ET23SWE0051 - Performance Assessment of Integrated Core Daylighting Technology
A previous CEC funded project ( https://cltc.ucdavis.edu/sites/g/files/dgvnsk12206/files/media/documents/LDA-June-2022-Edit.pdf) has demonstrated the potential of a fiber optic, solar-tracking core daylight system to deliver natural lighting into the building at ~620 lumens/watt at a maximum length of 300 feet when direct sunlight is available. While this system efficacy is significantly higher than the best SSL technology available today, this technology requires extensive refinement in the area of electric lighting controls and photometric integration if these savings are to be realized in practice. This CalNEXT project is focused on advancing the viability of solar-tracking core daylighting systems as energy efficiency measures through the refinement of the electric lighting controls integration, optical system integration, and on evaluating the potential for the solar tracking core daylighting system efficacy to scale upwards of 1000 lumens/watt based on a previously unevaluated version of the product.
Core daylighting technologies, particularly fiber optic systems like the solar-tracking fiber optic lighting system (STF) studied in this project present an innovative solution for enhancing natural light access within buildings, especially in areas unreachable by conventional daylighting methods. These systems are potentially more impactful in regions like California, where energy efficiency and sustainable building practices are increasingly prioritized due to the state's environmental goals and its high number of clear, sunny days. The team installed the STF system on the south side of the research building. This installation aimed to leverage the lessons learned from the previous deployment while improving overall system performance and scalability. The installation site was chosen to maximize exposure to sunlight throughout the day, ensuring minimal shading from surrounding structures.
The STF system delivered high-quality daylight with stable spectral characteristics, with a CCT of 4,623 to 5,158 Kelvin and a CRI of 88; however, luminous output varied substantially across fibers (396 lumens to 729 lumens), falling below the manufacturer’s stated maximum of approximately 700 lumens per fiber. Dust accumulation measurably reduced delivered light and required either rain or manual cleaning to restore performance. The hybrid fixtures achieved CRI values of 85 to 88 across operating modes, with color temperature shifting predictably as the LED to daylight ratio changed. Temporal stability analysis showed that fluctuations in daylight produced by cloud transients rarely created perceptible dimming events when the STF system was integrated with electric lighting. After filtering for human perception thresholds, the probability of an occupant noticing any event within a one-hour period remained low: 1.0 percent (partly cloudy), 0.7 percent (cloudy), and 3.4 percent (overcast). Among controllers tested, faster response and smaller deadbands reduced event frequency. During occupied daytime hours (9:00 a.m. to 5:00 p.m.), when most office lighting energy use occurs, the STF combined with a controllable electric lighting system achieved a 40 percent reduction in energy usage when compared to the installed LED baseline. This results in annual savings of 75 kilowatt-hours, assuming that the system is operated during business hours Monday through Friday, with 10 days of holidays hours.
Project findings include an identified need for improving collector uniformity, lens alignment, and fixture optics would increase delivered lumens and reduce variability. Dust mitigation measures—such as coatings, improved housings, or automated rinsing—should be pursued to stabilize long-term performance. Cost reductions are essential, with the continued development of low-cost plastic fiber, simplified collector geometries, and modularized tracker assemblies representing the most impactful levers.