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Use of Natural Refrigerants in Supermarkets

1  Summary and conclusions

1.1 Introduction

On the basis of projects previously completed under the auspices of the Danish Energy Agency’s CO₂ programme (journals nos. 731327/97-0164 and 731327/99-0199) and the Danish Environmental Protection Agency (file no. M 128-0428), a project was run at Fakta Beder, supporting and fully developing the use of propane and CO₂ as refrigerants in supermarkets.

The previous projects document the excellent thermo-physical and thermo-dynamic properties of CO₂ and propane.

This project (Use of natural refrigerants in supermarkets) focuses on the energy consumption and installation costs of using propane and CO₂ in supermarkets.

To optimise installation costs, we have to use the components familiar to fitters. The project was therefore run using copper pipes and other components known from the commercial refrigeration industry. The individual parts were combined into a unit and sold as a unit to Fakta.

The unit is a product with the same energy consumption as a traditional R404A (90 kg) system, but which reduces the direct equivalent CO₂ emission contribution by 342 tonnes.

Fakta is a chain of supermarkets with about 250 outlets. The chain has expanded and developed over the past 25 years. The energy consumption usually reflects the age of an outlet.

The outlets can be grouped according to type of refrigeration system and energy consumption with sufficient numbers in each group to establish a well-defined basis for a comparison between the propane/ CO₂ system and conventional systems.

To be energy neutral, the propane/CO₂ refrigeration system must have the same level of energy consumption as that of the latest systems.

1.2 Objective

The objective of this project is to fully develop and commercialise the technology underlying the use of natural refrigerants, thus making it more accessible to a greater part of the industry. Moreover, the technology will be demonstrated on a full scale (Fakta outlet), where the previous system stood in a less commercial outlet (local outlet in Odense, file no. 731327/99-0199). The technology needs to be demonstrated in an outlet of a size with which the industry can identify.

1.3 Results

The project shows that:

• the energy consumption is within the consumption of an optimised traditional Fakta outlet;
  
• the installation costs are 10-20% above the cost of an optimised Scroll solution with R404A in a standard Fakta outlet;
  
• the installation costs of the brine circuit are significantly lowered when hoses are used, and that string control valves are not required on systems of this size;
  
• the injection function of the cascade heat exchanger must have a time-limited start-up function followed by traditional PID control to provide stable, uniform operation;
  
• the propane circuit safety requirements are operational and manageable; and
  
• the choice of an adjusting strategy for secondary cooling circuits is a condition for energy-optimised operation of refrigeration compressors.
  

1.4 Conclusion

The conclusion is that the propane/CO₂ solution can be implemented in many installations in the Danish commercial refrigeration industry. Energy-wise, the technology is neutral at present, with potential optimisation. Installation costs will rise by 10-20% initially, but costs may also be optimised. Fitters must adjust their ability to handle CO₂/propane and brine. Hoses can considerably enhance the installation of the brine circuit, and string-adjusting valves are not required for systems of this size. The brine circuit flow should be constant. The injection valve on the cascade heat exchanger must be fitted with a controller with time- limited start-up function followed by traditional PID control. Applying the Pressure Equipment Directive (Order no. 743) and later the ATEX Directive as the design basis will achieve the optimum safety, an issue that is sure to be pivotal also in future. The project reveals that the refrigeration industry is able to offer refrigeration units using propane/CO₂ as refrigerants, thereby eliminating the direct equivalent CO₂ emission contribution.

1.5 Background

The restrictions and bans of the Montreal Protocol (1987) on the use of CFCs and HCFCs, the hardened line of the Kyoto Protocol on greenhouse gases (HFCs) and the Danish EPA’s proposal for taxes on industrial greenhouse gases will lead to the extensive use of natural refrigerants in the future and attempts to reduce the energy consumption of refrigeration systems.

Today, many outlets use R404A (HFC), which is a mixture of R143a, R125 and R134a with a direct greenhouse potential of 3,800 at a time horizon of 100 years. Because outlet refrigeration systems normally have a lifespan of 10-15 years, and yearly leakage represents 10-20% of the charge, the leakage from a large number of R404A systems will continue contributing to global warming for many years to come.

Denmark has about 2,200 supermarkets and department stores selling refrigerated and frozen foods. The refrigeration systems are primarily made up of remote systems where the refrigerated display cases (refrigerated and freezer islands) are connected to a central refrigeration system located in an engine room far away from the display cases. The remote system in retail trade is characterised by long pipe stretches and large amounts of refrigerants. Typically, the systems have only been minimally optimised, because focus is on operating safety, function and appearance, all aspects that can lift sales in the outlet. Hence, energy consumption is typically relatively high, as are refrigerant leakage rates.

When leakage occurs, the refrigerant gradually seeps from the system onto the cellar floor, shop floor and surroundings, but major leakages from burst pipes in transport lines also occur in certain cases. Leakages are discovered when system function becomes critical, which is often not the case until more than half the refrigerant has seeped out of the system.

On average, the charge of refrigerant is about 50 kg in each supermarket, corresponding to a total of 118,000 kg. New systems using HFC are estimated to have annual leakage rates of 10% (11,800 kg), equivalent to 44,840 tonnes of CO2 emission a year when using R404A refrigerant, whose equivalent CO2 emission is 3,800 kg of CO2 per kg.

Supermarkets’ refrigeration and freezing systems account for about 1.5% of the total energy consumption in Denmark (600 GWh per year). Given 0.78 kg of CO2/kWh, the annual CO2 load is 468,000 tonnes per year. Consequently, electricity consumption must be weighed heavily in the endeavours to phase out unwanted gasses from the commercial market.

1.6 The system

Several of the natural refrigerants cannot be used directly in the supermarket due to their undesirable properties as regards flammability and toxicity. For this reason, indirect systems need to be designed where these refrigerants exchange heat with a secondary refrigerant (brine), which is then pumped from the engine room to the refrigerated and freezer display cases.

Measurements and analyses made so far of this type of refrigeration system for supermarkets have proved, however, that the “price” of replacing unwanted gases will be an increase in energy consumption of 5-10%. Using a gas like CO2 directly in the freezer display cases and an indirect system in the refrigerated display cases will leave energy consumption unchanged compared with today’s optimised systems using R404A.

Se billede i fuld størrelse

The refrigerated and freezer display cases shown on the left are mounted in the supermarket itself, while other system components (compressors, pumps, heat exchangers, vessels, etc.) are installed in a compressor rack located in a separate engine room.

The combination of propane and CO₂ is an optimum solution because refrigeration companies are versed in the technology of soldering copper pipes and therefore have no difficulty in building systems using the new refrigerants. Moreover, using CO₂ on the low-temperature part sharply reduces the amount of propane added. It should be possible to get below 10 kg even in major supermarkets, where today’s charges of HFC stand at 60-120 kg.

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