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Researchers Study Demand-Responsive Dimmable Lighting

Demand responseElectric bills paid by commercial building owners often have a consumption and a demand component. The consumption component is the amount of electric energy, measured in kWh, that the building consumes in a given month. The demand component reflects maximum demand, measured in kW, that the building uses over a given time period. Peak demand is the most expensive power that a generator produces and can represent a significant part of the electric bill.

Utilities, Independent System Operators (ISOs) and other power providers servicing commercial buildings share a common interest with their customers to reduce peak demand. This is because shaving the peak enables these organizations to satisfy customer demand while avoiding the high cost of building new capacity or having to buy very expensive power from other markets during an emergency or demand spike. Besides charging more for power used during peak demand periods, a number of utilities and ISOs offer financial incentives to building owners to curtail load on request, usually during an emergency grid event such as during brownout or imminent blackout conditions.

Role of code and standards

Demand response is now beginning to be required by codes and standards—in particular, at present, California’s Title 24-2008 energy code and ASHRAE 189.1 standard for designing high-performance green buildings.

California’s Title 24-2008, which became effective January 1, 2010, requires demand-responsive lighting controls in retail buildings that have a sales floor area larger than 50,000 sq.ft. The code defines demand-responsive lighting control as “control that reduces lighting power consumption in response to a demand response signal.” In this case, the lighting controls must be able to uniformly reduce lighting power by at least 15%. The requirement does not apply if the building’s lighting already has 50% or more of the total lighting wattage controlled by daylight harvesting controls.

Section 7.4.5.1 of ASHRAE 189.1 requires peak electric load reduction capability. The building must contain automatic systems capable of reducing peak electric demand by at least 10%, not including standby power generation.

The role of demand response in codes and standards is likely to intensify in the future. According to the Department of Energy, about 281 gigawatts of new generating capacity will be needed by 2025 to satisfy growing demand for energy, much of which will be allocated solely to satisfy peak demand. This is nearly a thousand 300MW power plants.

Role for dimmable lighting

To reduce peak demand, we can turn equipment off, turn it down or use it more efficiently. Strategies include equipment downsizing, duty cycling, thermal storage, improved maintenance, commissioning. Lighting, at first glance, has a small role to play. While we can use it more efficiently, it is difficult to turn lighting off in routinely occupied spaces for obvious reasons and cannot be turned down in many spaces without installing dimmable ballasts. But what if we did just that—replace every fluorescent ballasts in a commercial office with dimmable ballasts? It is commonly accepted that typical levels of automatic dimming, occurring in strategies such as a daylight harvesting, is unlikely to be noticed or found irritable by occupants. Researchers at the National Research Council Canada – Institute for Research in Construction (NRC-IRC) put this notion to the test, conducting a study to determine how far, how fast and over what period lighting can be dimmed before occupants notice and are adversely affected.

The researchers conducted two laboratory studies in full-scale office mockups where various dimming scenarios were studied with typical office workers performed office tasks, and then designed a field study, conducted during the summer, that included an open-plan office with 330 dimmable light fixtures and a college campus with 1,850 dimmable fixtures in several buildings. Load shedding was undertaken during afternoon hours over several days. The rate of dimming spanned one to 30 minutes with dimming reductions up to 40%. Occupants were warned that an experiment would be conducted over the summer involving afternoon dimming, but were not told which days.

In the field study, lighting loads were able to be reduced by 14-23% without occupant complaint. Based on this data coupled with the lab study data, NRC-IRC developed several recommendations.

Stage 1: This type of demand response involves dimming by amounts that are not noticed by the large majority of occupants. Dimming can occur rapidly, over as little as 10 seconds, by 20% with no daylight, 40% with low prevailing daylight, and 60% with high prevailing daylight. If dimming occurs slowly, over 30 minutes or more, and with no immediate expectation of dimming occurring, levels may drop by 30% with no daylight and 60% with high prevailing daylight.

Stage 2: This type of demand response involves more load reduction, with steeper reductions in light levels but still acceptable to a large majority of occupants. Dimming can occur rapidly, over as little as 10 seconds, by 40% with no low daylight and 80% with high prevailing daylight. If dimming occurs slowly, over 30 minutes or more, and with no immediate expectation of dimming occurring, levels may drop by 50% with no daylight and 80% with high prevailing daylight.

The researchers emphasize that these recommendations relate only to situations where load shedding is performed to alleviate the effect of temporary—and infrequent—grid stress events, with dimming lasting a few hours at most. The recommendations are not intended to replace current lighting practice and support daily load shedding.

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