The accumulation of frost or ice on an evaporator coil always presents a challenge in home appliances such as refrigerators, freezers and air conditioners. The accumulation of frost or ice insulates the air circulating through the evaporator unit, which makes maintaining the overall temperature difficult. Also, atmospheric conditions that lead to the formation of ice block the heat exchange: the thicker the frost, the worse the efficiency. Figure 1 shows an efficiency comparison with different frost thicknesses based on one model of refrigerator.
Figure 1: Comparison by cooling down a fixed amount of water
Solving these problems requires a defrosting system that is operatively associated with the heat exchanger. The question is when to activate the defrost cycle. If you activate the defrost cycle too early, when there is not much frost on the evaporator coils, defrosting will occur too frequently and the heater will consume too much energy. If the evaporator coils have too much frost, the system efficiency will be too low (as shown in Figure 1) when defrosting is finally activated, and will also consume extra electrical power. A reliable and accurate way to sense frost or ice, reducing the number and duration of defrost cycles to the bare minimum, is fundamental to improving system efficiency.
One easily implemented method is to include a clock, which at some pre-selected interval shuts the compressor off and turns on a thermal heating device adjacent to the evaporator coils, melting the frost or ice. The disadvantage of this technique is that frost or ice does not always accumulate at a constant rate, depending on the ambient humidity and air temperature. Defrosting on a regular time cycle, whether necessary or not, is inefficient and wastes electrical power.
Another popular method uses temperature-sensitive devices such as a negative temperature coefficient (NTC) thermistor to measure the temperature of the refrigerating medium as one parameter. Using a timer as another parameter, an algorithm estimates the amount of frost or ice and begins the defrost cycle upon a predetermined temperature rise. But the formation of frost or ice is a function of time and environmental parameters such as temperature, humidity or air flow. Estimating frost or ice is quite complicated and difficult, making the reliability and accuracy of this method low and resulting in energy waste and low efficiency.
Other techniques simply shut the compressor off at predetermined intervals in order to allow the frost or ice to melt. However, this method is also inefficient, because turning off the compressor causes the refrigeration temperature to rise; thus, the compressor has to work harder to maintain the desired temperature.
The most effective method is to directly sense the quantification of frost and ice buildup on the cooling body and initiate countermeasures at the proper time. A controller, operatively coupled to a frost detection sensor (or sensors) and to the defrosting system, selectively activates the defrost system to initiate a defrost cycle of the evaporator in response to a signal that indicates the presence of frost formation on heat-exchange tubes and heat-transfer fins.
Frost or ice detection based on capacitive sensing
Capacitive sensing enables a more reliable solution for measuring the thickness of frost or ice. The Texas Instruments (TI) Capacitive Frost or Ice Detection Reference Design, Resolution of <1mm, Temperature Drive <0.25% detects the amount of frost and ice accumulated on the surface of a cooling body using capacitive-sensing technology. The capacitance of a sensor changes according to the thickness of frost or ice because of the variation of the dielectric constant between air and frost or ice. Let me explain the basic principles.
Capacitance is the ability of a capacitor to store an electrical charge. For a parallel plate capacitor, Equation 1 calculates the capacitance as:
where C is the capacitance related by the stored charge, Q, at a given voltage, V.
The capacitance (measured in farads) of a parallel plate capacitor (see Figure 2) consists of two conductor plates and is calculated using Equation 2:
where A is the area of the two plates (in meters), is the dielectric constant of the material between the plates, is the permittivity of free space (8.85 × 10-12 F/m) and d is the separation between the plates (in meters).
Figure 2: Parallel plate capacitor
Capacitive sensing is a technology based on capacitive coupling. A basic capacitive sensor is anything that is metal or a conductor and detects anything that is conductive or has a dielectric constant different from the air. Capacitance can be detected to sense different applications by changing one of the parameters while keeping the rest constant. Table 1 shows the dielectric constant of some materials.
Table 1. Material dielectric constants
The accumulation of frost and ice on the surface of a cooling body causes the capacitance between two parallel plates (the metal surface of the cooling body and a specifically designed sensor) to change due to the equivalent dielectric constant change, as shown in Figure 3. When a properly designed sensor composed of a conducted material is fixed on the surface of a cooling body, the parameters for d, and A remain constant. The capacitance will be a function of the equivalence of , which is the basic principle of frost and ice detection.
Figure 3: Model for frost and ice detection based on capacitance
Table 1 shows that ice has a dielectric constant three times that of air and an even greater dielectric constant than that of water. When ice and water are positioned between the parallel plate capacitor, the capacitance increases with the thickness of frost growth.
Figure 4 shows an example of the expected capacitance change curve based on a copper mesh sensor. In Stage 1, the capacitance remains a constant value when there is no frost or ice on the dry surface of a cooling body. At the beginning of Stage 2, the refrigerator or air-conditioner compressor starts to work and frost or ice accumulates on the surface of the cooling body. Due to the dielectric constant change from air to ice (see Table 1), the capacitance increases based on the thickness of the frost or ice. The defrost cycle activates when the desired thickness has been detected. The capacitance experiences a sharp change when ice (= 3.2) turns to water (= 80), shown in Stage 3, and then returns back to the original value after the water drops from the cooling body.
Figure 4: Capacitance change in principle
I built a test platform based on a refrigerator. Figure 5 shows the frost formation process on the surface of the evaporator, and Figure 6 shows the capacitance-change curve during one cycle. The six pictures labeled in Figure 5 correspond to the six points, also labeled, in the curve of Figure 6. This system uses capacitance values to calculate the amount of frost.
Figure 5: Frost formation process
Figure 6: Sensed capacitance during the frost formation process
To measure the capacitance, I used TI’s FDC2214, which is a high-resolution capacitive-to-digital converter. The devices employ an innovative narrow-band based architecture to offer high rejection of noise and interferers while providing high resolution at high speed. The test results verify that this is a robust and cost effective solution which could solve the problem of frost or ice detection in refrigeration system and significantly improve the system efficiency.
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