Natural refrigerants are considered substances that serve as refrigerants in refrigeration systems (including refrigerators, HVAC, and air conditioning). They are alternatives to synthetic refrigerants such as chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), and hydrofluorocarbon (HFC) based refrigerants. Unlike other refrigerants, natural refrigerants can be found in nature and are commercially available thanks to physical industrial processes like fractional distillation, chemical reactions such as Haber process and spin-off gases. The most prominent of these include various natural hydrocarbons, carbon dioxide, ammonia, and water. With the current technologies available, almost 75 percent of the refrigeration and air conditioning sector has the potential to be converted to natural refrigerants.
Background
Synthetic refrigerants have been used in refrigeration systems since the creation of CFCs and HCFCs in 1929. When these refrigerants leak out of systems and into the atmosphere they can have adverse results on the ozone layer and global warming. CFC refrigerants contain carbon, fluorine, and chlorine and become a significant source of inorganic chlorine in the stratosphere after their photolytic decomposition by UV radiation. Released chlorine also becomes active in destroying the ozone layer. HCFCs have shorter atmospheric lifetimes than CFCs due to their addition of hydrogen, but still have adverse effects on the environment from their chlorine elements. HFCs do not contain chlorine and have short atmospheric lives, but still absorb infrared radiation to contribute to the greenhouse effect from their fluorine elements.
In 1987 the Montreal Protocol first acknowledged these dangers and banned the use of CFCs by 2010. A 1990 amendment included agreements to phase out the use of HCFCs by 2020 with production and import being eliminated by 2030. HFC refrigerants, which have a negligible impact on the ozone layer, were seen as viable replacements, but these too have a high impact on global warming. The Kigali amendment of 2016 calls for these HFCs to be cut back by 80% over the next 30 years. Natural refrigerants are one of the potential options for replacement of HFCs, and are growing in usage and popularity as a result. The natural refrigerant industry is expected to have a compounded annual growth rate of 8.5% over the next 4 years, and is expected to become a US$2.88 billion industry by 2027.
Sustainability metrics Refrigerants are typically evaluated on both their global warming potential (GWP) and ozone depletion potential (ODP). The GWP scale is standardized to carbon dioxide, where the refrigerant's value is the multiple of the heat that would be absorbed by the same mass of carbon dioxide over a period of time. This is generally measured over a 100-year period. ODP measures the relative impact of a refrigerant to the ozone layer, standardized to R-11, which has a value of 1.
GWP and ODP vary greatly among the different refrigerants. CFCs are generally the highest impact, with a high GWP and ODP. HCFCs have similar GWP values and medium ODP values. HFCs again have similar GWP values but a zero ODP value. Natural refrigerants have low to zero GWP values and zero ODP values. Natural refrigerants are therefore gaining increased interest to replace HFCs and offer a more sustainable option for refrigeration. +ODP and GWP of Select Synthetic and Natural RefrigerantsClassificationRefrigerantOzone Depletion PotentialGlobal Warming PotentialCFCR-12110,900R-5020.334,657HCFCR-220.0551,810R-1230.0677HFCR-23014,800R-320675NaturalR-170 (Ethane)06R-744 (Carbon Dioxide)01R-717 (Ammonia)00R-718 (Water)00
Refrigerants
Hydrocarbons as refrigerants Pure hydrogen compounds see moderate use in refrigeration. Hydrocarbons are a viable option as refrigerants because, besides providing cooling properties, they are also plentiful and energy efficient. They are rated to be up to 50% more energy efficient than synthetic refrigerants. Hydrocarbons are also environmentally friendly, as they exist in nature and rank low on the global warming potential (GWP) scale. Historically, hydrocarbons have mainly seen use as a refrigerant for industrial chilling and refrigeration, but with the current shift towards natural refrigerants they are starting to see an increase in use in other areas of refrigeration.ASHRAE Position Document on Natural Refrigerants. July 21, 2011. Expires July 21, 2014. They are the favored refrigerant of many European countries.
Hydrocarbons used as refrigerants include: Methane (CH4) [R-50] Ethane (CH3CH3) [R-170] Propane (CH3CH2CH3) [R-290] Ethylene (CH2CH2) [R-1150] n-butane (CH3CH2CH2CH3) [R-600] Isobutane (CH(CH3)3) [R-600a] Propylene (CH3CHCH2) [R-1270] Pentane (CH3CH2CH2CH2CH3) [R-601] Isopentane (CH(CH3)2CH2CH3) [R-601a] Cyclopentane ((CH2)5)
Flammability The main detriment of using hydrocarbons as refrigerants is that they are extremely flammable at higher pressures. In the past, this risk was mitigated by turning hydrocarbons into CFCs, HCFCs, and HFCs,PROKLIMA, German Federal Ministry for Economic Cooperation and Development, Natural Refrigerants, Sustainable Ozone- and Climate-Friendly Alternatives to HCFCs, 2008. Retrieved 2022-04-07. but with the increasing avoidance of such substances, the problem of flammability must be addressed. Refrigeration systems work by pressurizing the refrigerant to a point where it begins to display refrigerant properties, but with the risk of pressurizing hydrocarbons there is a higher level of caution needed for the internal pressure. In order for hydrocarbons to combust, there must first be a release of hydrocarbons which mix with the correct proportion of air, and then an ignition source must be present. The range of flammability for hydrocarbons lie between 1 and 10%, and an ignition source must have an energy greater than 0.25 J or a temperature greater than 440 °C.
Current safety measures regarding the usage of hydrocarbons are outlined by the Environmental Protection Agency (EPA). EPA guidelines of hydrocarbon usage as a refrigerant include specifically designating pressure ranges for hydrocarbon refrigerant systems, ensuring the removal of potentially fire-starting components from hydrocarbon refrigerant systems such as electrical components prone to sparking, and placing standards on the construction of the systems to ensure a higher level of safety. Installing ventilation such that the concentration in air would be less than the flammability limit and reducing the maximum charge size of the refrigerant are other viable safety measures. Technological advances to reduce the total refrigerant charge amount have been recently obtained using aluminum mini-channel heat exchangers.
Applications and uses Hydrocarbon refrigerant markets have been growing as a result of increased concern for environmental effects of typical synthetic refrigerants. According to ASHRAE, available equipment that utilizes hydrocarbon refrigerant includes the following:
Systems with small charges such as domestic refrigerators, freezers, and portable air conditioners Stand-alone commercial refrigeration systems including beverage and ice-cream machines Centralized indirect systems for supermarket refrigeration Transport refrigeration systems for trucks Chillers in the range of 1 kW – 150 kW
Carbon dioxide as a refrigerant (R-744) Carbon dioxide has seen extensive use as a refrigerant. Carbon dioxide's main advantage as a refrigerant stems from the fact that it is classified as an A1 refrigerant by the EPA, placing it in the least toxic and hazardous category for refrigerants. This makes carbon dioxide a viable refrigerant for systems that are used in areas where a leak could cause exposure. Carbon dioxide sees extensive use in large-scale refrigeration systems, sometimes via a cascade refrigeration system. It is also used sparingly in automotive refrigeration, and is seen as favorable for uses in domestic, commercial and industrial refrigeration and air conditioning systems. Carbon dioxide is also both plentiful and inexpensive. These factors have led to carbon dioxide being used as a refrigerant since 1850, when it was patented for use as a refrigerant in the United Kingdom. Carbon dioxide usage at the time was limited due to the high pressures required for refrigerant properties to manifest, but these pressures can be easily reached and sustained with current pressurization technology.
The main concern over the use of carbon dioxide in refrigeration is the increased pressure required for carbon dioxide to act as a refrigerant. Carbon dioxide requires higher pressures to be able to condense within the cooling system, meaning that it has to be pressurized more than the other natural refrigerants. It can require up to 200 atmospheres to achieve adequate pressure for condensation. Refrigerant systems using carbon dioxide need to be built to withstand higher pressures. This prevents old coolant systems from being able to be retrofitted in order to use carbon dioxide. However, if carbon dioxide is used as a part of a cascade refrigeration system, it can be used at lower pressures. Using carbon dioxide in cascade refrigeration systems also means that the aforementioned benefits of availability and low price are applicable for a cascade system.
There are also benefits to the increased required pressures. Increased pressures yield higher gas densities, which allow for greater refrigerating effects to be achieved. This makes it ideal for cooling dense loads such as ones found in server rooms. It also allows for carbon dioxide to perform well under cold (-30 to -50 °C) conditions, since there are very small reductions in saturation temperatures for a given pressure drop. Plate freezers and blast freezers have noted improvements in efficiency and freezing time using carbon dioxide. There are also propositions for improved thermodynamic cycles to increase the efficiency of carbon dioxide at higher temperatures. Equipment with carbon dioxide refrigerant is also not necessarily heavier, bulkier, or more dangerous than similar equipment despite its higher working pressures due to reduced refrigerant volume flow rates.
When the pressure of carbon dioxide is raised above its critical point of 7.3773MPa it cannot be liquidized. Heat rejection must occur by cooling the dense gas, which creates a situation advantageous to water-heating heat pumps. These are particularly efficient with an incoming cold water supply.
Ammonia as a refrigerant (R-717) Ammonia () used as a refrigerant is anhydrous ammonia, which is at least 99.5% pure ammonia. Water and oil cannot exceed 33 and 2ppm respectively. Ammonia refrigerant is stored in pressurized containers. When the pressure is released it undergoes rapid evaporation causing the temperature of the liquid to drop until it reaches its boiling point of -33 °C (-28 °F), which makes it useful in refrigeration systems.
Ammonia has been used frequently in industrial refrigeration since it was first used in the compression process in 1872. It is used for its favorable thermodynamic properties, efficiency, and profitability. Ammonia is produced in massive quantities due to the fertilizer industry, making it relatively inexpensive. It has a GWP and ODP of zero, making the effect of ammonia leaks negligible on the climate. Ammonia is also tolerant to mineral oils and has low sensitivity to small amounts of water in the system. The vaporization heat of ammonia is high and the flow rate low, which requires different technologies to be used than other refrigerants. The low flow rate has historically limited ammonia to larger capacity systems.