A method for mapping color sky images of convective fair-weather cumuli to a scalar irradiance metric is presented. While basic interpretations of irradiance from color sky imagery are sufficient for many irradiance forecasting applications, sky images containing convective fair-weather cumuli can often contain gray-colored regions. Gray regions of a cloud commonly relate cloud thickness, indicating a region that has a high potential to attenuate surface irradiance from the sun. However, existing irradiance metrics are not found to translate such gray regions in accordance with their importance to a clear-sky index for short-term forecasting. This method exploits the special structure of sky images in three-dimensional red-green-blue (RGB) Cartesian space. By applying principal component analysis and basic clustering operations, a planar discriminant for cloudy-to-clear-sky classification is found. Once the {cloudy, clear-sky} identity of all image pixels is determined, the cloudy and clear-sky pixel distributions are separately remapped in RGB space so that gray pixels score a higher optical depth value than white pixels, and clear-sky pixels are mapped to low optical depth values. The resulting clear-sky index is simply the grayscale interpretation of the remapped image. The method is described and demonstrated with two actual sky images, and errors for sequences of several hundred images are tabulated and discussed. Remapped images are shown to represent gray cloud regions with a relatively high cloud optical depth in comparison with white cloud regions. The proposed metric is shown, by comparison with experimentally measured irradiance time series, to produce a more accurate clear-sky index in comparison with other methods.
Skip Nav Destination
Article navigation
January 2020
Research Article|
January 22 2020
A novel clear-sky index for color sky imagery used in short-term irradiance forecasting
Jeff Manning
;
Jeff Manning
a)
Department of Electrical and Computer Engineering, University of Texas
, Austin, Texas 78712, USA
Search for other works by this author on:
Ross Baldick
Ross Baldick
Department of Electrical and Computer Engineering, University of Texas
, Austin, Texas 78712, USA
Search for other works by this author on:
a)
Electronic mail: jeff.manning@rastrac.com
J. Renewable Sustainable Energy 12, 013701 (2020)
Article history
Received:
October 13 2019
Accepted:
December 17 2019
Citation
Jeff Manning, Ross Baldick; A novel clear-sky index for color sky imagery used in short-term irradiance forecasting. J. Renewable Sustainable Energy 1 January 2020; 12 (1): 013701. https://doi.org/10.1063/1.5131521
Download citation file:
Sign in
Don't already have an account? Register
Sign In
You could not be signed in. Please check your credentials and make sure you have an active account and try again.
Sign in via your Institution
Sign in via your InstitutionPay-Per-View Access
$40.00