The current answer is “nothing for sure yet.” But a number of options are being looked at - that became evident based on a number of published research papers presented at the 13th International Refrigeration and Air-Conditioning Conference held this summer at Purdue University.
In the broadest brushstroke, here is the current research:
HFOs - specifically 1234yf and 1234ze(E) - for stationary applications are drawing increasing attention. Their global warming potentials are considerably lower than HFCs, although it is possible they would carry a newly designated A2L “slightly flammable” safety rating falling between the A1 rating of HFCs and the A2 ratings of HCs. R-1234yf has already been designated for use in automotive a/c in Europe in place of HFC-134a.
Manufacturers are also experimenting with what are currently just being called “developmental refrigerants” which try to address issues related to global warming potential (GWP), flammability, glides, and performance.
CO2 continues to draw interest especially in ways for it to work in transcritical applications as well as the already established approach in cascade systems.
Ammonia in smaller applications also continues to draw supporters and research.
The four-day, every other year conference drew hundreds of researchers, scientists, and engineers from throughout the world. It was held concurrently with the 20th International Compressor Engineering Conference and 1st International High Performance Buildings Conference.
The following summarizes some of the findings presented in two sessions under the general heading “Refrigerant Assessments.”
HFOsHFOs were the dominant topic among refrigerants. For example, a paper from DuPont said, “A new class of refrigerants has been developed initially for use in automobile air conditioning systems. The class is partially fluorinated olefins, and the best-known example is hydrofluoroolefin 2,3,3,3-tetrafluoro- prop-1-ene, or HFO-1234yf. While this molecule has been found to be a good refrigerant in medium temperature applications, its volumetric capacity properties are not optimal for other applications, including stationary air conditioning and low temperature commercial refrigeration. While it has excellent environmental properties, the fact that it can be made to burn, albeit with limited energy release and limited flame propagation rate, will require some regulatory and building code review and modification to define safe use conditions.
“If regulations allow use of these new refrigerants in conventional equipment designs, it should be possible to retrofit existing equipment to use these reduced GWP fluids. Such an approach could allow users to transition more quickly into using environmentally sustainable, reduced GWP refrigerants and reduce the LCCP of existing systems. The candidates with reduced GWP and low or zero temperature glide are especially attractive with respect to facilitation of transition away from high-GWP refrigerants.”
The GWP aspect was noted by researchers at the University of Maryland who said 1234yf had a GWP of 4 compared with 1430 for R-134a. That university’s research paper specifically focused on the potential for the refrigerant in systems originally designed for either R-134a or R-410A.
The U of M research said, “In general, the thermo physical properties of R-1234yf are very similar to those of R-134a, and not as similar to those of R-410A. This trend is most readily observed in the pressure curves of these refrigerants. Sample heat exchanger simulations have shown that R-134a and R-1234yf have similar results for outlet refrigerant temperature and heat load. However, the pressure drop does vary and changes to the heat exchanger design and piping in these systems may be required.”
Another DuPont paper looked at 1234yf in beverage coolers. “The question arising given the success in automotive is whether HFO-1234yf can be used in other applications where R-134a is currently employed. In particular, vending machines and beverage coolers account for a substantial portion of R-134a use. There are over 10 million vending machines installed worldwide and a significant number of these are beverage coolers.”
At the end of the report, it was noted, “Test results from beverage coolers optimized for R-134a, HFO-1234yf and CO2 demonstrate that coolers can be designed for HFO-1234yf that demonstrate similar energy efficiency and cooling capacity compared to R-134a. Only minor modifications were required to achieve equivalent performance.”
There was also research from Hitachi Ltd. regarding 1234yf in room air conditioners.
In part, it said, “We have evaluated the performance of room air conditioners using HFO-1234yf. To meet the properties of (that refrigerant), we modified a room air conditioner that had been using R-410A. The main modifications were doubling the number of paths of the heat exchangers, triplicating the inner-diameter cross-sectional area of the gas-side connecting pipe, and installing the oil separator. The ratios of HFO-1234yf/R-410A COP are 97/88 percent at the rated/medium cooling capacity and 93/98 percent at the rated/medium heating capacity. Therefore, the ratio of the annual performance factor is 95 percent. The decrease in the COP ratio at the medium cooling capacity is because of the refrigerant accumulation in some paths under the large number of paths of the outdoor heat exchanger. Equalizing the refrigerant distribution of paths is then required. The increase in the diameter of the gas-side connecting pipe involves challenges such as increasing the workload of the installation.”
Beyond R-1234yf, the Purdue Conference also had information of R-1234ze(E). Researchers from Kyushu University in Japan wanted to see what would happen if they took a heat pump designed for use with HFC-410A and replaced it with HFO-1234ze(E) and with a mixture of 1234ze(E) and HFC-32. They noted that 1234ze was originally developed as a “cover gas for casting process of magnesium alloy.”
When using straight 1234ze(E) in the heat pump at 1.6kW, the COP value was said to be 20 percent lower than that of R-410A at 2.8kW. “These results prove that the heating effect and COP of HFO-1234ze(E) can be improved by adding HFC-32 as the second component.” The report said the “pressure drops in both evaporator and condensing sides of 1234ze(E) and its mixture with HFC-32 are almost at the same level as that of R-410A though the refrigerant flow rates are lower than that of R-410A.” Based on that, the report suggested larger heat transfer tubes and connecting tubes in a heat pump that might be developed for 1234ze(E) and 32.
“At the present stage, it seems that mixtures of HFO-1234ze(E) and HFC-32 are strong candidates for replacing HFC-410A in domestic heat pumps.”
Another paper, from Honeywell, looked at both 1234yf and 1234ze in a vending machine instead of R-134a.
HFOs “have potential in applications such as small commercial and residential refrigeration systems and other areas where a medium pressure refrigerant can be efficiently employed and where low global warming refrigerants are needed or desired.”
In the case of a vending machine, “Overall results show that comparable performance to R-134a can be achieved without significant hardware modification. The thermal stability and good interaction with POE oils used in these applications has also been demonstrated.
“HFO-1234ze efficiencies were lower than R-134a and HFO-1234yf. This was mainly due to pressure drop losses in the evaporator and compressor penalties (suction passages and electric motor sized for R-134a). Nevertheless, HFO-1234ze had a COP of 1.11 (efficiency), which is above the minimum level (1.0) mentioned by previous studies. We believe that this loss of efficiency can be recovered using a compressor properly sized and designed for HFO-1234ze as well as some minor modifications to the evaporator.”
DRsSeveral manufacturers discussed what were just described as “developmental refrigerants” as possible eventual alternatives to HFCs. Research was preliminary and information was often woven into papers that also were looking at R-1234.
In one DuPont report, it was noted “DR-11 is a developmental refrigerant under consideration as a potential replacement of HFC-134a in both existing and new centrifugal chillers. It is a nonflammable azeotrope containing HFO-1234yf with low toxicity, attractive environmental properties, thermo- dynamic properties that match closely those of HFC-134a, excellent stability, and broad compatibility with chiller lubricants and materials of construction. It has a higher volumetric cooling capacity and COP and approximates HFC-134a chiller performance more closely than HFO-1234yf, another potential low GWP refrigerant for chiller applications.”
In a presentation from Honeywell, it noted three developmental refrigerants with what it said was 75 to 80 percent lower GWP and slight flammability, which indicated such refrigerants would fit into the 2L safety category.
CO2Testing also continued with a broader range of applications for R-744 (CO2), as in a report from Federal University of Santa Catarina in Brazil.
The research focused on “CO2 thermodynamic cycles in order to explore design alternatives for light commercial refrigeration systems.” The study looked at a “ variable expansion device coupled to an active control of the discharge pressure with that obtained by a conventional system, assembled with a capillary tube.”
It was shown that “there is a single combination of capillary tube and refrigerant charge that provides the ideal discharge pressure and guarantees the proper filling of the evaporator. Systems with capillary tubes are not able to self-adjust in response to the variations in the operation conditions, and therefore they are penalized in terms of performance when such variations occur. It was also shown that a needle valve in series with a capillary tube is an effective means to control the evaporating pressure and that with such a configuration the system was less affected by the operating conditions. Furthermore, it was shown that the discharge pressure can be properly controlled by the refrigerant charge, with some impact on the evaporator superheating. The variable expansion and discharge pressure-controlled system provides a higher cooling capacity and COP than the original capillary tube system, for all the operating conditions.”
NH3A paper by Andy Pearson of Star Refrigeration Ltd. focused on “how ammonia systems might be adapted to make them more suitable for use on installations that traditionally adopted CFC and HCFC refrigerants in the range of 100 to 600kW cooling capacity for warehouses, blast freezers, and food factory applications.”
It also factored in some of the issues raised with HFCs and those beyond HFCs. Pearson said the paper also reported, “The excellent thermodynamic properties of ammonia set it apart from (some of) the recently developed (fluorine) refrigerants, which are likely to require 30 to 50 percent more electrical consumption for the same job. The dual safety concerns of toxicity and flammability can be fully addressed by designing the ammonia system for low charge and locating the equipment outdoors close to the evaporators.
“The low charge also means that the system is not subject to the complex management regulations, which apply to larger ammonia systems. The low pressure receiver system, which has been used with ammonia in Europe for 20 years, offers an alternative to end users who are required to get rid of existing R-22 plants, but do not want to switch to traditional pumped ammonia systems.”