Energy Advisor
Your guide to energy products for commercial buildings
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Energy Advisor |
Office Equipment: Smart Power Strips
The modern office is full of energy-using equipment—computers, printers, copiers, task lights, fans, and other devices—much of which is left on, unnecessarily, throughout the workday and, in some cases, for 24 hours a day, 365 days a year. The U.S. Environmental Protection Agency (EPA) estimates that 80 percent of the printers used in offices, 70 percent of copiers, and 20 to 30 percent of computer monitors and task lights are left on overnight. The result is wasted money for building owners and operators.
To reduce energy consumption, "smart" power strips are available that combine an occupancy sensor with a power strip and surge protection. These products sense the presence or absence of office workers and turn the attached equipment on and off accordingly, saving enough electricity to enable payback periods of less than two years in many cases. Manufacturers suggest using the products to control a variety of devices that can be turned off when not in use, including calculators, coffee warmers, space heaters, and computer monitors. Devices that must stay on—fax machines, modems, computer central processing units, and certain printers—can simply be plugged into outlets that are not controlled by the occupancy sensor.
Here are some of the main features you'll encounter when shopping for smart power strips:
Bundled together or add-on power strip. Some products come with the power strip and occupancy sensor as an integral unit. These products also often provide two sets of outlets, one controlled by the occupancy sensor, the other uncontrolled. This configuration allows equipment that must be kept on all the time, such as fax machines, to be plugged into the same strip. Other products feature a sensor and a relay device that can be plugged into a separate power strip to cover multiple devices. For companies that already own power strips, this could be the best choice.
Monitors and timers. Smart power strips can turn office equipment on and off based on the calendar, clock time, or equipment activity instead of, or in addition to, occupancy. In some cases it does not make sense to control equipment based on occupancy, but energy savings can be achieved through these other means. For example, if an infrequently used laser printer or copier requires a long warm-up period and is kept in an area that lacks frequent foot traffic, the productivity lost while people waited for the printer or copier to warm up would quickly negate the savings from reduced energy use. Turning the device on and off based on a simple clock would ensure energy savings overnight, and a calendar function would enable weekend and holiday savings as well.
Other controllers don't require users to program time schedules. Instead they determine for themselves when equipment is in use or in an idle state by monitoring equipment activity—for example, the flow of data to a printer—or by monitoring the flow of current to the equipment. If the equipment is idle for a user-specified period, the equipment will be shut off or put into a sleep mode and awakened when activity resumes. These options can also be used together: a clock might turn a printer on at the same time each morning, and an activity monitor or occupancy sensor might turn it off when activity stops at the end of the day.
Passive infrared (PIR) or ultrasonic sensors. PIR sensors are the most common type and are able to "see" heat emitted by occupants. Triggering occurs when a change in infrared levels is detected, such as when a warm object moves in or out of view of one of the sensor's "eyes." PIR sensors are quite resistant to false triggering. They are best used within a 15-foot range for two reasons: There are potential "dead" spots between their wedge-shaped sensory patterns that get wider with distance (see Figure 1) and, being passive, they do not send out any signal; instead, they depend on the intensity of the heat output of the moving part of the subject, which drops with the square of the distance.
Ultrasonic sensors can detect motion at any point within the contour lines. Infrared sensors see only in the wedge-shaped zones, and they don't generally see as far as ultrasonic units. The ranges are representative; actual sensors may be more or less sensitive.
Source: Platts
Ultrasonic sensors emit a high frequency signal (over 20,000 cycles per second), above human and animal audibility ranges, and listen for a change in frequency of the reflected signal. By emitting a signal instead of only receiving it, they are able to cover larger areas than PIR sensors and are noticeably more sensitive. They are also more prone to false triggering. For example, air motion created by a person running past a doorway or the on/off cycling of an HVAC system can cause false triggers.
Determine if smart power strips are cost-effective. Many buyers of smart power strips base their purchase decision on a simple visual observation that office equipment is left on longer than necessary. But to determine whether or not an investment in this type of equipment will really be cost-effective, buyers can monitor particular groups of equipment to see how much power they draw, how many hours they are left on, and how often they could be turned off. One company offers a Plug Load Analyzer that can be combined with a sensor to track energy usage and occupancy and estimate the savings that can be achieved through control of plug loads. Loggers can also be used to determine office equipment power draw and time of use. Once the equipment power draw, times of use, and occupancy schedules are known, a simple calculation, such as that shown in the sidebar, can be performed to determine the payback period of a smart power strip.
Pick the right sensor type. Ultrasonic sensors are more sensitive to movement than PIR devices, but they're also more prone to false triggering. Both will work in the typical office setting if they are installed carefully, as described below. PIR sensors are a better choice in a typical workstation, where the line-of-sight requirement of a PIR sensor can be easily meet, and the PIR's resistance to false triggering gives it an edge over ultrasonic technology.
Install sensors carefully. As is the case for wall- and ceiling-mounted occupancy sensors, the sensors that come with smart power strips are easily visible and can potentially be improperly adjusted, stolen, vandalized, or fooled into perceiving a human presence when a space is unoccupied. For continued energy savings, users should take the following precautions:
• Involve building personnel in planning for the sensors.
• Train maintenance personnel and office occupants to keep sensors operational rather than disconnecting them when problems occur.
• Position sensors carefully so that they only see the area that you want them to see; the biggest cause of false triggering is incorrect sensor positioning.
Decide on an area of coverage. Some smart power strips come with small-range sensors that only sense a presence in the workstation; others cover a wider area and detect the presence of people in the office around the workstation. Choose the latter if you want equipment to turn off less frequently and turn on sooner.
Pick a product with an adjustable time delay. Smart power strip products may come with either adjustable or factory-set time delays that determine how long a period of nonoccupancy must be before the equipment is turned off. A product with an adjustable time delay allows adjustment of the smart power strips to suit individual work habits.
Purchase Energy Star equipment in addition to smart power strips. The Energy Star program, a joint effort of the EPA and the U.S. Department of Energy, encourages manufacturers to produce more efficient equipment and to incorporate low-power sleep modes in many devices. These features can reduce energy use in office equipment to some degree, but they are best viewed as supplements to smart power strips rather than alternatives. That's because not all office equipment is available with a sleep mode (task lights, for example), and even sleep modes use some power, which can be eliminated with a smart power strip. Also, Energy Star features don't work for all situations, such as when manufacturers or equipment users disable Energy Star features.
The next step for smart power strips is integration into a total workspace controller. One company recently moved in that direction with a product that consists of an occupancy sensor, an infrared transmitter, and controls for outlet switching and fluorescent dimming. It not only controls office equipment based on occupancy, but it also allows occupants to control a variety of circuits themselves without leaving their seats. It can operate with any of the company's smart power strips, enabling the user to start or stop any device that is plugged into a controlled outlet. It can also be used to dim overhead fluorescent fixtures linked to an infrared controller.
Copyright © 2004 - Platts, a Division of The McGraw-Hill Companies, Inc.
