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Solid-state reactions for thermal energy storage

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Improved energy storage smooths the way for renewable technologies

Successful transition to a low-carbon economy is linked to an increase in renewable energy sources and an improvement in energy efficiency. One key element is thermal energy storage (TES), which can help ensure a constant power supply by resolving disparities between energy supply and demand.

Climate Change and Environment

Currently, three types of processes are considered for storing thermal energy based on sensible, latent and thermochemical storage. The greatest amount of energy can be stored via thermochemical processes. However, this is accompanied by an increase in the complexity of the TES system with a subsequent increase in cost. The SOLSTORE project was undertaken with the support of the Marie Skłodowska-Curie programme. Under this project, researchers investigated the development of new materials and technologies for the cost-effective, ultra-compact storage of thermal energy at high temperatures (300-800 °C). This included the study of reversible solid state chemical reactions that can be used in real world applications, such as concentrated solar power. The goal was to simplify TES technologies and reduce costs.

Multiple benefits

Using solid-solid reactions has a number of advantages like simple reaction mechanisms when compared to other types of thermo-chemical storage that involve gas-solid reactions. ”They contribute to the design of simpler storage systems and possible direct contact of the storage material with heat transfer fluid, requiring no heat exchange resulting in lower cost,” says Dr Stefania Doppiu, associate researcher at the Energy Cooperative Research Centre (CIC Energigune), Spain. Researchers identified and evaluated several promising solid state reactions for use in TES experimental studies that work under a wide range of temperatures. The in-depth study of two systems with different natures (metallic and salts-based) highlighted both the potential and limitations of these reactions, which are strongly dependent on the nature of the reacting materials. The study also revealed the link between microstructure and reactivity, identifying the best microstructural conditions to maximise reactivity in the solid state. SOLSTORE’s researchers will next focus on studying the materials at a larger scale to test their behaviour under more realistic conditions. The handling of large amounts of material for integration into a TES system will also contribute to lower costs.

Improved efficiency

Results demonstrated high storage capacity, good thermal conductivity, mechanical and chemical stability, and complete reversibility of the charging and discharging cycles. ”We identified a very promising reversible reaction working at around 500 °C having high thermal energy storage capacity (200 J/g) fast kinetics, good cyclability and stability,” Dr Doppiu explains. Integrating TES into conventional power plants can contribute to enhanced electricity grid management, providing greater regulatory capacity and higher levels of operational reliability and security of supply. According to Dr Doppiu: “In concentrated solar power plants TES is a key element to improving energy efficiency and cost effectiveness and to stabilise solar power generation.” SOLSTORE could also play an important role in industrial heat processes. “TES may improve on-site combined heat and power management and efficiency in steam generation processes, and it can also contribute to waste heat recovery and re-use in exo/endothermic batch processes,” Dr Doppiu points out.

Keywords

SOLSTORE, thermal energy storage (TES), reactions, solid state, thermochemical, concentrated solar power, low-carbon economy

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