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.