Perspective on compressed CO2 regenerative braking systems for passenger cars

• Historical significance of Peugeot Citroën's Hybrid Air in demonstrating pneumatic regenerative braking for urban and low-speed driving. • Viability of using compressed CO₂ as a working fluid in regenerative braking systems, particularly for passenger cars in urban environments with frequent braking. • CO₂-based systems offer superior energy density, faster response times, and better thermodynamic efficiency compared to traditional air-based pneumatic systems. • CO₂-based PRBS aligns well with hybrid vehicles using high-efficiency hydrogen internal combustion engines, offering a promising solution for sustainable energy recovery. The growing demand for energy-efficient and sustainable transportation has led to significant advancements in regenerative braking systems (RBS), which recover and reuse kinetic energy lost during braking. While battery-based RBS dominate the market, alternative technologies such as pneumatic regenerative braking systems (PRBS) offer simpler, cost-effective, and more durable solutions, particularly for heavy-duty and commercial vehicles. This study investigates the feasibility of using compressed carbon dioxide (CO₂RBS) as a working fluid in PRBS, specifically in hybrid vehicles powered by hydrogen internal combustion engines. A comparative analysis is conducted between CO₂RBS and battery RBS, evaluating their efficiency, energy density, lifecycle sustainability, and real-world applicability. The study also revisits Peugeot Citroën's Hybrid Air Technology to assess its potential role in advancing CO₂-based RBS solutions. Simulation-based efficiency comparisons across various driving cycles (WLTP, NEDC, and a custom urban cycle) reveal that CO₂RBS achieves round-trip efficiencies ranging from 45 % to 60 %, compared to 70 % to 85 % for typical battery RBS. Despite this lower efficiency, CO₂RBS offers significant advantages: projected system costs are 30 % to 50 % lower, recyclability is estimated at over 95 % of system components (compared to estimated 50 % for typical battery with current technologies), and preliminary lifecycle emission analysis suggests a potential reduction of 15 % to 25 % compared to lithium-ion battery-based systems, depending on the electricity generation mix. The findings indicate that CO₂RBS could be a viable alternative for cost-sensitive and environmentally conscious applications, particularly in sectors prioritizing robust, maintenance-friendly, and recyclable energy storage solutions. By addressing the material sustainability challenges associated with lithium-ion batteries, this research highlights CO₂RBS as a promising pathway toward more sustainable regenerative braking solutions for future transportation systems.