(208d) Reaction Characteristics of Copper Oxides for Solar Thermochemical Energy Storage

1. Introduction

The growing demand for energy utilization drives the research of clean-efficient-sustainable alternative energy sources. Solar thermal power is regarded as potential clean energy but owns disadvantages of instability and intermittency, hence achieving efficient storage to ensure sustainable supply of the thermal energy is the key of solar thermal power generation technology. Thermochemical heat storage is based on the rupturing and restructuring of the chemical bond of the reversible chemical reaction to achieve heat charging and discharging^[1], which has the advantages of large storage density and long storage time. According to different reactants, thermochemical energy storage is divided into metal hydride system, redox system, inorganic hydroxide system and organic system, ammonia decomposition system and carbonate system and etc^[2]. The redox system makes use of the conversion of different valence states of metal oxides to realize the storage and release of the thermal energy. In this system, air acts both as reactive material and heat transfer fluid^[3], leading to a more simplified structure to reduce the cost. Currently correlational researches mainly focus on several potential metal oxides such as the oxides of cobalt, manganese, copper,barium etc^[4], and to improve the thermal storage capacity and enhance the cycle stability. CuO/Cu₂O system (Equ.1) is getting extensive attention because of its low cost and suitable reaction temperature (≈1000℃) for next generation CSP plants, but at the same time the research of reaction kinetics characteristics of CuO/Cu₂O are relatively insufficient. Beyond that, pure CuO/Cu₂O redox couple suffers low re-oxidation rate and poor cycling stability. This paper focuses on improving the characteristics of reaction kinetics of CuO by TGA through quantitative analysis, and explores the possibilities for improvement of the redox reaction by means of combination of cheap inert metal oxides.

2. Experiment setup and methodology

The CuO mixed with 10, 20, 30, wt% Al₂O₃ were synthesized by citrate sol-gel method. The materials were identified by XRD (X-Pert, PANalytical B.V.) and field emission scanning electron microscopy (FESEM, SIRON, FEI). The TGA was performed in a HITACHI STA 7200. A Netszch STA 449 Jupiter F3 instrument was employed for DSC. All measurements were performed under synthesis air mixture of 20% oxygen and 80% nitrogen with a 50mL min^-1 gas flow. The TGA test was performed within the temperature range of 700℃-1100℃ with heating/cooling ramp rate of 5℃、10℃ and 20℃.

3.Result and discussion

3.1 Redox kinetics analysis of CuO/Cu₂O system

Kinetics analysis of pure CuO with heating/cooling rate of 10 °C/min is shown in Fig.1. Reduction reaction begins at ~1030°C and almost completes within ~180 seconds, and then the curves of weight change are “smooth” implying that the material undergoes no further redox reaction. And the weight loss rate is 9.85%, slightly less than theoretical rate of 10%. Compared with reduction, re-oxidation is much slower and there are two successive increments of sample mass for re-oxidation process, lasting for ~957 seconds and 1134 seconds, respectively. During period of cool-down, re-oxidation occurs when temperature drops to ~985°C. The re-oxidation reaction stops when the temperature is lower than 826 °C and could restart at higher temperature. In these two parts of re-oxidation processes, the mass increases by about 6.56% and 2.53%.

To deepen the understanding of reaction mechanism, the kinetics analysis of the reduction and oxidation are carried on basing on the linear fitting of reaction kinetics equation. For reduction, when the conversion rate changes from 0.2 to 0.8, activation energy increases with it in the range of 684 kJ/mol^-1 to 1285.35 kJ/mol^-1, and correlation coefficient R-square are greater than 0.999, which means linear fitting has good correlation. The average activation energy is 941.46 kJ/mol.

3.2. Effect of the lower limit of temperature to pure CuO/Cu₂O systems

The reaction rate of oxidation is too slow to finish the re-oxidation within limited time. It is obvious in redox reaction at heating/cooling rate of 20 °C/min (Fig.2a) that re-oxidation doesn’t react further when the temperature is lowered to a critical value (about 838°C), and restarts when the temperature rises above the critical value. To improve the kinetic characteristics of oxidation reaction, the lower Limit of temperature is increased from 700 °C to 900 °C and 1000 °C (Fig.2b), which are higher than critical value. Temperature control program consists of 3 successive phases, namely the decreasing phase, stabilization phase and the rising phase. Compared with the stabilization phase of 900°C, both temperature rising phase and decreasing phase show higher reaction rates, indicating that rising temperature is beneficial to improve the re-oxidation rate. The re-oxidation is almost constant in 3 successive temperature fluctuation phases when the stabilization phase is set as 1000°C, and then all these experiment data are compared as shown in Fig.2b. Overall, for the increasing of the lower limits of temperature chosen in Fig.2b, temperature procedure of stabilization phase of 900°C is better than that of 1000°C because of the discharge of heat in oxidation reaction. Thus re-oxidation is so sensitive to temperature that an appropriate temperature range should be chosen for charge and discharge.

3.3 Reaction characteristics of Cu-Al oxides

Al₂O₃ is a low-cost material with advantages of high melting point and hardness. In order to reduce the gap between reduction and re-oxidation, samples of Al₂O₃-doped CuO were made and compared with pure CuO by TGA. As shown in Fig.3, The temperature fluctuation procedure consists of 2 successive phases, rising from 700°C to 1100°C and decreasing from 1100°C to 700°C at the heating/cooling rates of 5, 10, 20°C/min. According to the mass changing curve, the initial reaction temperatures of reduction and oxidation are almost the same. There is no observable impact on reduction for CuO but doping causes remarkable changes in the reaction rates of re-oxidation. It is observed that the reactions of mixed samples are faster than that of pure CuO during discharging, as the re-oxidation of mixed samples is almost completely achieved in the cooling phase. For all these tests with heating/cooling rates of 5, 10 and 20°C/min, the improvement is the most significant for the experiment of 20 °C/min, which means heating rate plays an important role in controlling re-oxidation reactions of mixed and pure CuO. It should be noted that all doped samples show similar kinetic characteristics in spite of different content of Al₂O₃. XRD diffractogram depicted in Fig.4 indicates that all mixed samples contain 3 physical phases: CuO, Al₂CuO₄ and Cu₂Al₄O₇, and much less content of Al₂O₃ appears in the mixed samples than that of pretreatment samples. After calculation of lattice constant, Metal solid solution of CuAl₂O₄ controls the growth of crystal along b axis and facilitates the growth along a axis and c axis, leading to the distortion of crystal structure so as to reduce the activation energy, which is probably the reason for re-oxidation improvement.

References

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