Next Generation Refrigerants for High-Efficiency HVAC&R Equipment

Summary

In response to worldwide initiatives to phase down working fluids that have a high index for trapping of infrared radiation and may be affected by future environmental concerns (e.g., Polyfluorinated Alkylated Substances), the refrigeration and air-conditioning industry is searching for fluids to replace those that are currently in use.  Moreover, the new replacement working fluids are produced by the patent-holding US companies whereas the legacy refrigerants are not. The switch to new refrigerant needs to be done while maintaining high system efficiency and safety.  Consequently, this project will benchmark the heat transfer properties, flammability, and system performance of the proposed possible replacement candidates for vapor compression systems.

To meet the simultaneous requirements for good thermodynamic and fluid mechanic performance (coefficient of performance COP and volumetric capacity Q_vol), low toxicity, low-global warming potential (GWP), and low flammability, industry is going to use blends of compounds. Since the number of components blended may be more than five, the number of permutations is very large. To find optimum blends, existing methods are used to analytically predict other properties; however, no predictive tool exists to rank flammability.  Burning velocity is used by industry as a metric for flammability.  This project will develop experimental techniques for measuring and analytical tools for predicting the burning velocity of refrigerant blends.  These techniques will allow a ranking of different refrigerants with respect to their flammability, optimization of blends, and improved understanding of the full-scale fire behavior of refrigerants.

Industry is moving towards these new refrigerants, but they tend to be more flammable, which is undesirable.  The laminar burning velocity is a good metric for flammability but measuring it for fluorinated refrigerants is difficult and the current standard tests are inaccurate.  The ability to better measure and predict burning velocity would allow industry to more readily develop non-flammable blends, rank their flammability, and understand their full-scale behavior.

The HVAC&R industry relies upon NIST for fundamental refrigerant property data, which includes heat transfer coefficients, thermodynamic parameters, and cycle performance.  The extension of that database to include data to predict burning velocity is appropriate and useful.  NIST is a world leader in developing chemical kinetic models for hydrofluorocarbon (HFC) reactions at high temperature and has unique capabilities to measure and predict the needed parameters. Also, over many years, NIST has developed deep insight into the flame behavior of HFC compounds and can pass that insight onto industry to aid in their safe deployment of these new, mildly flammable compounds as well as to help develop accurate standard tests for flammability. 

Description

OBJECTIVE:

To develop heat transfer, flammability, and system performance information that will assist in the selection and implementation of the best replacements for existing legacy refrigerants that will be phased down per the Kigali agreement.

TECHNICAL IDEA:

The existing HFC refrigerants will be phased down, and new next generation refrigerants are needed. To achieve the desired thermodynamic and thermophysical properties of the refrigerant, blends will be used by industry, and the components of the blends will be varied to optimize performance while minimizing flammability.  For these new compounds, NIST will develop experimental data and predictive methods for optimizing the heat transfer and cycle performance.  For flammability, however, the predictive tools are more rudimentary.  Hence, the capability to predict the flammability of pure compounds as well as blends would be very useful for the optimization of blends and development of new ones.  The laminar burning velocity is a good metric for characterizing flammability because it is a fundamental combustion parameter that can be calculated from first principles, is correlated with quenching diameter, lean flame extinction, minimum ignition energy, and overall chemical rate, and it is used as a scaling parameter for turbulent flame speeds and as an input to full-scale explosion models.  It is also used as a metric in existing and developing codes and standards for refrigerant flammability.  Unfortunately, the present methods of measuring the laminar burning velocity in the standards are inaccurate, cumbersome, and do not allow for inclusion of flame curvature, radiation, and humidity in the air, which are important parameters affecting the burning velocity.

The technical idea in the present work is to improve the experimental methods and predictive tools for heat transfer, cycle performance, and burning velocity.  The emphasis in FY2025 will be on flammability; the primary methods for achieving these are through development of improved data reduction tools for the experiments as well as detailed numerical models of the laminar burning velocity.  Comparisons of the experimental results with the numerical predictions, followed by parametric analyses and sensitivity analyses will provide great insight into both the experiments and the models, facilitating their improvement.  Both literature values of the laminar burning velocity as well as those obtained in experiments of the present project will be used as needed. Industry is continuously developing new refrigerants for inclusion into blends.  The present project will update the NIST kinetic mechanism for refrigerant burning velocity prediction so that the newest compounds can be included in the predictive and experimental data reduction tools.      

An important output of the project will be input into standard groups as well as ongoing ASHRAE research projects on refrigerant flammability.  The laminar burning velocity of select blends of compounds will be measured and predicted numerically.   Modeling of the burning velocity will provide an understanding of the physical properties of the compounds that influence their behavior in both test methods and in full-scale fires, facilitating their safe use.  The insights obtained from these detailed simulations are unique and are of value to the industry as they try to improve their understanding of the fire behavior of these new compounds. 

RESEARCH PLAN:

The plan for FY25 is to improve the experimental techniques for burning velocity measurement so they are accurate and tractable; expand, validate, and distribute the kinetic model and data reduction tool for refrigerant flammability; help ASHRAE/AHRI in planning and implementing their research program in refrigerant flammability; and address new topics in refrigerant flammability of interest to industry as they arise.

NIST is collaborating with researchers at five universities (who have their own funding) to improve the kinetic models, experiments and data reduction for refrigerant flammability to make them more accurate.  That collaboration will continue.  Moreover, because of NIST’s central role in that research, representatives on ISO 817 SC8 (Refrigerants and refrigeration lubricants) have requested that Dr. Linteris act as a liaison between the ISO group and these researchers and he has agreed to do so.  

The 2 liter Constant Volume Combustion experiment for measuring the burning velocity was reviewed in FY23, and based on that review, safety improvements were made to the high-voltage ignition system.  After that review, it was additionally identified that it is desirable to add an HF scrubbing system to the exhaust of the chamber so that this dangerous product is immediately neutralized after each experiment and not released into the duct system.  This HF scrubbing system will be added, tested, and de-bugged in FY25.

Higher temperature refrigeration systems, e.g., chillers, need a low-GWP working fluid, and R-1336yf is promising.  In FY24, we developed a rudimentary kinetic model for its combustion.  In FY25, we will test this mechanism and use it to understand the flammability of this compound, alone and in blends, and improve it if necessary.

Recently, we have discovered that the very common refrigerant R410A may be very close to being flammable.  If this is true, it may change how we view mildly flammable refrigerants in general. In FY25, we will conduct simulations to understand the flammability of R410A, using data available in the literature for validation.     

Dr. Linteris will continue his active role on ASHARE/AHRI project management sub-committees related to refrigerant flammability research, to ensure that the research is accurate and comprehensive.

US and global HVAC industries are clearly adopting hydrocarbons as working fluids for HVAC&R systems. Dr. Linteris will provide technical input to industry regarding flammability of these compounds as compared to other low-GWP refrigerants as they come on-line.