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Calculation method of split flux

asked 2017-04-03 03:47:06 -0500

keiichirot gravatar image

updated 2017-08-07 12:42:52 -0500

According to the Input-Output reference, DElight, one of the daylighting calculation methods of EnergyPlus, uses a radiosity method to calculate the effects of light reflection inside a zone. How about split flux, the another method? Does it also use a radiosity method or a ray-tracing method as like Radiance?

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answered 2017-04-03 11:19:57 -0500

The split flux (not a registered trademark) method is kind of a step backwards from DElight, actually. It's basically a simple daylight factor interpolation, and nowhere in the equation is there a room surface interreflection term. It's a classic example of "you get what you pay for", and in this case, the currency is cpu cycles. Split flux is really very fast, but makes no real account of the facts beyond the notion that the sun rises and sets each day, and that clouds happen. In reality, the interior has spatial articulation and variety of surface value, and light actually does a whole bunch of things once it gets past the window.

DElight tries to capture the interior photometric chaos with radiosity, and with that there is a commensurate increase in simulation time. Unfortunately DElight still uses the EnergyPlus building model for the geometry, which is based on the thermal zone boundaries, which rarely match up with the actual architecture. Radiosity also assumes all surfaces in the model are lambertian (100% diffuse) reflectors, which means we cannot look at daylight redirecting devices that typically tend more toward the specular side of things.

Which brings me to OpenStudio, which DOES use ray tracing, and a more spatially-honest geometry model (you see what I did there?). If you are looking to add "real" daylighting simulations to your whole building energy modeling workflow, I recommend looking at using the Radiance daylighting measure with OpenStudio, or other tools that leverage this combination such as Honeybee.

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Asked: 2017-04-03 03:47:06 -0500

Seen: 10,986 times

Last updated: Apr 03 '17