The drastic change in climate has triggered an imbalance in ecology, socio-economic structure, energy security and sustainable development. Apart from CO2 emissions, use of toxic refrigerants having global warming and Ozone layer depletion potential has accelerated this maleficence in environmental equanimity. This book reflects our sombre concern in this regard. Following are the motivations behind this book: Use of natural/green refrigerant is a useful and vital step in pacifying the climate change. Mixed hydrocarbon refrigerants proposed seem to be potential in phasing out commercial toxic refrigerants. Carbon dioxide (CO2) capture and sequestration or utilization (CCS/CCE) has taken the center stage globally due to the increasing adversaries of CO2 emissions. Utilizing CO2 as refrigerant may be a miniscule drive in the direction of CO2 utilization and encouraging use of natural refrigerants in cooling devises. Present book simulates the performance of AC machine and cryocooler using hydrocarbon and carbon dioxide based mixed refrigerants. All the simulated results published in this book need to be piloting.