Thermodynamics of doped calcium manganite for thermochemical energy storage in concentrated solar power plants

Abstract

The U.S. Department of Energy Sunshot Initiative has suggested a cost target of $15 per kWhth for storage materials in cost-effective concentrated solar power (CSP) plants. This study explored the potential of redox cycles of highly reducible perovskites derived from earth-abundant elements for thermochemical energy storage (TCES) to meet these targets. The TCES capacities of perovskites based on doped CaMnO3 were analyzed by measuring changes in the oxygen non-stoichiometry (Δδ) in low O2 partial pressures (PO2), heats of reaction (Δ hreac), and specific heat capacity (Cp) over temperature ranges relevant for a central-tower CSP plants. This study focused on using earth-abundant dopants to stabilize the CaMnO3 structure during redox cycles without compromising its high degree of reducibility. Over 15 compositions were screened with thermogravimetric analysis (TGA) for high Δδ between high temperatures (Thot) of 890 °C and PO2 ≈ 10-4 bar and low temperatures (Tcool) of 490 °C and air to simulate possible conditions favorable for a CSP plant driving a supercritical CO2 power cycle. Differential scanning calorimetry (DSC) was used in combination with TGA to get integrated values of chemical storage capacity as function of Δδ. By also characterizing the specific heat capacity for sensible energy storage, the total specific TCES capacity was determined for a range of compositions. For more favorable compositions, tests were done for a range of Thot up to 1100 °C to assess the effects of Thot on TCES capacity. For Tcool = 490 °C and Thot = 890 °C, CaCr0.10Mn0.90O3-δ and CaCr0.05Mn0.95O3-δ were identified as the most promising candidates with a total TCES of around 470 kJ per kg and 550 kJ per kg respectively. TCES for CaCr0.05Mn0.95O3-δ exceeds 750 kJ per kg at a receiver temperature of Thot = 990 °C. Based on the experimental TCES capacity for CaCr0.05Mn0.95O3-δ obtained within Tcool = 490 °C and Thot = 890 °C, a 20 MWe CSP plant was designed using NREL's System Advisor Model (SAM). The energy output obtained from this plant was compared to that from a molten salt based CSP power plant with the same size, within Tcool = 290 °C and Thot = 574 °C. Finally, the total storage capacity for CaCr0.05Mn0.95O3-δ based TCES system was compared to the inert-particle based storage system to conclude that TCES can be a possible solution for CSP to reach the $15 per kWhth target set by the DOE.