These stores often have large frontal glass areas, which could result in high peak solar effects (except for north facing stores). High heat loss can also occur on cold, cloudy days. This part of the store should be designed to offset these higher cooling and heating requirements. Entrance heaters may also be used in cold climates.
Air conditioning is required for both comfort and proper operation of refrigerated display cases - usually located along an entire wall of the store. In fact, the major difference in the HVAC system from retail stores is the refrigerated display case loads and their effect on the HVAC system. Most are glass-door reach in type that do not affect the HVAC as much as open cases. Data for calculating loads for people, lights, motors, and other energy-consuming equipment should be obtained from the store staff, equipment manufacturers, and the ASHRAE Handbooks. The design of the HVAC system must compensate for the effects of these cold cases.
Many new convenience stores/service stations are using the geothermal heat pump (GHP) system, which can thermally connect space and water heating and cooling and refrigeration operations. It efficiently integrates the HVAC system with the ice maker, coolers and freezers. A closed loop ground heat exchanger located around the perimeter or under parking areas — away from the underground fuel storage tanks — can take advantage of the earth'span> relatively constant temperature year-round to provide most of the energy for summer cooling and winter heating. In addition, the display coolers, freezers, the walk-in cooler and ice maker — all of which reject a substantial amount of waste heat — can be tied into the system and contribute to a very high overall efficiency. In cold climates, the rejected heat can be used to preheat service hot water and water for a car-wash or a snow-melting system, and radiant heating in the car wash floor. Not only is maintenance lower but there is no exposed outdoor equipment to deteriorate due to weather or to be vandalized and the system allows design flexibility.
Alternatively, single-zone unitary rooftop equipment with prefabricated and matching curbs simplifies installations common in small convenience store/service station air conditioning. The use of multiple units to condition larger stores involves less ductwork and can maintain comfort in the event of partial equipment failure. These HVAC units may be either heat pumps or gas heating/electric cooling packages. This equipment is air-cooled and has a low first cost and ease of operation, which readily adapts them to small-store applications.
Water-cooled cooling equipment is available for small-store air conditioning, but many communities have restrictions on the once-through use of city water and require the installation of a cooling tower or other water-conserving equipment. Water-cooled equipment generally operates more efficiently and economically, but higher first cost with a tower and higher maintenance limits its application.
Retail facilities often have a high sensible heat gain relative to the total heat gain and equipment should provide the necessary sensible heat removal. The external static pressures available in small-store air-conditioning units are limited, and duct systems should be designed to keep duct resistances low. Duct velocities should not exceed 1200 fpm and pressure drops should not exceed 0.10 in. of water per 100 ft. Average air quantities range from 350 to 450 cfm per ton of cooling in accordance with the calculated internal sensible heat load.
Energy Saving Recommendations
Older inefficient systems should be investigated for upgrading or replacement, particularly if CFC refrigerants are used.
Programmable thermostats with night set back should be used on the "7 to 11" type stores.
Reflective film and/or some type of window treatment should be considered for the large glass area.
Energy conservation concepts 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 some convenience store/service station design is to incorporate a car wash, food preparation and food service functions; substantially increasing the usage of hot water.
Except when a geothermal heat pump system is used, 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. These systems can also be used to augment the GHP system.
Energy Saving Recommendations
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.
If the convenience store 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.
The interior lighting of the store will usually be T-12 open bottom fluorescent with magnetic ballast. The exterior lighting will usually be metal halide especially under the filling area canopy.
The larger gas station/convenience stores may even set their peak electrical demands at night. Almost all convenience stores are high hour operations running seven days per week. Most are open at least from 6:00 or 7:00 to 11:00 or 12:00, but many are open 24 hours a day.
Energy Saving Recommendations
All T-12 fluorescents should be converted to T-8 technology with electronic ballast. Reflectors should be added to the lighting system if they do not have them or if the existing reflectors are rusted and dull. Time clocks on the lighting system can only be justified in the "7 to 11" and if there is a history of the lights being left on at closing.
Investigate the load profile if the exterior lighting levels are high to determine if the store has an off-peak load. Time of use or off-peak rates may help with economic justification of measures needed at the location.
Convenience stores/service stations sell cold beverages and various perishable foods that require several temperature level 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. Typically there are display cases at 36 to 40°F for refrigerated product and at 0°F for frozen foods. In addition to refrigerated display cases, the larger stores have freezers and refrigerated walk-in coolers to store dairy and other perishable products, frozen food and ice cream.
The refrigeration equipment groups include icemakers, plus refrigerators for display and for storage; these are coupled to the mechanical refrigeration machines. The refrigeration systems range in temperature from the highest in the beverage cases (36 to 40°F) to the lowest in frozen food storage and display (0°F).
Prefabricated display cases are designed to attractively merchandise food at temperatures that vary with the product. They may be single- or multi-deck open, however most are glass door reach-in fixtures. They are not intended to cool the product, only maintain its temperature. The store's surrounding air temperature, humidity and movement significantly affect them.
Ice Makers may be self-contained or connected to the central plant, largely dependent on the level of ice use (if any) in the store.
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 store and multiplexed systems. Smaller stores may use roof-mounted condensing units.
Convenience stores 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.
Energy Saving Recommendations
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.
Periodic inspection of refrigeration gaskets and door locks should be conducted regularly. Mildew and moisture are good visual signs that a gasket needs to be replaced. Defrost heater timers should also be checked during an inspection.
In the past refrigerants CFC-12, HCFC-22 and R-502 (an azeotropic mixture of HCFC-22 and CFC-115) were used in convenience store 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 convenience store to phase-in replacement and include other steps to improve energy performance.