It’s a little ironic that there is a special term for non-artificial lighting (“Daylighting”), given that we have only had an electric alternative for the briefest flicker of our history. The rewards of daylighting are tremendous, particularly in commercial buildings, and they come in terms of dollars, productivity and contentment. Lighting can amount to 70% of electricity used in commercial buildings – making lighting efficiency the best way to reduce electricity use. It’s no wonder that so much scholarly effort is put to designing daylighting strategies. At WREF 2012, a Daylighting Technical Session illuminated great daylighting tips, and some of the contradictions between simulation and observation, design and practice.
The real trick to designing great daylighting is a problem as old as modeling (computer or physical): the dicrepancy between simulation and performance. The presenters each found a way around these obstacles. Another team from North Carolina State University found that computer modeling is inadequate to predict the behavior of the sun and clouds, and of surfaces in the building. So they stuck with physical models, replicated a 60-degree sector of sky in the lab and correlated its predictions with those of a scanner on the roof. The Australian team found that even standard measurements in the field often fail to reflect user perception. Of these measures, CIE Glare Index (CGI) agrees best with reported discomfort in open plan skylit offices and can be used to aid design.
Even when the daylighting solution is mocked up in full scale to the satisfaction of designers, the users may not take advantage of this free lighting. The University of Oregon team voiced the designers’ frustration: “The daylight is great, but the lights are on!” As with other best-laid architectural plans, occupant behavior, not design, drives energy use. The Australian team found that lights were often left on during the day just to satisfy the light consistency needs of the most demanding user of the space. Moveable dampers for daylight control were rarely used, and were left open or shut. Occupants need comfort and consistency in lighting, but occupants vary and few take advantage of controls beyond the lightswitch, so the default state of the system settles at “lights on.”
Successful daylighting designs must have controls that empower multiple user types and reset themselves to “lights off” so that daylight is the default. The system must be fast-operating – or at least fast enough to beat the user to the lightswitch. Simulation is a great way to test options, but there is no substitute for reality and for listening to the actual users of the space. The faster we get back to these basics, the faster we will return to lighting our world the old-fashioned way – for free.