The refrigeration and air conditioning industry has made tremendous progress over the past two decades in reducing the use of ozone depleting refrigerants. The original targets of the Montreal Protocol, established in 1987 to reduce emissions of ozone depleting substances, are being met and exceeded. Another consequence of these initiatives is that during the 1990s and the early part of the present century, there was considerable uncertainty regarding future refrigerant options. Now a new path has started to appear, defined by the global agenda on climate change and global warming. Energy security, miniaturization, cost and social demands have also become emerging issues for the refrigeration and air conditioning industry.
Selection Criteria of Refrigerants
Selection of suitable refrigerant for specific application is very critical as it depend on various factors, which can be broadly categorized as: (i) Environmental and personal safety, (ii) Thermodynamic properties, (iii) Chemical and physical properties, and (iv) Cost and system compactness. Environmental and personal safeties are the primary issues to select refrigerant. Under increasing pressure to address global warming concerns, the industry is spending more effort to understand the environmental impact of air conditioning systems using different refrigerants and technologies. The environmental performance of air conditioning or heat pump systems is partially defined by life cycle impacts on climate, including the direct impacts of refrigerant emissions, the indirect impacts of energy consumption used to operate the heat pump system, and the energy to manufacture, transport, and safely dispose of the system, all expressed in terms of CO2 equivalent emissions. Two environmental impacts, ozone layer depletion (measured by ODP) and global warming (measured by GWP) had made nearly all the conventional synthetic refrigerants unfit for long term use. Details of phase-out schedule of various synthetic refrigerants are shown in Figure 1. Personal safety is mainly related to toxicity and flammability. Detail of personal safety classification is shown in Figure 2.
Figure 1: Phase-out schedule of various synthetic refrigerants
Figure 2: Safety classification of refrigerants
As discussed, there are many criteria to be considered in the selection of an appropriate refrigerant for use in a refrigeration or heat pump system. Briefly, the refrigerants are expected to meet the following conditions:
- ozone- and environment friendly,
- low boiling temperature,
- low volume of flow rate per unit capacity,
- vaporization pressure lower than atmospheric pressure,
- high heat of vaporization,
- nonflammable and nonexplosive,
- non-corrosive and non-toxic,
- non-reactive and non-depletive with the lubricating oils of the compressor,
- non-acidic in case of a mixture with water or air,
- chemically stable,
- suitable thermal and physical properties (e.g., thermal conductivity, viscosity),
- commercially available,
- easily detectable in case of leakage, and
- low cost.
Next-Generation Refrigerants
As discussed earlier, the selection of suitable refrigerant for long term use in any refrigeration, heat pump or air-conditioning application may be governed by many parameters, which are graphically represented in Figure 3.
Figure 3: Various governing parameters for refrigerant selection
However, the next generation refrigerants (future refrigerant for long term use) have been selected mainly based on environmental criteria; those refrigerants, which have (i) zero ODP (Ozone layer Depletion Potential) and (ii) low GWP (Global Warming Potential). Limit for low GWP refrigerants is not well defined; according to most of the literatures, the low GWP range is up to 150; however R32 (having GWP of 675) has been also considered as low GWP refrigerant. Properties of next generation refrigerants for vapor compression system are shown in Table 1,
which can be classified as (i) Inorganic natural refrigerants, (ii) Organic natural refrigerant, (iii) Hydro-Fluoro-Carbons (HFCs) and (iv) Fluorinated ethers (HFOs). Within listed refrigerants, the new refrigerants R32, R1234yf and R1234ze have introduced very recently. Main advantages of these refrigerants compared to others are mild flammability (better than HCs), mild toxicity (better than R717) and comparable performance (better than R744). Many reputed manufacturers have recently launched various new refrigeration and air-conditioning products based on R32 and R1234yf. Table 2
summarizes the various past, present and future applications of next generation refrigerants for compression refrigeration and heat pump systems. New refrigerants R32 and R1234yf are discussed in details in the proceeding sections.
Characteristics of R32
R32 is difluoromethane (methylene fluoride) and it is an HFC type refrigerant. R32 has been used for many years as a component of both R407C and R410A; however, it has been introduced as pure refrigerant recently. It is flammable on its own, but not when mixed with the other components of these blends.
The relative merits of R32 can be summarized as:
- Zero ODP
- One third of R-410A GWP.
- Considerably lower refrigerant cost than R410A and potentially better affordability
- Available now in high volumes globally since it is 50% of R410A composition
- Lower vapor density and lower mass flow rate, hence lower pressure drop expected
- Required less charge as it has a 20% higher volumetric capacity.
- Similar saturated pressure and pressure ratio hence development is easier.
- Higher critical temperature hence higher COP.
- Single component of gas, therefore it is easier to be produced and managed.
- Better heat transfer properties at same mass flux, hence lower evaporator and condenser size
•No glide and potential to optimize heat exchanger with smaller volume for charge reduction
The disadvantages are cited below:
- A2L mild flammability rating (difficult to find a Low-GWP A1 non-flammable fluid)
- Higher compressor discharge temperature from higher vapor specific heat
- New oil likely required since existing polyolester (POE) oil is not miscible with R32
Overall, R32 seems to offer more advantages than disadvantages. Its lower cost provides incentive for investing development time for mitigating its disadvantages through compressor and system design optimization. R-32 can reduce electricity consumption up to approximately 10% compared to that of air conditioners using refrigerant R-22.
Characteristics of R1234yf
R-1234yf, a new environmentally-friendly refrigerant, has received final approval from the US Environmental Protection Agency (EPA) in 2011. R-1234yf stands for a specific compound: 1 – double bond, 2 – hydrogens, 3 – carbons, 4- fluorines, yf – position of the fluoro atoms. The biggest benefit of R1234yf is that it breaks down faster in the atmosphere than R-134a, leading to negligible GWP value. It is considered mildly flammable and has thermodynamic properties similar to R-134a. It has best balance Of properties and performance.
Relative advantages of R1234yf:
- Excellent environmental properties (Very low GWP, Zero ODP, Low TEWI, Favorable LCCP, Atmospheric chemistry determined and published)
- Low toxicity, similar to R-134a (Low acute and chronic toxicity, Significant testing completed, Has a favorable ATEL (Acute Toxicity Exposure Limit) value
- Mild flammability (significantly better than 152a), which is manageable
- COP and Capacity are very similar to R-134a, Operating pressures and refrigerant characteristics are pretty close to R134a, Better COP than R152a & CO2
- Thermally stable and compatible with R-134a components, similar compressor wear
•Complying to European and US Norms, Suited to all climates across the world
R1234yf Implementation issues:
- Flammable and may require additional safety measures during implementation and use
•R1234yf is a more complex and costly refrigerant gas to produce than R134a.
- Additional part (internal heat exchanger) is required to match the performance
- Many components have to be re-developed (New oil type, New ports)
Conclusions
The next generation refrigerants are presented in this article with special focus on R32 and R1234yf. All of these refrigerants have some individual merits and demerits. Hence, it not correct to say that the searching of suitable refrigerants is at the end and these refrigerants will serve forever. Any major demerit may be detected with some of these refrigerants in future (which, we are not able to understand today) and have to be banned. One the other hand, some new refrigerants may be introduced in future. It is a never-ending process.
AUTHORS CREDIT & PHOTOGRAPH
Dr Jahar Sarkar
Associate Professor,
Department of Mechanical
Engineering,
Indian Institute
of Technology, Varanasi
