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Solid-state reactions for thermal energy storage

Periodic Reporting for period 1 - SOLSTORE (Solid-state reactions for thermal energy storage)

Reporting period: 2017-03-15 to 2019-03-14

The building of a low carbon society is linked to the increasing of renewable energy sources together with the improvement of energy efficiency. The transition towards renewable energy sources causes situation where, time to time, the production of energy can exceed the demand and vice versa. The storage of the energy is the solution to overcome this problem allowing a constant supply of energy when energy demand and supply do not much. In particular, thermal energy storage (TES) is a key component for the sustainable and efficient use of energy resources involving heat. The development of suitable materials for TES, fulfilling a large number of requirements (high storage capacity, good thermal conductivity, mechanical and chemical stability, complete reversibility in charging/discharging cycles, affordable cost etc.) is a fundamental aspect in TES technologies. For this reason, basic research challenges in designing materials (materials improvement/new materials development) and development of related technologies (advanced storage systems) are the most important issues to be faced.
The project SOLSTORE proposes the development of a new kind of materials, based on solid-state reactions, with high potential for cost-effective and compact TES at high temperature (300-800°C). The storage of heat through reversible chemical reactions is very attractive due to the possibility to store great amount of thermal energy in form of chemical energy. The development of TCS based on solid-state reactions is a new storage concept with the potentiality to contribute to the development of low-cost, compact heat storage technologies. The expected advantages of using solid-state reaction are: i) simple reaction mechanisms compared to other types of thermo-chemical storage (e.g. gas-solid reactions), ii) in contrast to gas-solid reactions, no phase separation is required, contributing to the design of simpler storage systems, iii) possible direct contact of the storage material with the heat transfer fluid (no heat exchanger/lower cost).
The overall objectives of the projects include i) the identification and theoretical analysis of solid-state reactions of interest for TES; ii) the development of a strategy to maximize the reactivity in the solid state; iii) the evaluation of the performances of the materials developed in terms of thermal energy storage for the possible integration in real applications.
The project included the search and the theoretical analysis of different types of solid-state reactions in metallic based systems and salts based systems. In the selections of the most promising materials all the essential technical, economic and environmental aspects were taken into account discarding toxic, too expensive and low available materials. For the selection, different strategies were followed including, on one side, the analysis of the databases available for thermodynamic properties, on the other side, the use of software of modelling, based on the CALPHAD method, determining the key thermodynamic properties (e.g. enthalpy of reaction, specific heats, densities, etc.) fundamental for the selection criteria.
The results of the theoretical investigation allowed achieving a list of reactions (13 possible candidates) with promising theoretical volumetric energy densities. The second step of the investigation was focused on the experimental study of the systems selected, being the most promising candidate in terms of theoretical energy density and reaction temperature, and the evaluation of their possible use as TES materials.
One of the major challenges of the project was to develop materials with suitable reactivity (yield/kinetic/reversibility). Solid-state reactions are governed by the atomic diffusion in the solid state for this reason a synthesis strategy was defined to maximize the reactivity in the solid state. This included the synthesis of materials with different microstructure (nanostructured materials) to increase the diffusion in the solid state by i) decreasing the atomic diffusion path length (small grain sizes as well as small particle sizes), ii) introducing structural defects (dislocation, grain boundaries step, kink and corner atoms etc) and iii) promoting high intermixing degree (high contact area between the reagent). The effect of the synthesis parameters on the microstructure and reactivity of the powders have been analysed and optimal synthesis conditions have been established. As a result of this study, materials with tailored microstructures were synthesized with the objective to guaranty the maximum yield of the reaction (complete conversion of the reagent into the products and reversibility).

The main results achieved during the project can be summarized as follow:

• The work carried out allowed to identify several promising solid-state reactions that could selected for experimental investigation as possible candidate for the application in thermal energy storage.
• Different solid-state reaction were/are studied experimentally and their performance as TES material evaluated.
• The link between microstructure and reactivity was proven, with the identification of best the microstructural conditions to maximize the reactivity in the solid state.
SOLSTORE explores a new idea in the field of TES at high temperature, providing a low-cost solution for ultra-compact storage achievement in a large spectrum of relevant and challenging applications. At temperatures between 300°C to 800°C, TES in thermal power plants (including renewable power plants) could be the most significant application. For instance, large-scale TES can readily contribute to reach successful and cost-effective solutions for highly flexible fossil fuel power plants, capable of meeting demand peaks while mitigating the negative effects of cycling operation. Integrating TES in conventional power plants can also contribute to the grid management, providing greater regulatory capacity and higher levels of operational reliability and security of supply. In concentrated solar plants, TES is a key element to improve energy efficiency and cost effectiveness and to stabilize solar power generation within the fence of the solar plant and not at the expense of all other grid members. The materials developed could also play an important role in industrial heat processes. TES may improve on-site CHP management and efficiency in steam generation processes, and it can also contribute to waste heat recovery and re-use in exo/endothermic batch processes.