From The Top
Carbon Dioxide Refrigerant Makes a Comeback

Dick Topping, director of Appliance Research, TIAX, LLC

Before you assume this is another boring article on new refrigerants, what if I told you that air, water, and other everyday compounds can run your air-conditioner or refrigerator quite well? That’s a thermodynamic fact, and one of those everyday chemicals is so common that it constitutes what we all exhale—carbon dioxide (CO2).

At first, the phaseout of chlorine-containing refrigerants such as CFCs and HCFCs led the industry toward another class of fluorocarbon refrigerants, hydrofluorocarbons (HFCs) that did not contain chlorine and thus did not harm the ozone layer. However, in the 1980s, scientists identified global warming as a major environmental threat, and the global warming impact of HFCs came under scrutiny, leading many researchers and manufacturers to reconsider “natural” refrigerants such as CO₂, hydrocarbons, and ammonia, because these substances have negligible direct global-warming impact and ozone-depletion potential. The signing and ratification by many countries of the Kyoto Protocol has provided greater impetus to look for alternatives to fluorocarbon refrigerants; several European countries have already begun restricting their use and are planning for an eventual phaseout.

Carbon dioxide is non-flammable and non-toxic in contrast to other natural refrigerants—hydrocarbons (flammable) and ammonia (flammable and toxic). Furthermore, it is inexpensive, widely available worldwide from numerous suppliers, and not subject to venting restrictions. The high operating pressures of CO₂ also provide a potential opportunity for system size and weight reduction. The major challenge, however, is to design a cost-effective, efficient, reliable system that accommodates the unique characteristics of CO₂, most significantly, five times the typical system operating pressure and a low critical temperature that requires cooling a supercritical fluid rather than condensing a two-phase mixture.

The application areas attracting the most interest today for CO₂ are those where current system refrigerant leakage rates are high enough to attract regulatory attention, as well as in high-temperature heat-pump applications and in military cooling systems because of special logistics considerations.

Centralized refrigeration systems used in supermarkets are prone to leakage due to the large number of refrigerant line joints, long runs of refrigerant piping, and frequent thermal cycling. Carbon Dioxide can be used efficiently in these systems, and some leakage can be tolerated. The same is true for vehicular air-conditioning, where considerable engineering effort has been expended by the major automobile manufacturers to develop prototype CO₂-based air-conditioners for cars and trucks. (Another potential advantage of the CO₂ cycle for vehicles is a heat-pump mode that delivers instant heat in winter.) In Japan, CO₂-based heat-pump water heaters have been commercialized, and design efforts are underway in the U.S. These heat pumps take advantage of the high-temperature heat rejection from the transcritical CO₂ cycle.

However, there is a downside to using CO₂ as a refrigerant. Many studies, both theoretical and experimental, have demonstrated that the thermodynamic efficiency of transcritical CO₂ cycles is lower than that of conventional fluorocarbon-based vapor compression systems, particularly at high ambient temperatures. This decrease in system efficiency could negate part or all of the environmental advantage of the CO₂ system by increasing its indirect contribution to global warming due to the higher energy consumption. Furthermore, it would likely be unacceptable from a marketing or regulatory standpoint to introduce new air-conditioning and refrigeration systems with lower efficiencies than existing units. Therefore, an approach to improving the efficiency must be found in order to spur commercialization. Fortunately, such an opportunity exists by recovering the losses that occur during the expansion process as the refrigerant leaves the high-pressure gas cooler and enters the evaporator.

In theory, recovery of energy lost during the expansion process in a vapor compression cycle is of interest for any refrigerant. However, the relatively large expansion losses attributable to the high operating pressures of CO₂ make a work-recovery device particularly important. Design studies at my company have found that a reasonably efficient CO₂ expander based on scroll technology can improve the efficiency of a CO₂-based system to parity with fluorocarbon-based equipment while achieving the aforementioned environmental benefits described.

With these potential benefits, why aren’t we seeing more research or accelerated efforts by manufacturers to get CO₂ on the market sooner? The answer, of course, is that history has shown us that introducing new refrigerants is never easy. However, expect to see systems that accommodate the unique characteristics of CO₂ as a “green” refrigerant in the years ahead.

Dick Topping
Dick Topping is director of Appliance Research at TIAX LLC (www.tiaxll.com). He can be reached by phone at 617/498-6058, by fax at 617/498-7206, or e-mail at Topping.R@tiaxllc.com. From the Top appears bimonthly in APPLIANCE ENGINEER®.