Dynamic characteristics and control strategies of an 873.15 K-class high-temperature CO₂ heat pump tailored for load-following operation in Carnot battery energy storage systems

The increasing demand for power system flexibility, driven by the high penetration of renewable energy sources, has highlighted the importance of fast-response energy storage technologies. Among these, Carnot battery systems have attracted growing attention, in which ultra-high-temperature CO₂ heat pumps serve as key power-to-heat conversion units. However, it remains unclear how thermal inertia governs the ramp-rate capability and thermal-output response of ultra-high-temperature CO₂ heat pumps during load-following operation. Therefore, a system-level, high-fidelity dynamic model of an 873.15 K-class high-temperature CO₂ heat pump is developed for energy storage applications. Two regulation strategies, namely compressor-speed modulation and inventory-tank regulation, were investigated to identify the load-following characteristics of the system within their respective regulation limits of 16.45% and 49.41%. Owing to thermal inertia, the average downward ramp rates reached −5.39 and − 11.19% min⁻¹, respectively, while higher upward ramp rates were observed because of the reduced thermal inertia under low-load conditions. Compressor-speed regulation is primarily limited by the delayed thermal response of the gas cooler and gas heater, while inventory-tank regulation is governed by the coupled effects of CO₂ inventory migration and recuperator re-equilibration. Under an AGC-like power command, compressor-speed regulation is better suited for small-amplitude, high-accuracy tracking, with an upward ramp rate of 7.70%·min⁻¹, whereas inventory-tank regulation favors deeper and faster modulation, achieving 29.01%·min⁻¹ during load recovery. At the Carnot-battery level, the proposed heat pump enables estimated round-trip efficiencies of 64.40–64.75% for Greenfield sCO₂ deployment and 60.90–61.15% for Brownfield steam-Rankine repurposing, indicating its potential as a flexible charging interface for renewable power absorption.