Daylighting Controls

Daylighting systems, which use natural lighting to supplement electric lighting, offer the potential to cut energy use, reduce peak demand, and create a more-desirable indoor environment. Some daylighting systems work well, cutting lighting energy use by 20 to 80 percent and providing an aesthetic work environment. But many systems fail to live up to expectations—often because of shortcomings with daylighting controls. The key to getting more systems to live up to their potential lies in combining good design of the control system with commissioning, effective coordination of the efforts of many building disciplines, and training building occupants on how to use the systems.

What are the options
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Daylighting installations are categorized as top-lit and side-lit systems (Figure 1 and Figure 2). Top-lit systems use skylights and/or monitors (a raised section of roof that includes a vertical window) to bring in light from the top of a building. It is far easier to get top-lit systems to work well than side-lit systems. In a recent study of 36 top-lit installations in California, the Heschong-Mahone Group, a Fair Oaks, California consulting firm, found that the systems were providing 98 percent of the expected savings. The systems also showed very good persistence of savings—they ranged in age from 2 to 10 years and there did not appear to have been any reduction in benefits over time.

Figure 1: Simple daylighting techniques
This schematic shows a mix of top-lighting and side-lighting, light shelves, high-reflectance ceilings, and wall diffusion to provide fairly uniform deep-plan daylighting without the glare of direct sunlight.
Figure 2: Daylighting in action
Both top- and side-lighting let in daylight at this airport facility.

Side-lit systems, in which daylight is brought in through windows on the side of a building, are more complicated. In a top-lit room, the balance of brightness between horizontal work surfaces and walls or partitions is similar to what it would be with electric lighting alone, because the light comes from above. But in a side-lit space, the light balance may be very different between walls and work surfaces, because the daylight has a greater horizontal component than does electric light. With side-lighting, it's also difficult to get the daylight to penetrate deeply into the interior of a building and more controls are needed to prevent glare.

For either approach, the choices for controls are open-loop and closed-loop. Open-loop systems monitor the incoming daylight but not the electric lights, which they adjust in response to the daylight signal. In a closed-loop system, photosensors monitor both daylight and electric light, and the signals from the sensors are used to automatically adjust the electric lights.

Light levels may be adjusted by the use of dimming controls or on/off switching of banks of lights. On/off switching offers a lower initial cost, a simpler design, and is easier to commission, but the changes in light levels can be more jarring to occupants. Dimming is more expensive, but can be more acceptable to occupants.

How to make the best choice
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Achieving an effective daylighting system requires careful planning and installation as well as post-installation measures such as commissioning and training. A number of programs offer assistance in the design and installation of daylighting systems (Table 1).

Table 1: Daylighting programs
Several programs, including these, offer assistance in the design and installation of a daylighting system.

Make commissioning part of the process from the beginning. Commissioning a daylighting system consists of adjusting photosensors and ensuring proper sensor placement so that the electric lighting system responds properly to the presence of daylight. Unfortunately, with today's analog sensor technology, calibration is more art than science. Typically, a technician with a screwdriver stands on a ladder and adjusts the sensitivity of each sensor. There are generally no markings, so the technician simply adjusts the system until it works—but not necessarily until it works optimally. The technician keeps coming back until there are no more complaints. The process is complicated by the fact that the same signal from photosensor to ballast will produce different levels of dimming with different manufacturers’ products. New products are now entering the market that can be calibrated remotely or that have a self-calibrating capability that should help make commissioning easier.

Keep it simple. Keeping a daylighting system as simple as possible may sacrifice some portion of potential savings, but simple systems are more likely to work properly. Typically, “simple” implies using fewer sensors, choosing open-loop rather than closed-loop controls, and sometimes using on/off switching rather than dimming. Top-lit systems are generally simpler than side-lit systems and can more often make effective use of simple on/off controls rather than dimming.

That said, on/off switching can make changes in light levels more noticeable to occupants. Multi-lamp fixtures, in which individual lamps may be turned off, make on/off switching less noticeable than switching whole fixtures at once. Another way to take advantage of on/off capabilities without drawing too much attention is to use indirect rather than direct lighting fixtures. With indirect light, on/off switching is less noticeable because the light distribution doesn’t change as much when one or more lamps are turned off. On/off switching also enables the use of less-expensive, more-efficient instant-start ballasts. But be wary of one apparent simplification—units that combine occupancy sensors and photosensors in a single package are becoming more common, but the two types of sensors often have different positioning requirements.

Coordinate the efforts of all design professionals. To wind up with a good daylighting system, all members of the design team—architects, interior designers, mechanical engineers, and commissioning agents—need to coordinate their efforts. For example, if the interior designer isn’t on the team, you may wind up with interior furnishings that are too dark for the planned daylighting system. If mechanical and electrical specialists aren’t aware of the benefits of a planned daylighting system, the HVAC equipment may not be sized to take advantage of the reduced cooling loads and the controls are less likely to work properly.

Require good analytical models. Given that the direction of incoming daylight changes over the course of every day and throughout the year and that the intensity of the light is affected by changing weather conditions, most experts recommend using computer simulations in the design of daylighting systems. Researchers from the National Renewable Energy Laboratory point out that there are several areas that designers should pay careful attention to in order to make their models as accurate as possible. In working with daylighting designers, check that they simulate glass properties correctly; accurately specify frames, mullions, and window screens so that the incoming quantities of light can be correctly estimated; and consider the effects of exposed structural elements such as columns and beams as well as the eventual contents of the space. It is also important to make sure that models are kept up to date as a building's design evolves by periodically reviewing such parameters as window size, glazing, shading, partitions and other obstructions, and surface reflectance values.

Work closely with the design team. The more the design team knows about your needs and the better informed you are about the daylighting system, the more likely you are to support the system and use it properly. Ask for briefings that will help you understand what has been done and why. In addition, ask about visible manual controls that provide the option of overriding the system—that addition may decrease the potential savings, but can increase overall satisfaction with it.

Supplement daylighting with task lighting. Strategic use of task lighting can enable deeper dimming of ambient lighting.

Work closely with manufacturers, building operators, and contractors. Working with building operators, contractors, and equipment manufacturers can help avoid problems such as installing too few or too many sensors or improper calibration. One study of top-lit installations found that the best results were obtained in installations where the controls supplier had a continuing relationship or service contract with the building owner.

Ask for good controls documentation. Controls documentation should include a daylighting controls narrative and a set of instructions for occupants and building operators.

What’s on the horizon
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New products are emerging that can be calibrated remotely or that have a self-calibrating capability that should help make commissioning easier. In addition, new technologies are being developed to bring daylight deeper into buildings—into spaces that may not even have windows. Just coming into the market, these systems use mirrors that track the sun and focus the sunlight, and fiber-optic cables that transport the sunlight to where it can be used.

Who are the manufacturers?
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Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.
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