Natural refrigerant-based subcritical and transcritical cycles for high temperature heating

Theoretical analyses of subcritical/transcritical heat pumps using four natural refrigerants, carbon dioxide, ammonia, propane and isobutane have been carried out for high temperature heating applications at different heating outlet temperatures and heat sources using computer models. The compressor discharge pressures have been optimized for transcritical and subcritical (with near critical operation of condenser) cycles. Results show that for subcritical heat pumps, use of subcooling is efficient for heating applications with a gliding temperature. Results also show that although propane yields better coefficient of performance (COP) in low temperature heating applications, ammonia performs the best in high temperature heating applications. Finally, design aspects of major components of all the four heat pumps for high temperature heating have been discussed, particularly with reference to suitability of available lubricants to the newly evolved operating conditions.