The NIST project is said to be a response to U.S. industry interest in a new generation of alternative refrigerants that already are required for use in the European Union.
The refrigerants now used are mainly hydrofluorocarbons (HFCs). HFCs are now being phased out in Europe because they remain in the atmosphere for many years, yielding a high GWP. A compound's GWP is defined as the warming potential of one kilogram of the gas relative to one kilogram of carbon dioxide. For example, R-134a has a GWP of 1,430, much higher than the GWP of 150 or less now mandated for automotive air conditioning use in Europe.
Promising low-GWP chemicals include fluorinated olefins, which react rapidly with atmospheric compounds and thus will not persist for long periods.
“What industry is trying to do is be prepared, because moving from a GWP in the thousands or tens of thousands to a GWP of 150 is an enormous challenge, both economically and technologically,” said NIST chemist Michael Frenkel. “We decided to leverage the tools NIST has been developing for the last 15 years to look into the whole slew of available chemicals.”
The new NIST method estimates GWP by combining calculations of a compound’s radiative efficiency (a measure of how well it absorbs infrared radiation) and atmospheric lifetime, both derived from molecular structure. Additional filtering is based on low toxicity and flammability, adequate stability, and critical temperature (where the compound’s liquid and gas properties converge) in a desirable range. The method was applied to 56,203 compounds and identified 1,234 candidates for further study. The method, which was validated against available literature data, is accurate and fast enough for virtual screening applications. The approach is similar to the large-scale virtual screening and computational design methods for discovering new pharmaceuticals.
The screening is the initial stage of a larger study funded by the U.S. Department of Energy. The next step will be to further narrow down the candidates to a couple dozen suitable for detailed investigation in refrigeration cycle modeling.
Publication date: 10/8/2012