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Public Interest Energy Research Program: Final Report
Investigation of Secondary Loop Supermarket Refrigeration Systems

Publication Number: 500-04-013
Publication Date: March 2004

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Executive Summary

The largest single use of energy in a supermarket is refrigeration, accounting for half of total energy use, on the order of 2-3 million kilowatt-hours (kWh) annually for a 35,000-square-foot store. Refrigeration is typically provided by direct expansion air-refrigerant coils that are located in the display cases and walk-in coolers. The compressors are located in a machine room in a remote part of the store, such as in the back room area or on the roof. As a result of using this layout, the amount of refrigerant needed to charge a supermarket refrigeration system is very large. A typical store will require 3,000- 5,000 pounds of refrigerant. The large amount of piping and fittings used in supermarket refrigeration leads to significant refrigerant leakage. On average, a supermarket refrigeration system can be expected to lose 30 to 50 percent of the total charge annually.

With increased concern about the impact of refrigerant leakage on global warming, new supermarket system types requiring significantly less refrigerant charge are now being considered. Of these advanced systems, the secondary loop refrigeration system employs central mechanical systems and can reduce the total refrigerant charge to approximately 300 - 500 lb. The anticipated refrigerant leak rate for the secondary loop refrigeration system is much less than that of the direct-expansion systems, because all refrigerant is contained within the chiller system. Refrigeration is provided to display cases by a secondary fluid pumped between the cases and the chiller system. Secondary loop systems, however, have tended to use more energy than multiplex systems.

The overall goal of this project was to determine if an advanced, high-efficiency secondary loop refrigeration system could have a lower operating cost than a state-of-the-art multiplex refrigeration system, considering both energy and refrigerant use. This would make the environmentally benign secondary loop system economically attractive.


Project Approach

Southern California Edison and its subcontractor, Foster-Miller, Inc., obtained support from Safeway, Inc. to provide a supermarket where an advanced secondary loop refrigeration system could be installed and tested. After design and analysis work was performed, a specification for the advanced secondary loop system was prepared and several refrigeration manufacturers were asked to propose and bid on its construction. Hill PHOENIX, Inc. of Conyers, GA was selected as the system manufacturer and installing contractor based on their response to this solicitation.

The advanced secondary loop refrigeration system was installed and instrumented for evaluation of its performance. At that same time, a second supermarket operated by Safeway was identified that employed a state-of-the-art multiplex refrigeration system. The multiplex system was also instrumented for performance measurement and both sites were monitored for approximately 9 months.


Project Objectives and Results

The specific technical objectives of the project were to build and test a secondary loop refrigeration system, which:

  • Consumes approximately 14% less electricity than a state-of-the-art multiplex refrigeration system (baseline system) installed in a comparable store.

The modeling results for the secondary loop refrigeration system designed for this project predicted annual energy savings of 145,894 kWh, or 14.9%, when compared with a state-of-the-art multiplex refrigeration system with air-cooled condensing. The multiplex system with an air cooled condenser is the most common configuration found in supermarkets.

In the field test, the secondary loop system was compared to a multiplex system with evaporative condensing, which is more efficient than air cooled condensing. This was appropriate because the secondary loop system also used evaporative condensing. Modeling results predicted annual savings of 6,130 kWh, or just under 1% for this comparison, but the actual savings achieved by the secondary loop refrigeration system were 37,266 kWh/yr, or 4.9%. This suggests the benefit of the secondary loop system as tested over an air cooled multiplex system would have been greater than 14.9%.

The savings achieved by the secondary loop refrigeration system can be attributed to energy-saving features incorporated in its design. The annual savings achieved by each of these features compared to a more conventional secondary loop refrigeration system were:

  • Multiple, parallel brine pumps - Estimated annual savings versus single large pumps are 99,718 kWh.

  • Subcooling from warm brine defrost - Estimated annual savings versus no subcooling are 49,570 kWh.

Additional savings were also achieved by the use of a minimum difference between the display case discharge air and refrigeration saturated suction temperatures, the use of a low-viscosity secondary fluid, and evaporative condensing as noted previously.

  • Has a refrigerant charge that is ten times less (less than 500 lbs.) than the baseline system.

The advanced secondary loop refrigeration system as tested had an initial refrigerant charge of 1,400 lb., which is considerably larger than the stated goal. The reason for the larger refrigerant charge is that the secondary loop system also had heat reclaim capabilities for both hot water and space heating. The added charge was needed for piping and heat exchangers associated with heat reclaim. Heat reclaim is of great value to the operation of the supermarket since it is capable of displacing all energy use associated with hot water heating and space heating for the store (Estimated to be approximately 7% of the total store energy use). Without the heat reclaim equipment the refrigerant charge would have been approximately 500 lbs. Further design enhancements such as including the heat recovery equipment within the factory-made chiller equipment room could result in a reduced overall refrigerant charge.

  • Loses annually no more than 15% of its refrigerant charge due to leakage.

Service records allowed refrigerant leak rates for the 2 stores to be calculated, using the methods provided by the U.S. Environmenetal Protection Agency. These calculations showed that the refrigerant leak rate for the secondary loop store was on the order of 14.8%/yr, or 206 lb/yr. The multiplex system was estimated to have a leak rate of only 12.4%/yr, but because of its much larger charge this amounted to 370 lb/yr. The refrigerant data for the multiplex store were very limited stemming from a change in maintenance contractors. It is likely that a full year of data would show a larger loss of refrigerant for the multiplex refrigeration system. Over time, and as equipment is serviced, the more scattered refrigeration apparatus of the multiplex system will probably begin to leak more, which would increase the relative benefit of the more centralized secondary loop system.

The ability of the two refrigeration systems to maintain product storage temperature was also assessed. The comparison for single-deck meat cases showed that the multiplex and secondary loop systems maintained the product at acceptable temperature levels. The case associated with the multiplex system had a lower and more uniform product temperature than was seen for the case operating in the secondary loop system. The multiplex display case had to operate a much lower rack SST (2.3 degrees F for multiplex vs. 14.1 degrees F for secondary loop) in order to achieve this condition. For the multi-deck produce cases, the multiplex and secondary loop systems operated at similar rack SST values. The resulting average product temperature was approximately the same for both systems, but the product temperature of the multi-deck case in the secondary loop system was more uniform in value.


Conclusion

Based on this research, we can conclude that secondary loop refrigeration systems are a viable option for supermarket refrigeration because:

  • With efficient design the energy consumption of a secondary loop system can be less than a multiplex system. The project system consumed 4.9% less electricity than the baseline multiplex system.

  • Refrigerant use is reduced in proportion to the quantities of refrigerant, piping and fittings employed. Refrigerant leaks are difficult to find and repair, and after years of repairs and modifications the multiplex system will probably leak more than the compact and centralized secondary loop system.

  • The somewhat higher first cost for the secondary loop system will probably be mitigated over time by lower operating and maintenance costs.

Benefits to California

This project contributed to the PIER program objective of reducing the environmental costs of California's electrical system, by developing an alternative refrigeration system which uses significantly less refrigerant than conventional systems. It also contributed to the PIER program objective of improving energy value of California's electricity by lowering electrical consumption of supermarket secondary loop refrigeration systems.


Abstract

Present multiplex supermarket refrigeration systems consume approximately 1 to 1.5 million kWh/yr. and may lose annually as much as half of the 3,000 to 5,000 lb system refrigerant charge. The secondary loop refrigeration system employs fluid loops and a central chiller to provide refrigeration to the supermarket display cases requiring just 300 to 500 lb of refrigerant for operation. An advanced secondary loop system was investigated, taking advantage of energy-saving features to reduce energy consumption. Modeling of the advanced system indicated energy savings of 14.9 and 0.3% versus multiplex systems with air-cooled and evaporative condensing, respectively. Results of a field test comparison of the secondary loop system, a multiplex refrigeration system with evaporative condensing developed energy savings of 4.9% for the secondary loop refrigeration system. Savings produced by the secondary loop refrigeration can be attributed to: minimum temperature differences between display case discharge air and saturated suction temperatures; the use of multiple parallel brine pumps; low-viscosity secondary fluid; and subcooling produced by the warm fluid defrost system. An evaluation of product temperature for similar display cases at each site was conducted. The secondary loop refrigeration system maintained the same or lower product temperatures than the product temperatures achieved by the multiplex refrigeration system.



Table of Contents

Preface

Executive Summary

Abstract

1.0 Introduction

1.1 Present Supermarket Refrigeration System

1.2 Low-charge Refrigeration System Options

1.3 Project Objectives

1.4 Project Organization and Approach

1.5 Report Organization

2.0 Project Approach

2.1 Design of the Secondary Loop Refrigeration System

2.1.1 Description of the Secondary Loop Refrigeration System

2.1.2 High-Efficiency Features for Secondary Loop Refrigeration

2.1.3 Display Cases Designed for Use with Secondary Fluid

2.1.4 High-efficiency Refrigeration Compressors

2.1.5 Close-Coupling of Compressors and Evaporator

2.1.6 Multiple Parallel Pumps

2.1.7 Evaporative Heat Rejection

2.1.8 Low Viscosity Secondary Fluid

2.1.9 Refrigerant Subcooling from Warm Brine Defrost

2.2 Analysis of the Secondary Loop and Multiplex Refrigeration Systems

2.2.1 Multiplex Refrigeration Model

2.2.2 Modeling of Secondary Loop Refrigeration

2.3 Field Testing of the Secondary Loop Refrigeration System

2.3.1 Description of the Secondary Loop Refrigeration Test Site

2.3.2 Measurement Plan to Monitor Secondary Loop Refrigeration

2.3.2.1 Refrigeration Energy Consumption

2.3.2.2 Refrigeration Supplied

2.3.2.3 Refrigeration System Operating State Points

2.3.2.4 Brine Pump Operation

2.3.2.5 Brine Defrost Operation

2.3.2.6 Display Case Operation

2.3.2.7 Ambient Conditions

2.3.2.8 Data Collection at the Secondary Loop Store

2.3.3 Field Testing of the Multiplex Refrigeration System

2.3.3.1 Description of the Multiplex Refrigeration Test Site

2.3.4 Measurement Plan for the Multiplex Refrigeration System

2.3.4.1 Refrigeration Energy Consumption

2.3.4.2 Refrigeration System Operating State Points

2.3.4.3 Display Case Operation

2.3.4.4 Ambient Conditions

2.3.5 Data Collection at the Multiplex Refrigeration Test Store

3.0 Project Outcomes

3.1 Analysis Results

3.2 Field Test Results

3.2.1 Energy Comparison

3.2.2 Power Demand

3.2.3 Refrigeration Efficiency

3.2.4 Product Storage Temperatures

3.2.5 Refrigerant Leakage Comparison

4.0 Conclusion & Recommendations

5.0 References

List of Tables

Table 1. Comparison of Operating Temperatures for Direct-Expansion and Secondary-Fluid Refrigerated Display Cases

Table 2. Transport Properties of Propylene Glycol-Water and Organic Salt-Water (Dynalene) at Operating Temperatures of Secondary Loop Refrigeration

Table 3. Low Temperature Circuits for the Secondary Loop Refrigeration Test Store

Table 4. Medium Temperature Circuits for the Secondary Loop Refrigeration Store (Loop B1)

Table 5. Medium Temperature Circuits for the Secondary Loop Refrigeration Store

Table 6. Low Temperature Circuits at the Multiplex Refrigeration Store (Rack A)

Table 7. Medium Temperature Circuits at the Multiplex Refrigeration Store (Rack B)

Table 8. Medium Temperature Circuits at the Multiplex Refrigeration Store (Rack C)

Table 9. Medium Temperature Circuits at the Multiplex Refrigeration Store (Rack D)

Table 10. Secondary Loop Modeling Results

Table 11. Multiplex Modeling Results - Air-Cooled Condensing

Table 12. Multiplex Modeling Results - Evaporative Condensing

Table 13. Multiplex Refrigeration Average Daily Energy Consumption

Table 14. Secondary Loop Refrigeration Average Daily Energy Consumption

Table 15. Performance of Multiple Pump Arrangement for Secondary Fluid Pumping

Table 16. Peak Power Demand of the Multiplex Store

Table 17. Peak Power Demand for the Secondary Loop Store

Table 18. Multiplex Refrigeration

Table 19. Secondary Loop Refrigeration

Table 20. Comparison of Secondary Loop and Multiplex Refrigeration Using Normalized EER Values, Multiplex Store Refrigeration Loads and Ambient Wet-bulb Temperatures

Table 21. Comparison of Secondary Loop and Multiplex Refrigeration Using Normalized EER Values, Secondary Loop Store Refrigeration Loads and Ambient Wet-bulb Temperatures

Table 22. Comparison of Display Case Performance for the Multiplex and Secondary Loop Refrigeration Systems

Table 23. Refrigerant Additions at the Secondary Loop and Multiplex Stores

Table 24. Estimated Annual Refrigerant Leak Rates for the Secondary Loop and Multiplex Test Stores

List of Figures

Figure 1. Layout of Refrigerated Display Cases and Walk-in Coolers in a Typical Supermarket

Figure 2. The Multiplex System is the Most Common Type of Refrigeration Used in Supermarkets

Figure 3. Flow Diagram for a Secondary Loop Refrigeration System

Figure 4. Layout of the Secondary Loop Refrigeration Test Store

Figure 5. Piping Diagram for the Low Temperature Secondary Loop Refrigeration System (Patent by Hill PHOENIX, Inc.)

Figure 6. Piping for the Medium Temperature Secondary Loop Refrigeration System (Patent by Hill PHOENIX, Inc.)

Figure 7. Layout of the Multiplex Refrigeration Test Store

Figure 8. Piping Diagram for the Low Temperature Multiplex Refrigeration

Figure 9. Piping Diagram for the Medium Temperature Multiplex Refrigeration

Figure 10. Store and Total Refrigeration Energy use for the Multiplex and Secondary Loop Test Sites

Figure 11. Multiplex Refrigeration System Energy Consumption

Figure 12. Secondary Loop Refrigeration Energy Consumption

Figure 13. Peak Power Demand for the Multiplex and Secondary Loop Stores

Figure 14. Relation between Ambient Wet-Bulb Temperature and Low Pressure Refrigeration EER

Figure 15. Relation between Ambient Wet-Bulb Temperature and Medium Temperature Refrigeration EER for the Multiplex System

Figure 16. Relation between Ambient Wet-Bulb Temperature and Medium Temperature Refrigeration EER for the Secondary Loop System

Figure 17. Discharge Air and Product Temperature Profiles for the Single-Deck Meat Case - Multiplex Refrigeration

Figure 18. Discharge Air and Product Temperature Profiles for the Single-Deck Meat Case - Secondary Loop Refrigeration 

Figure 19. Discharge Air and Product Temperature Profiles for the Multi-Deck Product Case - Multiplex Refrigeration 

Figure 20. Discharge Air and Product Temperature Profiles for the Multi-Deck Product Case - Secondary Loop Refrigeration

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