Application of Chemical Looping for Solar Thermal Energy Storage

Abstract

In the recent years, Thermal Energy Storage (TES) systems have received significant attention because they can offer a low-cost Concentrating Solar Power (CSP) plant. Moreover, TES systems increase the economic viability of CSP plant for dispatchable power production over other renewable energy technologies. A state-of-the-art thermal energy storage system utilises commercially available molten salt as a medium for sensible thermal energy storage. Nevertheless, the operating temperature of molten salt is limited to 600˚C, which gives a thermodynamic efficiency of approximately 35% in a Steam Rankine cycle. Higher cycle temperatures would offer greater efficiency but new receivers, heat transfer fluids and storage media need to be developed for this to be achieved. Thermochemical energy storage with multivalent metal oxides provides high operating temperatures (600˚C to 1100˚C) and high storage capacity, depending on the storage media, through reduction and oxidation (RedOx) reactions. The RedOx reactions of multivalent metal oxides are based on Le Chatelier’s principle in which the spontaneous shift for rebalancing the reaction can be brought on by changing the temperature and/or pressure of the system. Nevertheless, both temperature and pressure swing systems have their own advantages and technical challenges. Different metal oxides have been assessed for their potential to store thermochemical energy in solid phase by changing the temperature of storage media during RedOx reactions. The CuO/Cu2O, Co3O4/CoO, and Mn2O3/Mn3O4 pairs are promising storage media for thermochemical energy. However, one of the main challenges of thermochemical energy storage is the thermal hysteresis that results from the temperature difference during RedOx reactions. This leads to a decreased energy efficiency because the charging temperature is higher than the discharging temperature. Many efforts have been made to address this issue by preparing a porous structure, a composite structure, and a binary system. One plausible way to address this issue is to perform a chemical looping cycle with pressure swing for thermal energy storage. This study assesses the potential of suitable metal oxides for thermochemical storage by pressure swing thermodynamically and experimentally through phase diagram and Thermogravimetric Analysis (TGA), respectively. The results showed that CuO/Cu2O, Co3O4/CoO, and Mn2O3/Mn3O4 pairs have a strong potential for chemical storage by pressure swing and excellent reversibility over ten successive RedOx cycles. Moreover, advantages and technical challenges of chemical storage by pressure swing have been identified. Liquid Chemical Looping for Thermal Energy Storage (LCL-TES) has been proposed to increase the operating temperature of a TES system by liquefying the storage media. It has also been proposed to improve the storage capacity of a solar-thermal energy system by combining sensible, latent and thermochemical storages. The thermodynamic performance of a LCL-TES system has been assessed for copper oxide. However, it has not been assessed for alternative metal oxides. Hence, the present study compares the thermodynamic feasibility of alternative metal oxides in the LCL-TES system. Moreover, a thermodynamic selecting procedure for the potential liquid metal oxide needs to be developed. This procedure provides a back-to-back comparison between various metal oxides with the purpose to identify the most suitable ones for the LCL-TES. A series of experiments at different oxygen partial pressures (0.05 bar to 0.8 bar) were performed by TGA to develop an understanding of the technical challenges of LCL-TES and to prove the thermodynamic assessment of copper oxide in a LCL-TES system. Results revealed that copper oxide has a strong potential for application in RedOx reactions in liquid state. However, a key challenge is to identify suitable combinations of metals and refractories with sufficiently low reactions to withstand multiple cycles of successive oxidation and reduction. The performance of LCL-TES with a Gas Turbine Combined Cycle (GTCC) was assessed and resulted in a 44.9% thermal efficiency which improved to 50% when an after-burnner was added to the system and the operating temperature increased to 1700˚C. The potential of chemical looping cycles by pressure swing for TES applications was demonstrated in this project. The CuO/Cu2O, Co3O4/CoO, and Mn2O3/Mn3O4 pairs were found to be suitable storage media for chemical storage by pressure swing at temperatures up to 1000˚C. In addition, the kinetics of the reaction were improved from two hours to fifteen minutes by pressure swing. The results show that the rate of oxidation can be increased with increasing the partial pressure of oxygen from 0.2 bar to 0.8 bar. Thermal hysteresis of metal oxides in RedOx reactions were eliminated by this cycle. The operating temperature of the thermal energy storage system was also improved by liquefying the metal oxide. The LCLTES system can improve energy density by combining sensible, latent and chemical storage in a process. The energy density of the LCL-TES system with copper oxide was estimated to be approximately 5 GJ/m3, which is six-times higher than that of molten salt (0.83 GJ/m3). Therefore, this system can be considered as an alternative plausible storage system for future CSP technologies.