Do thermal zone walls limit daylight secondary zones in CBECC

asked 2016-04-07 11:45:48 -0600

I'm doing detailed geometry for an office building for CBECC t24 2013 that is currently a shell with interior spaces tbd. I want to take full credit for daylighting and lighting. The windows have a head height of 10-11 feet so the secondary sidelite zone will be 20-22 ft into the perimeter. General guidance is to set perimter thermal zones at 15 feet. In Sketchup Open studio setting the thermal zones at 15ft creates solid walls when I generate the SDD file for import into CBECC. How do I best configure the model to maximize the daylight? Is there a way to designate thermal zone walls that allow light transfer that will carry through into the CBECC model? would I be better off setting my thermal zone boundary walls at the 20 ft mark instead of the 15 ft mark? other approaches? Or does cbecc ignore the thermal zone walls when determining daylight zones?

answered 2016-04-07 16:22:12 -0600

rpg777
6970 ●6 ●15 http://openstudio.net/

Yeah, this is kind of a problem with energy modeling tools and daylighting. The energy model is essentially thermal zone-centric, and geometric coherence kinda-sorta ends at the building perimeter. Those thermal zone boundaries are indeed opaque to the daylighting calculation in EnergyPlus, regardless of whether those boundaries are physical or logical.

With CBECC, EnergyPlus is doing all of the modeling, so you need to make do. ASSuming that extending the perimeter thermal zone to a depth of 20' does not skew your HVAC reality too much, that'd be the way to go. You can then have EnergyPlus do the daylighting calculation, and realistically you're not gonna get good side penetration of daylighting beyond 20' anyway, even with that head height of 11' (unless the ceiling is higher still, and you are using daylight redirection devices, and...). Just make sure you use two daylight sensors, locate them sensibly, and you fairly assign the percentages that each one is controlling for each zone.

And, if I may shamelessly shill for it: OpenStudio allows you to set these interior thermal zone boundaries as "air walls", which when simulating the daylighting in Radiance will not be exported, thus giving you an honest architectural model for the daylighting simulation. Not an option for CBECC, but...