Survey Summary on Salts Hydrates and Composites Used in Thermochemical Sorption Heat Storage: A Review

Abstract

To improve the proficiency of energy systems in addition to increasing the usage of renewable energies, thermal energy storage (TES) is a strategic path. The present literature review reports an overview of the recent advancements in the utilization of salt hydrates (single or binary mixtures) and composites as sorbents for sorption heat storage. Starting by introducing various heat storage systems, the operating concept of the adsorption TES was clarified and contrasted to other technologies. Consequently, a deep examination and crucial problems related to the different types of salt hydrates and adsorbents were performed. Recent advances in the composite materials used in sorption heat storage were also reviewed and compared. A deep discussion related to safety, price, availability, and hydrothermal stability issues is reported. Salt hydrates display high theoretical energy densities, which are promising materials in TES. However, they show a number of drawbacks for use in the basic state including low temperature overhydration and deliquescence (e.g., MgCl2), high temperature degradation, sluggish kinetics leading to a low temperature rise (e.g., MgSO4), corrosiveness and toxicity (e.g., Na2S), and low mass transport due to the material macrostructure. The biggest advantage of adsorption materials is that they are more hydrothermally stable. However, since adsorption is the most common sorption phenomenon, such materials have a lower energy content. Furthermore, when compared to salt hydrates, they have higher prices per mass, which reduces their appeal even further when combined with lower energy densities. Economies of scale and the optimization of manufacturing processes may help cut costs. Among the zeolites, Zeolite 13X is among the most promising. Temperature lifts of 35–45 °C were reached in lab-scale reactors and micro-scale experiments under the device operating settings. Although the key disadvantage is an excessively high desorption temperature, which is problematic to attain using heat sources, for instance, solar thermal collectors. To increase the energy densities and enhance the stability of adsorbents, composite materials have been examined to ameliorate the stability and to achieve suitable energy densities. Based on the reviewed materials, MgSO4 has been identified as the most promising salt; it presents a higher energy density compared to other salts and can be impregnated in a porous matrix to prepare composites in order to overcome the drawbacks connected to its use as pure salt. However, due to pore volume reduction, potential deliquescence and salt leakage from the composite as well as degradation, issues with heat and mass transport can still exist. In addition, to increase the kinetics, stability, and energy density, the use of binary salt deposited in a porous matrix is suitable. Nevertheless, this solution should take into account the deliquescence, safety, and cost of the selected salts. Therefore, binary systems can be the solution to design innovative materials with predetermined sorption properties adapted to particular sorption heat storage cycles. Finally, working condition, desorption temperature, material costs, lifetime, and reparation, among others, are the essential point for commercial competitiveness. High material costs and desorption temperatures, combined with lower energy densities under normal device operating conditions, decrease their market attractiveness. As a result, the introduction of performance metrics within the scientific community and the use of economic features on a material scale are suggested.

Review of Technologies and Recent Advances in Low-Temperature Sorption Thermal Storage Systems

“…Impregnating the salt hydrate helped control the water uptake by varying the pore size and chemical nature of the salt hydrate. It was concluded that the sorption behaviors involved in these composite materials were liquid absorption, solid adsorption, and chemical reaction, which could also overcome some of the drawbacks of the base materials [91,92]. The host matrix should have a high porosity to accumulate more active materials.…”

Morphological Observation of LiCl Deliquescence in PDMS-Based Composite Foams

“…The general idea is to embed the salt into porous matrices (such as carbon foams, expanded natural graphite, zeolite, vermiculite, silica gel, etc.) to prevent deliquescence and improve water transport into the materials [8,16,17].…”

Heat Release Kinetics upon Water Vapor Sorption Using Cation-Exchanged Zeolites and Prussian Blue Analogues as Adsorbents: Application to Short-Term Low-Temperature Thermochemical Storage of Energy

“…Low-temperature thermochemical storage based on solid-vapor adsorption offers a promising alternative to numerous concepts tested in the scientific and technical literature. In this frame, the envisaged objective is to harvest solar energy during a sunny period and store this energy until the next period of "darkness" [1][2][3][4][5]. In the case of daily or short-term storage, it is potentially possible to benefit from the thermal energy released during the discharging step as the auxiliary heat source, which may be activated on demand when the targeted indoor temperature cannot be attained by the main heating system.…”

“…The quantity of heat released upon discharging and its constancy in time with repeated charging-discharging cycles, as well as the thermal stability of the working materials under specific conditions of the charging step, are key criteria evaluated in the selection of appropriate adsorbents [5,6]. Fundamental laboratory studies or experimental tests performed with different prototypes of heat storage unit have revealed that such classical adsorbents as silica gels, amorphous mesoporous silicas, and zeolites of different types are capable of producing a sufficiently high heat effect while keeping their structural and textural integrity simultaneously (e.g., [5][6][7][8][9][10] and references therein). It appears clear that various materials attain their optimal performance under particular operating conditions, but none of them really stands out significantly above the others.…”