), and evaluation from satellite images.

3. Procedure proposed
Time series
Hourly, monthly
What do I need? Global, DNI
Maps
For what: Report, modeling
No
Satellite information Measurements?
Any other approach
Yes
Ok?
Solar resource knowledge

4. Solar radiation characteristics
 Solar energy reaches the earth in
a discontinuous form, showing
cycles or periods:
 Daily cycle: accounts for 50% of
the total availability of daily hours.
 Another effect of the daily cycle is
the modulation of the received
energy throughout the day.
 Seasonal cycle: modulation of the
received energy throughout the
year.

5. Solar constant and solar geometry
 Is the amount of solar energy
incident in 1 m2 of surface
perpendicularly exposed to
the solar rays and placed at 1
AU of distance.
 Changes slightly with time, but
can be considered as constant
 Ion = 1367 W/m2.(WRC).
Solar geometry is well known
We can estimate with high accuracy the solar irradiation at the top
of the atmosphere at every moment and every place

6. Interaction of solar radiation with
the atmosphere
Radiation at the top of atmosphere
Absorption (ca. 1%)
Ozone.……….…....
Rayleigh scattering and absorption (ca. 15%)
Air molecules..……
Scatter and Absorption (ca. 15%, max. 100%)
Aerosol…….………..…...……
Clouds………….……….. Reflection, Scatter, Absorption (max. 100%)
Water Vapor…….……...……… Absorption (ca. 15%)
Direct normal irradiance at ground

7. Solar Geometry
 The position of the Sun can be calculated suing
the following trigonometric equations:
ZENITH
 Cenital angle (θz) or its
SOL
TRAYECTORIA SOLAR
complementary solar angle (α)
(+) MAÑANA W 1
(-) ESTE
θz z z cos sin sin cos cos cos
-ψ α
ψ
S N
0
+ψ
 Azimutal angle (ψ):
PROYECCION DE LA
1
TRAYECTORIA SOLAR sin cos sin / sin z
E

8. Solar radiation components
RADIATION REFLECTED BY CLOUDS
GROUND ALBEDO
ABSORPTION
SCATTERING
DIRECT NORMAL RADIATION
DIFUSE RADIATION

9. Ley of Beer
In I 0 e( k L)
I 0 e( m)
I0 T
In In d I 0 e( k L)
d ISC e m
Clear sky models or transmitance models
Bn I CS (TRToTgTwTa 0.013) Yang
C
Bn ICS exp[ 0.8662 TLAM 2 mp R ] ESRA

10. The concept of optical mass
Aproximation to plane-
parallel
1
m
cos
Karsten equation
1.253 1
m (sin 0.15( 3.885) )

11. Air mass: variability
35
30
25
Masa relativa de aire
20
15
10
5
0
4 6 8 10 12 14 16 18 20

12. Sensibility of ESRA model to TL
Influence of TLINKE and altitude above sea level on DNI for clear sky
Dia juliano=200, z=500, Lat=37º N Long=-2º E TL=4, dia juliano=200, Lat=37º N Long=-2º E
1200 1000
TL=2 z=0 m
TL=4 900 z=500 m
1000 TL=6 z=1000 m
800
700
800
DNI (Wh m-2)
600
DNI (Wh m-2)
600 500
400
400
300
200
200
100
0 0
0 2 4 6 8 10 12 14 16 18 20 22 24 0 5 10 15 20 25
Hora Hora

13. Components and non-dimensioanl indexes
Components of solar radiation in horizontal surface
IG IB cos ID
Clear sky or transparency index
IG
kt
I0
Difuse radiation fraction
ID
kd
IG
Beam radiation transmitance
IB
kb
I0

14. Estimation of beam solar radaition
Correlations to estimate difuse radiation fraction
G (1 kd )
Ib 1.0 0.09kt kt 0.22
sen( ) kd 0.9511 1.1604kt
0.165 kt 0.8
4.388kt 2 16.638kt 3 12.336kt 4 0.22 kt 0.8
Correlations to estimate beam transmitance
Ib kb I o kb 0.002 0.059kt 0.994kt 2 5.205kt 3 15.307kt 4 10.627kt 5

15. Measuring Solar Radiation:
Pyrheliometers EKO MS-54
 Measures direct beam irradiance
 Typically used for calibration transfers Middleton DN5
 Normally defined with an opening angle of 5
 If used in conjunction with pyranometers, the optical
flat protecting entrance should match the optical
material of the pyranometer domes
 Relatively easy to characterize
 4 major manufacturers:
 EKO Instruments (Japan)
 Eppley Instruments (USA)
 Kipp & Zonen (Netherlands)
 Middleton Solar [Carter Scott Design] (Australia)
 Normally mounted on passive or active solar
tracking systems

16. Measuring Solar Radiation: Pyranometers
Tilted Irradiance
 Most pyranometers use a thermopile as means of converting solar irradiance into
an electrical signal.
 Silicon cell pyranometers are also available, but are not recommended by WMO.
 Advantage of the thermopile is that it is spectrally neutral across the entire solar
spectrum (domes may have spectral dependencies).
 Disadvantage is that the output is temperature dependent and the instruments
must ‘create’ a cold junction.

17. Measuring Solar Radiation: Silicon Pyranometers
 Instrument’s spectral response is non-linear and does not match solar spectrum.
 General calibrations are through comparison with pyranometers, therefore there
are spectral mismatch problems.
 LiCor is the primary instrument manufacturer and recognizes these problems:
 “The spectral response of the LI-200 does not include the entire solar
spectrum, so it must be used in the same lighting conditions as those under which
it was calibrated.”
–Pyranometer sensors are calibrated
against an Eppley Precision Spectral
Pyranometer (PSP) under natural daylight
conditions. Typical error under these
conditions is ±5%. (LiCor)
–Similar problems arise when using
sensors calibrated in one climate regime
and used in a different regime.

18. Rotating Shadowband Radiometer RSR2
 LI-COR Terrestrial Radiation Sensors
 Irradiance Inc. (www.irradiance.com)
 LI-200 Pyranometer is a silicon photodiode
calibrated from LI-COR ±5%
 RSR2 Head unit includes a moving shadowband
that momentarily casts a shadow over a LI-200
pyranometer
 Motor controller contains circuit to control the
exact movement of shadowband LI-200 Pyranometer
 Correction provided by Algorithm
 Measurement:
 Global Horizontal Irradiance
 Diffuse Horizontal Irradiance
 Calculation:
 Direct Normal Irradiance
RSR2 Headunit RSR2 Motor Controller

19. Measuring Solar Radiation:
Typical BSRN-like stations

20. Measurement recomendations
• Know exactly what temporal reference of the masurements
you are using (TSV, GMT, Local etc)
• Register with enough temporal resolution, almost 10 minutes
to register the dinamic of cloud transients.
• Follow BSRN recomendation for maintenance of instruments.
Cleaning every day radiometers, calibrate once per year each
instrument,…
• Secure the relation G=B cos θ + D. Some solar trackers have
embeded this filter in its program to activetes realtime alarms
when measurement is worng.

21. Meteorological Satellite population

22. Satellite classification
According to the type of orbit :
Polar satellites: placed in polar
orbits, modifying its perspective
and distance to the earth. The
resolutions of these satellites are
around 1m to 1km.
Geostationary satellites: placed in the geostationary orbit that is, the place in the
space where the earth's attraction force is null. It is an unique circumference
where all the geostationary satellites are situated in order to cover the whole
earth's surface. The resolutions of these satellites are higher in the sub
satellite point on the equator, and go decreasing in all directions.

23. Meteosat Satellite coverage
Meteosat Prime Meteosat East
 Spatial resolution 2.5 km at sub satellite, eg. About 3x4 km in Europe
 Temporal resolution 1h.
 Current Coverage: Meteosat Prime up to 1991-2005,
Meteosat East 1999 - 2006

24. Solar radiation derived from satellite images
Satellite to irradiance: general procedure
Meteosat – Goes - Mtsat
60’, 30’ or 15’ images in the visible
position assessement geometric
corrections – pixels averaging model to
obtain global irradiance

25. AOD (Aerosol Optical Depth estimations)
Estimations from MODIS (Moderate Resolution Imaging
spectroradiometer) on NASA’s Terra satellite
http://earthobservatory.nasa.gov/
AOD and water vapor vertical content estimations from satellite

26. Radiometric Databases
• Baseline Surface Radiation Network (BSRN)

27. Radiometric Databases
• Baseline Surface Radiation Network (BSRN)
• World radiation data centre (WRDC)
• Meteonorm

28. SSE
Radiometric Databases: SSE from NASA
http://eosweb.larc.nasa.gov/sse /
• Surface Meteorology and
• Solar Energy (SSE) Datasets
• And Web interface
• Monthly data
• Free upon
registration
Growing over the last 7 years to nearly 14,000 • 1ºx1º (120x120
users, nearly 6.4 million hits and 1.25 million
data downloads km) resolution

29. Solar radiation derived from satellite images
SWERA Project
The SWERA project provides easy access to high quality renewable energy resource information
and data to users all around the world. Its goal is to help facilitate renewable energy policy and
investment by making high quality information freely available to key user groups. SWERA
products include Geographic Information Systems (GIS) and time series data

30. Comercial data from satellite
• Irsolav
• Solemi (DLR)
• 3Tier
• Solargis
• ….

31. Some measurements in India

32. Some measurements in India

33. Some measurements in India

34. IrSOLaV activities
 Ciemat promoted a spin-off company for solar resource
characterization services (www.irsolav.com). Thus IrSOLaV
interacts with the industry needs and supply data and
consulting services on solar resource and also collaborates
with Ciemat in R&D.
 IrSOLaV and Ciemat develops R&D programs in the solar
resource field and collaborates with international scientific
groups (DLR, NREL, NASA, JRC, CENER, Universities…)
through European projects (COST project) or other initiatives
(Task 46 SHC/IEA)
 Within Spain IrSOLaV and CIEMAT collaborates with
universities (UAL, UJA, UPN) and support the industry through
agreements for doing specific research on solar resource
knowledge (forecasting, model improvements, atmospheric
physics, etc)

35. THANKS FOR YOUR ATTENTION !