Solar Thermal Energy Storage Using La_0.2Sr_2.8MnO₃ Perovskite Oxide Redox Chemistry ^†

Abstract

The potential of La_0.2Sr_0.8MnO₃ (LSM28) perovskite oxide for thermochemical energy storage (TCES) is assessed by analyzing its thermochemical performance. The TCES capacity of LSM28 was measured using a non-stoichiometric and van’t Hoff analysis at various reduction temperatures ($T_{red}$) and oxygen partial pressures ($P_{O_2}$). O₂ release and the associated non-stoichiometry (δ) increase with $T_{red}$ and decrease with $P_{O_2}$, according to the results, reaching a maximum δ of 0.101 at 1473 K and 0.0001 atm. The van’t Hoff research also showed that LSM28’s TCES capacity fluctuates greatly with δ, peaking at 37.1 kJ/kg under ideal circumstances.

1. Introduction

As global energy demand rises amid growing climate concerns [1,2], renewable technologies are increasingly developed to ensure sustainable power generation [3,4,5,6]. Among them, Concentrated Solar Power (CSP) utilizes reflective surfaces to focus sunlight onto a receiver, producing heat to generate steam and drive turbines for electricity [7]. However, the intermittent nature of solar radiation, especially during nighttime or cloudy conditions, limits continuous operation [8]. Integrating thermochemical energy storage (TCES) systems offers a practical solution, enabling CSP plants to store excess heat during sunny periods and release it when sunlight is unavailable, thereby ensuring reliable power generation and improving economic feasibility [9].

TCES has emerged as a promising high-temperature storage method for CSP applications [10]. It relies on reversible redox reactions of metal oxides (MOs) [11], where solar energy drives endothermic reduction, and the subsequent exothermic oxidation regenerates heat during discharge. TCES provides higher energy density and longer storage duration [12], improving both efficiency and sustainability of CSP systems.

Perovskite oxides (ABO₃) have gained attention for TCES due to their high oxygen mobility and excellent redox cyclability [13,14,15]. Their reversible reaction can be expressed as per Equation (1), where ${\delta }_i$ and ${\delta }_f$ represent initial and final O₂ non-stoichiometries during reduction and oxidation, respectively. The compositional flexibility of perovskites allows tailoring of O₂ non-stoichiometry and ionic/electronic conductivity [16], enhancing cyclability and TCES capacity. This study presents a detailed non-stoichiometric and van’t Hoff analysis of La_0.2Sr_0.8MnO₃ (LSM28) to estimate its maximum TECS potential under varying thermodynamic conditions.

$$AB{O}_{3-{\delta }_i}\leftrightarrows AB{O}_{3-{\delta }_f}+{\displaystyle \frac{{\delta }_f-{\delta }_i}{2}}{O}_2$$

(1)

2. LSM28-TCES: Non-Stoichiometric Analysis

The reversible redox reaction for the LSC28-driven TCES is presented below:

$${La}_{0.2}{Sr}_{0.8}Mn{O}_3+\Delta H\leftrightarrow {La}_{0.2}{Sr}_{0.8}Mn{O}_{3-\delta }+{\displaystyle \frac{\delta }{2}}{O}_2$$

(2)

The amount of O₂ released is calculated in response to the changes in $P_{O_2}$ and $T_{red}$ to assess the TCES capacity of LSM28. As $T_{red}$ rose, the amount of O₂ emitted steadily increased. For example, the amount of O₂ released during the reduction step, often referred to as the energy storage phase, increased dramatically from 0.00001 mol at 873 K to 0.01377 mol at 1473 K at a $P_{O_2}$ of 0.001 atm. The maximum O₂ emission across all assessed $P_{O_2}$ was placed at 1473 K.

As shown in Figure 1, the non-stoichiometry (δ) associated with the reduction reaction by looking at the quantity of O₂ released by LSM28 was estimated. It was found that δ exhibited a commensurate increase at all $P_{O_2}$ as the O₂ release rose with growing $T_{red}$. For example, δ rose dramatically from 0.00002 to 0.02755 at a $P_{O_2}$ of 0.001 atm. Notably, at a temperature of 1473 K, the highest δ values were observed across all $P_{O_2}$ values.

3. LSM28-TCES: Van’t Hoff Analysis

The ΔH study is performed after examining the equilibrium non-stoichiometry associated with the thermal reduction of LSM28. The TCES capacity of LSM28, specifically for the maximum δ (δ_max), is calculated in kJ/kmol and kJ/kg. It is important to note that the δ_max values obtained for $P_{O_2}$ between 0.21 and 0.01 atm are deemed to be too low and are thus not included in this analysis. To conduct the Van’t Hoff analysis, for δ values of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, we have approximated $T_{red}$ values for different $P_{O_2}$ ranging from 0.01 atm to 0.0001 atm. The results show that the necessary $T_{red}$ decreased as the $P_{O_2}$ lowered for every δ. For example, $T_{red}$ dropped from 1675 K to 1182 K at δ = 0.1, as $P_{O_2}$ dropped from 0.01 atm to 0.0001 atm. As expected, at a $P_{O_2}$ of 0.0001 atm, the lowest $T_{red}$ required to reach the predefined δ was found.

Linear fits are applied to transform the data into van’t Hoff form after estimating $T_{red}$ to reach the predefined δ values. This allowed us to provide iso-stoichiometry curves in the $P_{O_2}-T$-specified plane (Figure 2). We estimated the ΔH (slope) and ΔS (intercept) related to the reduction reaction by conducting a linear regression of the van’t Hoff plot. For the maximum δ attained at $T_{red}$ of 1473 K, the ΔH values obtained—represented in kJ/mol of O₂—were further converted into the corresponding values in kJ/mol and kJ/kg of LSM28.

The integrated values for each δ_max are shown in Figure 3. When $P_{O_2}$ was placed at 0.01 atm (with δ_max at 0.009), the TCES capacity of LSM28, which equates to 3.30 kJ/kg, excluding sensible heat storage, was at its lowest. The maximum TCES capacity, on the other hand, was recorded at a $P_{O_2}$ of 0.0001 atm (where δ_max reached 0.102), which corresponds to 37.1 kJ/kg of LSM28.

4. Summary and Conclusions

This work used thorough non-stoichiometric and van’t Hoff investigations to evaluate the TCES potential of La_0.2Sr_0.8MnO₃ (LSM28). As $T_{red}$ increased, the non-stoichiometry (δ) rose as well, peaking at 0.102 at 1473 K and $P_{O_2}$ = 0.0001 atm. The TCES capacity increased significantly as a result of this increase in δ. TCES capacity of LSM28 ranged from 3.3 kJ/kg (δ = 0.009 at 0.01 atm) to a maximum of 37.1 kJ/kg (δ = 0.102 at 0.0001 atm), according to van’t Hoff study. To evaluate the LSM28’s practical viability, more research is presently being performed to examine its long-term cyclability and real-world performance.