This chapter first reviews the evolutionary course of thermodynamic systems over the past few centuries, tracing their technological advancements and functional expansions in industrial, energy, and daily-life scenarios. Traditional thermodynamic systems are confronted with inherent critical challenges, with particular focus on their high dependence on fossil fuels. Against this backdrop, a promising alternative solution is proposed, which focuses on carbon dioxide (CO₂)-based thermodynamic cycles. The transcritical Rankine power cycle based on CO₂ is first proposed worldwide, and it is confirmed to possess higher efficiency and better applicability according to experiments. Furthermore, CO₂ refrigeration systems have been explored in depth. Starting from the consideration of refrigeration that crosses the triple point, to the construction of transcritical CO₂ vapor compression system, and finally to the CO₂-based integrated cooling and heating technology that is practically applied at the Beijing Winter Olympics, these developments fully demonstrate the application potential and value of CO₂ in the field of refrigeration.​ In addition, CO₂ also plays a role in other energy systems, such as CO₂ energy storage systems, CO₂ capture and utilization energy systems, and CO₂ jet systems. These diverse energy systems have greatly expanded the application scope of CO₂ in the field of energy systems. It is argued that CO₂ thermodynamic cycles, when integrated with renewable energy sources, will play a crucial role in promoting the transformation of the global energy system toward a cleaner, more sustainable, and low-carbon direction.