Refrigeration, Display Cases, Thermal Mass, Grid Services, Peak Demand
Refrigeration systems in supermarkets and convenience stores operate continuously to maintain proper product storage conditions, and these refrigeration systems can account for 40% or more of the electrical energy consumption of a supermarket or convenience store.Â Peak refrigeration system electricity demand typically occurs during the afternoon and summer months, corresponding to when the general demand and price for electricity are the highest.Â In order to reduce peak demand and its associated costs, it may be possible to shift refrigeration system electrical energy use by utilizing the thermal mass of the stored product. Refrigeration system capacities vary from 30-60KW in small store to over 400KW for large stores with annual energy consumption varying from 1-1.5 million kWh. During early morning hours, when electricity demand and price are lower and refrigeration system efficiency is greater, the temperature set point of the refrigeration system can be reduced in order to pre-cool the stored products to below their normal storage temperature.Â Subsequently, refrigeration system operation can be reduced during the mid to late afternoon when electricity demand and price are high, and product temperature may be allowed to drift upwards.Â This operating strategy is particularly feasible with products that are not adversely affected by variations in temperature, such as water and canned or bottled beverages. The key to utilizing the thermal storage is to understand the storage potential and associated time constants of the display cases. To determine the feasibility of reducing peak demand by shifting the refrigeration load to off-peak times, experimental and analytical analyses were performed.Â Simulated product, consisting of one-pint containers filled with a 50% ethylene glycol and 50% water solution, were stored in a medium-temperature vertical open refrigerated display case.Â Product temperature rise as a function of time was determined by turning off the refrigeration to the display case, while product temperature pull-down time was subsequently determined by turning on the refrigeration to the display case.Â It was found that the thermal mass of the product in the display case was such that during a 2.5 hour period with no refrigeration, the average product temperature increased by 10Â°F.Â In addition, it took approximately 3.5 hours for the product to recover to its initial temperature after the refrigeration was turned on. Using transient heat conduction analyses for one-dimensional objects (such as infinite slabs, infinite cylinders, or spheres), cooling or heating times of various objects may be estimated.Â For example, analytical methods predict that heating a cylindrical product by 10Â°F (from 30Â°F to 40Â°F) would take approximately 2.2 hours, which is in good agreement with the experimental results obtained in this study.Â From the analysis, it appears that the thermal mass of the stored product in refrigerated display cases is sufficient to allow product temperatures to safely drift for a significant time under reduced refrigeration system operation.Â Thus, strategies for shifting refrigeration system electrical demand can be developed.Â The use of an advanced refrigeration system controller that can respond to utility signals can enable demand shifting with minimal impact.