Experimental and numerical investigation on the dynamic characteristics of a lab-scale transcritical CO2 loop

Abstract

Supercritical carbon dioxide (sCO2) Brayton cycle attracts much attention due to its high efficiency, compactness and low water consumption. In a sCO2 cycle, a transcritical CO2 (tCO2) dynamic looping is unavoidable during processes of startup, shutdown and unexpected conditions, which brings many challenges in the system design and operation due to the drastic physical property change of CO2 near the critical point, and its effects on the system dynamic characteristics are still not clear. In this study, we carry out experimental and numerical studies to explore the dynamic characteristics of a simplified tCO2 loop. The numerical model is developed by combining heat exchangers, a pump and a back-pressure valve with a detailed consideration of the coupled CFD and heat transfer processes. We validate the model by comparing the simulation results with the experimental data and obtain a small root-mean-square error (<3%). We investigate the system response and dynamic characteristics in the case studies, where the developed model has a higher accuracy in capturing the dynamic characteristics compared with the experimental sensors, and the change of ambient temperature resulted in a significant fluctuation of gas source pressure between 3.8 and 8.0 MPa. We also observe a pressure overdose of > 1.0 MPa in a short time with the frequency rising from 20 Hz to 50 Hz, which is about 40% of the target pressure change. We find a large acquisition time gap could provide stable results but may lost transient characteristics during the operation.