Validation and Optimisation of Common Clear Sky Models with a use case for North-East Bulgaria

  • Svetlozar Kirilov Zahariev Technical University of Varna
Keywords: Clear Sky model, Global Horizontal Irradiance, Methodology, Validation, optimization


One of the most significant error contributors to preliminary design tools for Photovoltaic power systems is related to the simple parametric Clear Sky models. Therefore, this paper focuses on providing a methodology and a more sophisticated open-source tool for 3 commonly used Clear Sky models. This includes all relevant steps involved in the process - from filtering the raw meteorological data, identification of Clear Sky regions, data redistribution to genetic optimization of selected model parameter, etc.use case is built upon a multiyear dataset obtained from TU Varna meteorological station between 2012-2016. A significantly higher density distribution of Clear sky segments was identified during the summer through the Clear Sky Identification algorithm. To avoid the risk of overfitting the models to purely summer months and poor model fits in winter months, which was found to be the case with the legacy model, the underrepresented clear sky regions (based on θ) were replicated until uniform distribution is attained.  Subsequently, a genetic optimization was applied to selected parameters in the Clear Sky algorithms and the updated models showed a significant improvement in low winter months (θ) and even overall performance boost RMSE / MAE /R2. Furthermore, such validations and optimizations are recommended prior to any design or real-time PV-system analysis for the specific location.


Badescu, V., Gueymard, C. A., Cheval, S., Oprea, C., Baciu, M., Dumitrescu, A., Iacobescu, F., Milos, I., & Rada, C. (2012). Computing global and diffuse solar hourly irradiation on clear sky. Review and testing of 54 models. Renewable and Sustainable Energy Reviews, 16(3), 1636-1656. 
Duffie, J. A., Beckman, W. A., & McGowan, J. (1985). Solar Engineering of Thermal Processes. In American Journal of Physics (Vol. 53, Issue 4). 
Engerer, N. A., & Mills, F. P. (2015). Validating nine clear sky radiation models in Australia. Solar Energy, 120(September 2015), 9-24. 
ESA. (2017). Sentinel-5 Precursor Calibration and Validation Plan for the Operational Phase Reason for change Issue Revision Date.
Gueymard, C. A. (2008). REST2: High-performance solar radiation model for cloudless-sky irradiance, illuminance, and photosynthetically active radiation - Validation with a benchmark dataset. Solar Energy, 82(3), 272-285. 
Hottel, H. C. (1976). A simple model for estimating the transmittance of direct solar radiation through clear atmospheres. Solar Energy, 18(2), 129-134. 
Ineichen, P., & Perez, R. (2002). A new airmass independent formulation for the Linke turbidity coefficient. Pierre Ineichen CUEPE - University of Geneva, ASRC - State University at Albany.
Kasten, F. (1980). A Simple Parameterization of the Pyrheliometric Formula for Determining the Linke Turbidity Factor,". Meteorologische Rundschau, 33.
Google Scholar
Liu, B.Y.H., and R. C. J. (1962). Daily insolation on surfaces tilted towards the equator. Transactions of the ASHRAE. Vol. 67, 526-541.
Google Scholar
Myers, D. R. (2013). Solar radiation - Practical Modeling for Renewable Energy Applications (Vol. 2). CRC Group.
Reno, M. J., & Hansen, C. W. (2016). Identification of periods of clear sky irradiance in time series of GHI measurements. Renewable Energy, 90(May), 520-531. 
Reno, M. J., Hansen, C. W., & Stein, J. S. (2012.). SANDIA REPORT Global Horizontal Irradiance Clear Sky Models: Implementation and Analysis, Sandia National Laboratories,
Robledo, L., & Soler, A. (2000). Luminous efficacy of global solar radiation for clear skies. Energy Conversion and Management, 41(16), 1769-1779. 

Total number of hits on abstract = 1 times

Downloads for 2021

Download data is not yet available.
How to Cite
Zahariev, S. (2021, June 9). Validation and Optimisation of Common Clear Sky Models with a use case for North-East Bulgaria. ANNUAL JOURNAL OF TECHNICAL UNIVERSITY OF VARNA, BULGARIA, 5(1), 1-12.
Bookmark and Share