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Study finds cabinet sensors are wrongly placed

SWEDEN: ​Swedish researchers have found that the incorrect placing of temperature sensors in refrigerated display cabinets with fitted doors is leading to inaccurate control and wasted energy.  

The chance discovery by researchers at Chalmers University of Technology in Gothenburg led to a collaboration with the German supermarket chain Rewe and the repositioning of 6,519 sensors in 454 of its supermarkets. This simple procedure is said to have resulted in emission savings of approximately 18,889 tonnes of CO2eq.

During his doctoral project on supermarkets as virtual batteries in demand response systems, Tommie Månsson at Chalmers University of Technology found that the temperature sensor in a store refrigerator is generally incorrectly placed, leading to over-cooling.   

The report accepts that by introducing doors on refrigerated display cabinets, the energy demand is substantially decreased. However, it found significant discrepancies in temperature readings between visually identical refrigerated display cabinets equipped with doors. By using CFD simulations combined with laboratory experiments it was found that these discrepancies were due to a thermal gradient in the area of the return air temperature sensor. 

Variations

The study, based on 12 refrigerated display cabinets with a total of 61 doors in an operational supermarket in Hamburg, found significant variations in performance. This was found to be the case even among cabinets connected in the same line. For example, one cabinet had a 0.64ºC higher difference between the return air temperature and discharge air temperature than a cabinet connected to it. The difference corresponds to a 32% higher sensible heat extraction rate which the author finds dubious for two connected cabinets of the same brand and design, exposed to the same ambient condition and at times of no interaction.

It was subsequently discovered that there were differences in the placement of the return-air temperature sensors. Some cabinets had sensors placed closer to the front and others further back towards the evaporator. Also, a variation in heights above the cabinet floor was noticed. 

Thermal gradient

A possible explanation for the differences was that the return air temperature sensor was located in the area where a thermal gradient exists. Consequently, the temperature readings depended largely on the position of the sensor. It is thought that a heated air layer is formed along the interior side of the glass doors of the cabinet. This heated air layer then follows the path of the airflow through the return air grille, creating a thermal gradient in the area of the return-air temperature sensor. As a result, a sensor that is placed at a low height above the cabinet floor would falsely indicate higher temperatures due to the warmer air layer.

From the field study investigating the occurrence, it was found that 80.5% of the 221 reviewed refrigerated display cabinets had sensors placed in a zone where a thermal gradient exists.

When the researchers moved the thermometers to a more suitable position in the refrigerators, they noticed that the refrigerators on average consumed about 5% less energy.   

Patent

The discovery resulted in an EU patent for a holder for the thermometer in store refrigerators, which makes the thermometer easy to move and reposition inside the refrigerator, and a corporate spin-off – ChillServices – to launch and implement the innovation in the market. 

The article Exploratory investigation of return air temperature sensor measurement errors in refrigerated display cabinets was published in Springer Journal.

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