[ << Previous | Contents | Next >> ]

Commercial refrigeration is a broad equipment category that includes:

• Central refrigeration systems for supermarkets. These systems consist of a central refrigeration system located in a mechanical equipment room, connected to the variety of refrigerated and frozen food display cases that are used in typical supermarkets.
• A wide variety of self-contained refrigerated and frozen food display cases, reach-in refrigerators and freezers, beverage merchandisers and vending machines, and other specialized configurations for large and small retailers and for food service establishments.
• Walk in refrigerators and freezers
• Ice machines

While the category is broad, the essentials can be represented by two commonplace configurations:

• Supermarket central refrigeration systems based on several sets of rack-mounted compressors, connected via long runs of liquid and suction vapor lines to the refrigerated/frozen food display cases in the store. The centralized supermarket system is a unique configuration and is treated in detail in this section
• At the risk of oversimplifying, the various self-contained products are similar to domestic refrigerators and are not treated separately in this study. It should be recognized, however, that significant differences between domestic refrigerators and the self-contained commercial counterpart include the range of refrigerated storage volumes, refrigeration capacities and pull-down characteristics, the range of refrigerants used, and applicable energy efficiency regulations, as discussed further in 8.5.

Focusing on central supermarket systems, the prototypical system includes various refrigerated and frozen food display cases, connected to a central refrigeration system, typically located in a mechanical equipment room or a rooftop enclosure. The typical direct expansion (DX) central refrigeration system consists of several sets of rack mounted compressors that independently serve a portion of the refrigeration load in the store. Often there are two racks for medium temperature, fresh food loads and two racks for low temperature, frozen food loads, but the exact configuration varies depending on the store size and other factors. Traditionally, the long runs of liquid and suction vapor lines connecting the display cases with the central compressor system in the DX configuration have been a source of refrigerant leaks, due to the large number of tubing joints and the significant movement caused by thermal expansion during hot gas defrosts. Pre-Montreal Protocol, CFC-12, CFC-502, and HCFC-22 were the refrigerants used; as the CFC phase-out date has passed, a complicated transitional regime of refrigerants is in use that still includes CFC-12 and CFC-502 from some existing equipment, along with HCFC-22, HCFC-22 based blends, and HFC blends that replace CFC-502, including R404A and R507. In the post-ODS phase-out context of this study, the relevant baseline refrigerants are the HFC blends, most commonly R404A or R507. Alternate refrigerants include ammonia and carbon dioxide (there is very little use of CO2 in this application on a commercial basis), although modified system configurations are needed:

• Due to significant safety concerns, ammonia and hydrocarbons would only be used in a centralized system located in an equipment room with appropriate safety features, with the refrigeration capacity delivered by a secondary heat transport fluid. Ammonia with a secondary loop is considered in this study.
• Carbon dioxide potentially could be used in a direct expansion configuration, but the cycle would be a transcritical cycle. To obtain reasonable efficiencies, evaporatively cooled condensers would probably be needed, along with mechanical subcooling. This option has not been investigated extensively for commercial refrigeration, nor have the safety issues associated with handling a large charge of high-pressure CO2. Due to the inherently low efficiency of this cycle, it has not been considered further in this study.
• Alternate, vapor cycle based configurations include:
  • Water-cooled distributed systems. In these systems smaller refrigeration units are distributed among the refrigerated and frozen food display cases. Each unit rejects heat to a central water cooling system
  • Secondary loop systems, where a secondary coolant or brine is circulated from the central refrigeration system to the display cases

A major part of the rationale for the two alternate vapor cycle systems is to significantly reduce the refrigerant inventory, and to minimize the length of refrigerant tubing and number of fittings that are installed in the field.

While not subject to efficiency regulation, common practice in supermarket system design has been to design for high efficiency. This is an economically driven practice, owing to the high duty cycle of the equipment and the fact that supermarket energy costs are comparable to bottom line profits.

Recent developments in the rapidly evolving food retailing business have obsoleted many of the underlying assumptions of the TEWI-3 study. This subsection provides an updated estimate of typical energy use, based on input supplied by Hussmann [Thomas, 1999]. The most significant change, that began around 1990, is the increase in the size of the average supermarket that is being built. From the average size of 25,000 square feet for existing supermarkets cited in the TEWI-3 study, the average size of newly constructed supermarkets today is approaching 60,000 square feet. Given the 8-10 year remodeling/renewal cycle of the industry, this will be the average store by the mid to later part of this decade. Based on an average of six recent Hussmann installations throughout the U.S., a "typical" U.S. supermarket and its refrigeration system can be characterized by the following assumptions [Thomas, 1999]:

• Average store size is 60,000 square feet
• Average design load for low temperature is 330,000 Btu/hr, average 80 horsepower
• Average design load for medium temperature is 1,150,000 Btu/hr, average 175 horsepower
• R-404A and R-507 are the predominant refrigerants for both low and medium temperature. However, HCFC-22 is broadly used in medium temperature, as well as some low temperature systems.
• Average connected electric load is 440 kW, 55%-57% of which, or 245 kW is used to operate refrigeration equipment
• 1.2 million kWh/year is consumed by refrigeration equipment (direct expansion equipment 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, the efficiency losses due to the heat transfer DT 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 DT 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.

On the basis of the preceding, 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
• 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.

Refrigerant emissions are a more significant contribution to the LCCP in conventional DX systems in supermarkets than in smaller, factory assembled self-contained equipment, so the assumed charge sizes and emission rates have a significant impact on the calculated LCCP.

The GWP and manufacturing impact for the refrigerants of interest are summarized in Table 8-1.

Table 8-1: GWP and Manufacturing

LCCP estimates for U.S. supermarkets have been prepared based on the energy consumption discussed in 8.2 and the following assumptions about refrigerant charge size and emissions [Thomas, 1999].

• With current practice, the typical refrigerant charge of a DX system in pounds is 6% of the floor area in square feet --- 3,600 lb. for a 60,000 square foot store
  • Secondary loop refrigerant charges are 11% of DX
  • Distributed system charges are 25% of DX

• Refrigerant loss rates, in percent of charge loss per year, are:
  • 15% for DX in an optimum installation
  • <5% for distributed , 4% is assumed
  • 2% for secondary loop systems

Historically, leakage rates of 30% have been cited for DX systems, but over the past decade considerable effort has gone into improving this. The most recent draft of the UNEP TOC reports indicates that leakage rates of 10-15% are economically attainable and cites instances of leakage rates having been reduced to 5%.

100% EOL refrigerant recovery is assumed and an average system life of 15 years is assumed. Table 8-2 summarizes the resulting LCCP for four configurations:

• DX with R404A or R507 refrigerant
• Secondary loop with R404A or R507 refrigerant
• Secondary loop with ammonia refrigerant
• Distributed system with R404A or R507 refrigerant

Table 8-2: LCCP for Refrigeration Alternatives in Typical 60,000 sq. ft. U.S. Supermarket Constructed in 1999

For direct expansion and distributed systems, that place the refrigerant charge throughout the store, the amount of refrigerant charge that could potentially be released into the store is large and the use of flammable or high toxicity refrigerants is not feasible. Store operators in the U.S. and some other countries will not accept the safety and legal risks and safety codes prohibit such large quantities of flammable refrigerant to be used in a publicly occupied space.

With secondary loop systems, potentially hazardous refrigerants such as ammonia and hydrocarbons could be used, but additional costs of safety precautions will be incurred.

A wide-range of self-contained commercial refrigeration equpment is manufactured, ranging from beverage vending machines, beverage merchandisers, beer coolers, undercounter refrigerators, refrigerators for commercial food service, ice machines, and drinking water coolers, to name a few. Most of this equipment uses welded hermetic compressors (some semi-hermetics are used as well) and brazed joints to connect the refrigerant tubing and components in the refrigeration system. Foam insulation is used in most of this equipment. From a global warming perspective, this equipment is similar to domestic refrigerators. Refrigerant emissions are minimal because of the welded hermetic construction of the systems and foam blowing agent stays largely contained in the foam. Warming impacts are predominately caused by the energy consumption of the equipment, which is considerably higher than for domestic refrigerators due to higher duty cycles. (e.g., frequent door openings, large beverage cool down loads, ice maker throughput). The major opportunity for reduced warming impact is improved energy efficiency.

An important distinction between domestic refrigeration equipment and commercial refrigeration equipment is the typical refrigerant charge size. While domestic refrigerators typically have fluorcarbon refrigerant charges under 150 grams, self-contained commercial refrigerator charges range from 300 grams to several kilograMs. Consequently, a parallel cannot be drawn between self-contained commercial refrigerators and the adoption of isobutane as a refrigerant for domestic refrigerators in Europe (with isobutane charges of 50 grams or less). The potential safety hazard, based on charge size alone, is significantly greater. Moreover, many of the design measures used to ensure the safety of hydrocarbon based refrigerators --- cold wall evaporators, location of all electricals outside the refrigerated space --- cannot be applied readily to most types of self-contained commercial refrigerators.