Daylighting and Energy Savings

Figure 1: SAGE Electrochromics Inc.

Daylighting is the art of bringing daylight into the interior of buildings and managing it in order to provide a well-day lit, thermally and visually comfortable environment for occupants and save energy through turning off electric lights.

Before the invention of electric lighting, buildings had to be designed to maximize the admission of daylight since that was all the light that was available (but for candles), and so, for example, narrower buildings and designs around courtyards were prevalent. With the advent of electric light came the ability to design deeper floor plans and, in prior decades, many an office space has been lit predominantly by electric light with minimal access to daylight or view through a window.

Over the last few years however, there has been mounting evidence of the negative impact on human health of lack of daylight
[1]. The human circadian rhythms are regulated by access to light-and not just by any light-it has to be the right color, the right intensity, and at the right time of the day. Daylight is, it turns out, the perfect source, as it changes color and intensity throughout the day, just as our bodies’ needs change, and it entrains our Circadian rhythms perfectly. In the morning, blue light from the sun suppresses the production of melatonin, making us alert and active. In the evening the red of the sun, and following darkness, supports production of melatonin which helps us sleep. Melatonin is important for other functions of the body such as metabolism, mood, and the immune system, which explains why data shows that circadian rhythm disruption increases the risk of some cancers and can cause weight gain and depression [1]. Today, the EPA reports that we spend 90 percent of our time indoors [2], and so it is unsurprising that according to epigeneticist, Debra Burnett, “Light is the most essential design element of advanced 21st Century Buildings” [3].

Figure 2: Photograph ©Paul Schiefer Photography, courtesy of Lutron Electronics Co.

A good daylighting design can also save energy-a lot of energy in fact. By implementing continuously dimmable lighting controls, electric lighting energy can be offset by using daylight when available. And, since electric lights act as mini heaters in a building, lighting controls also reduce the cooling load. Figure 1 shows the typical shape of curves for building energy as a function of window to wall ratio (WWR) with and without lighting controls. Without lighting controls, the building energy increases as window area increases because the solar heat gain and U-factor of fenestration is generally higher than an opaque wall. However, with lighting controls, the building energy starts to reduce as window area increases because of the electric lighting and HVAC energy savings. At a certain window area, when daylight has saturated the space and the lights are turned off completely, the building energy reaches a minimum. Increasing the window area further cannot save any more electrical energy and so the building energy starts to increase as the effective solar heat gain and U-factor of the envelope increases. The US Department of Energy
(DOE) has estimated that if the entire nation’s building stock were outfitted with dimmable lighting controls together with highly efficient fenestration with dynamic solar control, 2.6 Quads
(10¹⁵ BTUs) of energy could be saved annually [4]. Since buildings use about 40 Quads annually that is a significant saving for just making changes to the façade performance.

Figure 3: Photograph ©SAGE Electrochromics Inc.

To achieve these kinds of energy savings, a good daylighting design with windows positioned appropriately on the envelope to maximize the depth of daylight admission while minimizing solar gains and glare, is needed. With a good daylighting design, dimmable lighting controls and high-performance fenestration, the minimum of the typical curve (figure 1) can be moved to higher window areas. Indeed, with careful designs, the window to wall ratio can exceed 50 percent and even higher window areas can still result in energy efficient designs. Some best practices for daylighting design are:

• Position windows higher up e.g. use clerestory, or split façades into upper daylighting and lower “view” windows (see for example the split façade zoning in figure 3).
• Use light redirecting techniques like light shelves with sloped ceilings to move light deeper into the building.
• Think carefully about interior design-use light colored surfaces to effectively reflect light, use low partition heights and, if you must have perimeter offices, use glass walls.
• Minimize luminous contrast-be careful when using punched opening windows which can cause contrast glare since they are bright compared to the adjacent wall. More continuous glazing, ribbon or curtain wall, is preferred (see for example the ribbon window in figure 2).
• Use a dynamic response for glare-this is essential for achieving optimum energy and occupant benefits of a daylighting design. Manual solutions, such as mini-blinds, are operated when the glare condition is present and are then left in place, blocking daylight admission and views, long after the glare condition has passed, negating the daylighting benefits. One recent study demonstrated that blinds were moved on average only 1.7 times a week and only 43 percent of the full window area is unobstructed over the course of the year [5]! Figures 2 and 3 show two alternative solutions for dynamic glare control-automated shades or electrochromic glazing.
• Manage heat gain and loss-be cognizant of glazing orientation (minimize east and west fenestration) and select high performance fenestration to minimize HVAC loads and maximize thermal comfort. The DOE suggests that dynamic solar control provides the most energy efficient way to manage the heat flows into the building [4]. Dynamic solar control can be provided by automated exterior louvers, between the pane shading systems, or dynamic glazing. Figure 4 shows an example of the use of electrochromic (EC) glazing to provide effective solar control as well as good daylight admission in a highly glazed application in Miami where the sun management is very challenging.
• Make use of top lighting where possible (but be careful to manage the heat gain and the glare). Figure 5 shows an example of a skylight that was used to naturally light an indoor space. After glazing with low-e glass with a 50 percent frit pattern, the space was found to be too hot and too bright and so it was re-glazed with electrochromic glass to provide comfortable daylighting with more effective solar and glare control.
• Use continuously dimmable lighting controls to 10 percent with and “off” state to optimize energy performance and minimize occupant distraction.

Figure 4: Photograph ©SAGE Electrochromics Inc.

Designers should also be careful about specifying glass with too high a visible light transmission (VT). High transmission does not mean good daylighting, since glare and accompanying heat gain must be managed appropriately. Finally, one of the most important aspects of a daylighting system is commissioning and post-occupancy feedback. The best daylighting designs can be circumvented if not commissioned well or adjusted based on occupant feedback. One study, by the Energy Center of Wisconsin showed that energy savings of up to 60 percent could be captured just by effective commissioning [6].

Daylighting done well is one of the single most effective ways of improving the energy efficiency of buildings, and it can also create indoor environments that are pleasant to live and work in and which support the long term health and well-being of occupants. Furthermore, from a purely economic standpoint, given that staff salaries are far and away the largest costs in operating a building, reducing sick days and improving retention and productivity by only a small amount has a significant return on investment.

Figure 5: ©Susan Fleck Photography, Courtesy SAGE Electrochromics.

References:

[1] See for example: (i) R. Blau, “The Light Therapeutic”, Intelligent Life Features, The Economist, 14 June 2014; (ii) S. Ulrich, “View through a window may influence recovery from surgery”, Science, April 27, 1984, v224, p420. (iii) E. Schernhammer, F. Laden, F. Speizer, W. Willett, D. Hunter, “Rotating Night Shifts and Risk of Breast Cancer in Women Participating in the Nurses’ Health Study”, Journal of the National Cancer Institute, Vol. 93 (Issue 20) p1563-1568.

[2] http://www.epa.gov/iaq/pubs/insidestory.html

[3] D. Burnett, “Knowledge to Practice: Epigenetics and the Built Environment”. Presentation at VELUX Daylighting Symposium, Copenhagen, May 2013.

[4] D. Arasteh, S. Selkowitz, J. Apte, M. LaFrance, “Zero Energy Windows”, Proceedings of the 2006 ACEEE Summer Study on Energy Efficiency in Buildings, August 13-18, 2006. LBNL report number 60049.

[5] Office fédéral de l’énergie (OFEN) of Switzerland, report by Société Estia SA., Lausanne, “Global Lighting Performance”, 10^th June 2014.

[6] The Energy Center of Wisconsin, “Commissioning for Optimum Savings From Lighting Controls”, http://www.ecw.org/project.php?workid=1&resultid=494

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Dr. Helen Sanders is vice president, technical business development at SAGE ELEctrochromics Inc., Faribault, Minn. Sanders has more than 17 years’ glass industry experience and more than 11 years’ experience in dynamic glass technology and manufacturing. She is active in codes and standards development and leads SAGE’s technical services organization. To learn more, visit www.sageglass.com.