Gendaymtx -5 for deterministic PV assessments

radiance-general
1

Hi all,

I’m trying to do a deterministic PV assessment with no inter-reflected light. My idea is to create solar sources whose intensity is equivalent to the direct sun AND the diffuse sky intensity. I don’t need to know the relative contributions from the sun and sky, only the total.

This would be similar to the 5-Phase method, except that there won’t be any sky patches at all.

The undocumented gensky -5 diam option works for the sun component, but does not account for the solid angle of the sky patch, since it was never meant to be used for this kind of exercise.

I suppose I could write some python script that converts the diffuse patch intensity of the sky matrix into an equivalent ‘diffuse solar source’ intensity by adjusting for the difference in solid angle between the sky patch and the solar disk.

Is there a better way of achieving this with some clever combination of Radiance tools?

Thank you for your help

-Axel

2

Hi Axel,

Nice to see a question coming from you for once – usually you are the one answering people!

Unfortunately, I can only respond with more questions. What form do you want the sky contribution in, exactly? Are you looking for a 180° source with a distribution like you would get from gensky, or do you just want an average, uniform value over the hemisphere? Or, do you not want a hemisphere glow at all, rather something concentrated?

Related to the previous two questions, how do you plan to account for the cosine factor on the incident radiation?

Cheers,
-Greg

3

Hello Greg,

since my PV assessment needs to provide per-month (and eventually also per-hour) sensor irradiance data, I would like to use a straight-forward rfluxmtx/dctimestep approach, but without sky – only ‘solar disks’. I’ll probably go with a Reinhart of 4, i.e. 2305 suns.

With the very simply 2-Phase method, we smudge out the sun’s intensity across the 3 or 4 nearest sky patches.

I’m trying instead to do the reverse, and assign the intensity of a sky patch to the sun at the same location. No smudging, since the sun is at the centre of the sky patch already.

In other words: The Tregenza/Reinhart sky dome is represented by discreet suns, so I get exactly the same result every time I run the same assessment.

gendaymtx -5 dim will give me the solar contribution. I guess technically speaking, the full invocation would be gendaymtx -5 dim -d, but it seems the -d isn’t actually required.

If I could run gendaymtx -5 dim -s, and the output would be adjusted to the much smaller solid angle the sun at this spot vs the solid angle of the sky patch, this would work nicely. However, running it this way does not account for the solid angle of the sky patch. So the -d works, but the -s doesn’t. Well, not for this particular niche-application.

I am therefore running a normal gendaymtx (without the -5 dim option), and now need to adjust the sky matrix, which is meant to represent patches, so that it accounts for the much smaller solid angle of the solar disc.

To reply to your second question: The cosine factor and everything else would be taken care of by rfluxmtx -I+. It really is an almost-normal 2-Phase method, just without the sky dome.

Is this making any sense?

Many thanks

-Axel

4

OK, I admit to struggling a bit to understand. Sounds like you are trying to avoid the randomness of rfluxmtx with a standard Reinhart subdivision, but if you reuse the calculated matrix, why would you see any variance? Or are you rerunning rfluxmtx for different configurations, and trying to avoid any sampling noise? Because it seems to me like the standard 2-phase DC method would work fine with the tools as they are.

That said, I’m thinking you could get where you want to go using light primitives whose RGB values are the Reinhart patch solid angle over the solar solid angle. So, using the conveniently supplied “reinsrc.cal” file, your suns would be generated by:

cnt 2305 | rcalc -e "MF:4;ss:0.5" -f reinsrc.cal -e "Rbin=recno;ss:0.5;ssa:PI*(ss*DEGREE/2)^2;sr=Romega/ssa" -o solar.fmt > mysuns4.rad

where the file “solar.fmt” contains:

void light solar${recno} 0 0 3 ${sr} ${sr} ${sr}
solar${recno} source sun${recno} 0 0 4 ${Dx} ${Dy} ${Dz} ${ss}

Then, when you run rfluxmtx -I+, add the -V+ option so the ratios you’ve computed are applied to the contribution coefficients. You can create a list of modifiers to use with rfluxmtx -M with this:

cnt 2305 | rcalc -o 'solar${recno}' > solarmods.txt

Is this what you are after, or am I still confused?

Best,
-Greg

P.S. I have used 0.5° for the solar disk size, but you can obviously adjust this by changing the “ss” constant if desired.