Dynamic behavior and control efficacy in carnot battery systems: A comparative study of feedforward, PID, and combined control methods

Abstract

This study establishes a dynamic model of a Carnot Battery (CB) system to analyze the response of key parameters (evaporating pressure, superheat degree, and mass flow rate) to step and periodic heat source fluctuations under two control strategies (compressor input work control and ORC superheat degree control). Three control methods (feedforward, PID, and combined control) are comparatively evaluated in terms of system performance, response time, and stability. Results show that without control, the superheat degree is most sensitive to fluctuations, while mass flow rate is least affected. When the temperature step ratio is 6 %, under ORC superheat control, the superheat and mass flow rate of the ORC system increase by 198 % and 646 % respectively, thereby increasing the net output power by 880 %. Meanwhile, the HP system shows a 35 % and 28 % increase in evaporating pressure and superheat degree under compressor input control. The combined control method delivers the fastest dynamic response (11.53 s for HP, 33.06 s for ORC), outperforming PID by over 55 % and 30 %, respectively. Periodic fluctuation tests reveal increasing evaporating pressure overshoot in the HP system and decreasing overshoot in the ORC system with longer cycles. The superheat degree control strategy reduces levelized cost of storage (LCOS) by 0.0083 $/kWh and increases energy storage capacity (ESC) by 0.112 kWh/t, while enhances the available energy ratio (AER) by 49.7 t. These findings highlight the superheat degree control strategy, especially for combined control, as the most effective approach for enhancing the dynamic, economic, and environmental performance of CB systems.