Barbara Haviland Minor

Flammability characteristics of HFO‐1234yf

A new low global warming refrigerant, CF3CF=CH2 (HFO‐1234yf), has been developed to replace HFC‐134a in automotive air conditioning systems. HFC‐134a is being phased out in the European Union due to high global warming potential. HFO‐1234yf is highly energy efficient, exhibits low toxicity, and can potentially be used in systems currently designed for refrigerant HFC‐134a with minimal modifications. Significant work has been completed to assess the flammability characteristics of HFO‐1234yf. Though it is flammable per ASTM E‐681, it is significantly less so than HFC‐152a which has also been considered as an HFC‐134a alternative. HFO‐1234yf has a relatively high lower flammability limit at 6.2 vol% in air. Computational Fluid Dynamics (CFD) modeling has shown it is very unlikely the lower flammability limit will be reached when the refrigerant is leaked into the passenger compartment of a vehicle. HFO‐1234yf also has a high minimum ignition energy (MIE) from 5,000 to 10,000 mJ which indicates there may be very few potential ignition sources in a vehicle. It also has very low burning velocity at 1.5 cm/sec indicating low potential for damage should an ignition occur. Several automotive industry groups have completed extensive risk assessments which have concluded HFO‐1234yf is safe to use as a refrigerant in vehicles. This article will review flammability test results and challenges that were overcome to measure flammability of a mildly flammable refrigerant as well as CFD modeling work for vehicle leakage scenarios.

Nonflammable Low GWP Working Fluid for Organic Rankine Cycles

Regulatory pressure has been increasing globally to address the issue of climate change. In particular, there are plans to reduce the use of hydrofluorocarbon (HFC) based working fluids across many applications, as HFCs are forecast to be significant contributors to global warming in the future. Therefore, there is a need to find low global warming potential (GWP) fluids suitable for organic rankine cycles (ORCs) in those systems where HFCs have historically been preferred. These are usually systems that require a non-flammable working fluid. A new ORC working fluid, cis-1,1,1,4,4,4-hexafluoro-2-butene, also called DR-2 (cis-CF3CH=CHCF3) has been developed which is nonflammable with very low GWP of 8.9 and an ozone depletion potential (ODP) of zero because it contains no chlorine or other halogen atoms other than fluorine. DR-2 also has a favorable toxicity profile based on testing to date. DR-2 is thermally stable in the presence of lubricant and metals, air and oxygen up to the maximum temperature tested of 250°C. DR-2 has a boiling point of 33.4°C and a relatively high critical temperature of 171.3°C, which result in relatively low vapor pressures and high cycle energy efficiencies. It can enable more environmentally sustainable ORC platforms to generate electrical power from widely available heat at higher temperatures and with higher energy efficiencies than incumbent working fluids.Copyright