[Radiance-general] Daysim/Gendaylit

[Thomas Bleicher]

Ciao Roberto.

I'll start with the end of your e-mail:

> I don`t know if my way is right but I`d like to know
> something about it and what sky is better to use
> to get the horly internal illuminace values to link
> these ones to EnergyPlus or Trnsys.

So you really want to know internal illuminance values for a given
scene for the whole year? (I hope you know you can get the external
illuminance from the epw file.)

My summary of your problem:

Your idea is to use Daysim (or Radiance) with a climat data file to
calculate the internal illuminance values and plug these into E+ or
whatever. Your first step is to calculate external values in an empty
scene to "validate" your sky model. Doing this in Daysim and Radiance
(with gendaylit) you got two sets of results with significant
differences.

> The parameters used to do it are:
>
> -ab 4 -ad 1024 -as 256 -ar 512 -aa 0.1 -lr 2 -st 0.015 -sj 1 -lw
> 0.004 -dj 0.5 -ds 0.001 -dr 2 -dp 512

I hope you did have a -I option in there some where. You also only
need to increase your "-ab" to 1 to get the "diffuse" part.

Now the interesting bit:

If you look at your results or the global horizontal values you will
see that the Daysim values are roughly 25%-30% higher than those
calculated in Radiance. However, they are consistently higher (which
is good). I would expect that the difference is a result of the
approach the two applications have:

1) Daysim calculates a sky component and just maps the sky onto this
distribution to get a particular result.

2) Radiance used the sky directly in it's raytracing calculation.

Both sets of results are bound to be different from the actually
observed illuminance on the ground. You can extract the illuminance
values from the EPW file and compare them to the calculated results.
If you find that one of the sets is reasonably accurate go with that.

You can also use the measured values to 'calibrate' your calculations
and scale the internal results according to a reference calculation
outside. If your reference point value is different from the EPW
record you increase or decrease the calculated illuminance to
compensate for the overall difference.

You can do this either with the Daysim or Radiance results. I would
guess that a calculation for a whole year would be faster in Daysim.

Regards,
Thomas