Engine working condition effects on the dynamic response of organic Rankine cycle as exhaust waste heat recovery system

Organic Ranking Cycle (ORC) is paid more and more attention on waste heat recovery of internal combustion engines. The ORC system is usually designed under the rated working condition of engines, while the engine often works under different conditions, which means the ORC system always works at part-load conditions and unsteady state as well. Consequently, the research of ORC dynamic response process is very significant and it is useful to develop the control system which ensures the safety and efficiency of the ORC during the whole running process. ORC is nonlinear system and the dynamic response is not the same under different engine working conditions. Therefore, the dynamic math model of an ORC with a medium heat transfer cycle as waste heat recovery system for a natural gas engine of 1000 kW rated power is built by Simulink in this work. Since ORC is mainly controlled through working fluid pump speed, the dynamic response process to pump speed change of four main ORC parameters (evaporating pressure, working fluid enthalpy at the end of heating, superheat degree and condensing pressure) under various engine working conditions is compared to illustrate the engine working condition effects. The results show that the dynamic response of condensing pressure does not vary much under different working conditions, while the dynamic response of the other three ORC parameters mentioned above change a lot. Their variation magnitudes become large and their response speeds get slow as the engine working condition declines. Finally, the control system can be improved under low working conditions based on these regularities.