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Analysis of compressed air regenerative braking and a thermally enhanced option

Obtaining a net benefit from regenerative braking is difficult. Various forms of regenerative braking systems have been proposed. They include electric batteries, capacitors, flywheels, and compressed air. Some have been implemented on a limited scale. The benefits of regenerative braking systems are easy to qualitatively describe, but can be challenging to quantify. The potential benefits depend upon obvious things such as driving patterns and the often overlooked considerations such as additional weight and cost, the less than ideal charge-discharge efficiency and a strategy for energy storage management that requires a forecast of the future driving pattern, such as uphill or downhill or stopping or accelerating. This paper will analyze a hydraulically coupled compressed air storage system that has been proposed for heavy vehicles. It uses the hydraulic system for transferring energy between the compressed air and the vehicle. The paper will also consider the possibility of thermal enhancement of the compressed air with engine exhaust heat. With this enhancement the compressed air system becomes an Otto cycle engine. The expansion can occur at a higher pressure than the compression. Thus, thermally enhanced storage has the potential of recovering more work for the vehicle with the discharge expansion than was put in during the charging compression.