issue: August 2008 APPLIANCE Magazine
Refrigeration Defrost Control
Adaptive Demand Defrost Using Proximity Sensors
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by Gabriel Sanchez, systems engineer, Freescale Semiconductor
E-field sensor technology can be used to improve adaptive frost detection and replace timer-controlled defrost systems.
With worldwide emphasis on energy conservation and the search for long-term solutions against global warming, customers expect new products that are more energy-efficient, yet still deliver more functionality than ever. Modern kitchen appliances must meet stricter government energy regulations while still remaining attractive purchases for the customers. Recent studies have shown that the U.S population believes it is important to purchase earth-friendly home products, and almost 75% of the population would pay up to 10% more for these products.
Refrigerators are appliances where demand for both energy conservation and user functionality is increasing. Looking for new ways to improve energy efficiency in refrigerators is an important task that should not be ignored.
One area where refrigerators can improve is in the defrosting systems. Many refrigerator/freezer systems are capable of automatic defrosting. In freezers, as the coils that cool the air reach low temperatures, the humidity in the air condenses and freezes on the coils, building up frost. As coils accumulate this frost, it becomes more difficult for them to cool the air inside the freezer. This effect forces the freezer system to stay on for longer periods of time to keep the space cold.
This is why defrost systems are installed. In a typical application, a defrost system has a timer, a heater, and a terminator. At certain intervals, the timer turns off the condenser that is used to cool the system and turns on the heater, which heats up the coils to melt the frost. The terminator can be a thermostat that turns off the heater once the temperature rises to a certain level, or it can be another timer that turns it off after a certain time. Unfortunately, this kind of temperature cycling is not optimal for food preservation and, therefore, should be kept at a minimum.
These cycles also consume a large amount of energy by heating up the coils and then cooling them back down. Many companies have developed proprietary systems for adaptive defrost control, and several patents exist on the subject. However, there is no accurate measurement of how much frost may be present, so a freezer may try to defrost itself when in fact there is no frost on the coils.
This creates an opportunity for many engineers willing to try new ideas for frost detection. One proposal is to use electric field (e-field) sensing. E-field sensors generate a low-level electric field from an electrode and measure attenuations in the field loading caused by objects moved into the field. The basic principle of this can be seen in Figure 1.
The change in the field creates a “capacitor” between the driving electrode and the object within the field, each forming a “plate” that holds the electric charge. The voltage measured on the capacitor is an inverse function of the capacitance between the electrode being measured, the surrounding electrodes, and other objects in the electric field surrounding the electrode. Increasing the capacitance results in decreasing voltage.
Figure 1. Visual representation of e-field sensor.
Testing the Technology
To demonstrate how e-field sensing can improve defrosting systems, tests were developed that used an e-field sensor installed in a freezer. An initial test placed the e-field sensor around the coils of a top-freezer refrigerator. However, measurements at room temperature were unable to provide stable values due to the excessive noise created by the metallic spikes around the coil that created a “leak” in the electric field.
The next test used a metal plate placed on the wall of the freezer and another metal plate placed 1 in. perpendicular to the first plate. This configuration reduced the noise in the system to almost zero. Large items brought into the field of the plates changed the e-field readings based on the object’s size and its dielectric constant. Small items, such as water drops, had no major effect on the e-field value read.
Figure 2. Block diagram of defrost system with e-field sensor.
Measurements were then taken on the e-field sensor as the freezer was turned on with no items inside. Results showed a change in the electric field as the temperature in the freezer decreased and, to a smaller extent, as frost formed on the sides of the wall of the refrigerator where the plates were placed.
A metal ring electrode was then placed around the pipe that feeds the cooling coil to create a measurement point. The pipe was straight and smooth, and frost built up at this point at the same rate as the entire cooling coil. Measurements taken in this test showed a change in the capacitive field as frost built up on the coil (see Figure 3). The tests showed that as frost built up on the coil, the e-field sensor’s output voltage changed. Further tests showed that when the freezer temperature returned to room temperature, the e-field sensor returned to its initial readings. By monitoring the output voltage, the adaptive defrost system can better control defrost temperature cycling to improve cooling performance and energy efficiency.
Figure 3. Picture of third test setup and e-field input voltages.
An e-field sensor can be used as an innovative way to help improve adaptive frost detection and replace old-fashioned timer-controlled defrost systems. The use of these systems can help companies lower overall power consumption in their products, and therefore, sell refrigerator systems that are more attractive to their customers.
About the Author
Gabriel Sanchez is a systems engineer at Freescale Semiconductor (www.freescale.com) with more than four years of embedded application experience. His work has been mainly focused on power consumption sensitive applications. If you wish to contact Sanchez, e-mail firstname.lastname@example.org.