Na2CO3 promoted CaO-based heat carrier for thermochemical energy storage in concentrated solar power plants

Calcium looping (CaL) is a promising thermochemical energy storage (TCES) technology to convert solar energy to power in CO2 Brayton cycle. However, the energy storage density (ESD) of the CaO-based heat carries decays drastically over the CaL cycles, and the energy storage performance of the CaO-based materials in a close-loop CaL-TCES system is still unclear. In this work, a novel Na2CO3 modified method was proposed to enhance the ESD of the CaO-based materials over the close-loop CaL-TCES process, and the enhancing mechanism was investigated by the simultaneous thermal analyzer, X-ray diffraction, electron microscopy, N-2 physisorption, X-ray photoelectron spectroscopy. The "10Na(2)CO(3)/CaO " showed more promising ESD over the cycles, even though it suffered sintering. After 60 cycles, the "10Na(2)CO(3)/CaO " still held an ESD of 594 kJ/kg, which was 2.27 times that of "Neat CaO " when cycled at "850 ?degrees C-900 ?degrees C " mode. Increase the carbonation temperature from 650 ?degrees C to 850 ?degrees C, the cyclic energy storage densities of the "10Na(2)CO(3)/CaO " ascended rapidly, which was opposite to the "Neat CaO ". The optimum operating condition for the "10Na(2)CO(3)/CaO " is "850 ?degrees C-900 ?degrees C " when operated in the close-loop CaL-TCES system. The Na+ species that enriched on the surface of the CaO grains were in amorphous or melting state, and they would react with CaO to form Na-Ca solid solution over the cycles. The "10Na(2)CO(3)/CaO " has a tendency of sintering over the cycles, but Na2CO3 improves reaction kinetics. One suggested mechanism is that the Na+ species may break through the mass transfer barriers to absorb CO2 during the carbonation process. Therefore, although the Na2CO3 doped CaO materials had poorer surface topography, they held much higher ESD over the repeated cycles.