U.S. supermarkets strive for efficiency
Recent developments in the rapidly evolving food retailing business have made obsolete many of the underlying assumptions of the TEWI-3 study (“Energy and global warming impacts of HFC refrigerants and emerging technologies,” published in 1997).
The most significant change (which began about 10 years ago) is the increase in the size of the average supermarket that is being built, according to a new report, “Global comparative analysis of HFC and alternative technologies for refrigeration, air conditioning, foam, solvent, aerosol propellant, and fire protection applications,” prepared by Arthur D. Little, Inc.
From the average size of 25,000 sq ft for existing supermarkets cited in the TEWI-3 study, the average size of newly constructed supermarkets today is approaching 60,000 sq ft. Given the eight- to 10-year remodeling-renewal cycle of the industry, this will be the average store by the middle to late part of the next decade.
Based on an average of six recent Hussmann installations throughout the United States, a “typical” U.S. supermarket and its refrigeration system can be characterized by the following assumptions.
- Average store size is 60,000 sq ft.
- Average design load for low temperature is 330,000 Btu/hr, average 80 hp.
- Average design load for medium temperature is 1,150,000 Btu/hr, average 175 hp.
- R-404A and -507 are the predominant refrigerants for both low- and medium-temperature units.
- 1.2 million kWh/year is consumed by refrigeration equipment (direct expansion with an up-to-date design).
- The average duty cycle of the refrigeration compressors is 85% for low temperature and 55% for medium temperature.
- Compared to state-of-the-art direct expansion, the energy consumption of alternative systems is:
— Comparable or less for distributed systems; in essence, efficiency losses due to the heat transfer and pumping power of the heat-rejection loop are comparable in magnitude to the efficiency losses in the DX configuration due to low-side pressure drops (the long runs of suction line and EPR valves) and to suction line heat gain (i.e., non-useful superheat).
— The efficiency of secondary-loop systems is about 10% less than for DX systems due to the heat transfer and pumping power in the secondary loop, while being subject to heat gains in the secondary loop piping that are comparable to DX system suction line heat gains.
Based on this, representative refrigeration energy consumption in a typical, newly constructed supermarket is:
- For DX systems — 1.2 million kWh/year;
- For secondary-loop systems — 1.4 million kWh/year; and
- For distributed systems — 1.1 million kWh/year.
This is a representative level of energy consumption for comparing these alternatives as applied in a particular store. Obviously, many variables influence actual energy consumption.