Shedding Light on Efficiency
In the quest to achieve the ever-efficient office lighting environment, both ASHRAE and the United States Green Building Council (USGBC) are pushing the envelope to encourage lighting designers and building owners to continually improve lighting efficiency levels. For example, ASHRAE 90.1 2001—which in 2004 will become the default guideline for states that do not adopt their own energy co...
In the quest to achieve the ever-efficient office lighting environment, both ASHRAE and the United States Green Building Council (USGBC) are pushing the envelope to encourage lighting designers and building owners to continually improve lighting efficiency levels.
For example, ASHRAE 90.1 2001—which in 2004 will become the default guideline for states that do not adopt their own energy codes—declares that the maximum lighting power density for an open-plan office should be 1.3 watts per sq. ft. The USGBC, in its Leadership in Energy and Environmental Design (LEED) program, concurs, as it utilizes the ASHRAE standard as a prerequisite and then awards points based upon savings in building energy usage that are beyond 90.1 levels.
OK. So the question is how does one achieve such energy savings?
There are a number of techniques that may be utilized to achieve a reduction in the lighting power budget for a facility. The most obvious is to simply use fluorescent or high-intensity discharge lighting sources, thus avoiding the use of incandescent lighting. On the other hand, the use of 2-ft. x 4-ft. prismatic or parabolic fixtures, while an inexpensive method to light an area, is not particularly efficient and cannot meet the requirements of ASHRAE 90.1. Another obvious option is the use of T-8 or T-5, direct/indirect pendant fixtures, which can achieve some reduction in the lighting power budget. For example, using the direct/indirect fixtures in a typical office environment, in which 50 foot-candles is maintained on work surfaces, will result in 10% to 30% savings below 90.1 requirements.
But to get into real reduced energy usage levels, more sophisticated lighting techniques must be employed. One method is the use of controls. Some lighting controls include:
Occupancy sensors. As the name implies, occupancy sensors turn on the lighting only when the space is occupied. This control can be connected to selected fixtures so that the lighting level is lower when the space is unoccupied or brighter when people enter the space. An example of this is a warehouse application in which the aisle lighting operates at a low level to permit seeing into the aisle. Thus, when someone enters an aisle, the lighting level is increased to enable label reading to aid in selection of the proper items. Common areas in a building that can benefit from occupancy sensors include janitor closets, mechanical and electrical spaces, low-usage corridors, conference rooms and other areas that have limited occupancy.
Daylighting sensors. Another option is reducing the level of artificial lighting based on the amount of available daylighting. This can be accomplished in a number of ways, but basically, the controls will be either dimming or switching type. Both systems utilize a lighting level sensor to measure the amount of lighting in an area. As the quantity of daylighting is increased, the light fixture output is reduced by a corresponding amount to compensate for the daylighting entering the space. At a preselected daylighting level, the lights may be turned off entirely. The switched controls operate in a similar manner, except for the fact that they switch the light fixtures in stages. Care must be taken in setting up the control sequence to prevent short cycling of the controls—i.e., a case where the amount of daylighting increases, the controls reduce the artificial lighting component, and then the sensor sees that there is a lower lighting level in the space and increases the amount of artificial lighting to compensate.
Timed lighting switches. Significant savings may be realized by installing timed switches for selected areas of a building. These switches work well in large, open office areas where everyone leaves the area around the same time. After the last person leaves, the lighting is switched off and it stays off until either the timer turns it back on the next morning or a manual override switch turns on the lights when the first person enters the office the next day. Technically, these kinds of spaces could really use occupancy sensors instead of timers. But due to their size and the prevalence of open-office partitions, a large number of sensors would be required to cover the area, which would make the total installation cost rise to the point where the economics no longer make sense.
Multi-level switching. Switching to achieve multiple lighting levels in a particular area is a technique that has been utilized for many years. The principle behind this kind of design is that when occupants in the space require less light, they can turn off selected lamps in their light fixtures to accomplish the reduction. In practice, it is a rare individual who will say there is more light than needed and then turn off some of the lights. Much more common is the person who says that only when all of the lights are turned on is there enough light. Or this individual will say that it is not worth the effort to walk over to the switch and turn off the unnecessary lamps. Therefore, while multiple-level switching is still expected by the majority of owners and many engineers, it is doubtful if the extra expenditure will ever pay for itself, even if it includes only a few feet of conduit and wiring and one additional light switch.
As many have pointed out in the past, decreased lighting levels also reduce the requirement for air conditioning, which further reduces the electrical usage in a building. Less air conditioning can also result in smaller chiller, pump and fan sizes, which further reduces the overall construction cost of a project. This being the case, it is important that the electrical engineer communicate his intent to utilize a reduced energy lighting system early in the project. This will assist the mechanical engineer in downsizing the equipment early on, rather than being surprised just before the drawings are released for bidding. With careful attention to the prevention of overdesign, the construction cost for the mechanical and electrical systems in an energy conservative facility may very well achieve lower levels than that of a typical facility.
Ultimately, the real key to reduced lighting levels and more energy-efficient design is education more than a matter of specialized engineering. The engineer must make sure that the design architect and the end user both buy into the concept of an energy-conservative facility and understand the trade-offs that will likely occur. They should also expect to experience a low-energy utilization space so that they are not surprised at the lighting levels in the new or renovated facility. They also need to understand that the few individuals who require more light to work shouldn’t influence the lighting levels for the entire facility.
Building Area Type – Lighting Power Density (watts per sq.ft.)
|Performing arts theater||1.5|
Savings for Office Lighting Retrofit
In a recent lighting retrofit, a 2,400-sq.-ft. space originally serving as an ice house was converted to offices and artists’ studios.
Base building lighting in this high-ceiling, warehouse-like setting was a combination of 34-watt T-12 fluorescents in the spaces with lower ceilings and 400-watt high-bay metal-halide lamps in the spaces with higher ceilings.
Specifically, the space had eight 400-watt metal-halide fixtures with prismatic acrylic reflectors mounted 25 ft. above the floor. However, the lighting was uncomfortably bright for an office space and was not conducive for work on video displays.
Instead of attempting to work with the existing metal-halide fixtures, a task/ambient lighting scheme was designed to provide office lighting for intensive VDT usage, i.e. one or more displays at every desk. Two 320-watt pulse-start metal-halide fixtures were selected to provide ambient lighting by reflecting the light off the walls and ceiling.
Since we had a 27-ft. ceiling height, a canopy fixture that had a fairly symmetrical distribution was desirable. Normally, ambient lighting is provided by a fixture that has a wide, asymmetrical distribution with a distinct bat-wing profile. It is used in lower ceiling applications to spread the light out as much as possible. In this application, it was better to push the light toward the ceiling as much as possible due to the high ceiling and long return path.
While the indirect, ambient light achieved only eight foot-candles, both the quality and quantity of the light was quite adequate for circulation. For task lighting, a combination of 14-watt self-ballasted compact fluorescents and single lamp, 32-watt T-10 fluorescents were utilized to raise the lighting level on the work surfaces to around 40 foot-candles.
The office’s conference area, by contrast, is illuminated by five 20-watt, low-voltage decorative fixtures. The combination of the ambient light, the decorative lighting and daylighting from the single window in the area provides more than 40 foot-candles over the conference table.
Overall, the task ambient lighting at first appears much darker than a typical office environment. But over time, employee satisfaction has risen to the point that even the task lights are not always turned on. According to the building owner, the lighting is very comfortable and well suited for the intensive, VDT work environment. The key elements to making this installation successful were controlling glare and ceiling reflections.
The total lighting power density for this space is now 0.4 watts per sq. ft., which is a 69% reduction below the ASHRAE 90.1 standard for offices of 1.3 watts per sq. ft.