This article was published in the Fall 2013 issue of Illumination in Focus.
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Harvesting daylight is virtually a requirement in new buildings with growing concerns over energy use and the disproportionate amount of energy used for artificial lighting. Still, effective use of daylight is a difficult challenge for lighting designers and architects. You must understand the concept of daylight autonomy and associated metrics to deliver optimum daylight, and technologies such as light sensors, wireless networks, solar-adaptive software, and shade control can enable energy-efficient buildings with no glare in which employees are comfortable and productive.
A brief history
In the millennia before the introduction of electricity, buildings were designed to take advantage of daylight to illuminate interior spaces. The Egyptians used daylight control to temper the heat of their extreme climate, introducing lattice and screens with different-sized openings to allow for daylight penetration. In Rome, buildings were designed around courtyards surrounded by living space to maximize available daylight.
During the European Renaissance, light was revered as a practical design tool but also recognized for its ability to enhance the experience of a space. Baroque style relied on indirect light to create mystery and highlight the special qualities of a building.
As electric lighting was introduced and technologies improved, daylight became almost passé; but evolving building codes, new energy regulations, and a renewed emphasis on sustainability have encouraged today's architects, building owners, and lighting designers to once again embrace daylight as a practical, aesthetic, and symbolic element of good building design.
What is daylight autonomy?
The concept of "daylight autonomy" means designing a space such that it maximizes the amount of useful daylight, thereby minimizing the need for electric light. In mathematical terms, daylight autonomy is the percentage of annual work hours during which all, or part, of the lighting needs can be met through daylighting alone.
Exterior daylight is in constant flux, which also affects the indoor lighting environment. These unpredictable changes present two specific challenges when designing spaces to achieve greater daylight autonomy — namely controlling glare and maximizing daylight availability to save energy and preserve the view. Effective designs must embrace daylight while minimizing or eliminating glare.
Glare can cause eyestrain, headaches, and general discomfort. Vast amounts of daylight may eliminate the need for electric light during daytime hours, but they may also create expensive problems like decreased productivity or increased absenteeism. There are different categories of glare that need to be addressed by lighting design, which are fully defined in the book Architectural Lighting Design by Gary Steffy. "Discomfort glare" is a glare sensation experienced as a result of viewing a light source or reflection of sufficient luminance to cause discomfort, but the observer is still capable of seeing and performing tasks. "Disability glare" is a glare sensation experienced as a result of viewing a light source or reflection of such great luminance as to be visually disabling. In the case of disability glare, the observer cannot see or can see in only such a limited capacity that they are unable to perform the required tasks.
It is important to note that the term "light source" is not limited to light fixtures and lamps; it also includes the sun, the sky, etc. Sunlight is typically responsible for creating disability glare, while excessive ambient brightness of the room or sky is responsible for discomfort glare. Both types can create an environment where work is particularly difficult and unpleasant.
Still, we need to maximize daylight availability to take full advantage of this natural resource. Allowing ample daylight into a building can indirectly improve occupant comfort through views and directly increase building energy performance by reducing energy from electric light. As energy rates rise, energy savings becomes more and more important to the bottom line. Ample daylight reduces the need for electric light but can also cause heat gain, which can raise HVAC costs. Daylight has to be admitted judiciously to balance light and heat.
More than daylight harvesting
A consideration of daylight autonomy is independent of a daylight harvesting system, and it accounts for both daylight availability and glare potential. Clearly, adding a daylight harvesting system is required to achieve the lighting energy savings, but daylight availability has benefits related to occupant comfort, mood, and potentially health.
By understanding the way daylight autonomy is measured and implemented, architects and lighting designers can use shading and lighting control strategies that work together to create greater daylight autonomy while emphasizing aesthetics, maximizing owner investment, and minimizing energy use in their buildings.
Measuring daylight autonomy
The success of a building's daylight design can be evaluated using daylight autonomy metrics. Each metric has a slightly different method of performance evaluation, but they all are used to quantify either the daylight harvesting potential, glare mitigation, or both. Three metrics that are particularly useful will be addressed.
Spatial Daylight Autonomy (sDA): Percentage of floor space where the required light level (often 30 fc) can be met completely with daylight for 50% of work hours. Based on the idea that more light is better, this metric indicates quantity of daylight available. A higher sDA yields greater autonomy from electric lighting.
Annual Sunlight Exposure (aSE): Percentage of work hours during which the light level from direct sun alone exceeds a predefined threshold (often 100 fc). This metric indicates quality of light related to direct sun glare, the worst type of glare. It does not, however, address issues related to potential glare from bright sky and reflections.
Daylight Autonomy max (DAmax): Percentage of work hours that the light level exceeds 10 times the required light level (often 300 fc). This metric is another indicator of quality, this time related to all types of glare.
Only dynamic fenestration, such as automated shading solutions, will improve the latter two daylight autonomy metrics that evaluate quantity and quality of daylight. No passive systems can do this. For example, changing the tint on a window or adding overhangs and light shelves will reduce glare, but these methods will also reduce available daylight. These strategies can improve the aSE and DAmax metrics but do not improve the quantity of daylight penetration in the space characterized by sDA. Increasing window size will improve daylight availability but also increases the potential for glare and therefore only improves the sDA metric.
Automated shading systems
Automated shading (Fig. 1) allows the lighting system to respond to environmental factors related to both energy use and glare. The ideal solution will combine automated shade control with solar-adaptive software, and cloudy-day/brightness sensors that further enhance automatic shade adjustment by allowing the shading software to evaluate and respond to real-time daylight conditions (Fig. 2).
As related to energy use, automated shading control maintains a consistent light level in all environments, minimizing the need for electric light. When only manual shades are installed in a building, occupants are only likely to move the shades when they are uncomfortable. This leads to closing or nearly closing the shades. When the glare goes away, the occupant is unlikely to reopen the shades or move them to a new optimal position. This leads to reduced daylight autonomy as well as a reduced useful daylight zone or the area inside a space where enough glare-free daylight is available for daylight harvesting.
Manual shades can generally achieve a useful daylight zone of 10 ft inside the perimeter windows (Fig. 3). Because automated shades respond to real daylight conditions, they can extend the useful daylight zone to 20 ft inside the perimeter, allowing for greater daylight autonomy (Fig. 4). As such, automated shades were shown to save 83% more energy on lighting in a recent study conducted by Purdue University and Lutron Electronics.
The study analyzed the benefits and energy-saving potential of solar-adaptive, automated shading control systems. Results show that perimeter private offices with daylight harvesting strategies in place can further reduce lighting energy usage by 65% through the use of automated shades.
Design trends and building codes
Another powerful motivator driving daylight strategies is code compliance. As utilities struggle to meet the ever-increasing demand for power, federal and state agencies are stepping in to regulate energy use. New mandates strengthen building codes to support energy savings and sustainable building design. Daylight control strategies are critical to not only meeting but exceeding these codes.
Standards and guidelines developed by the American Society of Heating, Refrigerating and Air-Conditioning (ASHRAE) are now mandated by the US Department of Energy. As of Oct.18, 2013, all state commercial building codes must meet or exceed ASHRAE 90.1-2010 standards that generally include mandatory requirements for daylight harvesting technology, known as "Daylight Zone Control." Other allowable building standards, including the International Energy Conservation Code (IECC) and California Title 24, include similar daylighting requirements in their updated recommendations.
Buildings can meet all three codes with manual shades and standard technology, or buildings can exceed codes and realize their full potential with automated shades and wireless technology. By designing the same space with wireless technology and automated shades, the initial cost is slightly higher, but the return on investment is significantly better (Fig. 5).
As spaces are designed to use daylight more effectively, they will be fundamentally less dependent on electric light. Forward-thinking lighting design will increasingly call upon the well-documented benefits of sunlight to create spaces that are comfortable, energy-efficient, sustainable, and code-compliant. The masters of ancient architecture relied on building techniques that simultaneously captured and tamed the sun's potential. Automated shading systems bring daylight back into the mainstream of modern design.