A Closer Look at CO2

The need to reduce hydrofluorocarbon (HFC) refrigerant charge and emissions from supermarket refrigeration systems has becoming increasingly clear over the past 10 years. By the end of the last decade, the next generation of Hill PHOENIX Second Nature systems began entering the market. The key to these systems ‘success was their use of carbon dioxide (CO2) as a working fluid.

A number of reasons have factored into the search for alternatives to HFCs. Foremost among these were environmental concerns that led initially to international agreements (i.e., Montreal and Kyoto Protocols) and then to regulations such as Section 608 of the Clean Air Act that banned the venting of CFC (e.g., R-11, R-12, etc.), and HCFC (e.g., R-22, R-401A, etc.) refrigerants. More recently, proposed rule changes by governments at home and aboard (i.e., European Union F-gases regulations) have sought to even further reduce emissions of all synthetic refrigerants including HFCs (e.g., R-404A, R-410A, R-507, etc.). The impact of these regulatory pressures on system operators has added economic justifications to the move away from their use as not just the penalties for leaks has increased, but the taxation on their use (in Europe) has directly added to their cost.

Mark Your Calendars for 2015
As of 2010, no new equipment using R-22 could any longer be produced for sale in the U.S. By 2015, R-22 domestic production will be by limited by 90% to 1989 levels, all but curtailing the use of any equipment except with existing or reclaimed stocks of R-22. Finally, in 2020, what little remaining production of R-22 there is will cease. In the years ahead, these trends are only expected to intensify with a more aggressive phasing-out of HCFCs and phasing-down of HFCs likely.

Clearly, the overarching need for sustainable approaches to refrigeration has strengthened as these forces upon the market have intensified. Consequently, refrigerant manufacturers have responded with a number of alternative approaches intended to address the environmental challenges supermarkets face. But despite various advances made by new synthetic alternatives such as R-1234yf — that have no ozone depletion potential and very low global warming potential —costs for these options are expected to be high. For many, therefore, a more promising path points toward natural alternatives. CO2 (R-744) has several key characteristics that make it an ideal choice as a refrigerant. It’s colorless, odorless, and naturally available in the atmosphere. As of mid-2011, it was measured at 392 parts per million (ppm; 0.039% by volume). Approximately 50% heavier than air, it’s produced through natural processes including the carbon cycle, in which it occurs as a product of respiration in animals and from fermentation of organic compounds. The most common means by which CO2 is produced is through the liquefaction of air.

As a refrigerant, carbon dioxide’s high volumetric cooling capacity means that small pipe dimensions and small compressor swept volumes can be used. Because of its low liquid and gas viscosity, only small pressure losses occur in piping systems. Other R-744 benefits include high heat transfer coefficients and compatibility with most materials since it is noncorrosive and nontoxic. For companies that are focused on sustainability, responsible corporate citizenship, and smaller carbon footprints, CO2 is an attractive choice.

The rational for choosing CO2 includes:
• Colorless, odorless, and naturally occurring in the atmosphere
• Approximately 50% heavier than air
• Non-flammable and actually suppresses combustion
• High volumetric cooling capacity (small pipe dimensions and small compressor swept volume)
• Low viscosity in liquid and gas pipes (small pressure losses)
• High heat transfer coefficients
• Compatible with most materials: noncorrosive, nontoxic
• Companies are increasing sustainability, responsible corporate citizenship, and smaller carbon footprints

When compared to other refrigerants, CO2 is extremely affordable. At 2011 prices around $1 per pound, it’s truly an inexpensive refrigerant. Coleman grade (99.99% pure) used for refrigeration must be dry, but it can readily be obtained from welding companies and other industrial gas suppliers. As of late 2010, a 50-pound cylinder of Coleman grade CO2 from one supplier was priced at around $61.

Increased Development of CO2 Systems
Manufacturers have pursued a number of different approaches to using CO2 in commercial refrigeration systems. In addition to low-temperature, secondary systems like those already in use here, other approaches include those that use subcritical cascade and transcritical refrigeration. Subcritical systems have been around for more than 10 years in various configurations. More recently, European transcritical systems have grown from just a few in 2006, to more than a thousand since then. Transcritical systems, particularly booster systems, continue to gain in popularity.

CO2 secondary fluid systems have been used for low-temperature applications. Following on the success of these systems, subcritical cascade (DX) systems have seen increasing use in low-temperature applications and some retailers have now begun to look even further at transcritical booster systems.

Here in North America over the past decade, supermarkets have begun adopting some of these approaches to CO2. So far, CO2 secondary fluid systems have been used for low-temperature applications. Following on the success of these systems, subcritical cascade (DX) systems have seen increasing use in low-temperature applications and some retailers have now begun to look even further at transcritical booster systems. Early in 2012, for instance, Canadian retailer Overwaitea Food Group became the first user on the continent of a new generation of transcritical booster systems in one of their Vancouver, British Columbia stores.