Building Controls: Types of Energy Management Systems
By Lindsay Audin - January 2005 - Building Automation
While most buildings use a variety of simple controls to manage HVAC and lighting systems, many facility executives have not taken advantage of sophisticated options capable of wringing more savings out of facility operations. Tighter control may entail more points and programming, as well as greater levels of commissioning and maintenance to ensure continuity of savings. Getting the most out of controls also requires building systems capable of adjustment, such as variable speed drives or gradual dimming, as opposed to simple on/off control. Here are some measures that have saved many facility executives a good deal of money.
1. Varying air handler fan speeds reduces power consumption significantly during the many hours of the year when peak fan speed is not needed to meet comfort and air quality requirements. Even a slight reduction in fan speed of 15 percent can yield a noticeable drop in fan motor power demand — almost 30 percent — because of the inverse square law inherent in fan motor loading.
Fan speeds may be adjusted not only in response to heating or cooling needs but also to limit peak electrical demand. Most buildings exhibit thermal inertia, meaning that the mass of the structure and its contents tend to stabilize temperature changes even when heating and cooling systems work to alter them.
Some facility executives have taken advantage of this stabilizing effect by reducing air handler fan speeds and the cooling or heating inherent in circulating air for brief periods — for example, 10 minutes out of an hour — when power is most costly. By sequentially shifting this reduction among all air handlers, no one space feels the reduction long enough to result in a significant change to occupant comfort.
2. Cooling and heating coil water supply temperatures may be reset instead of remaining at constant levels without occupants sensing the change regardless of outdoor conditions. In many systems, the chilled water temperature or circulating hot water temperatures for baseboards and coils are set at a constant level designed to handle the hottest or coldest day expected. Temperature is controlled by either cycling flow or shunting it around heating and cooling coils. Peak conditions occur relatively few hours a year, so the energy use to produce such temperatures is higher than needed the rest of the time.
An outdoor air sensor may be used with appropriate programming to raise baseboard water temperature as outdoor temperatures drop, and lower it on moderate days. The same may be done for chilled water temperatures, though the need to maintain a defined humidity may place an upper limit on that water temperature.
Supply water temperatures also may be reset based on return water or return air temperature where they reflect the need for conditioning specific zones or on a time schedule to reflect occupancy. When no one is in a space, its temperature may be allowed to vary widely. For some systems where air is both heated and cooled, such as dual duct systems, mixed air temperatures may also be reset based on time or outside temperature. In all cases, energy used by boilers or chillers is reduced and distribution losses are cut when circulating-water temperatures are moderated.
3. Enthalpy economizer controls take into account the humidity content of return and outside air, adjusting dampers and fans to move more of whichever has a lower overall heat content. Doing so may allow relatively warm outside air to be used for cooling when it is drier than return air. This process expands on standard economizer cycles based solely on dry bulb temperatures.
4. Carbon dioxide sensors can be used to control outside air intake. As with temperature control, good HVAC design provides sufficient fresh air to deal with the worst-case scenario, including full occupancy in a zone such as auditoriums and cafeterias. Most of the time, however, spaces are only partially filled, so that far more outside air is brought in than required by code or comfort. Conditioning of outside air, especially when it is very humid, may account for nearly half the load on an air conditioning unit.
By measuring carbon dioxide in return air, a reasonable estimate of the number of occupants may be made, allowing for a reduction in outside air. Reducing outside air intake means less cooling, heating, dehumidification and exhaust fan speed, if it’s variable, thus saving energy several ways.
5. Carbon monoxide sensors can be used to control outside air intake for garages where cars emit carbon monoxide. Exhaust fan systems are typically designed to supply enough fresh air when many cars are running at one time. Even when such air is not heated, fans may run at full speed all the time to ensure that no health or comfort problems occur. By measuring carbon monoxide, however, a reasonable reduction in ventilation fan speed and power use for many hours of the day may be accomplished when vehicle traffic is less than peak.
6. Chilled and circulating hot water pump speed may be reduced, where variable speed drives exist, at primary and secondary water pumps. Primary pumps may be adjusted based on outside air temperature and humidity. Where zone coil water flow is maintained by secondary pumps and controlled by on-off coil valves, a change in pressure may be sensed when many are in the closed position, indicating that full flow is not needed at that moment.
This option does not work well where three-way valves exist, because such valves allow a bypass flow that could make it difficult to sense valve closure. Once again, a small reduction in pump speed yields a significant reduction in electricity use.
7. Cooling tower fan speed may be reduced during periods of low outdoor humidity or temperature. Two-speed cooling tower fans are now common, but appropriate controls are needed to shift between high and low speeds. In many parts of the country, high speed is needed less than 20 percent of the time, allowing major savings during the other 80 percent.
8. The speed of condenser water pumps feeding cooling towers may be reduced when low outdoor humidity or temperature conditions exist, as long as the temperature and flow of water leaving the tower remain appropriate for best chiller operation.
9. Cooling-tower water may be used in lieu of chilled water through the use of a heat exchanger during transitional seasons or when winter cooling is needed for such areas as computer banks. When outdoor conditions are right and when tower fan speed is at full, cooling-tower water may be sufficiently cool to allow chiller compressors to be shut off, taking full advantage of the evaporative cooling capability of the tower. While tower fan and pump electricity use may rise, the net result may be a drop in electric demand when the chiller compressors are off. This process is often called “free cooling” or “waterside economizer” operation.
10. Hot gas bypass on electric chillers may be minimized when consistent with chiller specifications and controls. Doing so improves overall chiller performance. Similar measures may be taken regarding refrigerant pressure in some packaged air conditioning systems. Under recent federal efficiency standards, many new chillers already incorporate this technology.
11. Lighting may be significantly dimmed in response to incoming daylight, occupancy or time schedules where dimming ballasts and appropriate sensing and programming have been installed. Even without dimming ballasts, a portion of lighting may be controlled as a result of on-off schedules, occupancy sensing, daylight response or dimming on a schedule, or in response to peak building power demand. Where many fixtures are present in open common areas, temporarily turning off every third or fourth fixture is rarely noticed. In office spaces, studies have shown that gradual temporary dimming by about 20 percent is either unnoticed or noticed and accepted by the vast majority of occupants. When done in conjunction with outdoor light entering a space, significant savings in both lighting and cooling may result.
12. Cycle minor process loads such as refrigerators and vending machines or shut them down when possible to minimize peak demand and cut overall consumption. This may be done with add-on devices that allow wider device temperature swings, consistent with maintaining product quality. It should be noted that a standard vending machine, one lacking internal cooling, remains on at all hours, costing facilities $200 to $300 a year at average national power pricing. Specialized occupancy sensors for such machines have been successfully used at many facilities.
13. Fume hood exhaust fan control can be deployed in labs and kitchens. Through add-on devices, closing of fume hood sashes reduces exhaust fan speed, cutting power use and outside air intake. Similar actions may be taken in kitchens to reduce fan speed during periods when ovens are not in use.
14. Building peak electric demand may be reduced when pre-defined loads are minimized or briefly shut off by an automated system that senses building demand and sequences load reductions to meet a set level. Such control may be applied to loads such as fans, lighting, heat pumps, packaged air conditioning units and electric heating coils.
By integrating some of these options in a programmed sequence, facility executives have developed automated procedures for cutting peak load when it is most cost effective to do so, thereby saving on peak demand charges, or when called upon to do so by the local utility. Some utilities also offer rate cuts to those having a defined portion of interruptible load.
By sequencing brief — 10 to 15 minute — service reductions in spaces that people continuously occupy and for longer periods in other areas, such rotation avoids or reasonably shares any minor discomfort.
15. Load shedding with a backup generator (when allowed under environmental and utility regulations) may be especially profitable where high peak demand charges exist. As peak demand approaches a defined limit, the generator is deployed to feed pre-defined loads that are disconnected from utility service. Alternatively, the generator may feed into the general building electrical system.
Additional options exist, such as converting to reverse-acting thermostats, replacing three-way cooling coil valves for two-way valves, and recovering exhaust heating and cooling, but may require significant system-wide upgrades. The payback for each may be sensitive to prevailing energy costs and tariff rate design. All require a well-operating EMS, multiple zones and appropriate controls.
For owner-occupied facilities, even greater savings may be achieved during extreme weather by educating occupants that it’s a good idea to dress appropriately.
Lindsay Audin is president of EnergyWiz, an energy consulting firm based on Croton, N.Y. He is a contributing editor to Building Operating Management.
School of hard knocks
Any effort to cut energy costs must be tempered with a heavy dose of reality lest excess zeal result in major complaints, making future efficiency efforts suspect. Following are a few tips to avoid problems.