Personal Control

By most estimates, lighting is responsible for 30-40% of electrical energy use in large office buildings. As such, in the drive towards sustainability, lighting has been a principal target for energy savings.

The National Research Council of Canada’s Institute for Research in Construction (NRC/IRC) has conducted studies that examined the energy savings from personal dimming controls in offices, and we compared these to the performance of other control options. The results, assessed alongside prior research, suggest that personal dimming control may be worth greater consideration as a sustainable lighting control option.

Laboratory study. In the laboratory study, participants were seated toward the back of a large private office in Ottawa, with large, low light transmission windows facing just east of south. The study was done during April to July.

The participants conducted typical office work during the day, but were prompted every 30 minutes to use software on their computer to choose their preferred light output from recessed parabolic luminaires. The maximum electric light on the desktop was around 700 lux, whereas the maximum daylight contribution was around 500 lux.

The mean desktop illuminance preference of the 40 individuals varied widely, from around 200 lux to 900 lux. But throughout the day, the choices of an individual varied almost as much. This finding is interesting because daylight harvesting systems are typically designed to deliver the same constant light level (typically 500 lux) to all, but this is clearly not what occupants choose for themselves.

The wide range of preferred illuminances meant a similarly wide range of lighting energy use on each day. However, on average, energy use with personal dimming control was 25% lower than if the same lighting system had delivered a constant 500 lux of electric lighting.

Field Study. In the field study led by my colleague Anca Galasiu, we monitored for a year the performance of a lighting system installed on four floors of a typical open plan office building. The workstation-specific, direct-indi- rect luminaires had two downlamps and one uplamp. The luminaires had integral occupancy sensors, integral light sensors for daylight harvesting, and personal dimming control accessed through the occupants’ computer screens to control the downlamps. The uplamp was on at full output during the work day.

The lighting system used 69% less energy than the conventional recessed parabolic system installed elsewhere in the building. Two-thirds of these savings were due to the lower lighting power density (LPD) (5.8 W/m2 vs. 10 W/m2)) and one-third was associated with the controls. Furthermore there were concomitant reductions in the daily peak power demand for lighting (on average, the maximum power draw of the lighting system was only 3 W/m2).

We were able to compare the relative energy savings potential of each control option. If used on their own and averaged across all 86 studied workstations, the occupancy sensors would have saved about 35% compared to the direct-indirect luminaires at full power. In comparison, daylight harvesting would have saved about 20%, and the personal controls about 10%. As expected, savings for both these control types would have been higher near to windows.

People typically used the personal controls when they were first available to pick a general preferred level, and then only rarely after that, on average about once every 50 days. This infrequent use is one reason why the savings due to personal control were lower in the field study than in the laboratory study. Another reason is that the field study included workstations that did not have windows.

Although every site will have its peculiarities, there is enough published research to be able to rank the various control types on several dimensions: energy savings, effects on occupants, and initial cost, as shown in the table. For a truly sustainable lighting solution, one should consider the merits of different control options in several dimensions and as they can be applied to the specific site, and not focus only on the option that is expected to save most energy.

Gary Newsham, Ph. D is Senior Research Officer at the NRC Institute for Research in Construction, Ottawa. E-mail guy.newsham@ nrc-cnrc. gc.ca

This article is an extract from one originally published in the Illuminating Engineering Society of North America (IESNA) LD+A magazine, October 2007.