[Radiance-general] Calculating scheduled solar gains for EnergyPlus' surfaces

[Andy McNeil]

Hi German,
If you're okay with spatial averaging of the solar gains over a group of
windows, you could create a BSDF using Radiance to represent the complexity
in E+. The only caveat is that instances where sunlight coming from behind
the facade can enter the facade (reflecting off of buildings across the
street, for example) can not be represented in a BSDF. So you site
obstructions are best represented as geometry in the energy plus model. But
overhangs and undulated microperferated screens can now be represented as
BSDFs.

Now I'll try to compare my approach from 2011 to the approach you detail. I
traced rays from the window into the space to see what surfaces the ray
hit. You're creating sensors on surfaces tracing rays from the surfaces to
the windows to calculate irradiance. Your approach probably works fine in
most cases, but mine is better :).  With your approach you're trying to
calculate the average irradiance over a surface. If you have one sensor
point your estimate of the average will not be very good. So you decide to
use a grid of sensor points, which improves your estimate, but a regular
grid will be biased. So you use random points and your estimate is very
good. Though if you have a complex model, with say furniture, then you need
to know what hits the furniture too. Good luck putting sensor on all the
furniture surfaces. So then the next question is what settings are you
using? where they high enough?

Instead if you send rays from the window into the space, like I did, every
ray is accounted for, so your total solar gains will be accurate. With my
approach, I know that the sum of the energy distributed to the surfaces
equals the sum of the energy transmitted by the window, error in my
approach is limited to relative distribution between surfaces. With you
approach you have to do a lot of work to try to make sure the total energy
in the space is accurate, and I don't know of a simple way to check that
the sum of energy attributed to surfaces equals the sum of energy
transmitted by the window.

So while my approach might be trickier to implement, once it's running you
know you're not destroying or creating energy. Your approach is simple to
set up, but you're never really sure that the total energy in the space is
correct.

Andy



On Sat, Feb 27, 2016 at 4:18 AM, Germán Molina Larrain <germolinal at gmail.com
> wrote:

> Hi everyone,
>
> I am working on a project that requires calculating solar heat gains
> through (very) complex building envelopes. I am pretty sure that EnergyPlus
> will not be able to do it...
>
> I know EPlus is capable of dealing with CFS by means of BSDF... but that
> assumes that the CFS is an extra layer in the facade. In my case, you could
> consider the facade to be just anything.... a moving (ondulated and
> microperforated) overhang, a tree, a complex exterior environment, etc.
>
> My plan is to use Daylight Coefficients and use
> SurfaceProperty:SolarIncidentInside object in E+. That is, I want to
> pre-calculate the incident solar radiation using Radiance. *Just the
> incident! interreflection will be calculated by EPlus (is that correct?)*
>
> I understand this could be done as:
>
>    1. Create an office with black walls, floor and ceiling... the
>    exterior and the envelope keep their properties.
>    2. Create a sensor file for each surface... sensors are normal to each
>    surface, pointing inside
>    3. Calculate Daylight Coefficients for each wall
>    1. cat Wall.pts | rfluxmtx [options] -I+ - white_sky.rad scene.rad >
>       Wall.dcmtx
>    4. Calculate irradiance over surface
>       1. cat weather.wea | dctimestep Wall.dcmtx | averageSensors >
>       WallSolarRad.txt
>
> Now... I am not sure if this is correct, because what ANDY SHOWED IN 2011
> <http://www.radiance-online.org/community/workshops/2011-berkeley-ca/presentations/day3/AM_Radiance_EnergyPlus.pdf>
> is sort of different. He calculated a wird View Matrix.... is that an old
> way of doing this same thing? What am I missing?
>
> Best!
>
> Germán
>
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>
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