Commercial refrigeration systems applying R744 as the only refrigerant still have a large potential in development regarding energy efficiency, heat recovery and cost efficiency. Special focus and emphasis has to be given to the system architecture with respect to increase the system efficiency when these units are operated at elevated ambient temperatures. The objective of this thorough theoretical study is to investigate the energy required for different R744 refrigeration systems at 25-50-75-100% cooling load conditions. All R744 system configurations are assumed to operate at high ambient temperatures (from 30 to 42 degrees C) which mean only transcritical operations are considered for the following system configurations. Some alternatives are sustainable and viable competitors to conventional HFC supermarket refrigeration systems, up to now applied in warm climates: Standard booster cycle (baseline) Expander cycle (expander ->electrical generator) R744 booster cycle with a mechanical subcooler (MS) unit: working fluid MS: hydrocarbon Economiser I cycle (with a flash tank, i.e. parallel compression) Economiser II cycle (without a flash tank; i.e. parallel compression) Ejector supported parallel compression system These different cycles are evaluated with advanced spreadsheets assuming realistic component performances.