Why are there only two daylight controls in OpenStudio? What is the role of daylight controls?

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The daylighting controls are used to simulate, uh, daylighting controls, in an actual building. Therefore they need to be used judiciously, in spaces (and areas of those spaces) where it's anticipated there will be sufficient daylight to allow for electric lighting to be dimmed or switched off.

As David said there are two controls available per space as this emulates the "capability" of EnergyPlus. The typical use case for these things is the modeler will place one control near the perimeter and one deeper in the core, and set each control to "control" a percentage of the total electric lighting load in the space. In reality, there is one daylighting control (photosensor, what have you) in a given daylit area and the lighting is zoned and dimmed proportionally. The point is that daylighting controls as they are implemented in EnergyPlus and even OpenStudio at this point are fairly well divorced from reality. The OpenStudio team is planning to improve the daylighting control simulation modality, as are the folks at the CBEI. In the meantime you should be aware of these limitations:

1. In EnergyPlus, daylight transfer from one thermal zone to another is not possible with concave spaces, and even for convex ones the method is pretty weak
2. The split flux algorithm used in EnergyPlus can be pretty inaccurate
3. In Radiance, only one space per thermal zone can be calculated
4. In Radiance, only the primary daylighting control is used for calculating the lighting load control
5. Despite the limitations of the current Radiance implementation, this is the best way in OpenStudio to derive the daylight metrics.

OpenStudio daylighting control points are translated to Energy plus Daylighting:Controls objects. We matched EnergyPlus's limit of two per zone with two per space in OpenStudio. If you have multiple spaces in a zone with more than two controls, only two can be associated with the zone.

You still have to calculate the design load and typical usage schedule for your lights. Think of the daylighting controls as an override that turns them off or partially off when there is enough natural light.

If you have multiple thermal zones then you can have more than two daylighting controls.