Design and development of a thermochemical energy storage system based on the redox couple Mn2O3/Mn3O4 for concentrated solar power plants

Abstract

The future energy grid based on renewable energy sources will require strong support from energy storage systems since renewable energy cannot be managed. Concentrated solar power is one of the leading technologies to produce electrical energy from the sunlight and thermal energy storage systems helps this technology to keep producing electricity in a cloudy day or after sunset. In order to increase the penetration of this technologies, decreasing the cost of concentrated solar power plants becomes of paramount importance to compete with fossil fuel-based energy sources. Advanced thermal energy storage technologies can play an important role in this challenge. Current commercial thermal energy storage systems store the energy using sensible heat with an energy storage density that can be multiplied by 10 if a thermochemical energy storage technology is properly developed. Thermochemical energy storage uses endothermic/exothermic reversible chemical reactions to store and release heat and in addition to their high energy density, they would allow increasing the operation temperatures of the plant and consequently, its efficiency, improving significantly the competitiveness of the technology. Nevertheless, they are still at an early stage of development, requiring mainly to avoid material degradation during cycling, proper design of high temperature components and design an optimal integration into the operation of the plant. Among high temperature thermochemical candidate materials, metal oxides can use air both as heat transfer fluid and reactant, avoiding the use of intermediate heat exchangers or the complexity derived of using other gases such as steam or CO2 required for other type of thermochemical materials. These advantages facilitate their integration into the concentrated solar power plants and therefore, they are one of the most studied thermochemical materials. Within this challenge, we have studied a new approach, for the first time to the best of our knowledge, based on Mn2O3/Mn3O4 thermochemical material. This material has been selected since it is nontoxic and rather available. The objective is mainly focused on improving the thermochemical material behavior during repetitive charge and discharge cycles, as it has been identified as the main drawback of the technology at material scale. The novelty consisted in finding a doping agent that properly mixed with the active material, creates a protective layer around the particles that prevent them from agglomeration and at the same time, contributes to improve the chemical reaction kinetics. Furthermore, a granulation technique has been developed to produce granules of several mm, with enough chemical and mechanical stability, which have been tested and validated in a packed bed configuration within a lab-scale thermochemical reactor.