Is Your EMS on Target For Energy Savings?
By Lindsay Audin - April 2004 - Building Automation
A variety of energy cost-cutting opportunities are available by using a building-wide energy management system (EMS). According to several studies, modern control systems have been able to cut energy, operations and maintenance, and repair costs by 10 percent or more, making them worth a close look.
Essentially a programmed computer connected to various sensors and actuators, an EMS may be part of a larger system that also covers occupant comfort, safety and security. An EMS can perform many tasks that would otherwise take several people to handle manually. When properly programmed and commissioned, such systems hold the line on operating costs for lighting, chillers, boilers, fans and other building equipment.
A 1999 study on cybernetic building systems by the National Institute of Standards and Technology found that an advanced EMS could save 37 cents per square foot per year in energy, operations and maintenance, and repair and replacement cost — based on a mid-range of 10 percent and national averages for these costs. That same study found that the cost for installation of such systems averaged about $1 per square foot.
The Center for Analysis and Dissemination of Demonstrated Energy Technologies (CADDET), an international energy agency supported by European governments, estimates typical lighting energy savings of 30 to 50 percent from an EMS in an office building.
The lion’s share of savings is in energy and maintenance, which is mostly labor from reduced staffing and service calls. While many EMS vendors try to include occupant productivity savings, such value is notoriously difficult to prove or quantify. In a multitenant building, any productivity benefits accrue to tenants, not the building owner.
But even leaving occupant productivity aside, EMS savings show that how a building is operated can have an impact on its costs almost as great as the efficiency of the devices installed. That’s important: Many believe that energy bills may be reduced by just installing high-efficiency equipment, but it is often the efficient operation of such equipment that can make or break an energy budget.
For all the benefits an EMS can offer, however, the systems are not always used to their fullest potential. Indeed, the range of EMS options can be an obstacle to the effective use of the system and make it more likely that useful strategies will be overlooked. A list of basic options offers a quick way to determine how well a facility staff is tapping EMS benefits.
Any list of options will vary somewhat depending on facility types, operating schedules, building age and local climate. Some of the strategies listed below may also be pursued with simple stand-alone systems or manual efforts, but an EMS centralizes control and monitoring while maintaining greater flexibility for upgrades and changes. Where sophisticated building controls and equipment exist, greater savings may be found by pursuing more advanced strategies, such as sequentially adjusting variable speed drives and daylight dimming.
Scheduling By Zone
Nothing saves more energy than shutting a device off, but it is often common for a whole building to be running even when only 10 percent of its space is occupied. Scheduling by zone or floor — taking into account daily operations, weekday, weekend, and summer and winter schedules — can capture many hours when one system or another need not be on. Most common schedules cover the operation of lights, fans, zone dampers and terminal units. More complex systems have built-in overrides using call-in, occupancy sensing or reaction to out-of-spec temperatures that turn on systems for zones having special needs. When applied to lighting, such scheduling may involve selective shutoff of a portion of lighting not needed, for example, during cleaning or restocking, or late-night hours for exterior lighting.
Cycling causes units, such as fans, elevators and certain building pumps, to shut down for a defined time period — 10 minutes out of each half hour, for example — to reduce overall energy consumption and peak demand. The latter is accomplished by ensuring that some units never run simultaneously when peak demand charges are highest. At times, comfort may be slightly affected, and wear on machinery may increase because of increased starts. This strategy has, in many cases, been supplanted by variable speed control.
Setting back schedule times on equipment has been common since the ’70s as a way to reduce off-hour and nighttime heating and cooling energy consumption. Because the rate of heat transfer is proportional to the temperature difference on two sides of a surface, energy is saved by allowing interior temperatures to drop in the winter or rise in the summer during unoccupied periods, thereby reducing those temperature differences and the rate of heat loss or gain.
The EMS “learns” the best time to start and stop an HVAC system to reach a defined internal temperature at a specified time based on a building’s thermal inertia, temperature conditions and other characteristics. This strategy minimizes operation of fans, boilers, and chillers and may yield better results than simple night setback.
Sweep Shutdown Of Lighting
This involves automatically turning off lights across a large area, such as open office spaces, after an initial warning flicker, unless an occupant calls the EMS operator — or touches a button — to request that lights on a floor or zone remain on until the next sweep occurs an hour later.
Using Standard Economizer Cycles
This allows increased use of outside air when the economizer’s dry bulb temperature is less than that of return air. Doing so offers a simple way to reduce cooling load, typically when outside air is cooler than about 57 degrees Fahrenheit. However, a standard economizer cycle does not take into account the humidity of outside air. Greater energy savings and maintained comfort are achieved with enthalpy economizers, though they cost more to install and maintain.
Reducing Building Static Air Pressure
Cycling fans and adjusting damper settings can cut fan motor loads and reduce exfiltration, potentially cutting uncontrolled exhaust of conditioned air. Comfort in some locations may vary slightly.
Zoning By Orientation
This reduces HVAC service to some parts of a building based on exposure to the sun or operation at certain times of the day. Additional valves or dampers may be needed.
Reducing Domestic Hot Water Energy Use
Shutting off pumps, boilers and other domestic hot water equipment or allowing water temperature to float during unoccupied periods reduces the energy used by a facility.
Elevator Dispatching Based On Need
This may trim electricity consumption and demand while reducing wear on components. Savings will be much greater with older systems that use direct current motor-generator sets than with more modern systems with solid-state elevator controls.
Dimming Non-Essential Lighting By Using Area-Wide Voltage Reduction
Such systems are installed at main lighting panels and do not require special ballasts. Both incandescent and many magnetically ballasted fluorescent and HID fixtures may be dimmed by about 25 percent, thus maintaining security and safety while trimming energy use.
Fuel Switching On Boilers And Chillers
This allows these systems to use the cheapest energy source at a given moment. Where boilers are able to use either oil or natural gas, many facilities switch between fuels only when called upon by their natural gas utility. Greater savings may occur, however, when that switch is based upon choosing the fuel that is cheaper on a daily or monthly basis, depending on how it is purchased. In the case of hybrid chiller plants, which use both electric and gas or steam-powered chillers, total chilled water costs may be minimized by dispatching chillers based on the time-of-use cost of power, again depending on how the energy is being purchased.
Proceed with Caution
Some energy-saving EMS strategies have the potential to have a detrimental impact on comfort. Although these actions must be approached with care, they may be appropriate for some facilities.
Outside Air Reduction
Closing outside air dampers or shutting off exhaust fans — either partially or completely — reduces heating and cooling loads caused by the intake of outside air. Where building codes require more outside air to maintain comfort and health conditions, this strategy has been somewhat limited in application. When used in conjunction with night setback, however, code violations are avoided because outside air is only reduced during unoccupied periods. The same is often true for spaces where dense occupancy, such as auditoriums, occasionally requires high outside air flow, but which may be unoccupied for extended periods.
Shutting Off Reheat
This strategy can save on heating or electric energy during the summer. Many constant-volume air systems that serve several zones are equipped with small heating coils designed to adjust for variations in loads, such as lighting, office equipment and solar input among zones. The cooling coil discharge temperature serving several zones may be designed to cool the hottest zone; reheat coils keep cooler zones from being overcooled by reheating cool air sent to them from that coil. Shutting off reheat may result in occasional overcooling but, because reheating is done with electric coils, significant electric demand savings may result.
Allowing Humidity Levels To Float
This can save on both heating and cooling energy, though comfort complaints may result. Where spaces are humidified — typically parts of the country where air gets very dry in winter — moisture may be added to the air stream through the use of steam or misting from boilers or electric heating coils. Allowing humidity to drop slightly may then cut boiler or electric loads. Where moisture removal due to standard cooling is insufficient to make a space comfortable or meet tight specs, cooling coil temperatures may be quite low to cause more moisture to condense out at the coil. In such systems, reheat is often needed to avoid overcooling. Allowing the humidity level to rise occasionally could then reduce both cooling and heating demands.
Care must be exercised, however, with spaces where narrow humidity limits must be maintained, such as computer rooms or rare book storage. And in the deep South, humidity control may be more important than in the upper Midwest to avoid mold and odors developing.
The option to let some comfort standards slip must be tempered with the reality that the business of a facility cannot be measurably hampered by doing so. Not all facilities are good candidates. In general, poorly operating HVAC systems already create comfort problems so letting them slip further is a bad idea. The same is true for very old or poorly maintained buildings. Single-glazed, leaky and poorly shaded facilities are already quite vulnerable to temperature swings that could affect comfort, so why worsen them with intentional actions?
On the other hand, a facility with a well-operating EMS, multiple zones and appropriate controls has some leeway to loosen limits. The key is to not do so for very long in the same zone. The facility executives who have the most success are ones who allow conditions to vary only for short periods — for 15 to 30 minutes where people reside — and longer in other areas. Rotating such variances across spaces shares any discomfort and often is not noticed.
The facility executive must exercise sophistication and logic when trying to squeeze more savings out of a facility. Start with the easy zones and times. Allow temperatures to float when spaces are unoccupied, which may mean learning more about how and when spaces are used. Be ready to re-adjust settings in vulnerable areas without reverting back in all areas. Experiment at non-crucial temperatures to assess temporary impacts. And make small incremental adjustments of 2 degrees to avoid major complaints that could reverse an entire program. Let occupants know that there’s an effort to try and save energy without jeopardizing comfort, and suggest that they can help out by being both informed and flexible when it comes to minor adaptations, such as sweaters on cold days and taking off suit jackets on hot days. If an existing system or building already has severe comfort maintenance problems, it’s time to fix them before worsening them. Merely upgrading controls could yield significant savings.
A Few Caveats
Practical experience with controls systems teaches an important lesson: Too often, control systems are disabled because of the inability of some personnel to understand how a system operates. An occupant complaint then results in “jumped out” control wiring to ameliorate the problem temporarily, meaning that later signals are sent but not received or acted upon, creating an illusion of control. A whole industry has grown up around recommissioning such systems to restore them to proper order. Unless the root problems of training, documentation and education are corrected, however, controls will continue to deteriorate.
Likewise, no EMS will correct poor management. During a recent cold snap in the Northeast, for example, an alarm for a broken water pipe went unheeded while an EMS operator was on vacation because no one was delegated to replace him during that period. The pipe broke because of a freezeup from a failed heating system, and water poured throughout a building closed during a school break.
Energy management systems are programmed with various energy savings strategies that start, stop, limit, adjust, report on and sound alarms concerning the operation of a wide range of equipment. When intranet or Internet communications are used, control and monitoring from multiple locations is possible.
As control systems migrated from pneumatic actuators that use compressed air to alter an operation to direct digital controls (DDC) that use electronics, motors and relays, the level of control sophistication has risen. While various stand-alone controls — night setback thermostats, occupancy sensors and timeclocks, for example — continue to serve many buildings, an integrated EMS using DDC has become the general standard for new or upgraded facilities.
Successful management of a building system generally requires monitoring and controlling a variety of “points” — locations at which something is to be measured or adjusted. A modern air handling system, for example, may require a dozen or more control points to measure fan speeds, static pressure, temperature, power use, variable speed drive set points, coil temperatures, humidity, damper settings and perhaps even carbon dioxide levels. A similar degree of control may be involved in running lighting, chillers, boilers, cooling towers and pumping systems, depending on the sophistication of the systems and their operators.
Algorithms — computerized equations — are programmed into the system to alter the settings of some points when other points change at scheduled times. A sequence of operations governing startup, shutdown and order of response is also usually part of the control process.
For More Information
For those new to EMS or seeking more information, here are several useful and unbiased resources.