A Fixed-Charge Control Strategy of Trans-critical CO2 Rankine Cycle

The trans-critical CO₂ Rankine cycle (TCRC) is an effective solution for waste heat recovery and Carnot battery applications, especially when combined with sensible heat storage and operating over a wide temperature range. However, controlling the cycle can be complex, as its performance depends heavily on selecting an appropriate high pressure. This study proposes a fixed-charge control strategy for TCRC. Instead of adjusting the high pressure in real-time, the optimal refrigerant charge is selected and maintained constant throughout operation, which is determined under an intermediate condition defined by the cooling water and thermal oil supply temperatures. The relationship between high pressure, running charge, and thermal efficiency is illustrated, and the optimum charge distribution under various conditions is presented. Then, the fixed-charge strategy is compared to the conventional optimum-high-pressure approach in a TCRC equipped with a liquid reservoir. Around the intermediate condition, both methods yield similar thermal efficiency. A refrigerant reservoir can be added between the gas heater and the expander to improve performance over a wider range of conditions. This modification reduces the maximum absolute deviation in thermal efficiency to only 0.19 %. This work supplies a simpler alternative control strategy for TCRCs, reducing system complexity while maintaining high performance.