A carbon dioxide energy storage system with high-temperature graded heat storage structure: Thermodynamic intrinsic cycle construction and performance analysis

A research team from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences has developed a groundbreaking carbon dioxide energy storage (CES) system featuring an innovative high-temperature graded heat storage structure. The system achieves a remarkable round-trip efficiency of 76.4% under typical design conditions, significantly outperforming existing technologies and establishing a new benchmark for compressed gas energy storage.

Published in Frontiers in Energy, the study addresses critical limitations in current energy storage solutions as the world transitions toward carbon neutrality. While renewable energy adoption accelerates, large-scale, long-duration storage remains a key challenge. Traditional pumped hydro storage faces geographical constraints, compressed air systems suffer from long construction cycles, and battery technologies have limitations in capacity and adaptability.

A Smarter Approach to Heat Management

The novel CES system leverages unique properties of supercritical CO₂—its high density, low viscosity, and excellent heat transfer characteristics—to create a highly efficient thermodynamic cycle. Unlike conventional CES systems that use single thermal storage media at low-to-medium temperatures, this breakthrough employs a four-stage graded heat storage architecture spanning over 400K.

"We recognized that no single heat transfer fluid could optimally handle such a wide temperature range," explained the authors. "By strategically dividing the thermal zones and using molten salt for high temperatures, thermal oil for intermediate ranges, and both pressurized and atmospheric water for lower temperatures, we minimized both cost and irreversible heat losses."

The system uses single-stage compression with a wide pressure differential to maximize heat storage temperature, while the graded structure captures and recycles waste heat with 95.9% thermal utilization efficiency—dramatically reducing energy waste during the storage and discharge cycles.

Theoretical Framework Enables Precision Design

A key contribution of the research is the development of a universal thermodynamic intrinsic cycle construction method that provides, for the first time, a standardized theoretical framework for CES system design and performance prediction.

"Previous CES development often lacked systematic guidance, resembling trial-and-error approaches," noted authors. "Our method, based on irreversible loss corrections to the Brayton cycle, clearly defines the thermodynamic boundaries for critical parameters like heat storage temperature and cycle efficiency, enabling engineers to optimize designs before implementation."

The theoretical model predicts a maximum practical efficiency limit of 73.5%, within 4% of the actual simulated performance—exceptional agreement that validates the approach. The model reveals that cycle efficiency is primarily governed by three factors: maximum compressor discharge temperature, heat exchanger temperature differences, and heat storage losses.

Engineering Performance and Competitive Advantages

Under typical operating conditions, the system delivers 334 kW of output power per unit mass flow rate while maintaining a relatively modest high-pressure storage of 6.8 MPa—significantly lower than competing designs that require pressures up to 25 MPa. This reduces material costs and safety concerns while maintaining superior performance.

Comparative analysis shows the system surpasses previously reported CES technologies:

1. 76.4% efficiency vs. 50-71% for existing systems
2. Achieves higher performance without expensive high-pressure storage
3. Eliminates need for unproven low-pressure adsorption materials

Path to Commercialization

The research identifies critical optimization pathways: increasing pressure ratio, reducing heat exchange temperature differences, and improving heat exchanger efficiency all positively impact system performance. Notably, operation near CO₂'s critical pressure (7.38 MPa, 30.98°C) creates "optimization points" where turbine output increases dramatically due to enhanced enthalpy differences.

"The graded heat storage concept transforms CES from a promising idea into a commercially viable technology," said the authors. "By using cost-effective materials like water for lower temperature stages and reserving expensive molten salt for only the highest temperatures, we achieve both economic and technical optimization."

Next Steps

The team acknowledges that experimental validation is needed, particularly for high-temperature, high-load compressors and turbines operating under these novel conditions. Future work will focus on prototype demonstration and refining the thermodynamic models to account for real-world equipment performance variations.

Publication Details The research article, "A carbon dioxide energy storage system with high-temperature graded heat storage structure: Thermodynamic intrinsic cycle construction and performance analysis," was published in Frontiers in Energy, 2025. DOI: https://doi.org/10.1007/s11708-025-0995-3