Performance Optimization of a CO2 Energy Storage System Coupled with Flywheel Waste Heat Utilization

Abstract

In the context of large-scale grid integration of renewable energy, conventional CO₂ energy storage systems face two core technical bottlenecks: first, poor tolerance to electrical power input fluctuations, making it difficult to match the intermittent and volatile output characteristics of renewable sources such as wind and solar power; second, high exergy loss in heat exchangers, which significantly restricts the potential for improving overall system energy efficiency. These issues severely limit scale-up applications in long-duration energy storage. To address the sensitivity of traditional CO₂ energy storage systems to power input fluctuations and the high exergy loss in heat exchangers, this paper proposes a coupled energy storage scheme featuring "Flywheel Dynamic Frequency Modulation + Waste Heat Synergistic Exchange." This approach achieves a synergistic solution to the two core problems by utilizing the flywheel to smooth power fluctuations and utilizing cascading waste heat to reduce exergy loss. An Aspen Plus thermodynamic model of the system was constructed, incorporating a Flywheel Permanent Magnet Synchronous Motor (PMSM) dynamic control module and optimizing the internal helical heat exchange structure of the rotating shaft to analyze and verify system performance for efficiency improvement and exergy loss reduction. The results indicate that under the operating conditions of an energy storage pressure ratio of 7.0 and a flywheel speed of 12,000 rpm, the system Round Trip Efficiency (RTE) reaches 77.3%, an increase of 34.7 percentage points compared to pure CO₂ energy storage. The energy storage density is 12.94 kWh/m³, and heat exchanger exergy loss is reduced by 28.6%. Under conditions of ±20% power fluctuation, the flywheel frequency modulation response time is ≤0.5 s, and the system output power fluctuation amplitude is ≤5%, enabling stable adaptation to the fluctuating output characteristics of renewable energy. This coupled system provides a highly efficient and stable technical solution for long-duration energy storage under renewable energy fluctuation conditions and possesses significant engineering application value.