Study on off-design performance of supercritical CO₂ cycles coupled with single-tank thermal energy storage under variable heat source temperatures and partial loads

A single-tank molten salt thermal energy storage (TES) with a supercritical CO₂ (S-CO₂) cycle is a key technology for concentrated solar power (CSP) plants to achieve efficient and cost-effective peak-shaving operation. However, the S-CO₂ cycle has to cope with complex operating conditions. This study analyzes the coupled impact mechanisms of heat source temperature and load variations on cycle performance, and optimizes parameter regulation method under off-design conditions. The benefits of the proposed cycle operation mode in enhancing TES energy utilization and improving CSP plant performance are indicated. Results show that allowing turbine inlet temperature below the design value extends operational duration by 140.74 % and increases power generation by 194.65 %, thus elevating the 24-h load fulfillment rate of a hybrid PV-wind-CSP system from 79.61 % to 99.30 %. Additionally, as turbine inlet temperature and load decrease, cycle efficiency generally experiences a downward trend, but the efficiency does not strictly decrease monotonically with the load. Under near-full load conditions, as turbine inlet temperature decreases, the minimum cycle pressure increase sharply so that CO₂ at turbine outlet remains in supercritical state, causing compressor power consumption to decrease significantly to meet high-load demands. However, these conditions raise the likelihood of compressor surge and potential safety risks.