7 Rules of Thumb to Estimate Potential Energy Savings in Supermarket Refrigeration Systems
By applying seven easy rules of thumb, you can quickly estimate potential energy savings for refrigeration systems in a supermarket—even if you’re not an engineer.
By Abtar Singh, President Singh360 Inc.
With these rules you can make quick estimates in your head or on the back of an envelope. Your estimates are likely to be pretty accurate, even without a detailed audit. This information can help you spot big opportunities for potential savings. You’ll feel smart, and you may impress your colleagues.
But be careful. These rules of thumb use averages for North American supermarkets. Even within North America, the numbers will vary for stores that have an unusual design or sell an unusual assortment of refrigerated foods.
Rule 1. To estimate average annual power consumption (in kW) for a store, multiply floor square footage in thousands by 6 kW.
Example: For a store of 50,000 square feet, multiply 50 by 6 kW. Average power consumption is about 300 kW.
Rule 2. To estimate average annual energy cost for a store, multiply $900 by the annual power consumption you calculated by applying Rule 1.
Example: For a store of 50,000 square feet, average annual energy cost is about $270,000. (That’s 300 kW times $900 per square foot.)
Note that Rule 2 assumes an average electricity cost of $0.10 per kWh. If your cost is higher or lower, adjust the $900 upward or downward, respectively.
Rule 3. To estimate the power consumption of refrigeration systems in a store, multiply average annual power consumption (from Rule 1) by 0.4. Alternatively, multiply 2.5 times the number of thousands of square feet of store area.
Example: In a store of 50,000 square feet, refrigeration systems consume, on average, about 120 kW (0.4 x 300 kW. Or 2.5 x 50). This estimate includes the energy use of compressors, condenser fans, evaporator fans, defrosters, case lights, and door heaters.
Rule 4: The average power consumption of refrigeration compressors is about 25% of the total power consumption of a store.
Example: For a store of 50,000 square feet, refrigeration compressors consume about 75 kW (0.25 x 300 kW).
Note: Use of this rule can be a little tricky. Stores vary in their mix of low-temperature and medium-temperature racks. Your estimate of total potential savings may be inaccurate unless you measure power consumption separately for each group of racks.
Rule 5: For a medium-temp refrigeration rack, you can save 1.0% to 1.5% of total compressor power by raising suction pressure 1 pound per square inch (psi).
Rule 6: For a low-temp refrigeration rack, you can save 2.0% to 2.5% of total compressor power by raising suction pressure 1 psi.
To estimate total savings for reduced suction pressure in a combination of both low-temp and medium-temp cases, use 1.5% per psi.
Example: If you raise compressor suction pressure by 10 psi, you can cut power consumption by about 15% (1.5% x 10 psi). For a store of 50,000 square feet, you can save about 11.25 kW (15% of 75 kW from Rule 4).
Rule 7: In both low-temp and high-temp racks, you can save 0.5% of total compressor power by lowering discharge pressure 1 psi.
Example: If you reduce compressor discharge pressure by 20 psi, you can cut power consumption by about 10% (0.5% x 20 psi). For a store of 50,000 square feet, the savings are about 7.5 kW (10% of 75 kW from Rule 4).
By applying these rules of thumb, you should be able to easily identify some of your bigger opportunities to cut the energy cost of refrigeration racks.
Please share your comments. We also welcome your questions about how and when to use these rules in estimating potential savings.
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