best model

The Engerer2 Diffuse Fraction Model (Global Radiation Separation Model)


August 2016 Update: You can learn how to compute the Engerer2 model in my Rpackage here!

This blog post explores the success of the Engerer 2 model as laid out in: Engerer, Nicholas A.  Minute resolution estimates of the diffuse fraction of global irradiance for southeastern Australia. Solar Energy. 116, June 2015, pp. 215-237.

[download the paper here]


One of the key outcomes from my PhD thesis was the validation of two different types of radiation models: clear sky and separation, against one minute resolution data. For the clear sky validation, I found suitable performance from several models for use in Australia, but the available separation models, however, did not have acceptable performance

The main issue with the available separation models (models that take a global radiation measurement from a pyranometer and separate it into its direct and diffuse components), is that they are regression based, with the original data being hourly averages of radiation.  At minute-resolution timescales, the relationship with global and diffuse/direct radiation is very different.  For one, there are very rapid fluctuations in the incoming radiation budget across these timescales.  Another big difference is the influence of cloud enhancement which is where radiation arriving at the surface exceeds the clear sky value because of non-linear interactions with some types of cloud decks. 

The Engerer2 model, with the diffuse fraction (Kd), plotted against the clearness index (Kt)

Making big changes, in the name of science

Thus, when I formulated my model, I knew that I had to make some significant advancements upon the existing methods/literature.  The principal improvements made with this model are four-fold:

  1. Inclusion of a physical model (REST2 clear sky model), making the model 'quasi-physical', much like the DISC model written by Eugene Maxwell (Maxwell 1987)
  2. The model is the only one of its class (as of the time of publication) that has been fit to minute resolution data (most other models have been designed for hourly data)
  3. There are two new variables, which have not previously been utilised in a separation model.  These are delta_Ktc (deviation of observed clearness index from clear sky value of clearness index)
  4. and K_de (the portion of the diffuse fraction that is attributable to cloud enhancement events)
The Engerer2 model formulation.  Please  read the paper  for more information.

The Engerer2 model formulation.  Please read the paper for more information.

independent assessment of the model: it works very, very well

The result is an impressive performance of the model against the current suite representing the state-of-the-art.  In a recent study, Gueymard and Ruiz-Arias 2015, radiation data from 54 sites around the globe were used to validate 140 separation models.  In this study, the Engerer2 model was the best!  Here it is, as described in the text:

“It is found that two models stand out over the arid, temperate and tropical climate zones: ENGERER2 and PEREZ2. These two models share two important features: (i) They include a variability predictor; and (iii) They leverage clear-sky irradiance estimates. The reason why ENGERER2 performs consistently better than PEREZ2 or other models is most likely because it was actually derived from 1-min data (compared to hourly data for PEREZ2 or most other models tested here). Based on the ensemble of statistical results obtained here, it is concluded that ENGERER2 has the best generalization skill, and can thus be considered a ‘‘quasi-universal” 1-min separation model, wherever and whenever low-albedo conditions prevail.”
Figure 2 in Gueymard & Ruiz-Arias 2015. Displaying the stations at which radiation data was used for model evaluation.

Figure 2 in Gueymard & Ruiz-Arias 2015. Displaying the stations at which radiation data was used for model evaluation.

And well... was that ever quite the compliment (especially coming from a scientist whom I've looked up to for so long)!  I am very pleased with this result, because now, my Engerer2 model is the ‘'quasi-universal' 1-min separation model" and has been accepted to be of global standard.  That makes my inner nerd quite happy, I'll admit. "Chuffed" as my Aussie friends would say :-).  And now I've have been given a reason to write a long overdue blog post about this research work.  As well as deliver a little surprise...

NOw, the Engerer2 model is in demand

As a result of this excellent outcome, I have several researchers in the community who would like to use my model, and I am quite happy to oblige.  So with this post, I am also announcing that a beta version of my Rpackage "anusolar" is now available, on request.  You can read more about this software at nickengerer.org/rpackage and where you can find out how to use the Engerer 2 separation model!  This package will allow you to do more than that, including PV simulation, KPV calculation and creating output from clear-sky radiation models.  So go check it out!

Solar 2014: Which Clear-Sky Radiation Model is Best for Use in Australia?


Update 2015: This research is now available as a paper in Solar Energy journal!

[download paper]

Citation: Engerer, Nicholas A and Franklin P. Mills. Validating Nine Clear Sky Radiation Models in Australia. Solar Energy. 120, October 2015, pp. 9-24.


Have you ever wondered which clear-sky solar radiation model you should use in your research project or solar energy simulation?  When I was designing my KPV method for estimating PV system power output I needed to figure out which clear-sky model would be the best one to use.  But there was a problem - I couldn't find a single validation study for clear-sky radiation modelling for Australia!

So in the paper, I had to do a quick model validation using one year of radiation data from Wagga Wagga, from which I decided on the Esra model.  But that simple validation left me wondering which model REALLY was the best?  

Thus I embarked on a scientific journey to discover which model was the best for use in Australia using the solar radiation data from 14 sites in Australia:

 

First, I set a few ground rules.  I wasn't going to use any radiation model that was overly complicated, nor was I going to use atmospheric variables that were difficult to obtain.

This meant using climatological values for input values such as the Linke Turbidity coefficient or ozone content - rather than using direct measurements from a photometer (because who honestly has spectral data?).  I think this is important, because a validation study should focus on models that are widely applicable so that it is widely useful.

In the end, I chose nine models from the options from both beam (direct) and global radiation: 

In the many Australian presentations and publications I've read and attended over the past few years, the most common clear-sky radiation model used is the Ineichen-Perez model.  

However, my research shows that the Ineichen-Perez model is not even in the top-three best choices.  So we really shouldn't be using it in our research as it is introducing unnecessary errors into our collective research knowledge.

What models are the best to use?  For the beam models, the top three choices are the Iqbal-C, Esra and REST2 models.  And for the global models, they are the Solis, Esra and REST2 models.

If we were to chose the best overall model, that would be the Esra model.  Which edges out the REST2 model, due to is large errors at high zenith angles.

You can read more about this in my Solar 2014 Poster Presentation [direct download] and the hopefully an upcoming article in Solar Energy journal (fingers crossed).

Until then, the quick answer is...

The best clear-sky radiation model for use in Australia is the Esra model!

 
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