基于有机朗肯循环的内燃机余热回收系统准动态模拟

Semi-dynamic Simulation of ORC Based on Engine Waste Heat Recovery

作为一种有效的提高内燃机整体热功转化效率的技术手段,基于有机朗肯循环(organic rankine cycle,ORC)的内燃机余热回收技术受到越来越多的关注。通常来讲,组织合理的ORC的最关键的技术在于选择合理的循环工质和合适的膨胀机。对于车用内燃机余热回收,同时需要考虑内燃机的运行工况以及由此带来的烟气流量和温度的变化,以对余热回收系统进行有效的控制,达到最佳的工作效率。采用数值模拟的方法对内燃机排气进行余热回收,在不同软件环境下建立一维详细内燃机子模型和有机朗肯理想循环子模型,并将其进行耦合,达到根据内燃机工况变化进行准动态模拟的要求。通过对水,R123a和R245fa三种不同循环工质的考察发现,利用水作为循环工质具有最高的热效率,然而由于水是湿工质,大多情况下不能产生过热蒸汽,因而不适于作为余热回收的工质。对于两种有机制冷剂,R245fa比R123a具有更高的循环效率。通过对WHSC循环准动态模拟显示,需要对ORC的工质流量根据内燃机工况进行控制。通过目前较简单的控制,循环总效率可提高8.1%。

As an effective technology which can potentially improve internal combustion （IC） engine efficiency significantly, Organic Rankine Cycle （ORC） based IC engine waste heat recovery has attracted increasing atten- tions. Generally, the key factors for organising an ORC lie on organic work fluid and expander selection. For waste heat recovery of vehicle engines, particular attention needs to be paid on the dynamic operation conditions, and corresponding exhaust gas flow rate and temperature change. The ORC system will need to be controlled following the engine operation conditions to maintain a high efficiency. A quasi-dynamic ORC based IC engine waste heat recovery model was developed by coupling three independent sub-models which were developed in different software environments, including a 1 - D engine model and an ideal ORC model. Water, R123a and R245fa were investigated as working fluid candidates. The simulation results suggest that despite water has the highest cycle efficiency, it is impossible to use it because as a wet fluid, water cannot be heated to superheated steam in most of the conditions. For the two organic refrigerants, R245fa is superior to R123a in terms of cycle efficiency. The results for quasi-dynamic WHSC cycle simulation suggest that the control of working fluid flow rate is necessary to maintain a high ORC cycle efficiency A preliminary optimal control can reach 8.1% fuel economy improvement throughout the whole cycle.