Electrical Demand Reduction in Refrigerated Warehouses

Abstract

Electric utilities are currently searching for ways to meet the ever-increasing demand for electricity. One available option involves reducing customer demand during the daytime and during summer months, when demand for electricity peaks. This research investigates one option for reducing the electrical demand of industrial refrigeration systems, which account for a large portion of total electric consumption. Industrial refrigeration systems are primarily used for processing and storage of perishable foods. Refrigerated warehouses may be able to utilize the thermal mass of the stored products to shift their electrical consumption to off-peak times. By pre-cooling the products during the night, the refrigeration equipment could remain idle during on-peak hours without causing significant changes in the warehouse environment. To test this type of operating strategy, a large frozen vegetable processing and warehousing facility in south central Wisconsin was chosen for this research. A literature investigation into the effect of temperature fluctuations on frozen vegetable quality indicated that the expected variations in storage temperature would not cause significant quality losses. The study began with the development of a computer model of the test warehouse. The thermal characteristics of the stored products and the physical attributes of the warehouse structure determine the temperature changes in the warehouse under different weather conditions. The model is used to predict the temperatures inside the ii warehouse during any time of year and using any type of operating strategy. The model was validated experimentally to ensure the accuracy of its results, then used to estimate the annual economic impact of implementing three possible demand shifting strategies. Among the three options studied, full demand shifting offers the greatest savings but requires the largest amount of installed refrigeration capacity. This strategy involves a complete shutdown of the refrigeration equipment during on-peak hours. By idling the refrigeration system during the test facility's 14-hour on-peak period, the equipment must be able to remove 24 hours of heat gain during the 10-hour off-peak period. Modeling indicates that the product temperature will vary between a minimum of ?12.5�F and a maximum of ?2.5�F. Full demand shifting will save approximately $82,000 annually over normal operation at this facility. Other demand shifting strategies either offered minimal savings or required an unreasonably complex control system for implementation. Due to a preexisting lack of refrigeration capacity at the warehouse investigated, no operating strategies were implemented during the course of this project. An interactive spreadsheet, which calculates the economics of each demand shifting option, was created for use by warehouse operators and utility employees in future decision-making.