As much as 15% of the power consumed by commercial and industrial refrigeration units is used to defrost the heat exchangers. In typical installations, the defrost cycle is initiated at regular intervals during the day by a timer clock that is typically set to accommodate the summer high-humidity conditions. Thus, the defrost cycle is often begun too soon and run too long.

The integration of a low-cost frost sensor into a defrost control system will start the defrost cycle only when necessary and stop it as soon as the ice has been removed from the heat exchanger. The frost sensor, designed to measure the heat transfer from a heat-exchanger fin as a function of ice formation, is inexpensive and can be easily mounted on existing refrigeration units. This same sensor could be used to monitor the level of dirt formation on the fins and be used in a preventative maintenance program for scheduling cleanings when necessary. The system’s controller will use signals from the sensor to initiate and terminate the defrost cycle as well to control the temperature in the refrigerated zone. The projected unit cost is about the same as that of the timer and thermostatic controller being replaced; the new system, however, could save 7% to 11% of all the power used by the refrigeration unit.

Benefits

• About 4% of all power consumption in the United States is used in refrigeration. An effective demand-based defrost system could result in a reduction of refrigeration power use by 7% to 11%.
• Improved temperature control would minimize the duration and frequency of defrost cycles, reducing the occurrence of damaging freeze-thaw cycles and saving energy.
• The control algorithms are capable of providing an alarm to indicate failure of components, such as a fan.
• The controller could be used to recognize when a specific evaporative coil requires cleaning (due to excessive dirt accumulation) based on a reduced heat flux immediately following a defrost cycle.

Applications
The simplest application is for small refrigeration units, such as those in convenience stores, that use a compressor and a single cooling coil. The inexpensive controller can directly initiate and terminate the defrost cycles for these units.

Refrigeration systems for industrial plants (chemical, food processing, concrete, etc.) and supermarkets have multiple evaporative coils that are supplied with refrigerant from compressors that may not be near the units. For these systems, multiple controllers can be used in concert to perform the defrost cycles for all of the evaporative coils without the risk of defrosting multiple cases simultaneously.

The system and/or patent-pending frost sensor could be integrated into the control networks of large industrial plants.

Source: U.S. DOE, Office of Industrial Technologies Energy Efficiency and Renewable Energy