This article explains that substances such as air, water, ammonia, hydrocarbons, and carbon dioxide may provide solutions to the problem of finding environmentally acceptable refrigerants. The search for new and environmentally benign refrigerants to replace the existing chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) has led to the introduction of hydrofluorocarbons (HFC). HFCs could be useful as short- and mid-term replacements, but may ultimately not be suitable, owing to their high global-warming potential (GWP). Natural refrigerants still have several technical and safety challenges to overcome, and each has its unique advantages and disadvantages. Refrigerant, carbon dioxide offers a clear advantage over CFCs and HCFCs from the environmental impact standpoint. In addition to its environmental advantages, carbon dioxide also offers certain attractive thermal characteristics that can help it provide substantial potential as a long-term replacement if energy efficiency challenges can be addressed.
Refrigerations and air conditioning play important roles in modern life. They not only provide comfortable and healthy living environments, but have also come to be regarded as necessities for surviving severe weather and preserving food. Unfortunately, accelerated technical development and economic growth in much of the world during the last century have produced severe environmental problems, forcing us to acknowledge that though these technological advances may contribute to human comfort, they also can threaten the environment through ozone depletion and global warming. Aside from cost reduction, these concerns are the biggest driving forces for technical innovation in the field of refrigeration and air conditioning.
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)-used as working refrigerants in refrigerators and air conditioners as well as blowing agents in foams are now being regulated because of their contribution to ozone depletion. Hydrofluorocarbons (HFCs) could be useful as short- and midterm replacements, but may ultimately not be suitable, owing to their high global-warming potential (GWP). Accordingly, a long-term solution will require the use of natural refrigerants. The refrigerator and automotive air-conditioning industries have already begun to address the challenges of replacing HCFCs, including R-22, and, eventually, HFCs on a global level.
Refrigerants and the Environment
Along with the worldwide effort to protect the ozone layer, global warming-the second environmental effect of refrigerants-has garnered a great deal of public attention, as it threatens to become the dominant environmental issue for decades to come. In 1985, the World Meteorological Organization's international conference concluded that in the first half of the next century a rise of global mean temperature could occur that would be greater than any in human history, caused by increasing concentrations of greenhouse gases. As shown in the table on this page, greenhouse warming occurs when carbon dioxide (C02), released mostly from the burning of fossil fuels (oil, natural gas, and coal), and gases such as methane (CH,), nitrous oxide (NzO), ozone (03), CFCs, HCFCs, and water vapor build up in the atmosphere.
The environmental effects of selected refrigerants are shown in the table at the top of page 97 . The direct contribution of HCFCs to global warming is smaller than that of CFCs, but, as seen in the table, it is still much larger than those of natural refrigerants.
There are two types of global-warming effects to be considered here. The first is the direct global-warming potential that is due to the emission of refrigerants and other pollutants. The second type is an indirect global warming potential, which results from the emission of carbon dioxide due to the consumption of energy obtained from the combustion of fossil fuels. The indirect global-warming potential is greatly dependent on the efficiency of refrigeration systems and the method used to produce electricity. The combined effect of the two global- warming potentials is known as the total equivalent warming impact. The relative contributions of direct and indirect global-warming potentials in different applications are shown in the table at the bottom of this page. In general, the indirect potential is much larger than the direct potential. In some cases, the indirect potential can provide as much as 99 percent of the total impact.
Policies to reduce global warming force industries to develop technologies that will reduce energy consumption. In the United States, federal minimum efficiency standards have already dramatically increased the efficiency of home appliances, such as refrigerators and air conditioning units. Consequently, it is now becoming more important to develop alternative refrigerants that have a lower direct global-warming potential and a lower, or at least equivalent, indirect potential.
The search for new and environmentally benign refrigerants to replace the existing CFCs and HCFCs has led to the introduction of HFCs. However, HFCs have a much higher global-warming potential and higher costs than natural refrigerants. These concerns have spurred calls for the investigation of alter natives to HFCs. Some environmentalists would like the refrigeration industry to bypass HFCs and employ natural refrigerants as soon as possible.
Natural refrigerants are working fluids based on molecules that occur in nature. Examples are such substances as air, water, anm10nia, hydrocarbons, and carbon dioxide. Nevertheless, the actual fluids used in refrigeration systems may very well be synthesized and will not necessarily be extracted from nature. Ammonia, for instance, is synthesized in large quantities, and hydrocarbons undergo an extensive chemical processing procedure. Still, the cost of these fluids is much lower than that of HFC refrigerants, and they do not affect the environment in an unknown way. Also, the amount of fluid produced is negligible compared with the amount available in nature.
Studies of natural refrigerants are already underway. For example, Annex 22 of the International Energy Agency implemented a three-year project, Compression Systems with Natural Working Fluids, in 1995. Air, water, ammonia, hydrocarbons, and carbon dioxide have a low or zero direct global-warming potential and zero ozone-depletion potential (ODP), as shown in the table at the top of page 98.
Development of Refrigeration Systems
Since the first vapor-compression refrigeration system was invented by Jacob Perkins in 1834, more than 50 chemical substances have been used as refrigerants in refrigeration and air-conditioning systems. The development of refrigeration systems using CO2 as the refrigerant started in 1866, when an ice production machine that used CO2 was invented by Thaddeus S.c. Lowe. In 1880, the first CO2 compressor was designed by Franz Windhausen. After the late 1800s, the use of CO2 refrigeration systems increased. As a result of continuous efforts to improve efficiency, two-stage CO2 machines were developed in 1889 by the J. & E. Hall Co. in Great Britain, and the multiple-effect CO2 cycle was developed by G.T. Voorhees in 1905.
Meanwhile, Thomas Midgley, Jr., and Albert Henne published a paper in 1930 on fluorochemical refrigerants, as a result of a search for stable, nontoxic, nonflammable, and efficient refrigerants. In 1931, dichlorodifluoromethane, CFC-12, was commercially produced. After the introduction of fluorochemical refrigerants, the early refrigerants, including CO2, were replaced by many other CFCs and HCFCs. This led to a drastic decline in the use of refrigerants other than CFCs and HCFCs after World War II. Only ammonia remained in use, and its application was confined to large industrial systems. In recent years, though, natural refrigerants have gained considerable attention as alternative refrigerants for mobile air conditioners in Europe.
TLV: Threshold limit value (the refrigeration concentration limit in air for a normal a-hour workday; it will not cause an adverse effect on most people).
IOLH: Immediately dangerous to life or health (maximum level from which one could escape within 30 minutes without impairing symptoms or any irreversible health effects).
Varieties of Natural Refrigerants
Although natural refrigerants were used extensively in the early years of refrigeration technology, a number of technical and safety challenges caused them to be readily abandoned when CFCs became available. These challenges still exist today for air, water, ammonia, hydrocarbons, and carbon dioxide.
Air is used extensively as a refrigerant in aircraft cooling. Its advantages are that open systems require fewer heat exchangers, aircraft have compressed air available already, and systems tend to be low in weight. Its efficiency, however, is quite poor. Nonetheless, German railways have installed air cycles in the latest generation of high-speed trains, because of weight concerns and, most importantly, because maintenance time is very short. There is no cumbersome and time-consuming refrigerant reclamation and no system evacuation is required.
Water has the potential to be a very efficient refrigerant, but because it requires operation in a deep vacuum, its vapor density is quite low. This leads to costly large-volume vacuum tanks that must house all the machinery, such as heat exchangers and compressors. Furthermore, water's pressure ratio is very high, imposing additional challenges for compressors that must operate in a deep vacuum.
The only applications where water is used as a refrigerant on a commercial basis are in large-capacity lithium-bromide water-absorption chillers. Over the last decade, a few large-scale water-vapor compression systems have been used commercially. One such system provides cooling to the LEGO factory in Denmark, another to a mine in South Africa. In both cases, open systems are employed, and the need for heat exchangers is eliminated by using direct-contact heat exchange. Thus, the chilled water that circulates through the facility is also used as the refrigerant. Although various research projects are underway worldwide, a demanding compressor technology, the need for vacuum pumps, and degassing remain great challenges in terms of cost and design.
Ammonia is also a very good refrigerant and is used to a significant extent in large warehouses. Ammonia is toxic and, under certain limited conditions, flammable and even explosive. However, with its intense, pungent odor, it is a self-alarming refrigerant. Ammonia has emerged as a refrigerant for water chillers in Europe. These units are entirely self-contained, including a gas tight cabinet that houses the entire unit and a water tank to dissolve any ammonia in case of a leak. These measures, to be sure, increase costs considerably.
Hydrocarbons are excellent refrigerants, but they are also flalm11able and explosive. In North America, any flammability risk is unacceptable, but some countries in Europe and elsewhere have less-stringent liability laws. Since the mid-1990s, virtually all refrigerator production in Germany has used hydrocarbons as the working fluid. Some heat pump manufacturers whose systems are installed entirely outdoors have followed suit, and some commercial installations have recently become publicly known. Nevertheless, the danger of fire remains an overriding concern. To address this challenge with safety features, the cost of a system would have to be increased by about one-third.
Carbon dioxide is a refrigerant that operates at very high pressures in a transcritical cycle for most operating conditions. Thus the refrigerant condenser of a conventional refrigeration system serves now as a cooler for supercritical fluid. Only after the expansion process is liquid carbon dioxide available to provide cooling capacity through evaporation. Because of the nature of the transcritical cycle, the efficiency of carbon dioxide is quite poor. However, this is its only disadvantage. All the other characteristics of carbon dioxide are very favorable. It is environmentally safe, has very low toxicity, and allows for extremely compact systems. The vapor pressure of CO2 is approximately seven times higher than that of R-22. Moreover, the supercritical CO2 has a higher density than subcritical fluids, so there is potential to reduce the size of hardware. There are indications that with modern materials and technologies, the weight of CO2 heat exchangers can be reduced considerably, especially for tap water heating, with essentially the same performance.
Gustav Lorentzen and his colleagues revived research on the CO2 cycle in the 1990s, especially for mobile air-conditioning applications. This group has focused on the experimental evaluation and thermodynamic modeling of mobile air-conditioning systems. Meanwhile, a group of researchers in Germany has focused on railway air systems. In 1994, European automobile manufacturers launched a joint research project, Refrigeration and Automotive Climate Systems under Environmental Aspects (RACE), to investigate an air-conditioning system with the natural refrigerant CO2. Most efforts conducted so far with respect to CO2 have focused on experimental evaluation and thermodynamic modeling for R-12 and R-22 replacement.
Investigations of prototypes for automotive air conditioning and some other applications show that well-designed systems can actually perform at levels reached by other refrigerants. One method that would greatly enhance the efficiency of CO2 cycles is the use of an expander instead of an expansion valve. An expander produces work that can be fed back into the compressor or used otherwise. In conventional systems, the expander is beginning to emerge in large-capacity units. However, any kind of machinery added to a cycle will increase cost to an unacceptable degree. Therefore, most research and development projects underway today do not include an expander.
R-22 and Carbon Dioxide
Thermophysical properties of R -22 and CO, are compared for the saturation temperatures of -25°, -10°, and 5°C in the table at the bottom of page 98. The evaporation pressure of CO2 is four to eight times higher than that of R-22. The saturated liquid density of CO2 is approximately 70 percent that ofR-22, while the saturated vapor density of CO2 is approximately five times that of R-22. The higher density offers the opportunity to reduce heat exchanger size and weight. CO2 has better heat- transfer characteristics (higher latent heat, specific heat, and thermal conductivity) and lower viscosity than R-22.
The first approach to comparing the refrigerants is to compare their ideal cycle performance. An ideal cycle 1S defined as a refrigeration cycle that has zero approach temperatures and minimum or no pressure drops in the heat exchangers. It should also have isentropic compression, no subcooling (for R-22), and no suction superheating: This ideal cycle is applied to an R-22 refrigeration simulation model (REFSIM) and a CO, simulation model (CO,SIM). Overall, the ideal-cycle coefficient of performance (COP) of CO2 is only 50 to 60 percent of R-22's at various chilled water temperatures and various gascooler cooling water temperatures.
On the other hand, the COP of the actual cycle for the CO2 water chiller is 94 to 106 percent of the existing R- 22 water chillers, based on recent experimental work. This difference between the ideal cycle and actual cycle is analyzed by using refrigeration cycle models. The actual cycle is different from the ideal cycle because of the irreversibility of the actual process, which can be attributed to the following parameters: compressor efficiency, approach temperature in heat exchangers, pressure drop in heat exchangers, and degree of subcooling and superheating. In a comparison of the effects of these parameters on the cycle COP change for both refrigerants, the compressor efficiency for both refrigerants has the largest impact on the COP drop. Moreover, this parameter has more impact for R-22 than C02, in absolute terms. The effects of the suction gas heating by the motor, as well as mechanical losses, are much less for CO2 than R-22, owing to the higher specific heat of C02. The approach temperature and degree of subcooling in the condensing process have the second and third largest impacts on the COP drop ofR-22. This difference is a result of the better heat transfer of CO,. The overall heat transfer coefficient of CO2 during the gas-cooling process is approximately double that ofR-22 during the flow-condensation process. Another important parameter is the sensitivity of temperature change to the pressure drop. CO2 is less sensitive than R-22. The more sensitive case has a larger approach temperature and COP drop.
Thus, it can be concluded that the beneficial thermophysical properties of CO2 contribute considerably to its surprisingly good overall COP. In this light, it is conceivable that CO2 may be a promising candidate as a viable refrigeration. Extensive research work is required to prove feasibility and, if successful, an industrywide development effort will be required to make this fluid succeed. At this juncture, any prediction of the winner is premature.
Natural refrigerants still have a number of technical and safety challenges to overcome, and each has its unique advantages and disadvantages. Although it is• not clear yet which one is the dominant candidate as an alternative refrigerant, CO2 offers a clear advantage over CFCs and HCFCs from the environmental impact (ODP and GWP) standpoint. In addition to its environmental advantages, CO2 also offers certain attractive thermal characteristics that can help it provide substantial potential as a long-term replacement if energy efficiency challenges can be addressed.