In-depth thermodynamic analysis of a novel middle temperature solar-fuel thermochemical hybrid hydrogen-electricity cogeneration system with inherent CO2 capture

Considering the formidable challenges of substantial energy consumption and complex system of CO2 capture in hydrogen production and power generation from fossil fuel, a novel solar-fuel hybrid hydrogen-electricity cogeneration system with chemical looping conversion was proposed to produce hydrogen and power by efficient utilization of solar energy and fuel with CO2 source capture. The NiO-doped Fe2O3 mixed oxygen carrier (OC) reduces the reduction temperature, enabling the use of mid-temperature solar energy to drive methane chemical looping, realizing the quality and efficiency improvement of intermittent unstable solar energy. Employing chemical looping reactions, the new system achieves efficient CO2 capture in the fuel reactor, high-purity hydrogen production in the steam reactor, and combined cycle power generation using the heat released from oxidation. Based on the established system model, experimental validation of the key processes in the chemical looping cycle with mixed OC has been conducted. By analyzing the reduction performance of mixed OC, solar and fuel energy flow conversion, system thermodynamic performance, and variable operating conditions of the system, the system key operational parameters, design and off-design performance were determined. Parameter analysis indicates that the mixed OC reduces the reduction temperature from around 800 °C to 650 °C, achieving complete CO2 capture under mid-temperature concentrated solar energy. Under the design direct normal irradiance (DNI) of 800 W/m2, the system achieves an energy efficiency of 66.82 %, a CO2 capture rate of 99.76 %, and a solar energy share of 32.17 %. The proposed system shows the advantages of flexible adjustment to varying solar radiation and wide range regulation of the electricity‑hydrogen output ratio, by controlling the OC circulation and steam feed rates. Under the design DNI, the system's adjustable electricity‑hydrogen ratio ranges from 1.52 to 4.26 kWh/Nm3, achieving flexible and wide range adjustment of system outputs. As DNI increases from 300 to 900 W/m2, the sensible heat of the OC entering the fuel reactor decreases, and more energy is available for power generation, increasing the system's electricity‑hydrogen ratio range from 0.95-3.28 to 1.63-4.46 kWh/Nm3. The promising results indicate that the proposed system provides a new option for integrating solar energy with chemical looping hydrogen-electricity cogeneration, expanding the utilization of solar energy, reducing fossil fuel consumption, and achieving CO2 capture without additional energy consumption.