Due to their very high storage density of >2GJ/m3 many salt hydrates are considered as promising candidates for thermochemical energy storage. However, kinetic problems with the hydration reactions limit the practical applicability of the pure (bulk) salts. Therefore, attempts have been made for a more efficient use of the heat effect in hydration reactions by dispersion and embedding of the salt hydrate in a porous host material. However, this is only possible at the expense of storage density which is now controlled by the porosity of the host material and the degree of pore filling with the embedded salt. On the other hand, technical applications using the heat of sorption of water vapor in microporous and mesoporous materials with large surface areas such as zeolites do already exist. The basic idea of this joint project is to use the advantages of both thermochemical storage techniques, i.e. water vapor sorption and salt hydration. For that purpose, composite materials with optimized porosities and pore sizes and an embedded inorganic salt hydrate will be prepared. Hierarchically structured mesoporous–macroporous materials will be used as host materials. Salts are embedded in the macropores of the material to make use of the reversible hydration reaction. The pore walls of the macropores contain interconnected mesopores and can be used for water vapor sorption storage. The synthesis of the hierarchically structured materials with adjustable sizes of mesopores and macropores and the preparation and optimization of the composite materials are major tasks within the scope of the project. Different host materials with adjusted pore sizes will be combined with different salts in order to maximize both storage density and cycle stability. Finally, a lab-scale prototype reactor is constructed and operated with the composite material in order to optimize several process parameters. The project is a joint collaboration of research groups at the University of Hamburg (Department of Chemistry, Inorganic and Applied Chemistry) and the Bauhaus University Weimar (Faculty of Civil Engineering, Chair of Chemistry of Construction Materials).
Achieving high storage density
The project is concerned with the development and optimization of a class of new composite materials for thermochemical storage of solar energy. The composites consist of a hierarchically structured mesoporous–macroporous host material and a salt hydrate that is embedded in the macropores of the host material. Thus, the material combines the advantages of both water vapor sorption (making use of the enthalpy of sorption in the mesopores) and reversible hydration of an inorganic salt (using the enthalpy of hydration). The overall objective of the project is the development of tailored composites for thermochemical energy storage.
In order to achieve a high storage density, the host materials must have high porosities, high thermal conductivities and a high degree of pore filling with the embedded salt. As crystals growing in a hydration reaction in confinement can generate substantial stress, the composites must also exhibit a sufficient tensile strength. Otherwise, the hydration pressure can cause mechanical damage limiting the cycle stability of the materials. The size of the macropores (i.e. the maximum size of the crystals in the pores), the degree of pore filling and the thermodynamic properties of the embedded salt have to be adjusted all together in order to ensure a rapid and complete hydration of the salt. Finally, the operation of a thermochemical energy storage reactor has to be adapted to the properties of the composite material. Several parameters such as the temperature and relative humidity during sorption and hydration have a strong influence on the achievable temperature rise of the storage unit.
Optimising the pore sizes
Within the scope of the project, new hierarchically structured materials will be synthesized. The water vapor sorption properties of these materials are controlled by the mesopores in the pore walls of the macropores. Hence, the mesopores must be interconnected, the total porosity of the pore walls must be high and the size of the mesopores must be adjustable. The use of the materials for reversible salt hydration is largely controlled by the macropores with the total macroporosity, the macropore size and the degree of pore filling with salt being the major parameters. In order to optimize the energy storage density of the composite materials, it will be important to maximize the macroporosity and to independently adjust and optimize the pore sizes of the mesopores and the macropores. The size of the macropores and the degree of pore filling also depend on the thermodynamic properties of the embedded salt and the kinetics of the hydration reaction. The final choice of the porosity parameters has to consider mechanical requirements as well. The tensile strength of the material must be sufficient to resist the hydration pressure that is generated against the pore wall by the growing hydrated crystals. Finally, the interaction of the parameters mentioned so far has a strong influence on the operation of a storage reactor and its practical use. The purpose of the experiments with a lab-scale reactor is to optimize, for a given composite material, the process parameters, e.g., sorption and desorption temperatures and relative humidities, air flow rates etc.
Economic viability and durability
Thermochemical energy storage with optimized energy density allows for seasonal storage of solar energy that can be used to (at least partially) replace conventional heating systems such as gas and oil boilers. Hence, there is a contribution to the reduction of CO2 emissions. The cost effectiveness of the new storage materials will largely depend on the potential industrial production of the porous host materials.
- Sub-project 1 of this joint research project is carried out at the Department of Chemistry of the University of Hamburg. One of the objectives within the scope of the project is the synthesis of the hierarchically structured mesoporous–macroporous materials. These materials comprise of a densely packed network of macropores the pore walls of which have interconnected mesoporosity. Such bimodal mesoporous–macroporous materials can be prepared by template-directed synthesis. Close packed arrangements of spherical polymer or silica nanoparticles are used as hard templates for the macropores in the final material. The interstitial voids in the close-packed structure of the nanoparticles are infiltrated with appropriate polymer or silica precursors in the presence of supramolecular soft templates. A network of polymer or silica is formed surrounding the nanoparticles. Finally, after removal of the hard and soft templates a bimodal mesoporous–macroporous material is obtained.
- Sub-project 2 of the joint project carried at the University of Hamburg (Department of Chemistry) is mainly concerned with a thorough investigation of the thermodynamics and kinetics of the hydration–dehydration reactions and the release of heat during hydration and sorption. Possible candidates of inorganic salts will be identified and their phase diagrams will be fully characterized. Next, the phase transition reactions of the most promising candidate salts embedded in the porous host materials will be studied. These investigations include both the kinetics of hydration and dehydration and the influence of confinement on the phase diagrams. Based on experimental results and model calculations, the properties of the host materials, i.e. pores sizes and degrees of pore filling, and the process parameters during the operation of a storage reactor will be optimized in close collaboration with the remaining project partners.
- Sub-project 3 of this joint research project is carried out at Bauhaus University Weimar (Faculty of Civil Engineering). A lab-scale prototype reactor with an optimized hierarchically structured composite material will be designed and operated to optimize process parameters such as sorption and desorption temperature and relative humidity, air flow rates etc. Other major tasks will be the calorimetric measurement of overall reaction enthalpies of the composite materials, i.e. enthalpy of hydration and enthalpy of sorption, and, the determination of the heat conductivities of the composite materials. Finally, the cycle stabilities of the new composites will be investigated.