Chill out! Frost sensor improves refrigerator and air-conditioner efficiency

Michael Seidl

Refrigeration makes everything from desert cities to medical miracles possible. But it also requires a lot of energy, both to chill the interior air and to remove frost buildup on cooling fins.

“Air conditioners and refrigerators are among the most energy-hungry items we have in our households,” said Michael Seidl, sector manager for appliances at TI.

Help is here.

Pairing the TI FDC2214 capacitance-to-digital sensor circuit with small copper grids on the cooling fins or evaporator coils dramatically reduces the energy wasted on defrosting cycles. It’s the world’s first proven technology that can directly measure the amount of frost on an evaporator coil. A reference design is now available that helps manufacturers reduce wasted heat and energy on unneeded or excessive defrosting cycles.

Frost harms efficient operation by making it harder for the evaporating coil and cooling fins to do their jobs. Just 7 millimeters of buildup can reduce evaporator efficiency by 40 percent. Although most appliances come with a built-in heating device that melts frost and restores normal operation, running the heater requires even more energy. Defrost cycles that shut down the compressor don’t expend more energy in the short run, but let the chilled air temperature rise.

An electric eye on frost

Until the FDC2214-based reference design, manufacturers had no reliable way to measure the presence of frost and had little to go on but guesswork. The most common solution uses a simple timer and defrosts every day or two. Others measure the temperature of the coils or count every opening of the refrigerator door. These methods are imprecise and usually result in the coils being defrosted too often.

“All of the existing approaches using timers or measuring temperature or humidity are indirect – they’re not measuring the thing they want to control,” Michael said. “We put ourselves in our customers’ shoes. What they really want to measure is whether there is frost or not.”

The reference design uses the extreme sensitivity of the FDC2214, which can detect a layer of frost less than 1 millimeter thick. The sensor is wired to a perforated copper mesh placed over the evaporator coil or fin. When frost collects, it slightly changes the capacitance of the copper. It’s small but measurable, and increases as the ice gets thicker. And although the sensor is also capable of detecting changes in capacitance from a stray droplet of water or the brush of skin, those fluctuations are much smaller. This means that a foreign object can’t accidentally trigger the defrost cycle.

Serious energy savings

To test the concept, our company purchased refrigerators from major appliance makers and replaced the defrosting timer circuit with the FDC2214-based solution. The results are already impressive, and there’s still room to grow.

“For a typical refrigerator in a typical environment, we see a 20 percent improvement in defrosting energy efficiency,” said Richard Wang, who created the reference design. “However, the savings are even higher when the difference between the exterior temperature and the desired interior temperature grows larger.”

An accurate frost sensor will help manufacturers design smarter appliances that defrost only when they need to. The way frost builds up on an appliance depends not just on the design of a refrigerator or air conditioner but on the internal temperature, room temperature and ambient humidity. Appliances in warmer or wetter environments need more frequent defrosting, but a factory-set timer has no way to account for significant differences in room conditions.

Our company’s solution addresses two significant challenges all cooling appliance manufacturers face: knowing when to start defrosting and knowing when to stop. The capacitance sensor is always working and knows when the last of the frost is gone, ending the defrost cycle. Defrosting occurs only when needed, eliminating wasted energy from unnecessary cycles.

“With this reference design from the TI Designs reference design library, manufacturers can shorten a typical refrigerator defrost cycle from 20 or 30 minutes to 10 minutes,” Richard said.

Fewer and shorter defrost cycles save energy and cut down on unwanted noise. This reduces operating cost and environmental impact, making the appliances cheaper and more attractive to own and operate.

“Our FDC2214-based defrost sensor is a good solution for our planet because it can reduce the carbon footprint of these appliances,” Michael said. “Refrigeration has been a mainstream technology for a century, and now our customers will be able to make some big advances in a short period of time.”