Future Trends of Natural Refrigerants: Selection, Preparation, and Evaluation ^†

Abstract

The field of refrigeration technology has played a pivotal role in modern society, providing essential cooling solutions for various industries, including food preservation, healthcare, and manufacturing. However, the conventional refrigerants used in these systems, such as hydrofluorocarbons (HFCs) and chlorofluorocarbons (CFCs), have been identified as major contributors to climate change and ozone depletion. Recently, the heightened environmental consciousness of the refrigeration industry paved the way for searching for natural refrigerants (NRs) as an alternative to the usual commercial and synthetic refrigerant (SR). Natural refrigerants are known to be substances that occur naturally in the Earth’s surroundings and were commonly used, while synthetic refrigerants took their place because of their known better thermal performance durability and safety. Despite challenges such as flammability and toxicity, these NR substitutes demonstrate competitive performance, urging a transition from traditional SR. In this review paper, commonly known NRs such as ammonia, carbon dioxide, air, and hydrocarbons, are presented in terms of their sustainable characteristics, historical origins, selection criteria, preparation techniques, evaluations, and impacts. To provide a sustainable and eco-friendly guideline for the advancement of refrigeration technology, this analysis examines the trends, selection criteria, preparation processes, and evaluation procedures of different NRs. Finally, the results presented in this paper will be useful baseline information for both researchers and scientists in developing a refrigeration system.

1. Introduction

As we move forward in an era that highlights environmental consciousness and a need for sustainable solutions, the refrigeration industry stands at the lead of innovation. After careful examination of the adverse impact that traditional synthetic refrigerants have on the environment, now the focus lies with natural refrigerants. This paradigm shift is a strategic response to mitigate climate change and reduce the ecological footprint of refrigeration systems.

Every industrial device, such as refrigerators, water heaters, dehumidifiers, freezers, air conditioners, heat pumps, and many more, contain refrigerants with distinct properties that are chosen for suitable cooling and heating applications [1]. Refrigerants serve as a medium fluid between the sinks and sources for the heat transfer to happen. This is typically utilized in a heat cycle which undergoes a reversible phase transition from liquid to gas and vice versa. Before electricity was discovered, refrigeration technology had been adopted by humans using ether in 1805 [2]. This concept was put into practice by Jacob Perkins in 1834 for his refrigeration machine. Natural refrigerants are known to be substances that occur naturally in the Earth’s surroundings and were commonly used, while synthetic refrigerants took their place because of their known better thermal performance and durability and safety back in the 1930s. The most widely used refrigerants between 1830 and 1930 were ether, HCs, CHCs, HCOOCH₃, SO₂, H₂O, CO₂, and NH₃ [3]. The major of these were hazardous, combustible, extremely reactive, and prone to incidents. However, the usage of synthetic refrigerants leads to depletion of the ozone layer in the stratosphere and strong potential for global warming, which has become a major concern due to its environmental impact. Growing environmental awareness has led to a renewed interest in natural refrigerants like ammonia, carbon dioxide (CO₂), and hydrocarbons (propane, isobutane).

Natural refrigerants have become increasingly popular as environmentally friendly and energy-efficient substitutes. These include hydrocarbons, ammonia, and carbon dioxide. In order to provide a sustainable and eco-friendly answer to the advancement of refrigeration technology, this analysis examines the trends, selection criteria, preparation processes, and evaluation procedures of natural refrigerants. This paper serves as a guide for businesses, researchers, and governments to embrace and realize sustainable refrigeration procedures as more than just an academic exercise. Academicians, researchers, and others will find this review useful and well positioned to learn about the upcoming developments in natural refrigerant selection, preparation, and evaluation.

2. Selection of Natural Refrigerants

Refrigerants are an indispensable part of cooling systems that cool air and water, which find applications in fields as diverse as air conditioning, heat pumps, and refrigeration. Picking the right refrigerant is very important because it can make a big difference in how well everything works together. The type of refrigerant employed affects things like how much energy it consumes, whether it is dangerous to people, and the impact on the environment.

With climate change and ozone depletion becoming more of a threat, today, it is particularly important for refrigerants to have low global warming potential (GWP) with no ODP. The ODP figures explain how much destruction a refrigerant can cause to the ozone layer, and GWP measures how many units of greenhouse gas are emitted over its lifetime. Natural refrigerants, which are available from nature or can be easily made without harming the environment (minimal GWP), such as carbon dioxide and propane gas, now need to find good substitutes for synthetic HFCs. This review examines the relevant literature to figure out just how good natural refrigerants are, before painting us an optimistic picture for long-term cooling.

2.1. Ammonia

Ammonia has fine thermodynamic properties that make it good for absorbing heat. It consumes less energy than other refrigerants [3]. For this reason, ammonia is an ideal gas for large-scale industrial air conditioners used in processing food or keeping things at a cool temperature.

One advantage of ammonia is that it has low global warming potential (GWP) and ozone depletion potential (ODP). Unlike synthetic refrigerants, ammonia released into the air does not add to global warming or ozone loss. This, therefore, makes it a good choice for environmentally friendly cooling systems. In its Cooling Report 2023, the International Energy Agency called on everyone to replace high-GWP options with low-GWP ones like ammonia to reduce, as much as possible, the harm that cooling technologies inflict on our climate.

The disadvantage of ammonia, however, is that it is toxic and foul-smelling [3]. Anybody who works with or operates ammonia-based systems must strictly follow safety rules and receive special training. It is also a bit harder to use for more people because it has greater system requirements and initial equipment costs than some other natural refrigerants.

2.2. Carbon Dioxide

Carbon dioxide is an excellent refrigerant because of its high cooling capacity. It is as effective at work as a normal R410a heat pump [4]. Compared to synthetic refrigerants, the advantage of CO₂ is that it has almost zero GWP and ODP, which means that releasing it into the air does not affect temperature or destroy ozone.

Carbon dioxide is also able to extract heat more efficiently, and this results in smaller equipment sizes than other refrigerants [5]. With that, CO₂ is an ideal refrigerant for large-scale industrial applications like cold storage and food preparation. CO₂ is also not flammable, but its high density makes it a safety risk because it can cause asphyxiation in small areas [6], making ventilation a very important factor to consider.

The downside of using CO₂ is that it operates at high pressure; therefore, it needs sturdy equipment which makes it more expensive and complicated compared to others. It is also important to make sure that materials work well together, and that the system is well designed so that CO₂ devices work safely and effectively [6].

2.3. Hydrocarbons

Hydrocarbons, such as isobutane, have become an eco-friendly option that can make upgraded systems work much better, making them a potential long-term choice. A mix of hydrocarbons can be used in car air cooling to obtain the best of both performance and safety [7]. There are many ways in which hydrocarbons benefit the environment. First, they have almost no global warming potential (GWP) and ozone depletion potential (ODP). These can be much more beneficial to the Earth than synthetic refrigerants as they do not increase climate change or reduce ozone. Hydrocarbons are estimated to cut greenhouse gases by over 90% compared with standard refrigerants [8].

Hydrocarbons are also thermodynamically better than synthetic refrigerants. They can be used to can cool more for less energy. This would yield a better coefficient of performance (COP). Hydrocarbons are reported to have a COP up to 20% above R2 [9], which is the most common refrigerant.

The disadvantage of using hydrocarbons as refrigerants is that they are flammable, they need to be carefully designed, and safety steps must be taken. Leak detection and ventilation must be properly considered when designing equipment that uses hydrocarbons as refrigerants [10], in addition to ensuring the compatibility of materials used and choosing suitable parts.

2.4. Air

Although it is not as effective as synthetic refrigerants, air has come to be known for its availability and good environmental impact. The GWP, as well as the ODP, of air is zero, which makes it a good option as an alternative refrigerant. Also, because air is readily available in the environment, its cost is much lower compared to other natural refrigerants [11]. However, using air is disadvantageous when it comes to the economy. It has a low thermal capacity and cannot cool as effectively as other refrigerants. Because of this, bigger systems and more energy are needed to produce the same cooling effect [12]. These problems are explored in the studies, which point out that new ideas and innovations are needed to fully utilize it as a refrigerant.

2.5. Water

Water is also an attractive alternative to synthetic refrigerants since it is a naturally occurring and abundant substance with zero ODP and negligible GWP. It is inexpensive, nonflammable, nontoxic, and stable, making it easier and safer to use. Its application includes chillers, ice generators, and high-temperature heat pumps [13].

Water requires low operating pressures compared to traditional refrigerants, which require specific compressor designs. Design considerations include impeller design, shaft sealing, and heat exchanger optimization [14]. It also requires a large amount of volume for operations which needs larger, more costly equipment. Moreover, corrosion and scaling are potential issues that must be addressed when using water as refrigerant [15].

Additionally, water has a high boiling point, making it suitable for cooling substances at higher temperatures. However, it also has a higher freezing point compared to other refrigerants, making it applicable only for operations that require temperatures to remain above 0 °C [13]. Incorporation of antifreeze agents like propylene or ethylene glycol is needed for it to be utilized in colder conditions to extend its refrigeration capabilities beyond its natural freezing point.

3. Preparation of Natural Refrigerants

Natural refrigerants have emerged as a popular choice in our quest for environmentally friendly solutions. Their appeal lies in their lower warming potential compared to previous refrigerants, but it is worth noting that working with them can present challenges due to their unique properties. Preparation of refrigerants plays a crucial role in various aspects of our lives, allowing us to create ice, preserve food, and maintain optimal temperatures for different purposes. One of the many reasons they are widely used is the abundance of available sources [16]. However, not all natural refrigerants are created equal in terms of efficacy and warming potential—it is heavily influenced by their individual properties.

The goal is to use gases that contain no chlorine or fluorine and do not emit CO₂. While carbon dioxide is considered a viable natural refrigerant due to its low global warming potential (GWP), the emphasis on this goal stems from a broader consideration of the overall environmental impact. This includes factors such as the refrigerant’s energy efficiency, which influences the system’s overall carbon footprint. Additionally, the focus is on refrigerants that can be used for low-temperature applications below −20 °C, such as in solar thermal energy systems, to improve living conditions in arid regions [10].

3.1. Hydrocarbon Refrigerants

The use of hydrofluorocarbon (HFC) refrigerants, such as R134a, in refrigeration systems has raised concerns due to their contribution to global warming. As a result, researchers have been exploring alternative refrigerants with lower environmental impact. One such alternative is hydrocarbon (HC) refrigerants, which are natural and have properties that make them suitable for use in refrigeration systems.

A combination of several hydrocarbon molecules, including propane (R290), butane (R600), and isobutane (R600a), is created to produce HC refrigerants. The generation’s optimal mass ratio for these chemicals is calculated to determine the necessary heating capacity [17]. The same cooler will be used for both training and testing, as per the test strategy. The refrigerator is equipped with the numerical data displayed in Figure 1. The temperature and humidity were regulated in a controlled setting when the determination was made. To lower the risk of fire and explosion, safety measures have been put in place [17]. Numerous metrics, including COP, lifetime utilization, and lifetime coefficient, were used to assess the freezing runs.

In compact freezers, HC refrigerant can be used instead of R134a, according to the findings in [18]. Forty percent of the weight of R134a is the recommended weight for HC refrigerant. Energy usage is decreased when HC refrigerants are used.

3.2. Water Refrigerants

The cooling frame is the only place where water can be used as a coolant. Still, worries about conservation have surfaced, and efforts have focused on evaluating the commercial and professional feasibility of steam heating. An alternative perspective on steam as an integrated heater is presented in this article, which also looks at the business potential of employing steam for heating.

In order to precisely identify the device and forecast the effectiveness and significance of subcooling at the time, numerous other genetic models were employed. Compression ratio and sidewall weight reduction are two key topics covered by these component models, which are associated with liquid cooling cycles [18]. Additionally, the foundation for water vapor cooling was found to be unappealing during the expansion phase, and it was not possible to create machines that were both successful and affordable [18]. Condensation and compressor discharge are two more financial factors that are investigated that affect cooling water efficiency. This concept highlights the significance of producing affordable machinery, particularly the capacity to create a frame for water vapor heating. Overall, this essay assists in addressing the financial and professional difficulties associated with heating using steam.

3.3. Ammonia Refrigerants

Ammonia has a broad range of applications as a coolant in engineering processes, and is crucial for the synthesis of nutrients, energy storage, and heat control. Although these panels are readily available and can be integrated effectively, their limited options are in specific applications like thermal insulation and water heating [19]. An important topic covered in this article is the legal and security considerations surrounding the use of ammonia as a radiator. Legal obligations vary from country to country, while France is renowned for having very stringent legal guidelines regarding this matter. These guidelines are designed to ensure the safe handling and application of alkali in mechanical settings [19].

4. Evaluation of Natural Refrigerants’ Impacts and Performance

Analyzing natural refrigerants’ impacts and performance is essential to determine whether they are a practical substitute for conventional refrigerants or not. This section will assess natural refrigerants on the basis of their safety, performance coefficient, and potential effects on the environment, such as potential for ozone depletion and global warming.

4.1. Efficiency Comparison of Natural and Conventional Refrigerants

The coefficient of performance is frequently used to assess the efficiency of a pump, air conditioner, or refrigerator. Since COP is a measure of the useful cooling or heating required to work, higher values indicate more energy-efficient systems. On the other hand, a lower COP would indicate less efficiency, which would result in a higher electrical energy consumption for the same amount of heating or cooling. Banjo et al. assessed how well HC600a performed in comparison to HFC134a. The results indicated that when 46 g of HC600a was used compared to 70 g of HFC134a, a 32.2% increase in COP was obtained [20].

Palm conducted a performance comparison between R134a and several other refrigerants (R290, R717, R152a, R1234yf). Remarkably, compared to R134a, R152a and ammonia showed a 10% higher COP, while R1234yf had a COP value of 7% less than the other three refrigerants [21]. Additionally, when analyzed under identical conditions, ammonia and R600a exhibited a lower COP value than R22, according to Vemu and Singathi [22]. However, R290 showed outcomes that were comparable to those of R22.

As cited in Emani and Mandal’s review [23], Mohanraj et al. assessed hydrocarbon refrigerants (R290, R1270, R600a, and R600) as well as R152a as a replacement for R134a. His analysis revealed that, in comparison to R134a, the COP values of R290 and R1270 were only slightly lower—by roughly 2.3% and 1.9%, respectively [24]. It is important to remember that different refrigerants work better in different situations and with different system configurations. Nonetheless, it is noted that natural refrigerants are still a good substitute because their COP values are still on par with those of conventional refrigerants.

4.2. Environmental Impacts of Natural Refrigerants

Natural refrigerants have low to zero ODP and GWP values, in contrast to traditional refrigerants that have high values.

According to Emani and Mandal’s examination [25], ODP values for CFCs and HCFCs are typically less than 1. Conversely, natural refrigerants, HFCs, and HFOs, have zero. In terms of GWP, CFCs, HCFCs, and HFCs typically have values that are noticeably higher than those of HFOs (GWP < 0–12) and natural refrigerants (GWP = 0).

Moreover, the results of Emani and Mandal are consistent with Pavkovic’s comparison. Pavkovic’s [25] work included compiling the ODP and GWP (based on 100 years) of various refrigerant types. His analysis demonstrated that the ODP values of CFC and HCFC refrigerants, such as R22/R115, dichlorodifluoromethane, trichlorofluoromethane, chlorotrifluoromethane, and chlorodifluoromethane, ranged from 0.33 to 1. In the meantime, their GWP, which ranges from 1700 to 11,700, is extremely high. In contrast, natural refrigerants like carbon dioxide, ammonia, isobutane, propane, and propylene are associated with zero ODP. Additionally, their GDP, which ranges from 0 to 20, is low [25].

These data show that traditional refrigerants may have a low potential to destroy ozone but a high tendency to contribute to global warming. On the other hand, there is little to no risk of ozone depletion and global warming associated with natural refrigerants, making them a more environmentally friendly choice.

4.3. Safety Concerns Regarding Natural Refrigerants

Certain natural refrigerants are known to have toxic and flammable properties, which raises concerns about their safety. For instance, it is well known that hydrocarbons and ammonia are highly flammable, with ammonia being especially toxic. Carbon dioxide, meanwhile, needs a high operating pressure [26]. Water is thought to be the safest natural refrigerant out of all of them; however, its use is limited to high temperatures, and its refrigeration would require a compressor with a very large capacity [23].

5. Future Perspectives

The markets for refrigerants worldwide are growing significantly and quickly, a trend that will be especially noticeable between 2010 and 2050. The demand for air conditioning is expected to increase by 4.5 times for non-OECD (Organization for Economic Cooperation and Development) nations and by 1.3 times for OECD countries during this timeframe, according to projections made by the International Energy Agency (IEA). Due to commitments made on a regional, national, and worldwide level, there is a significant market for new refrigerants and related products [27].

Future perspectives on refrigerants in the current refrigeration technology sector will be influenced by a combination of technological advancements, regulatory requirements, and environmental considerations. There is a notice able trend away from conventional fluorinated refrigerants and toward sustainable substitutes, particularly natural refrigerants like ammonia, carbon dioxide, and hydrocarbons. Policies such as the F-gas regulation promote the gradual phase-out and decrease in high-GWP refrigerants, encouraging the use of more environmentally friendly substitutes [28].

5.1. Advancements in Refrigerants: An Overview of Future Options

Natural alternatives to refrigerants are expected to dominate the market. The ecological effect assessment places natural refrigerants in the lead position because of their minimal environmental impact, with an emphasis on overall equivalent warming impact numbers. This points to a more general trend in the industry, where the continued research and development of natural refrigerants is expected to be crucial in determining how refrigeration and air conditioning technologies will develop in a sustainable manner. It evaluates the environmental effects and implications of refrigeration systems in a comprehensive manner, assisting in the mitigation of climate change through informed and sustainable energy decisions. Evidently, a study showed that emissions reach their peak when the system incorporates fluorinated refrigerants. Particularly, R410a, a hydrofluorocarbon (HFC), exhibited the highest levels attributable to its substantial refrigerant charge and elevated global warming potential (GWP) value [28]. In contrast, natural refrigerants showcase markedly favorable environmental outcomes owing to several factors. Their lower system charges, combined with GWP values approaching zero, render their direct impact on global warming negligible.

5.2. Limitations of Natural Refrigerants as a Future Trend

In a study conducted by Palm, he identified that ammonia, hydrocarbons, and carbon dioxide could substitute for synthetic refrigerants in the environment. Ammonia is a popular refrigerant with excellent heat transfer and pressure drop properties that improve system performance. Safety is the primary concern. Its flammability, toxicity, and need for specific equipment to prevent accidents complicates safety measures. Large industrial systems may utilize ammonia because it requires considerable amounts of concentration and energy [20].

In conclusion, carbon dioxide, ammonia, and hydrocarbons are promising refrigerants with established applications. However, problems still exist, especially in regions in which safe, nonflammable alternatives are needed [20]. To solve these issues, industries are required to develop processes which are more reliable, effective, and in line with regulations. Consequently, there is a need to find a balance between meeting consumer demands and ensuring sustainability.

6. Research Gaps

Although natural refrigerants are environmentally sustainable, their complexity requires continual research and improvement. For example, refrigerants expert Dr. Richard Powell mentioned in one of his interviews that natural refrigerants produced in a chemical plant (i.e., transforming feedstock methane to carbon dioxide and ammonia) may have environmental effects that could only be detected in the long run [29]. As a result, a research gap in the use of these refrigerants is the lack of thorough studies regarding their long-term effects on the environment. Overall, natural refrigerants still require further development in order to achieve the same reliability and efficiency as conventional refrigerants.