[Radiance-general] lighting simulation of blinds in Energy Plus vs. Radiance

[Rob Guglielmetti]

On Jun 5, 2008, at 11:34 AM, Svetlana Olbina wrote:

> Energy Plus also performs lighting simulations.  I wanted to ask  
> for your opinion about accuracy/validation of Energy Plus when it  
> comes to lighting simulation.
>
> I have read that Energy Plus has integrated DElight software  
> (radiosity based lighting simulation software) and that  
> measurements are within 10% error? Is this margin of error acceptable?
>


Depends on what you're trying to accomplish, but 10% error is  
perfectly acceptable for most architectural/lighting design  
simulation problems.  But to simply say that the program is within  
10% without stating assumptions regarding the various simulation  
parameters (which affect accuracy) is a gross misrepresentation (and,  
sadly, a common one with software manufacturer claims).


>
>
> Our building is very simple (10’x15’ in the plan), and it has a  
> 6'X6' window on a 10' X 11' wall. We expected sufficient
> levels of illuminance  for this room but we got low illuminance  
> value of 270 lux at the workplane close to the window.
>
> I am concerned is that either Energy Plus is not accurate enough or  
> we have some error in our input.
>


What time of day?  What sky condition?  Hoe close is "close to the  
window"?  Lots of questions...


> How would you compare Radiance and Energy Plus (or DElight) when it  
> comes to simulation of blinds?
>
>
>
> We are new to Radiance and work already in Energy Plus, so we want  
> to see if we can use Energy Plus for lighting simulation instead of  
> Radiance.
>
> Or Radiance is still the most accurate tool for lighting simulations?
>

Using a light-backwards ray tracing algorithm to solve light  
transport through blinds is a tough problem.  Radiance includes a  
tool called mkillum that can do an initial raytrace at the blind  
interface and convert that result into a direct distribution for the  
window/blind system.  This in turn is then used as a virtual light  
source to calculate the illuminance in the space.  It leads to faster  
and more accurate results in Radiance.  I cannot speak to Energy  
Plus' lighting calculation capabilities, but my experience with DOE-2  
is that building energy simulation programs are severely limited in  
their ability to accurately estimate annual daylight and electric  
light performance.  It is possible to precalculate the daylighting  
performance in Radiance and convert that to some sort of lighting  
schedule that can be plugged into the E+ model to at least get a  
single picture from E+ for the building's performance.

- Rob Guglielmetti
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