Preliminary tests on dynamic characteristics of a CO₂ transcritical power cycle using an expansion valve in engine waste heat recovery

CO₂ has been proposed recently as working fluid for power generation from low grade heat sources due to its good thermodynamic properties and natural feature. CO₂ transcritical power cycle (CTPC) is suitable for engine waste heat recovery since it has the advantage of miniaturization and better temperature matching. In this research, a constructed test bench of CTPC using an expansion valve was dynamically tested to recover exhaust energy from a heavy-duty diesel engine. CTPC system dynamic responses to mass flow rate and pressure ratio are presented in detail. Based on dynamic characteristics, a steady state detection method is proposed, which will give reference for experiments of waste heat recovery systems and make it more time-saving and economical. Results show that the CTPC system possesses good dynamic characteristics and the average transition time is less than 62 s. System high pressure or expander inlet pressure could be chosen as a representative indicator for steady state detection and experiment guidance. In addition, the test set-up is of high-accuracy and reliable after examining the heat balances over the heat exchangers and error propagation of the measurement uncertainties. CTPC is applicable for engine waste heat recovery although more efficiency improvements are needed. - Highlights: • Dynamic testing results of CTPC using an expansion valve are given in detail. • CTPC shows good dynamic characteristics and possesses short transition time. • A steady state detection method is proposed based on dynamic characteristics. • High pressure could be employed as an indicator for steady state detection. • CTPC is proved to be applicable for engine waste heat recovery by experiment.