The engerer2-separation-model from jamiembright

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Code for the Engerer2 diffuse fraction separation model following the newly parameterised version published in the Journal of Renewable and Sustainable Energy. The model estimates the diffuse horizontal irradiance from inputs of global horizontal irradiance, latitude, longitude and time.

MATLAB 66.17% R 15.23% Python 18.60%

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TJ clear-sky model implementations use sin instead of sind

I think all three implementations of the TJ clear-sky model in this repo are flawed in that the arguments to sin are interpreted as radians but should be interpreted as degrees. Here are the relevant lines of code:

Engerer2-separation-model/Engerer2Separation.py

Lines 128 to 130 in 8749240

    
           a_tj = 1160 + 75 * np.sin((360 * (day_of_year - 275)) / 365) 
        
           k_tj = 0.174 + 0.035 * np.sin((360 * (day_of_year - 100)) / 365) 
        
           c_tj = 0.095 + 0.04 * np.sin((360 * (day_of_year - 100)) / 365)

Engerer2-separation-model/Engerer2Separation.m

Lines 122 to 124 in 8749240

    
           A = 1160 + 75 .* sin((360.*(doy-275))./365); 
        
           k = 0.174 + 0.035 .* sin((360.*(doy-100))./365); 
        
           C = 0.095 + 0.04 .* sin((360.*(doy-100))./365);

Engerer2-separation-model/Engerer2Separation.R

Lines 97 to 99 in 8749240

    
           A = 1160 + 75 * sin((360 * (doy - 275)) / 365) 
        
           k = 0.174 + 0.035 * sin((360 * (doy - 100)) / 365) 
        
           C = 0.095 + 0.04 * sin((360 * (doy - 100)) / 365)

As an aside, I think the implementation in the clear-sky-models repo has the same problem: https://github.com/JamieMBright/clear-sky-models/blob/86fe607e9ba66226be6102895491b7d756473a7e/R/1-TJ.R#L26-L28

The 360/365 coefficient seems like it is intended to produce one period per year in degrees. Treating it as radians introduces an undesirable amplitude variation of period ~6 days, as these two screenshots show:

Changing the 360s to 2*np.pi seems to resolve the issue in the python implementation. I did not check the R or MATLAB versions.

As far as I can tell, these equations for A, k, and C do not come from the 1957 Threlkeld & Jordan paper but rather from G Masters' "Renewable and Efficient Electric Power Systems", which contains this example (Example 7.8). It only gives the correct answer if the argument to sin is interpreted as degrees:

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jamiembright / engerer2-separation-model Goto Github PK

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TJ clear-sky model implementations use sin instead of sind

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	a_tj = 1160 + 75 * np.sin((360 * (day_of_year - 275)) / 365)
	k_tj = 0.174 + 0.035 * np.sin((360 * (day_of_year - 100)) / 365)
	c_tj = 0.095 + 0.04 * np.sin((360 * (day_of_year - 100)) / 365)

	A = 1160 + 75 .* sin((360.*(doy-275))./365);
	k = 0.174 + 0.035 .* sin((360.*(doy-100))./365);
	C = 0.095 + 0.04 .* sin((360.*(doy-100))./365);

	A = 1160 + 75 * sin((360 * (doy - 275)) / 365)
	k = 0.174 + 0.035 * sin((360 * (doy - 100)) / 365)
	C = 0.095 + 0.04 * sin((360 * (doy - 100)) / 365)