US Patent 7238299 Heat transfer fluid comprising difluoromethane and carbon dioxide

Disclosed are heat transfer fluids which possess a highly desirable and unexpectedly superior combination of properties, and heat transfer systems and methods based on these fluids. The heat transfer fluid comprise from about 30 to about 70 percent, on a molar basis, of carbon dioxide (CO2) and from about 30 to about 70 percent, on a molar basis, of hydrofluorocarbon (HFC), preferably HFC having one to two carbon atoms, and even more preferably difluoromethane (HFC-32). The preferred fluids of the present invention have a vapor pressure of at least about 100 psia at 40° F. and are also preferably not azeotropic.