The US Department of Energy (DOE) Pacific Northwest National Laboratory (PNNL) has published a report detailing a study the lab performed to evaluate the energy efficiency of educational spaces lit to some published circadian lighting guidelines. Even given the inherent efficiency of LED sources, lighting designs that meet guidelines published by the WELL Building Institute, UL, and the California High-Performance Schools (CHPS) program would increase energy usage over the range of 10–100% compared to Illuminating Engineering Society (IES) recommendations for school lighting.
The report “Energy impact of human health and wellness lighting recommendations for office and classroom applications” was published initially in the academic journal Energy and Buildings. The intent was to examine the impact of lighting specification that meets WELL v2, UL Design Guideline 24480, and Collaboration for High Performance Schools Core Criteria 3.0 guidelines.
We have covered the WELL program numerous times and its application in commercial offices in a manner that comforts workers, extending to lighting and other environmental conditions such as air quality. UL 24480 is a more recent initiative that was focused specifically on developing a circadian lighting guideline and that work was led by Mark Rea of the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. We have not covered the California guideline in detail, but that document was released in 2019 and is broad based, covering lighting, water efficiency, and much more.
The PNNL research team modeled both an office and a classroom, taking into account things such as desk orientation, surface reflection, and more. The lighting installation was based on IES guidelines for light at the work plane, uniformity, and more. The team used it models and ran 45 unique simulation scenarios to gather the data in the report.
As one might expect, there is a somewhat fundamental disconnect between IES recommendations and those tied to circadian metrics. The IES recommendations are based on human visual sensitivity and units such as lumens (lm) and lux (lx). The circadian guidelines are based on evolving circadian metrics that characterize the response to light by the non-visual receptors in the eye — the intrinsically-photoreceptive retinal ganglion cells (ipRGCs). Research on the non-visual receptors and circadian and wellbeing impact is ongoing, but it’s pretty clear that intensity, duration of exposure, and the spectral power distribution (SPD) of light can impact mood, productivity, sleep patterns, and more.
One area of intense research has been in developing circadian-centric metrics. Both equivalent melanopic lux (EML) and circadian stimulus (CS) are used in the various circadian guidelines and recommendations. For more back ground on these metrics, see coverage from our 2018 Lighting for Health and Wellbeing Conference.
Want more details on fundamentals of circadian lighting? See the initial entries in a series of articles by LRC scientist Allison Thayer. Part 1 explains a shift in lighting vocabulary and Part 2 expands upon circadian lighting metrics.
The PNNL simulations took into account the SPD, which the team could vary via white-point-tunable luminaires. Metrics were captured at the desk levels and vertically at the eye level. Moreover, the models accounted for not just SPD but also spectral reflectance distribution (SRD) or the actual spectrum that reaches the eye directly from the source and from reflections of surfaces that shift the spectrum based on the color and texture of the surface.
The three guidelines that PNNL evaluated have varying requirements. WELL initially relied solely on EML, but now offers the option of using EML or CS. WELL requires a minimal performance 120 m-lux (EML) or a CS value of 0.3 or greater. A level above 240 m-lux earns more WELL points. The UL guideline requires a CS value of 0.3 or greater. The CHPS Core 3.0 guideline requires 250 m-lux or greater or a CS value of 0.3 or greater.
The simulations varied CCT, lumen output, reflective surfaces and more. The models were generally set to deliver IES illuminance recommendations of 300 lx on the horizontal work plane, and 50–150 lx in vertical illuminance at the eye. In reality, measured levels were higher due to factors such as reflectance. In the office model, only one simulation at the coolest CCT of 6500K delivered the circadian response defined in the three guidelines. Results were slightly better in the classroom model but also far less consistent, given variability in desk orientation.
The researchers concluded that light levels and/or exposure times would have to be increased to deliver on the circadian requirements. Both would increase the amount of energy used. The variation in energy use is considerable across different CCTs. In general, the report projects that 10–100% more energy than IES-guided baseline would be required to meet the circadian goals.
Looking forward, the report states that further research could reveal an optimal SPD that reaches the circadian requirements more easily. It further suggested varying the light distribution or optimizing desk locations might improve the results. But it appears energy usage will go up to some extent for educational facilities that want to operate lighting at recommended circadian guideline. PNNL has placed an archive of the full report for easy access on its website.
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