A novel high-efficiency solar thermochemical cycle for fuel production based on chemical-looping cycle oxygen removal

Solar-driven two-step thermochemical cycling is a promising means to convert solar energy into storable and transportable chemical fuel, in which hydrogen or carbon monoxide is generated by continuous reduction and oxidation reactions. However, the high energy consumption of deoxygenation in the reduction step is an important factor restricting its efficiency improvement. In this work, we propose a fuel production system with thermochemical cycles coupled with chemical-looping cycles, which uses the chemical-looping cycle oxidation reaction at relatively low temperature to absorb the oxygen produced by the thermochemical cycle reduction reaction. The oxygen-carriers in the chemical-looping cycle can be reduced by adding reductants or direct heating with waste heat from the thermochemical cycle. The coupled system can remove the oxygen in the thermochemical cycle reduction reaction and reduce the energy consumption in this process. In addition to the hydrogen production, waste heat from the thermochemical cycle can also be used in the chemical-looping cycle to generate extra electricity. Theoretical calculation results show that in the oxidation temperature range of 900–1100 °C, the energy consumption for separating oxygen from inert gas after sweeping in the traditional thermochemical cycle accounts for 30–57% of the total energy consumption, while the chemical-looping cycle part in the coupled system accounts for 26–36%. The coupled system can improve the solar-to-fuel efficiency by 45.9% to 20.9% and improve the solar-to-electricity efficiency by 104.1% to 14.6% without heat recovery compared to traditional thermochemical cycles when the reduction temperature is 1500 °C. Under the conditions of 20% solid-state heat recovery and 90% gas-state heat recovery, the coupled system achieves a solar-to-fuel efficiency of 28.1%. In addition, we also put forward a vacuum pump, inert gas and chemical-looping cycles combined oxygen removal method. Since the vacuum pump has high efficiency and fast deoxygenation speed in the low vacuum interval, the system efficiency can be further improved. Our research can provide a new solution to the high energy consumption of deoxygenation in thermochemical cycles.