The most energy-efficient refrigeration system is one that uses natural refrigerants, such as carbon dioxide, ammonia, or hydrocarbons, and recovers excess heat or cool from the refrigeration process. These systems have lower environmental impact, lower operating costs, and higher performance than conventional systems that use synthetic refrigerants, such as hydrofluorocarbons (HFCs) or hydrochlorofluorocarbons (HCFCs).
In this article, we will explain how refrigeration systems work, how natural refrigerants differ from synthetic ones, and what are the benefits and challenges of using natural refrigerants in refrigeration systems. We will also provide some examples of natural refrigerant systems that are currently in use or under development.
Refrigeration systems are devices that transfer heat from a low-temperature region (the cold reservoir) to a high-temperature region (the hot reservoir) by using a working fluid, called the refrigerant, that undergoes a thermodynamic cycle. The refrigerant absorbs heat from the cold reservoir and releases it to the hot reservoir, thus creating a cooling effect.
The most common type of refrigeration system is the vapor-compression cycle, which consists of four main components: the compressor, the condenser, the expansion valve, and the evaporator. The diagram below shows the basic layout of a vapor-compression cycle.
The vapor-compression cycle operates as follows:
The refrigerant enters the compressor as a low-pressure vapor and is compressed to a high-pressure vapor, which increases its temperature.
The high-pressure vapor then flows through the condenser, where it gives off heat to the surrounding air or water and condenses into a high-pressure liquid.
The high-pressure liquid then passes through the expansion valve, which reduces its pressure and temperature, and partially vaporizes it.
The low-pressure liquid-vapor mixture then enters the evaporator, where it absorbs heat from the cold reservoir and evaporates completely into a low-pressure vapor.
The low-pressure vapor then returns to the compressor, completing the cycle.
The coefficient of performance (CoP) is a measure of the efficiency of a refrigeration system. It is defined as the ratio of the cooling effect (the heat removed from the cold reservoir) to the work input (the energy supplied to the compressor). The higher the CoP, the more efficient the system.
How natural refrigerants differ from synthetic refrigerants
Synthetic refrigerants, such as HFCs and HCFCs, are man-made chemicals that have been widely used in refrigeration systems since the 1980s. They replaced the earlier generation of synthetic refrigerants, such as chlorofluorocarbons (CFCs), which were found to deplete the ozone layer. However, synthetic refrigerants have their own drawbacks, such as:
They have high global warming potential (GWP), which means they contribute to the greenhouse effect and climate change when they leak or are released into the atmosphere. For example, the GWP of R-134a, a common HFC refrigerant, is 1,430, which means it is 1,430 times more potent than carbon dioxide as a greenhouse gas.
They have high operating pressures, which require more energy and more robust equipment to operate. This increases the capital and maintenance costs of the system, as well as the risk of leaks and failures.
They are subject to strict regulations and phase-out schedules, which limit their availability and increase their prices. For example, the Kigali Amendment to the Montreal Protocol, which came into effect in 2019, aims to reduce the consumption and production of HFCs by 80% by 2045.
Natural refrigerants, on the other hand, are substances that occur naturally in the environment and have been used in refrigeration systems since the 19th century. They include carbon dioxide (CO2), ammonia (NH3), hydrocarbons (such as propane, butane, or isobutane), water (H2O), and air. Natural refrigerants have several advantages over synthetic refrigerants, such as:
They have low or zero GWP, which means they have minimal or no impact on the climate when they leak or are released into the atmosphere. For example, the GWP of CO2, NH3, and hydrocarbons is 1, 0, and 3-20, respectively.
They have high thermodynamic efficiency, which means they can achieve higher CoP and lower energy consumption than synthetic refrigerants. For example, CO2 can achieve CoP of up to 6.5 in transcritical cycles, while NH3 can achieve CoP of up to 8 in ejector cycles.
They are widely available and inexpensive, which reduces the operating and maintenance costs of the system. They are also not subject to any regulations or phase-out schedules, which ensures their long-term viability.
However, natural refrigerants also have some challenges and limitations, such as:
They have high flammability (hydrocarbons), toxicity (NH3), or pressure (CO2), which require special safety measures and standards to prevent accidents and injuries. For example, hydrocarbons need to be used in small quantities and in well-ventilated areas, NH3 needs to be isolated from the occupied spaces and equipped with leak detectors and alarms, and CO2 needs to be handled with high-pressure components and valves.
They have low compatibility with some materials and lubricants, which may cause corrosion, leakage, or degradation of the system. For example, CO2 and NH3 are incompatible with copper and brass, while hydrocarbons are incompatible with some synthetic oils and elastomers.
They have specific design and operational requirements, which may limit their applicability and performance in some conditions and applications. For example, CO2 has a low critical temperature of 31°C, which means it cannot operate efficiently in high ambient temperatures, NH3 has a high latent heat of vaporization, which means it requires large evaporators and condensers, and hydrocarbons have a low volumetric refrigeration capacity, which means they require large compressors and pipes.
What are the benefits and challenges of using natural refrigerants in refrigeration systems
Using natural refrigerants in refrigeration systems can offer significant benefits for the environment, the economy, and the society, such as:
Reducing greenhouse gas emissions and mitigating climate change, by avoiding the use of high-GWP synthetic refrigerants and lowering the energy consumption of the system.
Saving money and resources, by reducing the capital and operating costs of the system, as well as the dependence on imported and regulated synthetic refrigerants.
Improving performance and reliability, by increasing the CoP and the lifespan of the system, as well as reducing the risk of leaks and failures.
Enhancing innovation and competitiveness, by creating new opportunities and markets for natural refrigerant technologies and products, as well as supporting the development of skills and expertise in the field.
However, using natural refrigerants in refrigeration systems also poses some challenges and barriers, such as:
Increasing safety and compliance risks, by requiring more stringent and complex safety measures and standards to prevent and manage potential hazards and incidents.
Facing technical and operational limitations, by requiring more advanced and customized design and engineering solutions to overcome the specific characteristics and requirements of natural refrigerants.
Encountering market and policy obstacles, by facing low awareness and acceptance of natural refrigerant technologies and products, as well as lack of incentives and support from governments and stakeholders.
Examples of natural refrigerant systems
Despite the challenges and barriers, natural refrigerant systems have been successfully implemented and demonstrated in various applications and sectors, such as:
Supermarkets and convenience stores, where CO2 and hydrocarbons are used in refrigerated display cases, walk-in coolers, and freezers, as well as in heat recovery systems that provide hot water and space heating. For example, the Whole Foods Market in Brooklyn, New York, uses a CO2 transcritical system that saves 40% of energy and 2,000 tons of CO2 emissions per year, compared to a conventional HFC system.
Industrial and commercial refrigeration, where NH3 and CO2 are used in large-scale cold storage facilities, ice rinks, and food processing plants, as well as in cascade or booster systems that combine the benefits of both refrigerants. For example, the Coca-Cola Company has installed over 2.5 million CO2 coolers in more than 45 countries, reducing its carbon footprint by 52% and its energy consumption by 40%, compared to HFC coolers.
Air conditioning and heat pumps, where hydrocarbons and CO2 are used in residential and commercial split systems, window units, and water heaters, as well as in district heating and cooling networks that supply multiple buildings. For example, the Drake Landing Solar Community in Alberta, Canada, uses a CO2 heat pump system that provides 90% of the space heating needs of 52 homes, using solar thermal collectors and borehole thermal energy storage.
Natural refrigerants are substances that occur naturally in the environment and have lower environmental impact, lower operating costs, and higher performance than synthetic refrigerants. They are the most energy-efficient refrigeration systems, as they can achieve higher CoP and lower energy consumption than conventional systems. However, they also have some challenges and limitations, such as high flammability, toxicity, or pressure, low compatibility with some materials and lubricants, and specific design and operational requirements. Therefore, they require special safety measures and standards, as well as advanced and customized design and engineering solutions, to ensure their safe and effective use.
