Ammonia: The Natural Refrigerant of Choice
(An IIAR Green Paper)
II. ADVANTAGES OF AMMONIA AS A REFRIGERANT
Ammonia is a naturally occurring compound, made up of one atom of nitrogen and three atoms of hydrogen, with the chemical formula NH3. Ammonia is a key intermediary in the nitrogen cycle, and under normal conditions, is essential for many biological processes.19 Most of the ammonia in the environment comes from the natural breakdown of manure and dead plants and animals.20 Ammonia can be found in water, soil, and air, and is a source of much needed nitrogen for plants and animals.21 In fact, ammonia is among the most abundant gasses in the environment.22
Refrigerant-grade ammonia is 99.98% pure - free of water and other impurities. It is readily available, inexpensive, and capable of absorbing large amounts of heat when it evaporates. The operating pressures of ammonia are comparable with other refrigerants. Ammonia's ability to absorb larger amounts of heat per volume makes it possible to use smaller pipes and smaller components compared to other refrigeration systems while delivering the same amount of refrigeration.23
As a refrigerant, ammonia offers three distinct advantages over other commonly used refrigerants. First, ammonia is an environmentally compatible refrigerant because it has an ozone depletion potential (ODP) of zero and a global warming potential (GWP) of zero.24 Second, because of its superior thermodynamic properties, ammonia as a refrigerant requires less energy than other refrigerants when used in large industrial systems. Third, ammonia refrigeration has a proven safety record, in part because of the physical properties of ammonia, not the least of which is ammonia's well-recognizable and easily-detectable odor, compliance with voluntary industry standards, and an industry of well-trained operators.
Ammonia is an environmentally compatible refrigerant.
Ammonia does not harm atmospheric ozone. It is now well recognized that halocarbons released into the atmosphere will eventually reach the stratosphere and the ozone layer.25 Halocarbons are extremely chemically stable, with estimated life cycles of two to three centuries. When released into the atmosphere, this stability allows halocarbons to migrate through the troposphere and into the stratosphere.26 At this altitude, the intense ultraviolet rays of the sun break down halocarbon molecules, often releasing chlorine ions, which in turn act as catalysts to break down ozone molecules.27 This process reduces the ozone layer's effectiveness as a filter against ultraviolet radiation, resulting in higher amounts of ultraviolet radiation reaching the surface of the earth with harmful biological consequences.28 Increased radiation causes increased health risks in humans and damages the flora and fauna of the ecosystem.
The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, under the auspices of the United Nations Environmental Program went into force on January 1, 1989 and has since been revised and strengthened.29 The United States, Canada, Japan and the European Economic Community, major producers and users of CFCs, were among the signatories.30 The Protocol ensured that production of CFCs ceased at the end of 1996 and has led to the implementation of their gradual reduction in developing countries.31 Amendments adopted in 1992 and 1995 covered hydrochlorofluorocarbons (HCFCs), which have a much lower ozone depleting potential than CFCs and had been relied upon to speed up CFC reduction.32 Under the Protocol, HCFCs will be gradually phased out by 2030. Certain countries have set earlier target dates for phase out of HCFCs.33
Just as it does not damage atmospheric ozone, ammonia, with a life cycle in the atmosphere of less than one week, does not contribute to the greenhouse effect responsible for global warming. Global warming results from the short wave, near infra red radiation that reaches the earth from the sun.34 About fifty percent of the sun's radiation reaches the earth.35 This is absorbed by the earth's surface which re emits the radiation in longer, far infra red wavelengths. This re emitted radiation is partially absorbed by gasses known as greenhouse gasses.36 Greenhouse gasses are either natural (CO2, water vapor, etc.) or man-made (CO2, N2O, CH4, CFC, HCFC, HFC, etc.).37
The 1997 Kyoto Protocol to the United Nations Framework Convention on Climate Change controls emissions of greenhouse gasses.38 The Protocol defines six gasses or family of gasses: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); hydrofluorocarbons (HFCs); perfluorocarbons (PFCs); and sulfur hexafluoride (SF6).39 HFCs were developed in order to enable CFC and HCFC phase out.40 Within the framework of the Protocol, developing countries will reduce emissions of these gasses by at least five percent with respect to 1990 levels during the 2008 and 2012 period.41 CFCs and HCFCs, which are governed by the Montreal Protocol, are not included in the Kyoto Protocol.42
Under each protocol, each signatory nation establishes its own rules and procedures to meet the phase-out goals.43 The Clean Air Act Amendments of 1990 gave statutory recognition to the Montreal Protocol's phase-outs in the United States and established a comprehensive set of regulatory requirements for recovery, recycling, and disposal of CFCs when equipment containing them is serviced or discarded.44 The Amendments also directed the U.S. EPA to establish a program for the control or phase-out of substances harmful to the stratospheric ozone layer. Through the Significant New Alternatives Policy (SNAP) Program, the agency has identified ammonia as an acceptable substitute to ozone depleting substances in the major industrial sectors, including refrigeration and air conditioning.45
Ammonia has superior thermodynamic properties and as a refrigerant results in lower CO2 emissions and requires less energy.
Generally speaking, ammonia refrigeration systems cost 10-20% less to install than systems using alternative industrial refrigerants. Thermodynamically, ammonia is 3-10% more efficient than other refrigerants; as a result ammonia systems use less electricity than systems with alternative refrigerants. The cost of ammonia itself is significantly less than other industrial refrigerants and less ammonia is also generally required to do the job as compared to other industrial refrigerants. All of this adds up to lower operating costs for food processors and cold storage facility operators, and that means lower grocery bills for the average household.
Ammonia and its alternatives function as refrigerants in various types of systems that consume electricity or other types of energy during operation. Due to its highly favorable thermodynamic properties, ammonia used as a refrigerant requires less primary energy to produce a certain refrigeration effect compared to other commonly used refrigerants. Therefore, its indirect global warming effect due to CO2 emissions from electric power plants can be considered one of the lowest of all refrigerants.46
Proper environmental impact assessment of refrigerants and their systems requires consideration of both their direct and the indirect contribution to global warming. Refrigeration systems directly contribute to global warming through the greenhouse gas effect of their fugitive refrigerant emissions. They indirectly add to global warming through carbon dioxide emissions resulting from the conversion of fossil fuels to energy required to operate the systems.
The Total Equivalent Warming Impact (TEWI), is defined as the sum of these direct and indirect contributions. It is a concept developed in the early 1990's to help system design engineers assess refrigerants and their systems using a single number rating system. More recently, the concept of Life Cycle Climate Performance indicators (LCCP) was introduced to further refine the TEWI concept. The LCCP program was designed to include the two main elements of the TEWI calculation as well as the assessment of the environmental impact of refrigeration production, distribution and end-of-life disposal.
The TEWI and the LCCP concepts illustrate how important energy efficiency and low global warming potential (GWP) of refrigerant emissions are to refrigeration and air conditioning systems.
The Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) and the U.S. Department of Energy (DOE) funded TEWI studies using a systems approach to determine the overall contribution of refrigerants to global warming. The studies focused on:
- Technical options that could be implemented by the year 2000
- Energy and global warming impacts of next generation fluorocarbons as well as not-in-kind (NIK)/non-fluorocarbon technologies that could be developed or improved to replace CFCs and HCFCs
- Refrigeration and air conditioning technologies that can be commercialized during the phase-out of HCFCs
The studies demonstrate the importance of assessing both the direct and indirect contributions of all refrigeration alternatives when selecting the best environmental option. The studies conclude that energy efficiency is a powerful tool to mitigate future potential climate change.47
Ammonia refrigeration is inherently safe.
In any mechanical refrigeration system, leaks will occur. This fact is exacerbated when the leaks involve odorless refrigerants as evidenced by the abundant supply of CFCs in the atmosphere today. The inherent safety of ammonia refrigeration is explained in part by ammonia's characteristic odor, which signals even the smallest leak, at concentrations far lower than any dangerous level.48 Ammonia's safety record as a refrigerant is also explained by other physical characteristics such as its density and limited range of flammability, engineering advances for refrigeration systems, and the solid record of well-trained ammonia refrigeration systems operators.
Ammonia's self alarming quality is due to its well recognizable and easily detectable odor. Even the slightest traces of ammonia in the air can be detected. This allows for the safe and immediate repair of system leaks or sources of leaks. Contrasted to the penetrating odor of ammonia, other commonly used refrigerants like the halocarbons are odorless and their escape difficult to detect without mechanical systems. The pungent odor of ammonia will encourage individuals to leave the immediate area of release before harmful concentrations will exist.
The safety record of ammonia refrigeration is also due to the fact that ammonia is 1.7 times lighter than air and thus easily vented by mechanical means into the atmosphere.49 If a leak occurs in a refrigeration system under pressure, only the pressurized gas and, absent additional heat, a smaller amount of the liquid in that space will be released.50 Releases of liquid ammonia are rare. Because ammonia vapor is lighter than air, it will rise and quickly become diluted in the atmosphere.51 In the presence of moisture, a visible water vapor cloud will form.52 In contrast, halocarbons are heavier than air and will collect at ground level, displacing oxygen and posing a risk of suffocation.
Ammonia is difficult to ignite and exhibits a narrow range of flammability.53 Ammonia is flammable only at high concentrations and under extremely limited conditions. Because ammonia will not sustain a flame on its own, ignition of ammonia vapor requires an uninterrupted external flame source. Ammonia's burning velocity, at a maximum of 8cm/s, is substantially lower than other flammable refrigerants, and is not high enough to create an explosion. For these reasons, ammonia explosions are rare. Properly designed ammonia refrigeration systems that are well ventilated and free of open flames or ignition sources mitigate against potential explosion.54
Also significant to ammonia's safety record is the fact that individuals who work with ammonia refrigeration systems have specific training available to them. A wide range of education and hands-on instruction is currently provided by industry associates, contractors, and community colleges. Additionally, industry codes and standards along with applicable federal regulations, provide further operational and system design safeguards.
| 
