By: Bryan S. Rogers, Kerrwil Publications Limited
Tapping daylighting’s potential for office buildings has been receiving renewed attention on both sides of the Atlantic. Part of the reason, of course, is the potential for energy savings. But much research has been done recently into the proper daylighting techniques for worker comfort and illuminating offices with VDTs. The potential for energy savings and increased office worker comfort can be impressive.
In a recent issue of the CADDET (Centre for the Analysis and Dissemination of Demonstrated Energy Technologies) Newsletter from The Netherlands, for example, Swiss researchers Juliette Fong and Miklos Kiss report that in some cases, simple systems can improve daylight lighting levels by a factor of three. The two suic rules asggest some bas to how this can be achieved:
* High windows
* No suspended ceilings
* Use of light colours
* No 100% covering of windows with glare control.
They add that to get the most out of natural light, correct use of shading (venetian blinds) and a daylight-dependent control of artificial lighting are essential. The Swiss Federal Office of Energy’s Daylighting Project has provided useful information on the implementation of daylighting strategies in office buildings, and on the mistakes all too often made.
During the project, which has been carried out as part of Switzerland’s Energy 2000 program, a number of daylighting implementations were observed, measured, analysed and assessed. Say Fong and Kiss: “The study showed that in daylighting practice, the same mistakes were often repeated, although they could have been easily avoided.
The study of various buildings, all with allegedly good daylight usage, uncovered many shortcomings in the use of natural and artificial lighting. Fortunately, though, remedies were at hand and were easily implemented.” The researchers point out that computer work stations have completely changed the office world, and that it is not surprising that visual discomfort such as glare and reflections are the complaints most commonly mentioned.
The solution is simple: where practical, the monitor should be placed so that the viewing direction is parallel to the window. In larger offices, however, the problem becomes more complicated. Often, when natural lighting is provided by large window surfaces and no room dividers are fitted, most of the office has to be shaded to cut out glare.
The result? Artificial lighting is often switched on, as the lighting level is insufficient. Where glare is a major problem, workers have reported that they erect cardboard barriers around the screen to reduce reflections. The authors suggest that, ideally, a separate blind, acting as a light diffuser, should be fitted along with sunblinds to protect the worker(s) not only from glare but from heat as well.
“Completely independent of the prevailing weather,” say Fong and Kiss, “artificial lighting was switched on in eight of the nine buildings studied, and was left on all day. Not exactly what one expects from daylighting projects!” Possible causes for this situation included:
* lights were switched on in the morning, when daylight was insufficient;
* lighting was not switched off later when enough daylight was available;
* during changeable weather, employees did not want to be constantly getting up to switch lights on and off.
For a balanced lighting situation in the office, the researchers say that, in an ideal case, a room where work is carried out should have general, homogenous lighting (natural or artificial) with additional point sources providing lighter and darker zones. The addition of task lighting and desk lamps was also noted at several work places.
Here are some tips for small, but effective, changes in the office that the authors suggested after their study:
* Place desks and work places near windows
* Place PC monitors parallel to windows
* Individual glare protection for monitors, or on windows in the field of view
* Place furniture and room dividers so that natural light is not blocked
* Use modular, individually combinable furniture instead of large, inflexible, angled desks
* Fit light-coloured blinds with automatic or manual control
* Automatic control of artificial lighting, switching at least the window zone
* Continuous control with lighting levels of 300 to 500 lux is ideal
* Installed power should be 6 to 12 watts per square meter
* Use light-coloured walls and smooth ceilings
In their conclusion, Fong and Kiss stated that trivial but often repeated mistakes hinder the sensible use of daylight. “One certain imperative is that, on a sunny day, the artificial lighting of work places near windows should be switched off. Also, on cloudy days, unnecessary glare protection and shading should not cause lighting levels to deteriorate.” In other words, good glare protection, good shading and sensible light control are integral parts of an optimal natural lighting system. Closer to home, a U.S. lighting consultant, writing in the February issue of Energy User News, states that “Modern electric lighting, cheap electricity, and the interest in fully conditioned environments are responsible for the transition to a situation where daylighting has often been ignored in contemporary (North American) building design.
“However,” continues John L. Fetters, principal of Effective Lighting Solutions Inc., “with recent advances in microelectronics, high-performance glazing, electrochromics and fabrics, there has been a renewed interest in active daylighting.” He warns, however, that daylighting is not a total substitute for electric lighting, which is needed at night and when daylight levels are too low to provide general ambient and task lighting. Geographic location and local conditions, he points out, determine the extent to which daylighting can be implemented.
Geographic considerations include proximity to the equator and the orientation of the building. “One of the challenges of a daylighting design,” he says, “is to provide a uniform distribution of light. Those closest to the daylight source may have adequate light, but people located in areas farther away may not.
The design should either light the space evenly or compensate through shading devices or controlled electrical lighting.” Turning to these controls, Fetters explains that “active daylighting” is the term applied to the control of electric lighting to supplement daylight. The goal of active daylighting, he says, is to couple photosensors to luminaires in ways that will reduce energy use while optimizing illumination levels for worker comfort. New technologies have emerged in recent years that increase the opportunities to apply daylighting techniques cost-effectively, the author states. New sensors and electronic dimming ballasts provide distributed control that operates independently to detect conditions in small areas and controls the light from a small number of luminaires in that area. For example, in an active daylighting design for fluorescent lighting, a photosensor reads the ambient light.
If the daylight is sufficient to light the space, the connected electronic dimming ballast is signalled to reduce light output to its minimum value. As sky conditions change, the photosensor output will signal the dimming ballasts to increase the light output of their associated luminaires to supplement the daylight. Silicon photosensors are analog devices in which output signals vary in proportion to the amount of incident light, he goes on.
A fade feature is needed to ensure that lighting is not changed too abruptly. The fade control originally was provided by an interface circuit, but now can be integrated in the sensor. Multiple dimming ballasts with low-voltage control leads connect directly to each sensor. The dimming ballasts can dim down to 10 per cent. In many cases, the hardware for these systems is modular, allowing them to be plug and play. The low-voltage control leads can be plugged together in daisy-chain fashion.
“This flexibility allows control zones that are independent of the restrictions of the power zones. The keys to the effectiveness of daylighting controls are the careful placement of photosensors and proper calibration of controls during the commissioning phase of the installation.” A well-designed daylighting application may optimistically realize annual energy cost savings of 20 to 30 per cent, he concludes, compared to buildings without daylight design or controls.
Electrical energy used for lighting systems can be reduced as much as 70 per cent during peak natural light periods. “Predicting the economic effects of daylighting systems is difficult because of the many assumptions involved. According to the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, NY, a sample calculation shows that energy costs saved per year can be approximately 25 cents (U.S.) per square foot of daylighted floor area. This assumes that there are 260 working days per year, electricity costs 10 cents per kilowatt hour, the daylighting system turns off the lights five hours per day, and the connected lighting load is two watts per square foot.
Many installations will have a lower lighting load or a lower electrical rate. The calculation also assumes that there are no cloudy days or holidays. “With the initial cost of a new daylighting system and the energy cost savings, even these broad assumptions result in a longer payback period than many businesses seek.” Meanwhile, here at home, researchers at the National Research Council’s Institute for Research in Construction (IRC) in Ottawa are currently involved in assessing modelling tools that can help engineers, architects and lighting designers predict indoor illuminance from daylighting and assess the impact of daylight-linked control systems on energy consumption in buildings.
According to Dr. Morad Atif at IRC, daylighting is a preferred source of light that can be exploited in building design. The use of computer programs for the design and analysis of daylighting systems, he says, is becoming the preferred method, as this approach is faster and much less expensive than experimental testing. “However, despite the fact that there are several computer programs that can simulate daylighting, most of them have limitations that restrict their ability to replicate the complexity of daylight distribution in actual buildings.” This is where the IRC comes in.
Through its involvement in the International Energy Agency’s Task 21: Daylight in Buildings, IRC has undertaken the modelling of an existing daylit building using a computer package developed as part of this task group’s work. This work, which is supported by the Buildings Group, Natural Resources Canada, will not only determine the accuracy of the software, but also provide Canadian building designers with examples of how to achieve retrofits that conserve energy by using daylighting. IRC researchers monitored the daylight parameters in a building and used the data to create a computer model to simulate the impact of outdoor conditions on the building. They then compared the measured data with the results of the computer simulations in order to verify the accuracy of the computer program. Finally, they evaluated the energy savings achieved by using different daylighting design options (such as lighting control systems) or retrofit measures with the help of the computer model that they developed. “From a survey of the available daylighting software, only one program was found to have the capability to analyse the effects of replacing electric lighting with daylighting on the total energy balance of a building,” says Dr.Atif. “Although this program too had some limitations, by making a few adjustments IRC researchers were able to validate the model according to measurements and study energy-conserving retrofits for a real-life daylit building.” The results from this research will help building designers, not only in Canada but throughout the world, to optimize the potential of using daylight – both as a source of light and as a means of saving energy (and money).