Dynamic performance and control strategy of CO2-mixture transcritical power cycle for heavy-duty diesel engine waste-heat recovery

Carbon dioxide mixture transcritical power cycle (CMTPC) is considered as a new promising technology for waste heat recovery (WHR), while there are still critical challenges arising from the transient fluctuations of heat sources when it comes to the heavy-duty diesel engines (HDDEs) applications. This paper presents a dynamic model of CMTPC systems, which is carefully validated against experimental data. Constant temperature, constant pressure and optimal control strategies are proposed to realize stable and optimal operation. The dynamic performance of the system with different control strategies is predicted under slow step change and transient change conditions. The results demonstrate that under slow step change conditions, the stability of optimal control system and constant pressure control system is better than that of constant temperature control system. In addition, the net power output keeps maximum under the optimal control because of the compromise between operating pressure and temperature. However, under transient conditions the control performance of optimal control is not as obvious as under slow step change conditions. Compared with open loop system, the improvement of system power by optimal control under slow step change is 2.31%, while that is only 0.07% under transient change condition. By contrast, it is found that the constant pressure control achieves the best power improvement of 0.58% under transient conditions because of the satisfactory control effect on operating pressure and relatively small fluctuations of expander inlet temperature. In a word, different control strategies behave obviously different performance under various conditions.