Investigation of Engine Waste Heat Recovery Using Supercritical CO₂(S-CO₂) Cycle System

Primary energy consumption of diesel engines is increasing rapidly and strict emission standards are introduced by the government. Interests in engine waste heat recovery have been renewed to alleviate the energy shortage and emission issues. Supercritical CO₂ (S-CO₂) cycle has emerged as a promising method considering its compact structure and system safety level in addition to the environmental friendly characteristics. This paper explores the potential of using S-CO₂ cycle system for engine waste heat recovery. Both heat load from the low temperature jacket cooling water and the high temperature engine exhaust gas are intended to be recovered. In the original system, the jacket cooling water is used to preheat the S-CO₂ working fluid and the engine exhaust gas is utilized in the preheater. As an optimized scheme, system with two preheaters is presented. The engine exhaust gas is further cooled in a high temperature preheater after the jacket cooling water in the low temperature preheater. The available heat load from these two heat sources can be entirely recovered. However, the increasing preheating temperature suppresses the regeneration effect. A regeneration branch is then added in the system. Part of the S-CO₂ working fluid from the compressor goes into a low temperature regenerator and then converges with the other part from the two preheats. A deeper utilization of the regeneration heat load is achieved and performance enhancement of the S-CO₂ cycle system is expected. The maximum net power output of the system with regeneration branch reaches 82.8 kW, which results in an 8.5% increment on the engine power output.