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 CO 2 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 Na 2 CO 3 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 °C–900 °C” mode. Increase the carbonation temperature from 650 °C to 850 °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 °C–900 °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 Na 2 CO 3 improves reaction kinetics. One suggested mechanism is that the Na + species may break through the mass transfer barriers to absorb CO 2 during the carbonation process. Therefore, although the Na 2 CO 3 doped CaO materials had poorer surface topography, they held much higher ESD over the repeated cycles.