Managing Energy Costs in Groceries
Grocery stores in the U.S. use an average of 52.5 kilowatt-hours (kWh) of electricity and 38,000 Btu of natural gas per square foot annually. In a typical grocery, refrigeration and lighting represent about 47 percent of total use (Figure 1), making these systems the best targets for energy savings.
Energy costs account for 15 percent of a grocery store’s operating budget. Because grocery stores’ profit margins are so thin—on the order of 1 percent—every dollar in energy savings is equivalent to increasing sales by $59. You’ll be better able to manage your building’s energy costs if you understand how you are charged for energy. Most utilities charge commercial buildings for their natural gas based on the amount of energy delivered. Electricity, on the other hand, can be charged based on two measures—consumption and demand (Figure 2). The consumption component of the bill is based on how much electricity, in kWh, the building consumes during a month. The demand component is the peak demand, in kilowatts (kW), occurring within the month or, for some utilities, during the previous 12 months. Monthly demand charges can range from a few dollars per kW to upwards of $20/kW. Peak demand can be a considerable percentage of your bill, so care should be taken to reduce it whenever possible. As you read these energy cost management recommendations, keep in mind how each one will affect both your consumption and your demand.
Many grocery stores can benefit from low- or no-cost energy expenditure reductions.
It’s the simplest of ideas. Remember that every 1,000 kWh you save by turning things off equals $100 off your utility bill. (This assumes average electricity costs of 10 cents/kWh.)
Plugged-in devices. Computers, cash registers, bar-code readers, deli scales, and deli cooking equipment should be shut off when not in use. “Smart” power strips with built-in occupancy sensors are available to shut off plugged-in devices when no users are present.
Lights. Turn off lights when they’re not in use. Occupancy sensors can help; a less-expensive alternative is to train staff to ensure that switches are off when the lights aren’t needed. Stores that are open all night may want to install dual-level switching for overhead lights, allowing some fixtures to be turned off during low-traffic hours.
Some equipment cannot be turned off entirely, but turning it down to minimum levels where possible can save energy.
HVAC temperature setbacks. During closed hours, turn temperature settings down in warming seasons and up in cooling seasons.
Special-use rooms. Make sure that HVAC settings in warehouses, stockrooms, offices, and other special-use rooms are at minimum settings.
Check the economizer. Many air-conditioning systems use a dampered vent called an economizer to draw in cool outside air when it is available to reduce the need for mechanically cooled air. The linkage on the damper, if not regularly checked, can seize up or break. An economizer that’s stuck in the fully open position can add as much as 50 percent to a building’s annual energy bill by allowing hot air in during the air-conditioning season and cold air in during the heating season. Have a licensed technician calibrate the controls; check, clean, and lubricate your economizer’s linkage about once a year; and make repairs if necessary.
Check air-conditioning temperatures. With a thermometer, check the temperature of the return air going to your air conditioner. Then check the temperature of the air coming out of the register nearest the air-conditioning unit. If the temperature difference is less than 14° Fahrenheit (F) or more than 22°F, have a licensed technician inspect your air-conditioning unit.
Change filters. Change air-conditioner filters every month—more often if you’re located next to a highway or construction site where the air is much dirtier.
Check cabinet panels. On a quarterly basis, make sure that the panels to your rooftop air-conditioning unit are fully attached, that all of their screws are in place, and that the gaskets are intact so no chilled air leaks out of the cabinet. Such leaks can cost $100 per year per rooftop unit in wasted energy.
Clean condenser coils. Check condenser coils quarterly for any debris, natural or otherwise, that has collected there, and remove it. At the beginning and end of the cooling season, thoroughly wash the coils.
Clean evaporator coils. The buildup of dirt and ice on evaporator coils slows down the rate of heat transfer and causes the refrigeration system to use more energy to maintain the same temperature.
Check for airflow. Hold your hand up to air registers to ensure that airflow is adequate. If there is little airflow or dirt and dust are found at the register, have a technician inspect your unit and duct work.
Check the refrigerant charge. Incorrect refrigerant charge can compromise refrigeration equipment efficiency by 5 to 20 percent and raise the risk of early component failure. Have a licensed technician check the refrigerant charge of all refrigerated equipment annually.
Check refrigerated cases for air leakage. Every month, inspect and replace any worn seals and gaskets on the doors and inspect the door closers for proper operation.
Check temperature settings on refrigerated systems. Energy is wasted if refrigeration temperature settings drift too low. Periodically check to verify that the appropriate temperature settings are specified.
Add strip curtains to walk-ins. Simply adding strip curtains to the doors of a 240-square-foot walk-in refrigerator reduced the unit’s energy consumption by 3,730 kWh per year—about 9 percent of total consumption!
Replace incandescent lightbulbs with screw-in compact fluorescent lamps (CFLs). Whenever an incandescent lightbulb burns out in a fixture that is on for longer than two hours per day, replace it with a CFL. They are three times more energy efficient than incandescent bulbs, last 10 times longer, and—because they give off one-third as much waste heat—increase the efficiency of walk-in refrigerators and freezers. Specify low-temperature-rated CFLs for freezer applications.
Install occupancy sensors in walk-ins. By replacing light switches with low-temperature occupancy sensors, you’ll reduce lighting energy consumption by about half.
Although the actions described in this section require more extensive implementation, they can dramatically increase the efficiency of your store. Ask your local utility for more information about initiating such projects.
The optimization of refrigeration systems can reduce energy use by 24 percent relative to standard practice. The following measures yield the largest savings.
Floating head pressure. Taking advantage of lower ambient temperatures to reduce refrigerant temperatures is a form of free cooling. One approach is to allow the pressure of the vapor coming out of the compressor (the “head pressure”) to float—that is, to drop with reduced ambient temperatures. This requires an expansion valve capable of operating at lower pressures and flow rates, and such valves are now commercially available. In addition, refrigerant pressures must be kept high enough to avoid “flashing”—the unwanted vaporization of refrigerant. In one field test, operating a system with floating head pressure reduced annual electricity costs by 4.9 percent relative to operating with fixed head pressure.
Ambient and mechanical subcooling. Reducing the temperature of the liquid refrigerant below its condensation temperature is called subcooling. This can be done either by using ambient air or water to remove heat from the liquid refrigerant (ambient subcooling) or by using an additional refrigeration system (mechanical subcooling). Colder refrigerant means either more cooling per pound of refrigerant delivered to the display case or shorter compressor run times because less refrigerant is needed, both of which can decrease energy use. Ambient subcooling is often more cost-effective than mechanical subcooling because it requires less equipment.
Evaporative condensers. Most condensers in grocery stores are air-cooled, but it is also possible to use evaporative condensers, which are cooled by water spraying over the condensing coils. Evaporative condensers are more energy efficient than their air-cooled counterparts, but they do have a notable disadvantage: They require a water supply, which often means increased maintenance due to freezing, clogging, and mineral buildup. Evaporative condensers may be cost-effective in drier climates, but the added maintenance may make them unattractive in other climates.
Heat-recovery systems. Heat-recovery systems are available that capture waste heat from refrigerators to make hot water for use in the store. A 7.5-horsepower compressor can heat all of the hot water a midsize supermarket would use in its kitchen cleanup and bathroom sinks. Often, enough waste heat is also available to supply hot water coils for space heating in cold weather.
Display case shields. Aluminum display-case shields can reduce refrigeration load from the display case by 8 percent when applied overnight and by 40 percent when applied over a 24-hour holiday, relative to the load present without the shield. Products are kept colder when the shields are attached and remain colder for several hours after the shields are removed.
Evaporator-fan motors. Replacing existing shaded pole motors on evaporator fans with electrically commutated motors will reduce the energy consumption of refrigerator and freezer cases by 40 to 70 percent. Drop-in replacement designs have made this retrofit relatively simple for a technician to perform. Additionally, most evaporator-fan motors in walk-ins run continuously even though full airflow is usually required only about half the time. Consider introducing advanced controls that slow the fans when full-speed operation is unnecessary. Annual cost savings can result in about a one-year payback for the total cost.
Anti-sweat heaters. The latest anti-sweat heater controls sense humidity in the store’s ambient air and reduce the operation of their heaters in low-humidity conditions. They promise significant savings and quick payback, and they are relatively easy to install.
“Smart” defrost controllers. When installed in walk-in freezers, a smart defrost controller monitors several variables and optimizes the number of daily defrost cycles. Adding these kits can save hundreds of dollars a year, depending on the size of the freezer.
In humid climates, much of the energy used in air conditioning goes to removing moisture from the air. Desiccant dehumidification can be a cost-effective solution for removing this moisture because it uses natural gas instead of electricity. In some cases, air-conditioning equipment can be smaller sized because it is only used to cool dry air.
Lighting is critical to creating ambiance and making merchandise attractive to shoppers. High-quality lighting design can reduce energy bills and drive sales. If your facility uses T12 fluorescent lamps, relamping with high-performance T8 lamps and electronic ballasts can reduce your lighting energy consumption by 35 percent. Adding specular reflectors and new lenses and reducing the number of lamps can double the savings. Occupancy sensors or timers can add further savings in storerooms and other staff-only areas. Paybacks of one to three years are common.
Changing refrigerated display-case lighting to light-emitting diode (LED) light strips saves energy and has been shown to appeal to customers significantly more than linear fluorescent lamps. LEDs are more than 40 percent more efficient than T8 lamps, provide a more-even light distribution, are dimmable, and have a long lifetime. In stores that remain opened 24 hours a day, LEDs can be tied to occupancy sensors so the display cases are only illuminated when customers are present. The waste heat from LEDs can be dissipated outside the case, something fluorescent lighting can’t do, resulting in reduced refrigeration energy: For every watt in reduced energy consumption, there is an additional 0.48-watt savings from reduced refrigeration demands.
Grocery stores with high ceilings might want to consider using T5 lamps and indirect fixtures to boost both lighting quality and lighting efficiency. T5 lamps are far more energy efficient and offer better light quality than either the high-intensity discharge lights or the older-style T12 and T8 linear fluorescent lamps typically found in high-ceilinged stores.
Most parking lots are designed with far more lighting than the 1 foot-candle or lower average that the Illuminating Engineering Society of North America’s Lighting Handbook (2000) recommends. Using lower-wattage bulbs can actually increase the safety of your lot—an overlit lot can be dangerous to drivers if their eyes cannot adjust quickly enough in the transition from highly lit to dark areas. When designing lighting for a new parking lot, consider low-wattage metal halide lamps in fixtures that direct the light downward, rather than high-pressure sodium lamps. Even with a lower wattage, a grocery store could safely use fewer lamps if this choice is made. Metal halide is less efficient than high-pressure sodium in conventional terms, but it puts out more light in the blue part of the spectrum, which turns out to be easier for our eyes to see under low-light conditions.
LED lighting has emerged as an even more efficient parking lot option than high-intensity lighting. However, because the Energy Star program does not currently include parking lots in its list of acceptable LED applications and because LEDs’ high initial costs result in long payback periods, be sure to conduct a thorough analysis before you commit to LED lighting for your parking lot.
Painting the roof of a grocery store with white or other highly reflective paint can reduce the energy required for summer cooling by 25 to 65 percent and help trim peak demand, as well as increase the life of the roof. The U.S. Environmental Protection Agency has a list of suitable reflective roof coating products.