Walk-In Freezer Demand Defrosters Deflate Energy Costs

John Sustar, E Source Research Analyst -- Foodservice Equipment & Supplies, 3/10/2008 1:58:00 PM

Walk-in freezers, also known as cool rooms for storing food items in restaurants, with a 1-ton capacity typically cost about $1,700 per year to operate (assuming the cost of electricity is $0.08 per kilowatt-hour). A significant portion of the total energy these units consume runs an electric defroster to defrost the ice that accumulates on the evaporator coils during operation. Removing ice build-up decreases the load of the compressor by eliminating an insulating barrier on the coils.

However, these electric defrosters typically consume much more energy than necessary because the defrost cycles occur at regular intervals using a timer. Generally speaking, operators can plan on one hour of defrost out of every six hours of operation, rather than on an as-needed basis. On average, timer-based defrosters account for about 20 percent of the total energy consumption of walk-in freezers. New demand defrost systems, which initiate defrosts only when they are needed, can save significant amounts of energy by reducing the number of defrost cycles.

Independent tests show that the more advanced demand defrost controllers can reduce defrost cycles by as much as 40 percent compared to defrosters with timers. This can result in savings from $160 to $3,000 annually on energy costs depending on the size of the freezer. In addition, these controllers can help maintain the quality of products kept in the freezer because fewer defrost cycles translate into a more-constant temperature in the freezer.

Conventional demand defrost controllers have been around for more than 10 years and use simple control strategies to initiate defrost cycles based on freezer temperatures, pressure drops across the evaporator, frost accumulation, or by sensing humidity. Any of these methods are more effective than using a simple timer to initiate defrosting. Studies show that these conventional demand controllers can yield 1 percent to 6 percent energy savings as compared to timer-based defrosters.

Unlike previous demand defrost systems, the most recently commercialized demand defrost controllers use an advanced adaptive control algorithm to learn how a freezer operates and then allow the refrigeration equipment's controls to initiate defrosts only when they are needed, thus saving energy. Over the course of several days after installation, the controller develops a model of how the freezer operates based on several inputs, including the refrigerant line pressure and temperature as well as the air temperature outside the freezer. The controller establishes a refrigeration system performance baseline that is compared to ongoing operating conditions. Subsequent defrost cycles are allowed to occur only if system performance degrades, indicating ice buildup on the evaporator coils. As a safeguard, a minimum of one defrost cycle per day is allowed to occur. In some cases, these demand defrost controllers can learn and adapt to changes in the usage of a freezer.

One manufacturer reports that their advanced controller can save approximately $160 per year for small systems (less than 1-ton systems), $300 to $600 per year for midsize systems (2-ton) and $1,000 to $3,000 per year for the largest freezers (more than 3-ton), assuming an electric cost of $0.08 per kilowatt-hour.

In February 2007, Intertek, a third-party testing company, conducted a one-month test of one such advanced demand defrost controller with an adaptive algorithm. They installed the controller on a walk in freezer located within a restaurant in Georgia and noted the number of defrost cycles over one-week periods as compared to a walk in freezer with a timer-based controller. The results showed that the number of unnecessary defrosts was reduced to 16 cycles per week as compared to the typical 28 defrost cycles per week. This comes out to about a reduction of 44 percent with the demand-based defroster. For a 1-ton walk-in freezer, which typically consumes 4 MWh annually to run the electric defroster with a timer, the demand defrost controller will save approximately $150 annually in energy costs. Given that these systems typically cost about $400 to $600, the demand defrost controller will yield a payback of about two to four years.

Several refrigeration industry experts agree that controlling the defrost cycles based on ice buildup can save energy compared with defrost cycles based on time alone, but they note that past attempts to achieve that end have not found widespread success. The biggest problem has been that refrigeration technicians often disable this type of defrost control after a single malfunction that results in a service call, eliminating any future savings that could have been gained from the devices. Because the new demand controllers use an adaptive control strategy and monitor several variables in the refrigeration system, they could prove more robust than past controllers.