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

Sat Feb 27 16:35:54 PST 2016

Hi Andy,

thanks for such a very good answer. It is quite clear, or will be once it
sinks in.

I have one concern though. Let me elaborate.

My intention was to, actually, not use BSDFs at all. I would like to
account for the irradiance caused by the sky and sun, not using the 3-phase
method, but the 2 phase one. Accordingly, I send rays from the walls to the
binned sky, not to the window. In such case, I would say that your method
is impracticable since there is nowhere to send rays from, and with it it
is impossible to account for inter reflections on the (very weird) exterior
environment.

The idea of using black walls, floor and ceiling surfaces was not to
consider inter reflections in the interior space (I understand that energy
plus will account for that...?). Why do I need -ab 0? I actually want to
account for all the optical phenomena on the exterior.

Am I being clear? Am I totally wrong?

Kind regards!
El 27 feb. 2016 18:33, "Andy McNeil" <mcneil.andrew at gmail.com> escribió:

> Also, if you use your approach and use ab>0 then you need to multiply the
> irradiance on the surface by solar absorbance of the surface. If you don't
> then you will definitely create energy.
>
> Andy
>
> On Sat, Feb 27, 2016 at 1:28 PM, Andy McNeil <mcneil.andrew at gmail.com>
> wrote:
>
>> 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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>>> Radiance-general at radiance-online.org
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>>>
>>>
>>
>
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