Cooling Supermarkets Efficiently

Supermarkets are one of the most energyintensive types of commercial buildings. Significant electrical energy is used to maintain chilled and frozen food in both product display cases and walk-in storage coolers. Fig. 1 shows a representative layout for a supermarket showing refrigerated display cases and storage areas located generally around the store perimeter.

Typical supermarkets with approximately 2,000 to 11,000 square metres of sales area consume roughly 2 million kWh annually, and roughly half of it is for refrigeration.

Compressors and condensers account for 60 to 70% of refrigeration energy consumption. The remainder is consumed by the display and storage cooler fans, display case lighting, evaporator defrosting, and for anti-sweat heaters used to prevent condensate from forming on doors and outside surfaces of display cases.

The refrigeration systems also produce a large amount of rejected heat that can be recovered and used by heat pumps or other equipment to provide space and water heating for store requirements.

Thus, improvement in energy efficiency of supermarket refrigeration will affect the store’s bottom line of profit margin.

Conventional supermarket refrigeration system

The most commonly used refrigeration system for supermarkets today is the multiplex direct expansion system.

Fig. 2 shows the major elements of a multiplex refrigeration system. All display cases and cold store rooms use direct expansion air-refrigerant coils that are connected to the system compressors mounted on a skid, or rack in a remote machine room located in the back or on the roof of the store. Heat rejection is usually done with air-cooled condensers because these are the least cost to install and maintain.

The hot refrigerant gas from the compressors is cooled and condensed as it flows into the condenser. The liquid refrigerant is collected in the receiver and distributed to the cases and coolers by the liquid manifold.

The refrigerant is expanded turning a fraction of liquid into vapour before flowing into the evaporator. After cycling through the cases, the refrigerant returns to the suction manifold and the compressors.

Supermarkets tend to have one direct expansion system for ‘lowtemperature’ refrigeration (e.g., ice cream, frozen foods and so on) and one or two direct expansion systems for ‘medium-temperature’ refrigeration (e.g., meat, prepared foods, dairy, refrigerated drinks, etc.).

The large amount of piping and pipe joints required can result in large refrigerant losses – historically 30% or more of the total charge annually.

This system has historically been designed for ease and rapidity of service rather than low leakage. However, this practice is changing for new supermarkets with more emphasis on reducing leakage.

Advanced supermarket refrigeration systems

Conventional supermarket refrigeration systems require very large refrigerant charges for their operation, and several new approaches, such as distributed, secondary loop, low-charge multiplex, and advanced selfcontained refrigeration systems, are available that utilise significantly less refrigerant resulting in reductions of annual energy consumption and total equivalent warming impact (TEWI).

Distributed compressor systems

Fig. 3 illustrates typical system components. Unlike traditional direct expansion refrigeration systems, which have a central refrigeration room containing multiple compressor racks, distributed systems use multiple smaller rooftop units that connect to cases and coolers, using considerably less piping.

The compressors in a distributed system are located near the display cases they serve, for instance, on the roof above the cases, behind a nearby wall, or even on top of or next to the case in the sales area. Thus, distributed systems typically use a smaller refrigerant charge than direct expansion systems and hence have decreased total emissions.

The refrigerant is compressed in multiple parallel compressors and the superheated refrigerant gas is cooled and condensed in a watercooled condenser. The refrigerant is then expanded before entering the evaporator. It absorbs heat from the cooled products before returning to the compressors as a vapour.

The water that is heated by the condensing refrigerant in the condenser is sent to an evaporative cooler. It is cooled and pumped back to the condenser to repeat the cycle. To enable the cabinets to be located in or near the sales area, this refrigeration system employs scroll compressors because of their very low noise and vibration levels. Scroll compressors have no valves and in general are not quite as efficient as reciprocating units for refrigeration applications.

However, this efficiency disadvantage is offset by the fact that the no valve feature allows them to operate at lower condensing temperatures. The refrigerant charge required for a distributed refrigeration system is on the order of 30% or 50% of that required for a conventional air-cooled multiplex system depending upon whether water- or air-cooled condensing is employed, respectively.

Secondary loop systems

Secondary loop systems use a much smaller refrigerant charge than traditional direct expansion refrigeration systems, and hence have significantly decreased total refrigerant emissions.

In secondary loop systems, two liquids are used. The first is a cold fluid, often a brine solution, which is pumped throughout the store to remove heat from the display equipment. The second is a refrigerant used to cool the cold fluid that travels around the equipment. Secondary loop systems can operate with two to four separate loops and chiller systems depending on the temperatures needed for the display cases.

As shown in Fig. 4, the refrigerant is compressed in parallel compressors and the superheated refrigerant gas is cooled and condensed in a remote condenser.

The liquid refrigerant is then collected in the receiver, expanded in a throttling device, and evaporated by absorbing heat from a cold fluid (i.e., brine). The cooled brine is distributed in the sales area (refrigerated area) absorbing heat from the products before returning to the evaporator to repeat the process.

These systems employ chillers located in a remote machine room to refrigerate a brine solution that is pumped to each display case or cold store room.

Lowest energy consumption for secondary loop systems is achieved when the display case heat exchangers are designed specifically for the use of brine, so that the temperature difference between the brine and air is minimised.

Brine selection is also of importance, because energy consumption for pumping is a large component of overall energy consumption. The use of brines with high heat capacity and low viscosity at low temperature is desirable. The number of brine loops employed will also impact energy consumption. Typically, 2 loop temperatures are used, such as -30 and -7OC. If significant portions of the refrigeration load can be addressed by higher temperature loops, energy savings can be obtained.

Each loop requires its own controls and means of maintaining the loop temperature (separate chiller etc.). The central chiller systems are constructed similarly to multiplex racks, using multiple parallel compressors for capacity control.

Use of high-efficiency compressors is highly desirable to help offset some of the added energy consumption associated with brine pumping. Because of the location of the evaporator on the chiller skid, the compressors for the secondary loop system are considered closecoupled to the evaporator.

The pressure drop and return gas heat gain are minimised in this configuration. Both these factors help in reducing compressor energy consumption. These chiller systems can also be equipped with hot brine defrost where brine is heated by subcooling of the chiller refrigerant. Heat rejection can be accomplished with air-, water-, or evaporatively cooled condensers. Like distributed refrigeration, the use of evaporative heat rejection is recommended to reduce energy consumption. The system refrigerant charge will be on the order of 15 to 25% of that required for conventional air-cooled multiplex systems.

Low-charge multiplex systems

Low charge multiplex system (Fig. 5 illustrates typical system components) consists of control systems for condensers that limit the amount of refrigerant charge needed for the operation of multiplex refrigeration. The refrigerant liquid charge is limited to that needed to supply all display case evaporators.

This control technique can reduce the charge needed by the refrigeration system by 1/3 or more. Some liquid charge control approaches can also provide some energy-saving potential by enabling the compressors to operate at lower condensing temperatures. The minimum condensing temperature values suggested are 4.5 & 15OC for low and medium temperature refrigeration, respectively.

Self-contained refrigeration systems

In this type, each display case and storage room has its own compressor and condenser (usually water-cooled). A glycol loop is used to reject heat from the individual refrigeration units to the exterior of the store. For energy-efficient operation of a self-contained system, capacity control of the compressor is needed. The advanced selfcontained system consists of display cases or storage coolers each having their own compressor and water-cooled condenser with warm water pumped to the rooftop fluid cooler for heat rejection.

The self-contained system has advantages in extremely low refrigerant change (one tenth of rack systems), easy and low-cost installation, flexibility in time to order and remodeling. However, the self-contained system has some inherent disadvantages including high equipment cost and low efficiency due to heat transfer penalty of watercooled condensing.

Cascade refrigeration system

The cascade refrigeration system is shown in Fig. 6. Subcritical CO2 cycle is used in low temperature circuit. Due to the potentially high system pressure of the carbon dioxide loop during stand still, special precautions have to be taken to avoid excessive pressures.

In a typical supermarket system, which operates 24 hours a day, seven days a week, CO2 is simply blown off to the atmosphere, if pressures exceed the maximum allowable pressure during stand still. The low temperature loop is built as a cascade system to the Medium Temperature (MT) system. Employing a propylene glycol system for the MT cooling has a few advantages _ as compared to direct expansion systems.

Transcritical CO2 multiplex system

Fig. 7 shows the various components of transcritical CO2 multiplex refrigeration system, which uses two-stage compression and evaporations. Due to negative environmental effects of man-made refrigerants, natural refrigerants have been emerged as valuable alternatives.

CO2 is a strong choice in LT circuit for cascade system. Due to toxicity of ammonia and flammability of propane and isobutene, XP10 is also a good choice in both multi-evaporator and cascade systems due to its comparatively lower GWP (Table 1).

Conclusions and future options

Distributed compressor system using CO2 as working fluid in transcritical cycle or in secondary loop /cascade systems is the best layout for supermarket refrigeration _ in term of both minimum energy consumption and environmental effect (TEWI). However the various further improvement possibilities are there, such as use of microchannel heat exchanger, heat recovery and renewable energy sources. Use of distributed system may yield significant power saving and TEWI reduction compared to the baseline multiplex system.