9.3 Thermochemical energy storage principles

Thermochemical energy storage uses reversible chemical reactions to store and release heat. This method offers high energy density, making it great for long-term storage. It's like a chemical battery for heat, charging up with endothermic reactions and discharging with exothermic ones.

Sorption processes and chemical heat pumps are key techniques in thermochemical storage. These systems can upgrade low-grade heat to higher temperatures, making them useful for heating, cooling, and industrial applications. It's a game-changer for efficient energy use.

Thermochemical Reactions

Reversibility and Energy Transfer

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• Thermochemical reactions are reversible chemical reactions that involve the transfer of thermal energy between the system and surroundings
• Endothermic reactions absorb heat from the surroundings during the forward reaction (dehydration of salt hydrates, decomposition of metal hydroxides)
• Exothermic reactions release heat to the surroundings during the reverse reaction (hydration of anhydrous salts, synthesis of metal hydroxides from metal oxides)
• The direction of the reaction depends on the temperature and pressure conditions
• Thermal energy is stored during the endothermic reaction and released during the exothermic reaction, allowing for energy storage and release

Reaction Kinetics and Catalysis

• Reaction kinetics describe the rate at which thermochemical reactions proceed and the factors that influence the rate
• Temperature, pressure, concentration of reactants, and presence of catalysts affect the reaction rate
• Higher temperatures generally increase the reaction rate by providing more kinetic energy for reactant molecules to overcome the activation energy barrier
• Catalysts lower the activation energy of the reaction, increasing the rate without being consumed in the process (transition metal oxides, zeolites)
• Proper catalyst selection and optimization are crucial for efficient thermochemical energy storage systems

Thermochemical Storage Processes

Sorption Processes

• Sorption processes involve the binding of a gas or liquid (sorbate) to a solid material (sorbent) through physical or chemical interactions
• Adsorption is a surface phenomenon where the sorbate adheres to the surface of the sorbent (zeolites, activated carbon, silica gel)
• Absorption involves the incorporation of the sorbate into the bulk of the sorbent material (salt hydrates, metal hydroxides)
• Sorption processes are used for thermal energy storage by exploiting the heat of adsorption or absorption during the binding process and the heat of desorption during the release of the sorbate

Chemical Heat Pumps

• Chemical heat pumps utilize thermochemical reactions to upgrade low-grade thermal energy to higher temperatures
• The system consists of a reactor, condenser, and evaporator that operate in a closed-loop cycle
• During the charging phase, heat is supplied to the reactor, driving an endothermic reaction that stores thermal energy in the chemical bonds of the reaction products
• In the discharging phase, the reverse exothermic reaction occurs, releasing the stored thermal energy at a higher temperature than the input heat
• Chemical heat pumps can be used for space heating, cooling, and industrial process heat applications (ammonia-based systems, metal hydride-based systems)

Energy Density and Storage Capacity

• Thermochemical energy storage systems offer high energy densities compared to sensible and latent heat storage methods
• Energy density refers to the amount of thermal energy stored per unit mass or volume of the storage material
• The energy density depends on the specific thermochemical reaction, the storage materials used, and the operating conditions
• Higher energy densities enable more compact storage systems and reduce the required storage volume
• The theoretical energy density of thermochemical storage can reach several GJ/m³, making it attractive for long-term and seasonal thermal energy storage applications