Clear sky irradiance on plane of PV array (POA) is needed for calculating tilted clear sky index (CSI). The CSI can thus be used to access the variability within a large PV array. However, how can we obtain the clear sky irradiance on POA?
I have several ideas in mind:
1. Select the tilted measurements of clear sky situation (through Perez's sky's clearness and brightness, or using a sky camera). Then use a regression-based method to find an empirical formulation.
2. Calculate the global horizontal and diffuse horizontal irradiance components using a clear model, say Bird's model. Transposition model can then be used to estimate the irradiance on POA.
Can anyone share some references on such a practice? Thanks in advance.
I used the second approach to separate direct and diffuse from meaurements of global plane of array irradiance. See this publication:
https://www.researchgate.net/publication/256374705_Performance_ratio_revisited_is_PR__90_realistic
However I did not further validate this approach (contrary to what is mentioned in the paper ;-). For the overall results (clear sky in plane of array, correllation model to seperate direct and diffuse, calculate reflection losses and than compare pyranometer and sensor in plane irradiance) there was a quite high scatter in single time steps but low quite differences over longer time periods like a year.
Not sure if this helps. I was looking for something similar when I did this but didn't found a specialized model or publications on validation of such approaches.
Article Performance ratio revisited: is PR > 90% realistic?
Dear Dahzi
you can consider also the clear-sky model proposed by Gueymard SMARTS2 which proposes clear-sky irradiance calculation for a given plane orientation.
- Gueymard, C. 2001. “Parameterized Transmittance Model for Direct Beam and Circumsolar Spectral Irradiance.” Solar Energy 71 (5): 325–346.
- Gueymard, C. 2006. “SMARTS Code (version 2.9.5). User’s Manual.”
Another option would be indeed to use a clear sky model providing global and direct irradiance and then apply a transposition model to get global tilted irradiance. You can consider the excellent review of such transposition models provided by Ineichen (2011)
- Ineichen, P. 2011. “Global Irradiance on Tilted and Oriented Planes : Model Validations.”
For clear modeling, you can consider McClear: a freely available worldwide clear-sky product (historical dataset since 2004) incorporating MACC-ECMWF aerosol and ozone (re)analysis products. McClear is available at www.soda-pro.com (you will have to register, but it is free).
- Lefèvre, M., A. Oumbe, P. Blanc, B. Espinar, B. Gschwind, Z. Qu, L. Wald, et al. 2013. “McClear: A New Model Estimating Downwelling Solar Radiation at Ground Level in Clear-Sky Conditions.” Atmospheric Measurement Techniques 6: 2403–2418. doi:10.5194/amt-6-2403-2013.
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