Walk-In Coolers Part of C-Store Savings
Fortunately for XtraMart, the blow was lessened because three years earlier it had begun a large-scale energy conservation program. The program has reduced kilowatt-hour consumption by up to 25 percent and decreased demand charges up to 12 percent in its 80-plus retrofitted stores, according to those involved in the project.
"Our operating costs have taken a hit with rising energy prices," said Tom Sansoucy, operations manager for XtraMart. "But thanks to our energy-management system, we've been able to fight back."
To help implement the energy management program, Sansoucy retained National Resource Management (NRM), a Massachusetts-based company that designs, installs, and monitors energy management systems around the country.
NRM began with an energy audit that revealed a number of opportunities, especially with XtraMart's walk-in coolers.
COMMON SENSE EFFICIENCYA typical walk-in cooler is made up of an outdoor compressor coupled with one or more banks of evaporator fans mounted on the ceiling. The refrigeration systems are sized to a capacity sufficient to handle the hottest days of the year within the local climate. However, given the rarity in most areas of such extremes, there is considerable opportunity for energy savings in daily operations.
In the case of the XtraMart situation, the first step came with the assistance of free cold winter air. To take advantage of this phenomenon, NRM added a ventilation system (Economizer) that pipes in cold air when the outside temperature dips several degrees below the cooler set-point. In New England, there are approximately 1,900-2,300 hours when outdoor temperatures are low enough for the economizer to provide primary cooling. It virtually eliminated compressor run time during those cold periods. At 11 cents/kWh, that innovation has meant an average winter energy cost reduction of $900 per store.
"It's just common sense to take advantage of Mother Nature's freebie," said Emre Schveighoffer, president of NRM.
Schveighoffer explained that the economizer employs two blowers (intake/exhaust) and six-inch PVC pipes that are run through the wall of the walk-in where they terminate in a stainless-steel intake hood mounted to the outside wall. When the outdoor temperature falls 3° to 4°F below the set point, the control system automatically makes the economizer the primary cooling source while the refrigeration system becomes secondary or stage two. The control system turns on the economizer to bring in the free cold air through one pipe while expelling an equal amount of warmer inside air through the other pipe. This design satisfies the needed temperature while keeping the air pressure balanced.
Since humidity at this low temperature is so low, very little moisture is introduced into the cooled space. Because the cold air is free, it can be used to bring the temperature of the cooler several degrees below the normal set point to slightly overcool the contents allowing the compressor to coast during times when it is too warm outside to use the economizer.
NRM also saw an opportunity with the evaporator fans, which typically run continuously inside the walk-in. To minimize any possible heating issues, NRM installed high-efficiency GE ECM by Regal-Beloit evaporator motors.
The motor is of brushless DC design with permanent magnets mounted on its rotor. These features lessen the refrigeration load because the compressor doesn't have to work as hard to remove the heat generated by the evaporator motors. Each motor has its own microprocessor that enables it to deliver a programmed constant torque and speed in a range of static pressure conditions. Each motor's circuitry is equipped with a line-voltage regulator to level out any voltage irregularities from the power company.
NRM also abandoned the practice of running the fans continuously. Instead they were set to run only long enough to satisfy the temperature requirements of the compressor, thereby reducing total hours of run time.
"People question the wisdom of shutting off the fans," said Schveighoffer. "But we have thousands of installations across the country that demonstrate the effectiveness. The key is knowing when to safely turn them off and on, and how to cycle them to prevent stratification."
He said the motors are never off more than seven minutes at a time. The system will stir the air either with the normal compressor cycle or the system will periodically cycle the fans during the off cycle to keep the air mixed.
GLASS DOOR SAVINGSAnother energy-saving opportunity was discovered in the glass doors of coolers and freezers. Those doors and frames are heated to prevent condensation from forming and reduce fogging on the inside of freezer glass doors when opened. And like the rest of the cooler, those heaters are designed to handle the highest-possible humidity conditions in the store. NRM, however, discovered that doors and frames only require a fraction of the heat to stay clear.
The approach works by first measuring temperature and relative humidity in the store to determine the dew point. The cooler control system (called CoolTrolÂ®) was designed to send the right amount of electricity to the door and frame heaters using solid-state switches that pulse them on and off every second. As a result, the system can still keep the glass clear while reducing power consumption in cooler doors by 75-95 percent and 40-60 percent in freezer doors.
"We estimate that we save about $50 per door per year with our freezers, and about $100 per door per year with our coolers," said Sansoucy.
The cooler-control system serves to both monitor and control the system to optimize its efficiency. The system's built-in intelligence tracks temperatures and equipment run time to measure performance and analyze energy consumption. One of its purposes is to identify usage patterns so cooler operations can be adjusted to extend the life of the equipment and save energy.
The system also automatically alerts store personnel when predetermined high- or low-temperature limits are exceeded. Rapid response to potential problems reduces or eliminates loss due to spoilage. Users and technicians can bypass the system to diagnose problems or to service the cooler, resetting it once service is finished.
The system also facilitates a more efficient defrost cycle, based on coil temperature and compressor run time. Experience suggests that as compressor run time increases, it is more likely to produce icing on the coils, so the control will automatically adjust the defrost cycles based on compressor run time. If it is discovered that fewer defrost cycles are needed, the system can be adjusted locally or remotely.
The system includes a cooler "Load or Shutdown" button that allows local personnel to safely turn the cooling system (both fans and compressor) off for up to 20 minutes when products are being loaded or while working in the walk-in.
The system has a built-in scheduler that can be used to shut off novelty or visi-cooler soda and water coolers during times when a store is closed. Tests prove that the overnight shut off saves energy without affecting product quality or temperature.
REMOTE SITE MANAGERThe cooler control, however, is only one component in a master control system called the Remote Site Manager (RSM). This Web-based energy-management system saves energy and allows users to remotely manage store equipment. Besides cooling, it also monitors and/or controls, lighting, HVAC, status of cooler/freezer and main entry doors, freezers and other temperature-controlled merchandisers.
Each major load's amp draw is monitored. At some sites it is used to monitor the utility meter and provides real-time energy usage. Data from all of these control areas converge in a gateway device that relays information from the store to NRM's servers. XtraMart dispatchers and technicians can view real-time equipment operation from any store or remotely by using a Web browser. System performance can be viewed in real time and, if necessary, changes can be made to improve performance.
Remote Site Manager gathers and stores information on a report-by-exception basis, meaning that it only sends information based on change. For example, the system measures key data points (compressor amps, operational temperatures, humidity, equipment status, etc.) as frequently as every seven seconds.
The server application maintains the data history, sends and tracks alarms, provides the interface for changing settings in the controllers located at the stores, generates reports and trends on demand. As alarm conditions are detected, the server sends alarms to XtraMart dispatchers (each shift has one on duty) so maintenance personnel can be dispatched to prevent a catastrophic failure or product loss.
Said Schveighoffer, "We designed our system to be event-driven, recording changes in vital signs as they happen. Think of it as a check-engine light in your car."
He explains that because RSM measures so many variables, they can pinpoint energy-wasting culprits. For example, prior to RSM, store personnel might overcompensate on hot days by turning the thermostat down to 60°F. The new system sets the thermostat so it functions within a much narrower range, thereby saving energy and preventing a service call due to iced coils.
Lighting is another area that receives a lot of attention with the system. RSM is not affected by changing seasons because lights are automatically switched on or off based on the store's operating hours and input from the photocell.
MAINTENANCE COSTSThose sites with the energy-management system have realized an added bonus - a reduction in maintenance calls, according to officials. XtraMart also uses NRM's Remote Maintenance Manager software to track and manage all of their work orders and maintenance costs. "In the summer months, we used to lose a freezer practically on a weekly basis," said Sansoucy. "Now it rarely happens."
Sansoucy added, "Nuisance maintenance calls have been virtually eliminated. Savings were further enhanced by providing their outside service contractors access to the Websites and data, so they too can observe and make predictions about system performance and take corrective action before a catastrophic failure might occur. For example, if there are multiple compressors at a location, and the usage pattern indicates that one compressor is running 80 percent of the time while the other 20 percent, the contractor or technician can make adjustments to more evenly balance the work load which can extend compressor life."
Payback on energy-savings systems averaged a little over two years. Rebates from local utilities play an important role in the equation. NRM obtained rebates for XtraMart for the economizer, the CoolTrolâ„¢ control system, door heater controls, ECM motors and Remote Site Manager. While rebates vary from state to state, in some cases, the local utility rebated up to 80 percent of the cost and financed the rest at no interest.
Publication date: 07/03/2006