Take 10: Reap the Benefits of Natural Light

April 23, 2010

Q: Years ago, there was there an option to use fiber optics to bring in natural light. Is that still available?
A: The system I remember from the mid ‘90s is the Himawari Solar Lighting System, which was invented by the late Dr. Kei Mori. The company introduced a new catalog last year. (By the way … If anyone is interested in buying a rather large “pre-owned” Himawari system, please contact me at [email protected].) Today, there are several sunlight collection and fiber optic distribution systems on the market, such as the Parans system from Sweden and the Hybrid Solar Lighting system from Sunlight Direct, which combines sunlight and electrical illumination.

Q: Is HVAC system capacity decrease possible with daylighting because sunlight lumens produce less heat than manmade lighting? Why is there a reduction in HVAC system capacity with daylighting?
A: The general concept is that, by harvesting daylight, you are able switch or dim electrical lighting and reduce cooling load, thereby downsizing the size of the HVAC system. The preferred form of daylight is skylight rather sunlight since it has less heat for the same amount of lumens. Sunlight contains about 93 lumens per watt of radiant energy, while skylight from a clear sky has about 150 lumens per watt of radiant energy. As a point of comparison, a T5 fluorescent lamp is about 107 lumens per watt; an incandescent halogen lamp has about 20 lumens per watt.

Q: Isn’t using daylighting in museums harmful to fugitive organic materials?
A: All light causes cumulative, irreversible damage to fugitive organic materials whether produced by daylight or electric light. Shorter wavelengths, such as ultraviolet, are more “energetic” so they can cause damage at a greater rate. However, even longer wavelengths, such as infrared, can cause damage by raising the temperature of an object. Since we need light to see art objects, museums limit the duration and intensity of light exposure that objects receive based on wavelength and the object’s sensitivity.

Q: What would be a good ceiling height to get the most use of daylight from a side window?
A: The rule of thumb is that daylight penetrates twice the window head height, so you want to locate the window head as close to the ceiling as possible.

Q: Do have any rules of thumb that help determine which electric lighting control strategy is most cost advantageous?
A: Occupancy sensors provide the greatest savings, followed by dimming, then switching. Dimming has a higher upfront cost than switching due to the cost of dimming ballasts, so budget needs to be considered. Application is another consideration. Switching may work fine in a low-occupancy or non-critical application, such as a warehouse or lobby, and reap energy savings with low added cost.

Q: Is DALI still a viable option when it comes to daylighting controls?
A: It is still viable. Here’s a site you can get more info: http://www.dalibydesign.us/about.html.

Q: What software are you using for daylighting analysis?
A: Tanteri + Associates uses a combination of public and proprietary software, depending on budget and the complexity of the project. Daylighting-specific software includes Ecotect, Radiance, Daysim, Spot, AGI32, and Light Stanza.

Q: Can you explain light tubes, ways of using them, and their drawbacks?
A: The design of Tubular Daylighting Devices (TDDs) has come a long way in recent years. Each element of the system – collector, transmitter, and distribution element – has undergone some type of major improvement in optical design, functionality, or aesthetic appearance. TDDs are excellent in clear-sky climates when daylight in the form of sunlight needs to be delivered a distance down from the roof, or in something other than a straight line. TDDs are designed to maximize sunlight collection over a small diameter, so TDD quantity, spacing, and skylight-to-roof area has to be carefully considered, as well as related fenestration metrics (e.g. SHGC and U-value). Drawbacks can occur with too many, too few, or too-far-spaced TDDs. Like any other type of skylight, introducing a hole in a roof adds another layer of construction responsibility and involves a higher level of coordination.

Q: When do you think daylighting will be considered a renewable energy source that qualifies for incentives?
A: The daylighting incentives that I’m aware of are far and few. My hope is that more will appear soon from utilities and government agencies, and that they will become popular just like they have for solar power, solar hot water, wind turbines, and ground source heat pumps. My guess is that toplighting will be first and come by way of roof-mounted devices with well-calculated bottom line energy use and payback periods.

Q: How do you keep a nightlight that illuminates the egress path on a monumental stair under a skylight from affecting a photosensor?
A: Photosensors come in a variety of spatial distributions and spectral sensitivities, so one approach can be through photosensor selection and aiming. Another approach is to use a shield or mask over the device that prevents it from “seeing” the nightlight. These solutions assume you’re using a closed-loop sensor. Another solution is to locate an open-loop photosensor in the skylight well, or on the roof, that responds to daylight only.

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