e-News #83: Supermarket Refrigeration

December 21, 2011
 
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Supermarket Refrigeration: Efficiency Features & Modeling

On a square foot basis, the food store category, which includes supermarkets, grocery stores and convenience stores, has the second highest energy consumption of major building types in California, with electricity representing over 80% of this. Supermarkets are open for long hours, and the energy-intensive refrigeration system must operate continuously in any case.

This newsletter will focus on the refrigeration system as it consumes the greatest fraction of supermarket electricity use, with lighting and HVAC each about one-third as large. Also mentioned briefly are applicable utility incentives, proposed refrigeration requirements in Title 24, and the analysis of efficiency measures in energy modeling software.

Supermarket refrigeration systems and how they consume energy

Numerous components consume energy in a refrigeration system. To keep product cases cool, fans move air across an evaporator coil which causes the refrigerant inside it to boil, absorbing heat from the air circulating in the case. Heat is introduced to the cases from the warmer store environment, lights, defrost and anti-sweat heaters, and the fans themselves. The greater these heat gains, the greater the amount of work the entire system must do. In addition, three of four of these heat gains are caused by components which consume electricity themselves. This refrigerant is piped to compressor(s) which raise its pressure and temperature (consuming electricity). The hot gas from the compressor is piped to an outdoor condenser coil where a fan (consuming electricity) moves outdoor air across coil, cooling the refrigerant back to liquid so it can return to the cases to repeat this cycle. A typical supermarket will have many cases connected to a refrigerant "circuit" and each circuit will usually have multiple compressors. Three to six circuits will serve each market, each dedicated to different temperature cases or areas of the store.

Figure 1: Energy Common Large Supermarket Refrigeration Diagram
Large Supermarket Refrigeration Diagram
Source: Supermarket Refrigeration Codes and Standards Enhancement Initiative (CASE), Pacific Gas and Electric Company, March 17, 2011 pg. 71

Alternative refrigeration systems

Recently some variations on this design have emerged in response to concerns about the global warming contribution of the inevitable leakage of conventional refrigerants. These involve the use of CO2 or glycol "brines" in place of typical refrigerants in some components of the systems. As these systems are still rare, they are not detailed here. Recent analyses have shown that energy use of these variations range is equivalent or somewhat greater than conventional systems, but have savings in greenhouse gas emission (due to reduced refrigerant leakage) that more than offset the GGHG emissions associated with increased energy use. Many of the applicable EEMs for typical systems also apply to these variations. (E.g. reducing case heat gain will reduce energy use in any system.)

Energy Saving Opportunities

Given the interacting energy consuming components in supermarkets, many opportunities exist for both energy waste and savings.

Store Lighting and HVAC

The ambient store lighting is not covered in this E-Newsletter, as the applicable EEMs are generally the same as those used in other large retailers. For restaurant operations within supermarkets, the EEMs are similar to those for any restaurant operation.) Special HVAC systems are often used to dehumidify air for supermarkets, but simulation analysis has shown limited savings for conventional methods of providing this extra dehumidification in California.

Refrigerated Cases

The entire refrigeration system must be designed to remove heat that enters or is generated within the cases, so this is the first place to look for energy savings. Federal standards for new supermarket refrigerated cases will take effect in 2012, and some features which have been optional will become standard to meet the new standard.

Doors and Open Cases: Many case types never have doors, some may have doors, and others always have doors. On average, where the choice exists, choosing a case with doors will save energy, but care must be taken to minimize lighting and anti-sweat heater energy consumption in cases with doors. The thermal gains through doors can be reduced by utilizing better glazing and frames. Condensation on the inside of these doors may increase, but anti-fog films are available to minimize this. Night curtains will effectively reduce the loads on open cases during unoccupied periods, if any. Door gaskets deteriorate with time and should be replaced when this is apparent.

Anti-Sweat Heaters: In CA, these heaters, which prevent exterior condensation on doors and cases, should be needed only occasionally. Controls to make sure these heaters stay off as long as possible are typically installed but may not be commissioned or maintained properly. Some newer case doors are designed with low-conduction glass and frames, eliminating the need for these heaters, however the cases themselves will still typically have ASHs, and the store will still need to control these ASHs.

Case Lighting: Standard lighting in cases has been florescent, but both LED lighting and fiber-optic lighting systems are now available with comparatively less energy consumption. The fiber-optic lighting appears to reduce overall energy use to the greatest degree, in part because more of its heat is left outside the case, where it often beneficially warms the store, instead of adding to case heat gains. Any lighting can be switched off by a timeclock. LED lighting can be controlled by a motion sensor and be turned off when the area in front of a case is vacant, and this may be required in non-24 hr stores under the 2013 Title 24.

luminance.jpg
Figure 2 - Luminance Image Comparing the Same Case Lighted by Different Sources
Source: SCE Design & Engineering Services

Fans, Coils and Defrost: For cases with similar designs, lower design power for evaporators will generally minimize energy consumption. Exact determination of fan power for cases is hindered by lack of consistent ratings. EC ("Electronically Commutated") motors are readily available and are significantly more efficient than older PSC ("Permanent Split Capacitor") or shaded-pole motors. These motors can be retrofit to existing case fans and can be used with variable speed controls in walk-ins. Floating suction pressure control can raise the temperature of refrigerant in the coils when the load is light, saving compressor energy. This requires the use of electronic expansion controls on the case evaporators, connected to a computer control system. This has become relatively common in new stores and may be required in non-24 hr stores under the 2013 Title 24.

Compressors

Centralized supermarket systems are typically served by reciprocating compressors, with smaller distributed systems often served by scroll compressors. In all cases, multiple staged compressors typically serve each circuit. Compressors of similar sizes are typically of similar design efficiencies for a given temperature range. Part-load efficiency of various types of unloading may be examined by simulation, but simulations may not completely capture the systemic effects of staging.

Condensers

Condensers are available which cool refrigerant with either outside air alone or outside air and the evaporation of water. For recent new supermarkets in CA the market is about evenly split between the two. Under some utility programs, incentives are available for replacement of air-cooled condensers with evaporative condensers in dry climates. Particularly during hot, dry weather, evaporative condensers decrease compressor power demand by decreasing the temperature and pressure which the compressor must achieve. They should reduce annual energy consumption, but may not perform as well as expected at part loads. Condenser fan energy use is reduced by lower specific fan power (rejected heat/fan power) and by variable speed fans, and the proposed 2013 Title 24 will regulate both. Two less common condenser types are water-loop, where refrigerant condensers are attached to water loop cooled by central fluid cooler(s) and hybrid, where a condenser which can switch between evaporative cooling and air cooling.

A tradeoff is always made between the energy use of the condenser and the compressor, because the condenser fans must run harder to produce the cooler refrigerant condensing temperatures and pressures that allow compressors to run more efficiently. Because the compressors use more energy than the condenser, the most efficient systems use controls which adjust the pressure the condenser is trying to achieve in response to outside drybulb or wetbulb temperature, particularly when the fans are operated by VSDs.

Heat recovery is the use of additional refrigerant heat exchangers to reject heat to coils in the store's heating system or domestic hot water system. These typically save considerable heating energy, more than offsetting the small compressor power penalty which will occur while in this heating mode.

System and Piping

While most of the EEMs discussed above relate to individual components, the overall design of the store and systems will contribute to the energy consumption. The maximum achievable efficiency decreases with the temperature of the lowest temperature case on a system. So in general energy consumption will be minimized by maximizing the number of circuits as long as this is done so that the temperatures on each circuit are similar. Greater pressure drops through components will increase compressor energy consumption. Some piping designs may decrease efficiency of condenser operation. Systems are now being installed which distribute smaller compressor/condenser units around a store roof, closer to the case groups served, reducing piping runs and refrigerant inventory.

Liquid-to-suction heat exchangers (LSHXs) cool the hot refrigerant before it enters a case through heat exchange with cool refrigerant leaving the case. Simple LSHXs are common, but high-performance LSHXs significantly improve the efficiency of low temperature circuits, and some medium temperature circuits.

Mechanical subcooling (use of medium-temperature circuits to reduce the load on low-temperature circuits) is a common cost-effective feature in new stores and may be required by the 2013 T24.

Some components in refrigeration systems can be de-activated on a call for load shedding, qualifying for further incentives. Defrost can be deferred, and some case lighting can be switched off.

Retrofits

Energy cost savings can justify the replacement of some components of the refrigeration system. These are typically either case components or controls. Published results from a program in San Diego reported savings averaging 309,000 kWh per market. The distribution of those savings is shown in the following chart.

Figure 3 - Distribution of Savings from Retrofit Program*

savings-chart-large.jpg
Source: PECI, Supermarket Controls and Commissioning: Uncovering Hidden Opportunities, Diane Levin and Lawrence Paulsen, Portland Energy Conservation, Inc. 2006 ACEEE Summer Study on Energy Efficiency in Buildings

* ECMs = Electronically Commutated Motors; ASHC = Anti-Sweat Heater Control; FHPC = Floating Head Pressure Control; FSPC = Floating Suction Pressure Control

Annual Energy Simulation of Supermarket Systems

In CA, supermarkets have been modeled using the DOE-2.2R program to determine incentives under the Savings By Design program. Because of the complexity of refrigeration systems, this modeling is performed by a few select consultants.

The DOE-2.2R program has also been used to analyze measures for inclusion into Title 24. The DOE-2.2R program requires numerous specific inputs for refrigeration components. For instance, the representation of a fixture or case includes 130 input values, though not all would be needed for any given case. Some of these inputs are not clearly evident from manufacturers' literature. The program also makes assumptions about operation of controls that may not be always be achieved in actual operation.) All programs share these issues.

Other Programs

The EnergyPlus program has been updated to include detailed models of refrigeration system components, libraries of American components, secondary loops and new refrigerants. The EnergyPlus representation of components generally requires fewer inputs than the DOE-2.2R; for instance a case is represented by a maximum of 35 inputs. This program is being used at NREL to model prototype markets to develop Advanced Energy Design Guidelines for supermarkets and to model energy efficiency measures for specific supermarket design projects.

CyberMart is a Swedish program with simplified inputs and libraries of European components.

Training Highlights

California utilities offer outstanding educational opportunities that focus on the design, construction and operation of energy-efficient buildings. Listed here are a few of the many upcoming classes and events; for complete schedules, visit each utility's website.

Energy Auditing Techniques for Small & Medium Commercial Facilities
The intention of this three-day workshop is to develop the energy auditing skills of individuals associated with small-and mediumsized (under 500 kW) commercial facilities. We will provide an overview of building technologies with an emphasis on

distinguishing older, inefficient equipment from newer efficient systems. In-class exercises and a mock-audit at the end of the last day will allow the attendees to apply what they have learned. Though geared to the non-experts, attendees should have a fundamental understanding of building components and energy concepts.

January 9-11 (Monday, Tuesday, Wednesday), 8:30 am to 4:30 pm
San Francisco - PEC
register >

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e-News is published by Energy Design Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.

Savings By Design (www.savingsbydesign.com) offers design assistance and incentives to design teams and building owners in California to encourage high-performance nonresidential building design and construction.

Energy Design Resources and Savings By Design are funded by California utility customers and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison and Southern California Gas Company, under the auspices of the California Public Utilities Commission.

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