Many efforts of using carbon dioxide (CO₂) as the safest working fluid or refrigerant in areas of heat pump (HP), air conditioning (AC) and refrigeration  have been made. However, the ideal CO₂ refrigeration cycle has too large throttling loss to be ignored. Recently, the author have developed refrigeration theory where a new thermodynamic model for assessment of various refrigerants including CO₂ and their refrigeration cycles was proposed, and indicated that the future work will focus on the innovation in refrigeration technology including recovery of expansion energy (REE) to improve CO₂ refrigeration cycle performance for its  wide use.