The great thing about managing demand is the same strategies that reduce an electric bill by shaving demand peaks will enable a customer to develop control strategies to reduce his or her total electricity usage. If control strategies are in place, adding demand limits might cause a reduction in service, but according to the California study mentioned in part 2 of this series (January 2009, page 120), this may not be the case. More often than not, demand reduction (DR) won’t negatively affect the building’s functions; in fact, DR strategies will become overall efficiency strategies.
Setting up demand-reduction schemes depends in part on whether you’re working on an existing building or one under construction. For a new building, it’s important that DR planning be part of the design process from the beginning. The New York Times built a new headquarters building in the center of Manhattan and wanted to incorporate control systems that could be used for DR and that would also optimize electricity usage. The design elements that would accomplish this centered on lighting and heating, ventilating and air conditioning (HVAC) controls.
The walls of the 52-story building are all glass to allow the interior to be illuminated by sunlight. It is tricky to use natural light. To reduce glare and keep down the heat load, the sun’s effects are minimized by a combination of exterior ceramic shields and interior motorized shades, which respond to outside luminance sensors that track the sunlight. The lighting system uses a combination of dimming ballasts, daylight sensors, occupancy sensors, and wall controls (dimmers and switches) that work together to maintain the correct light level for the various spaces in the building. The building uses the following strategies:
• Light-level tuning: setting the appropriate target light level for each space
• Daylight harvesting: automatically dimming electric lights when enough daylight is present
• Occupancy sensing: turning lights off when space is vacant
The lighting control is a digital addressable lighting interface (DALI) system. At the Times headquarters, about 18,000 microprocessor-equipped ballasts can be individually addressed by the system controllers.
With digitally addressable dimming ballasts, lights are networked with the luminance sensors, occupancy sensors and wall controls and are integrated into the building management system. The lighting is segregated into zones based on the kind of work being done in each area. The DALI system allows for centralized control. At the same time, it enables each worker to adjust the dimming of his or her own lighting.
The newsroom is an open space with cubicles. Ceiling-located luminance sensors control the individual ballasts to keep a desirable overall light level that includes a mix of daylight and dimmable fluorescents. On a typical sunny afternoon, the ceiling lamps nearest the windows will be off, with artificial lighting levels increasing toward the room’s interior.
When DR is called for, the settings of the devices relative to each other can be kept constant while their absolute output levels are cut back. By using the DALI system, the best DR settings can be decided experimentally.
The second through fifth floors are cooled by air ducted underneath the floors and returned through the lighting diffusers to a ceiling plenum. The temperature is controlled by adjusting airflows and heaters. It was decided that DR for heating/-cooling would not be done by varying central equipment such as the rooftop chiller, but by widening the dead bands of the individual thermostat settings.
During commissioning, it was found that precooling the floors on a summer morning and cutting back on the cooling in the afternoons could be enacted without occupant discomfort. Although this has no effect on total energy usage, it shifts the demand, making it possible to reduce the peak, which is usually high in the afternoons. In New York and many other places, electricity is priced lower in the morning than in the afternoon, so this can bring down total cost.
The bottom line
There are three main motives for instituting demand reduction systems:
1. Saving money by reducing electricity use during high price periods
2. Optimizing the working environment for the building occupants
3.Helping stabilize the electricity delivery system to avoid brownouts and power failures
Incorporating control systems for lighting and HVAC allows building managers to achieve the first two. Once in place, these systems enable demand reduction strategies so the third can be achieved for no extra cost.
If a building such as the Times headquarters is designed in advance with building systems that can be tuned for best efficiency, it is relatively simple to add DR. An advantage of designing a building to include these strategies can be seen from the lighting load. Although the building was initially designed to use 1.28 watts per square foot of lighting power, it is using only 0.38 watts per square foot—70 percent less.
BROWN is an electrical engineer, technical writer and editor. He serves as managing editor for SECURITY + LIFE SAFETY SYSTEMS magazine. For many years, he designed high-power electronics systems for industry, research laboratories and government. Reach him at firstname.lastname@example.org.