Supermarkets are typically found in strip malls as the anchor store; some are stand-alone buildings. Many supermarkets are chain units with the chain owned by a single company that operates a number of stores, supplying them from central distribution warehouses. They are managed by a centralized professional staff that specifies standardized construction, layout and equipment size and type. It is unlikely that local supermarket management makes any equipment decisions.
There are new competitive trends in progressive supermarkets that repackage concepts for convenience of customers product- and lifestyle-oriented needs and desires. These include:
*Re-emphasizing traditional groceries (dried, frozen and non-food products) where there distribution competency is fully developed.
*Increasing competency in perishables (fruit, produce).
*Adding a prepared foods department that goes beyond the traditional deli.
The energy costs in supermarket operation are high -- about equivalent to the store's profit. Food storage cooling and refrigeration is usually the largest single element since refrigerated display cases operate 24-hours a day.
Air conditioning is required for both customer comfort and proper operation of the refrigerated display cases. Unlike most stores that focus on temperature control alone, supermarkets emphasize dehumidification. Experience indicates the optimum balance is where the relative humidity is maintained between 40 and 45%, a significant variation from the typical retail store that operates in the 50 to 56% range. This range reduces energy use while maintaining product appearance and quality, reduces frost, and keeps case glass surfaces clear.
Humidity control in the supermarket is a key element in the energy consumption of the supermarket. If the humidity is too high, the refrigeration evaporators frost up quickly and anti-sweat heaters on case doors and other surfaces operate longer, increasing refrigeration energy use (as well as the energy used in anti-sweat heaters). If the humidity is too low, food produce wilts, requiring more water spraying and thereby raising the humidity again. While the frost buildup tends to control moisture in the air, it is an expensive way to provide dehumidification. The most efficient humidity control is through the air conditioning system.
The major difference in the HVAC system from variety stores is the refrigerated display case loads and their effect on the HVAC system. Data for calculating loads for people, lights, motors, and other energy-consuming equipment should be obtained from the store personnel, equipment manufacturers, and the ASHRAE Handbooks.
The principal complaints of store personnel and many patrons are cold aisles due to the spill-over of cold air from the refrigerated cases. Ironically, this spill-over can require operation of the heating system even on warm summer days in mild climates. The design of the HVAC system must compensate for the effects of these cold cases and include:
* Increased heating requirement, due to removing heat, even in summer, when only dehumidification is needed.
* Deduct latent and sensible refrigeration effect from normal air conditioning load. Equipment selection should take into account this load reduction and the change in sensible to latent load ratio.
* Provide for temperature and humidity control in open refrigerated case versus other areas.
* Offset heat removed by open display cases with specifically designed air distribution.
These cases increase the heating need and often at a time not normally expected. For example: Assume a desired 75°F store temperature and a heat gain or loss of 15,000 Btuh per degree F difference in outdoor-indoor temperature, and heat removed by the refrigerated displays of 190,000 Btuh of which latent is 19% of the total. This leaves 154,000 Btuh of sensible. This will cool the store 154,000/15,000=10°F; that is this will cool the store 10°F below outdoor temperature summer and winter. Thus, in mild climates, heat must be added to the store to maintain comfort conditions. (Ref: ASHRAE 1995 Applications Handbook 2.3).
An interesting result of the interplay of case coolers, HVAC, and anti-sweat heaters is the actual resulting kW and kWh of a superstore. The traditional load factors (LF) and effective full load hours (EFLH) used to size transformers, predict cost, etc. don't work well. Obtain the utility histories of two or three similar stores from the chain if a new store is being built and get accurate projections on size.
While conventional system designs can be used, they are not designed to produce the optimum supermarket humidity conditions, as outdoor air is mixed with return air and then cooled and dehumidified. When forced to control humidity, their energy performance is usually poor, as they are typically run to cool all the air to a lower temperature to remove moisture. This supply air is then reheated, often using refrigeration waste heat reclaim, to avoid overcooling the store.
To better handle these issues, special equipment is often considered, including:
* Improved single path electric system, where a smaller volume of air is cooled to lower than conventional temperatures (about 40 to 45°F compared to 52 to 56°F), and the remaining air bypasses the cooling coil and is remixed before delivery.
This bypass design also permits a smaller compressor since the lower air flow compensates for the larger temperature drop through the coil. Using bypassed air also lowers reheat energy needs, while the smaller ducts and power wiring reduces first cost.
* Dual-path electric system, uses two cooling coils, one each to separately condition the incoming outdoor and return air. The hot and humid outdoor air and, in some cases, return air is directed to a primary coil for dehumidification, reducing moisture at the source by cooling air to 40 to 45°F. The secondary coil furnishes the sensible cooling of the relatively cool and dry return air as and if needed.
These two air streams are then mixed, reducing the reheat energy needs, and supplied to the supermarket. This system is more energy efficient, however there is a first cost premium since two coils and compressors are used.
* Heat pipe heat exchanger enhancements are available to improve either the improved single-path or dual-path systems. The heat pipe's transfer of heat directly from the entering air to the low-temperature air leaving the cooling coil saves both cooling and heating energy.
A heat pipe is used between the warm air entering the cooling coil and the cool air leaving the coil, transferring sensible heat to the cold exiting air, thereby reducing or even eliminating the reheat needs. Also the heat pipe precools the air before it reaches the cooling coil, lowering the system cooling energy use. The use of the heat pipe adds to the first cost and to the fan power to overcome its resistance, however the equipment size can be reduced in some cases. These factors can offset each other.
* Ice storage system used in conjunction with either the improved single-path or dual-path system. The ice made during off-peak periods is used to provide cooling during peak electric demand periods. This system can be quite economical where the electric utility offers relatively attractive off-peak electric rate schedules. This type of ice storage system is also an area where a utility rebate or incentive can frequently be justified.
* Electric desiccant hybrid system which uses a moisture-attracting desiccant to lower humidity, and regenerates the desiccant with condenser waste heat off the compressor cycle. These systems are used in conjunction with conventional electric air conditioning systems for supermarket use.
The cold, dry air escaping from the refrigerated cases is usually captured by return grilles and ducts under the cases to minimize patron discomfort in the aisles. Since much of the return air is already cool and dry, the airflow rate needed with either the improved single-path or dual-path system is typically about half that typically used in conventionally cooled supermarkets -- only 0.5 cfm per square foot instead of 1.0 cfm per square foot of floor area. This can reduce the costs of the installed ductwork.
Recommendations/Energy Services Opportunities
Existing stores can be retrofitted to one of the more modern systems discussed above. Other opportunities include:
* Older inefficient systems should be investigated for upgrading or replacement, particularly if CFC refrigerants are used.
* Energy conservation concepts discussed above not in use, or antiquated or inappropriate control systems all represent energy service opportunities.
While water heating is not a major energy user with uses typically for hand washing and cleaning purposes, the trend in supermarket design is to incorporate food preparation and food service functions, substantially increasing the usage of hot water. Peak usage is usually associated with cleanup periods, often at night, with a total consumption of 300 to 1000 gallons per day.
Most water heating is done separately from the building heating system using direct resistance or gas heaters, and in some cases, point-of-use heaters.
Recommendations/Energy Services Opportunities
If existing systems are inefficient or inadequate, replace with modern efficient equipment Also add better insulation on storage tanks, or timer controls. In some cases, refrigeration equipment heat reclaim can be used to preheat service hot water. The ASHRAE Applications Handbook Chapter on Service Water Heating publishes typical hot water use data as well as estimating procedures.
The trend in supermarket design is to incorporate food preparation and food service functions. This requires the installation of some cooking equipment, such as ovens, broilers, fryers, griddles, ranges, steam cookers, and warmers; discussed individually in Commercial Cooking.
Supermarkets sell all types of perishable foods that require a variety of refrigeration systems to preserve and attractively display the variety of items. The general purpose of a refrigeration system is to cool and store food, thereby preserving its shelf life.
In addition to various temperature refrigerated display cases, there are refrigerated meat processing rooms and walk-in rooms. These rooms include meat cutting and storage, produce and dairy product coolers, frozen food and ice cream storage. Meat, fish and poultry should be in separate rooms to prevent transfer of odors. Cracked and crushed ice is used to display some products.
The refrigeration equipment groups include refrigerators for display, for storage, for processing; these are coupled to the mechanical refrigeration machines. The refrigeration systems range in temperature from the highest in the meat processing room to the lowest in frozen food storage and display. Open ice cream display cases may have a refrigerant temperature as low as -40°F.
* Prefabricated display cases are designed to attractively merchandise food at temperatures that vary with the product. Fresh meat, poultry, and fish displays should be held as cold as possible without freezing the product (typically between 35 and 37°F with minimum air flow). Cases may be single- or multi-deck open, island type, or glass door reach-in fixtures. They are not intended to cool the product, only maintain its temperature. They are significantly affected by the store's surrounding air temperature, humidity and movement. Nominal design is at 75°F dry bulb and 55% relative humidity. Conditions in excess of this will seriously impair performance; lower humidities result in significant energy savings, i.e. at 35% rather than 55% savings range from 5% for a glass door reach-in to 29% for multi-deck deli cases. Lighting and other radiant heat sources also adversely affect performance, temperature and meat discoloration.
* Frozen food display cases are similar however they are special versions designed to hold product at 0°F or below, and have modified defrost.
* Processing rooms should not be too dry. The faster the processing the less critical temperature and load. Low velocity fan-coil units are typically used to cool these rooms to 45 to 55°F for personnel with a maximum of 10°F difference between room and refrigerant temperature. As this is not low enough for meat storage, product should stay in this room only long enough for cutting and packaging.
* Storage rooms may be adjacent to the processing room and should be kept at 28°F to 32°F for best results. Straight-time or time-initiated, time or temperature terminated hot gas or electric defrosting is generally used.
* Produce and dairy coolers are usually in the 35 to 40°F range with moisture conditions kept in a narrow range to both mold and sliming and dehydration. Low velocity fan-coil units are typically used to cool these rooms with a 10°F difference between room and refrigerant temperature. Defrost is usually off-cycle.
* Icemakers may be self-contained or connected to the central plant, largely dependent on the level of ice use in the supermarket.
* The refrigeration system (high side compressor/condensing units) may span from the simple one-compressor and associated controls on one refrigerator to a complex central plant operating on all refrigerators in the supermarket and multi-plexed systems.
* Supermarkets usually use air-cooled condensing with low-ambient control so that the equipment can operate with the lowest possible head (condensing) pressure as required by expansion valves, hot gas defrosting, and heat reclaim if used. Condensers should be amply sized to reduce energy consumption with a typical 10 to 15°F temperature difference between design outdoor air entering the condenser and saturated condensing temperature. Condensers may be in an air-cooled machine room, remote outdoors or indoors, and used to heat portions of the building in winter.
* Low ambient control using fan cycling commonly is used down to 50°F. Below that condenser flooding arrangements and split condenser coils are typical options.
* Evaporative and water-cooled condensing is less popular due to the problems inherent with water, its treatment and blowdown disposal, and freezing.
Recommendations/Energy Services Opportunities
Refrigeration is often a significant steady use of year-round electricity since this equipment runs even when the building is unoccupied. Therefore, it is usually cost effective to install the most efficient refrigeration practical. Consequently, an energy utility's representatives should be asked to work closely with the energy customer during the early planning stages in order to help understand the options. Be alert to expansion needs and the potential replacement of old inefficient equipment with new, improved units as described above.
In the past refrigerants CFC-12, HCFC-22 and R-502 (an azeotropic mixture of HCFC-22 and CFC-115) were used in supermarket refrigeration. Production of CFCs has ceased and HCFCs are being phased out with a production freeze and no use in new equipment in 2010 and production ceases in 2020. One current replacement for R-22 and R-502 in low and medium temperature refrigeration applications is R-507 (an azeotropic mixture of HFC-125 and HFC-143a). As new refrigerants are developed, work with the supermarket to phase-in replacement and include other steps to improve energy performance.