Conference Agenda

R410A (GWP ~ 2088) is imminently being replaced mainly by R454B (GWP ~ 465) and R32 (GWP ~ 675) in residential and light commercial air-conditioning and heat pumping equipment. R454B is a binary blend of R32 (68.9%) and R1234yf (31.1%). However, there continues to be regulatory and societal pressure to further reduce the direct global warming potential (GWP) of refrigerants used in these products. One path to lower GWP in unitary-type products is to employ other compositions in the R454 (R32/R1234yf) series. In particular, the composition of 43.5% R32 and 56.5% R1234yf, known as DR‑4 (predecessor to newly designated R454D, 43%/57%), has a GWP ~ 300, identified at one time as a potentially practical long-term limit for unitary-type equipment. Additionally, the composition of 21.5% R32 and 78.5% R1234yf (R454C) has a GWP ~ 150, a limit set in early evaluations of alterna­tives to R134a in automotive systems and now an upper bound in newly passed future regulations.

This paper reports measurements of performance made on a commercial 10.5 kW (3 RT) rooftop unit (RTU) heat pump. The unit was run with R410A and R454B as baselines and with DR‑4 and R454C. The original compressor was replaced with a compressor of larger displacement for the DR‑4 and R454C tests. An AFD was also used to adjust the compressor speed to achieve similar capacities with all refrigerants. Measurements were made in both cooling and heating modes; only the cooling performance is presented here due to space limitations. The measured performance of the RTU with each refrigerant is compared with predictions based on a simple thermodynamic cycle model. The capacity and efficiency with R454B were about as expected relative to R410A. Performance with DR‑4 was close to the simplified predictions with deviations due mainly to performance differences between the original compressor and the larger replacement as well as slightly better evaporator performance with DR‑4. Performance with R454C fell short of the simplified model expectations, especially in cooling mode. This was mainly due to poor performance of the indoor coil operating as the evaporator. Overall, it was determined that DR‑4 is a very suitable replacement for R454B if the GWP limit were to be set at 300. R454C is a feasible alternative to R454B if the GWP limit were to be set at 150. However, R454C’s larger temperature glide and lower gas density present engineering design challenges to replacing R454B. Other fluids with GWP < 150 that have higher operating pressures and smaller temperature glides would be better candidates.

Ultra-low temperature refrigeration applications for medical or intermodal applications among others have mainly relied on R-23 or blends with R-116 forming R-508A or R-508B. All these options have very high global warming potential and the search for alternatives that can provide similar operating characteristics and performance with a lower GWP has been difficult. R-508B is used in the low temperature stage of the cascade refrigeration cycle of ULT freezer units used for medical storage applications for tissue samples or vaccine handling. R-473A has been proposed as an option for ULT applications with a GWP of less than 2000. Refrigerant blend evaluations in ULT systems have shown that R473A has superior performance characteristics down to evaporation temperatures of about -65C. However, many ULT applications require lower evaporation temperatures to maintain cabinet temperature levels of about -85C. Further development work has resulted in another developmental blend designed achieve these operating requirements while having a significant GWP reduction compared to R-508B.

This paper investigates the experimental evaluation of a medical cascade refrigeration ULT freezer system that utilizes R-449A in the high temperature stage and the developmental fluid in the low temperature stage. Characteristic performance and operating parameters such as bottom out temperature, pull down performance and energy consumption are evaluated and compared to a representative baseline system using R-508B in the low stage.

Trifluoroethylene (HFO-1123) emerges as a promising next-generation refrigerant for refrigeration and air-conditioning applications, attributed to its extremely low global warming potential (GWP) of less than one and zero ozone depletion potential (ODP). Nevertheless, it is known to cause a thermal decomposition, called disproportionation reaction, under extreme conditions of high temperature and pressure. Previous research investigated the marginal pressure for the propagation of the HFO-1123 disproportionation reaction. The addition of inhibitors proved an effective method for suppressing this reaction. This inhibition effect was confirmed through extensive laboratory experiments and further substantiated by validation experiments using an actual compressor. Results demonstrate that disproportionation reaction can be effectively suppressed even under the anticipated worst operational condition. However, comprehensive consideration of flammability, refrigerant performance, and GWP is essential for commercial application. Consequently, this study extends its scope to include validation experiments on new refrigerant mixtures, considering these additional factors. Results identify several combinations that effectively suppress the reaction. Moreover, a previous study demonstrated that propagation of the disproportionation reaction could be suppressed when passing through a narrow slit, which is attributed to heat conductivity at the surface of the narrow slit. This phenomenon resembles the quenching distance observed in the flame propagation through the parallel boards. Therefore, the internal design of refrigerant compressors is crucial to suppressing the propagation. This hypothesis is verified through the validation experiments. Findings from this study provide valuable insights into the practical implementation of substances with disproportionation reaction issues like HFO-1123 as a next-generation refrigerant. Furthermore, results also reveal that the input energy significantly influences the disproportionation propagation.

A hydrofluoro-olefin based (HFO) refrigerant with reduced direct global warming potential (GWP) and environmental impact, R-454B, is replacing the current widely-used refrigerant, R-410A, for residential and light commercial air conditioning and heat pump systems in the US and beyond due to its similar physical properties and performance to R-410A while meeting new US regulations that require new residential heat pump and air conditioning systems to use a refrigerant with GWP less than 700 after January 1, 2025. Although R-454B can be a long term-solution for many years to come and offers the closest performance to R-410A with a GWP of 466, there are frequent questions to explore the tradeoffs and impact on performance with refrigerants with even lower GWP.

Two new potential alternatives have been developed and optimized to provide cooling capacity and energy efficiency close to R-454B and R-410A with a GWP less than 300. R-455C has been developed to have similar cooling capacity within about 11% of R-410A with similar energy efficiency. R-454D has been developed to have cooling capacity within about 18% of R-410A with similar energy efficiency. Recent work has also been published with a refrigerant option with GWP less than 150 such as R-454C, and the performance comparisons and system optimizations for each of these options will be compared.

This paper explores the performance of these new lower GWP refrigerants in stationary air conditioning and heat pump applications. A commercially-available residential heat pump designed for R-410A was modified to optimize the performance of each of these options, including electronic expansion valves to control at equivalent amounts of superheat and increased compressor displacement for testing at equivalent cooling capacity. Data presented will include thermodynamic properties, performance test results, thermal stability, lubricant miscibility, and materials compatibility compared to the existing refrigerant. These new refrigerant blends could provide useful options to maintain similar safety and performance in today’s air conditioning and heat pump systems while reducing direct emissions to meet regulatory demands in the future.

Composition shift evaluation and analysis of difluoromethane (R32) and 2,3,3,3-tetrafluoropropene (R1234yf) mixture were experimentally conducted using near-infrared absorption (NIR) spectroscopy in a heat pump system with a pseudo accumulator. The NIR spectroscopy is a non-destructive direct detection method validated for the zeotropic mixture in our previous works. Molar attenuation coefficients at wavelengths of 1.59, 1.64, 1.70 and 1.78μm were obtained using 1.5 μm wavelength data as a reference spectrum, where the errors were calibrated by a Gaussian process regression in this work. The heat pump system was charged with refrigerants of R32 : R1234yf = 27 : 73 and 52 : 48 wt%, and increase in the volatile R32 component were detected when large retention was observed in the visualized accumulator. The correlation between the retention ratio to charged refrigerant and circulating composition measured in steady conditions showed good agreement with the analytical estimation based on the chemical properties of the components. However, the dynamic composition was dependent not only on the chemical properties but also on the refrigerant transition in the heat pump system. The transient temperature and pressure information of the accumulator circuit were identified as key parameters for improving dynamic detection of composition shift, as they have a qualitative impact on refrigerant retention in the accumulator and thus dominate the circulating composition.

This study investigates the dynamic shifts in refrigerant technologies driven by environmental regulations, particularly emphasizing low global warming potential (GWP). Moreover, there is a rising trend in the adoption of aluminum tubes with internal axial micro-fin structures in heat exchangers to reduce costs. The research focuses on the condensation process within an expanded axial micro-fin aluminum tube with a 5.96 mm fin-tip diameter. Various refrigerants are analyzed, including both single compounds (R-32, R-1234yf, R-1234ze(E)) and zeotropic mixtures (R-454B, R-454C, R-455A). Experimental procedures cover a range of condensation temperatures (35~45 °C), reduced pressures (0.21~0.55), and mass fluxes (150~350 kg/(m² s)), providing crucial data on heat transfer coefficients (HTC) and frictional pressure gradient (FPG). This data is particularly significant for high-glide refrigerants and is instrumental in the design of advanced air conditioning and refrigeration systems aimed at mitigating global warming.