Solar Analyst - effect of the diffuse proportion on the direct insulation.

[JulianWill]

Dear everyone,

I am using the Solar Analyst (ArcMap 9.3.1) to calculate the solar insulation on roofs. I have question to the effect of the diffuse proportion on direct insulation. I run the area solar radiation tool with the default settings and afterwards I increased the diffuse proportion to 0.4. The results showed that this changed the values of the diffuse radiation but not of the values of the direct insulation! That means that more clouds (higher diffuse proportion) does not affect the direct insulation in the Solar Analyst. I thought the direct plus the diffuse proportion are 1 and so higher diffuse proportion lowers the insulation of direct radiation.

In the description of the equations I could not find a connection between the diffuse proportion and the direct insulation. So it seems that the Solar Analyst does not consider the diffuse proportion in the calculation of the direct insulation.
Am I right with this assumption? If not can you please explain me the connection?

Best Regards,
Julian

[JulianWill]

Or if someone could explain me why the calculation of the direct radiation is not affected by the diffuse proportion! Because that sounds for me unrealistic and it falsified the calculation in my opinion.
I could not find any information about that. Thank you very much!
Best,
Julian

[RyanDeBruyn]

Julian,

Diffuse and direct do affect each other. However, Solar Analyst doesn�??t account for that automatically, instead, relys on the parameters.

You increased the diffuse proportion parameter (which affects diffuse),  however you should decrease the transmittivity parameter (which affects direct).

The parameters are defined int he help as follows.

diffuse_proportion:   The proportion of global normal radiation flux that is diffuse. Values range from 0 to 1. This value should be set according to atmospheric conditions. The default value is 0.3 for generally clear sky conditions.

Transmittivity:  The fraction of radiation that passes through the atmosphere (averaged over all wavelengths). Values range from 0 (no transmission) to 1 (all transmission). The default is 0.5 for a generally clear sky.


Hope this helps.
Regards,

-Ryan

[JulianWill]

Thanks Ryan!
I had read the help, but the problem is that the transmittivity also influence the value of the diffuse radiation. That makes it difficult to find representative settings for the time period, especially if it is for a whole year.

Existing there recommended settings for other atmospheric conditions besides the clear sky condition?

Because it is difficult to interpret the result when you change the diffuse proportion and transmittivity without having more thresholds for other atmospheric conditions, because you do not really know how strong the Solar Analyst values your changes.

Best,
Julian

[ChadSmith]

Ryan-

I have a similar question.

I have found a value for transmittivity for my location from NREL (.53). What is the correct way to calculate the diffuse proportion? I have seen studies that have divided the diffuse proportion of radiation by the total horizontal radiation to get a factor. Is this valid?

Thanks!