SPT heliostats atmospheric attenuation

- I have been observing the atmospheric attenuation at the SPT model in SAM for a while now and I seem to have a problem.  In the heliostats field page, the atmospheric attenuation section, when I set the polynomial coefficients to zero, the average attenuation loss display in SAM gives a zero percentage which is logic. In addition, when I use some polynomial coefficients that correspond to a higher value of Aerosols like in my case (0.3205 AOD for the annually averaged AOD that I assembled) the average attenuation loss in SAM gives 6.6% which is also a good sign as higher AOD must result in higher attenuation percentages.

The problem is that somehow for the scenario of zero AOD and an average attenuation loss of 0% I get way lower generated energy which is not logic. Using the coefficients that corresponds to a higher AOD value normally should give a lower generated energy, but in my case it does give higher energy despite showing higher average attenuation loss percentage in SAM display.

I used Jesus Polo mathematical model to calculate the polynomial coefficients based on the AOD values and it is indeed linear with the average attenuation loss in SAM as the higher the AOD value gets, the higher average attenuation loss percentage SAM displays, however the problem remains in the energy output. 

I had a look on the receiver incident thermal power for the zero AOD scenario and it is lower while it should the highest scenario out of all other scenarios as the generated energy..

I even did a parametric analysis for AOD values from 0 to 1.5, the range is linear with the average attenuation loss percentage displayed in SAM, however the generated energy is really random.

Note:
I attached:
- the SAM file that I am using.
- a CSV file with the polynomial coefficients for two cases, i.e. the zero AOD scenario and the 0.3205 AOD scenario.
- The weather file I am using. 
- Jesus Polo attenuation model is:
         a= 3.13 AOD³ – 1.9 AOD² + 1.6 AOD – 0.133 / 100
         b = – 14.74 AOD³ + 2.49 AOD²– 11.85 AOD + 0.544 / 100          
         c = 28.32 AOD³ – 7.57 AOD²+ 48.74 AOD + 0.371 / 100
         d = – 2.61 AOD³ + 3.70 AOD²– 2.64 AOD + 0.179 / 100

Dear Mohammed,

The reason you are observing a discrepancy in annual generation between the two AOD cases is that SAM is optimizing the field for each attenuation model. The field optimization uses  the COBYLA method, which is a local derivative-free optimization algorithm and may not always converge on a global optimal solution. Because the optimization determines a different tower height, receiver height, receiver diameter, and number of heliostats each time you change the attenuation assumptions, the resulting difference in annual energy changes is due both to the different attenuation and the different field layout. To isolate the effect of the attenuation model on annual energy output, you can turn off the receiver optimization. This approach should show that as AOD increases, attenuation losses increase and annual energy decreases.

Best regards,
Paul.

Dear Mr. Gilman,

Thanks a lot for your answer I am really grateful and amazed of how you saw it very quickly as I spent over 2 week to realize that it is the problem.

I had a wrong believe that the solar field is only optimized based in the DNI levels reaching the heliostats, that is why I started to doubt the model I am using for the introduction of the AOD into the third order polynomial.

Thanks a lot you are really of a great help.

Regards,
Mohammed 

Hi there Mohammed,

I noticed that you have divided each of the polynomial coefficients, which you found via the jesus polo model, by 100. 

I realised that I have to do this too, or else I get a % attenuation loss of a factor of 100 too large! 

Do you know why this is though, as I am struggling to work out why this is necessary even though it clearly is?

Kind regards,

Harry