This document discusses a method for estimating solar radiation using digital image processing of sky camera images. Sky camera images are divided into three areas and pixel values from clear and cloudy day images are analyzed. Correlations between pixel values and solar altitude are used to create databases for clear and cloudy conditions. Beam, diffuse, and global radiation values are estimated based on averages of pixel values from the three areas and compared to measured values, achieving reasonably low error rates. The method provides information that can improve solar plant performance and operation based on meteorological conditions.

1. Solar Radiation Estimation
based on Digital Image
Processing
HARSH VARDHAN MALL
16EC65R22

2. Solar Radiation Estimation
 Quantifying the total solar radiation incident in the solar field
 Forecasting cloudy situations.

3. Why Solar Radiation Estimation?
 Expansion in solar energy applications
 Information of atmospheric conditions is the major
requirement to improve system performance
 Information is used to adapt solar plant's strategic operation
to the meteorological conditions

4. Components of Solar Radiation
 3 components - Beam, Diffuse and Global Solar Radiation
 Beam Radiation – Solar radiation travelling in a straight line
directly to the surface of the earth
 Diffuse Radiation – Solar radiation reaching the surface after
being scattered by atoms and molecules in atmosphere
 Global Radiation – Total radiation reaching the surface. It
includes both Beam and Diffuse radiation

5. .
Courtesy –
www.cspalliance.org

6. Sky Camera
 Sky Camera is used for solar radiation estimation
 Hemispherical vision is represented in JPEG images
 Images collected over 1 minute periods when solar altitude is
higher than 5 degrees
 Dataset made with sky camera images and solar radiation
data

7. Image taken from sky camera
Courtesy - www.niwa.co.nz

8. Division of image into three areas
Distance Matrix

9. Division of image into three areas

10. Division of image into three areas
 Area around the sun is more saturated and dilutes as we
move away from "Sun pixel"
 This area varies according to the time of the day
 After image splitting, pixel values are studied for cloudless
and overcast situations
 Different databases were created for both situations
 Data of 4 clear and 4 cloud covered days (for each season)
was collected

11. Study of Image values
 Digital values of RGB(red, green, blue) and HSV(hue,
saturation, value) are collected for each pixel
 Beam, Diffuse and Global solar irradiance values are also
collected for each pixel
 These values are stored at the databases

12. HSV
Courtesy - infohost.nmt.edu

13. Study of image values

14. Obtaining data from sky camera

15. Range of RGB and HSV value recorded

16. Channel values based on solar altitude
 Correlation between different image and solar altitude was
analyzed.
 It is done for each type of sky condition and each area

17. Correlation of digital image levels with solar altitude

18. Plotting Solar Irradiance
versus ND values
For Beam, Diffuse and Global Solar Radiation Values

22. Estimation of Solar Radiation components
 Area 1 is considered the most representative in terms of Beam
solar radiation component
 Area 2 and 3 better define the diffuse and global solar
radiation components
 To estimate beam radiation, the values of beam radiation
from pixels of area 1 are averaged
 To estimate diffuse and global radiation, pixels from all three
areas are averaged

23. Errors
 RMSE (Root Mean Square Error)
 Normalized RMSE (nRMSE)
 MBE (Mean Bias Error)
 Normalizes MBE (nMBE)

24. Formulae Definitions

25. Test data
 Four years of data was accessed
 Out of total number of days – 318 corresponded to clear skies
(216,243 images)
 304 corresponded to partially cloudy skies (193,889 images)
 86 to overcast skies(51,161 images)

26. Component RMSE nRMSE MBE nMBE
Beam 214.17 20.64 169.01 16.28
Diffuse 44.12 5.85 6.77 0.10
Global 66.45 6.46 32.92 3.20
For cloudless sky conditions

27. Component RMSE nRMSE MBE nMBE
Beam 106.50 11.13 48.97 0.87
Diffuse 122.09 16.31 43.47 0.38
Global 152.72 11.81 55.64 0.66
For overcast sky conditions

28. References
 J. Alonso-Montesinos, F.J. Batlles "The use of a sky camera for solar
radiation estimation based on digital image processing" Energy
2015;90: Page 377-386
 Escrig H, Batlles FJ, Alonso J, Baena FM, Bosch JL, Salbidegoitia IB, et al.
"Cloud detection, classification and motion estimation using
geostationary satellite imagery for cloud cover forecast." Energy 2013;55
Page 853-859
 Rusen SE, Hammer A, Akinoglu BG. "Estimation of daily global solar
irradiation by coupling ground measurements of bright sunshine hours to
satellite imagery." Energy 2013;58 Page 417-425