• Energy Research

In this work, we evaluate 454 salt hydrates and 1073 unique hydration reactions in search of suitable materials for domestic heat storage. The salts and reactions are evaluated based on their scarcity, toxicity, (chemical) stability and energy density (>1 GJ/m3) and alignment with 3 use case scenarios. These scenarios are based on space heating (T > 30 °C) and hot water (T > 55 °C) to be provided by discharge as well as on heat sources available in the built environment (T < 160 °C) for charging. From all evaluated materials, only 8 salts and 9 reactions (K2CO3 0–1.5, LiCl 0–1, NaI 0–2, NaCH3COO 0–3, (NH4)2Zn(SO4)2 0–6, SrBr2 1–6, CaC2O4 0–1, SrCl2 0–1 and 0–2) fulfil all of the criteria. Provided a suitable stabilisation method is found additional 4 salts and 13 reactions (CaBr2 6-0, CaCl2 6-0, 6-1, 6-2, 4-0, 4-1, 4-2, LiBr 2-0, 2-1, 2-0, LiCl 2-0, 2-1, ZnBr2 2-0) From this selection, only 2 salts/reactions (NaI and (NH4)2Zn(SO4)2) have not been extensively studied in the literature. Moreover, many well-investigated salt hydrates, such as MgSO4 and LiOH, did not pass our screening. This work underlines the scarcity of materials suitable for domestic applications and the need to broaden the scope of future evaluations.

The advent of thermochemical energy storage (TcES), that is, storage of thermal energy by means of reversible chemical reactions, incites finding pathways of stabilization of thermochemical materials for thermal batteries of the future. Currently, salt hydrates such as LiCl·H2O, CaCl2·6H2O, and SrBr2·6H2O are being actively studied for TcES in buildings due to both high energy storage density (1-2.5 GJ/m3) and high storage duration. In this work, we report the core-shell composites "salt in hollow SiO2 spheres with mesopores"(salt = LiCl·H2O, CaCl2·6H2O, SrBr2·6H2O) for domestic TcES. The salt hydrates were encapsulated into submicrometer-sized hollow SiO2 (HS) capsules as confirmed by transmission electron microscopy (TEM) and N2 sorption analyses. High sorption/desorption rates due to mesopores of the shells were shown by thermogravimetric analysis (TGA). The sorption equilibrium for salt@HS was reported, and the applicability of the materials for domestic heat batteries was analyzed. As a result of almost the densest packing of salt@HS, the composites were shown to provide a state-of-the-art energy storage density up to 0.86 GJ/m3 on the bed level for the high-temperature lift of 32-47 °C, showing high energy storage capacity. The stability in at least 50 charging/discharging cycles was confirmed by TGA and TEM.

The solid-state hydration of salts has gained particular interest within the frame of thermochemical energy storage. In this work, the water vapor pressure-temperature (p-T) phase diagram of the following thermochemical salts was constructed by combining equilibrium and nonequilibrium hydration experiments: CuCl 2 , K 2 CO 3 , MgCl 2 ·4H 2 O, and LiCl. The hydration of CuCl 2 and K 2 CO 3 involves a metastable zone of ca. 10 K, and the induction times preceding hydration are well-described by classical homogeneous nucleation theory. It is further shown for K 2 CO 3 (metastable) and MgCl 2 ·4H 2 O (not metastable) through solubility calculations that the phase transition is not mediated by bulk dissolution. We conclude that the hydration proceeds as a solid-solid phase transition, mobilized by a wetting layer, where the mobility of the wetting layer increases with increasing vapor pressure. In view of heat storage application, the finding of metastability in thermochemical salts reveals the impact of nucleation and growth processes on the thermochemical performance and demonstrates that practical aspects like the output temperature of a thermochemical salt are defined by its metastable zone width (MZW) rather than its equilibrium phase diagram. Manipulation of the MZW by e.g. prenucleation or heterogeneous nucleation is a potential way to raise the output temperature and power on material level in thermochemical applications. © Copyright 2019 American Chemical Society.

Thermochemical materials K2CO3, MgCl2 and Na2S have been investigated in depth on energy density, power output and chemical stability in view of domestic heat storage application, presenting a critical assessment of potential chemical side reactions in an open and closed reactor concept. These materials were selected based on a recent review on all possible salt hydrates, within the frame of a thermochemical heat battery and considering recent advances in heat storage application. Judged by gravimetric and calorimetric experiments in operating conditions and worst-case-scenario conditions, K2CO3 is recommended for both an open and closed system heat battery. The compound is chemically robust with a material level energy density of 1.28 GJ/m3 in an open system and 0.95 GJ/m3 in a closed system, yielding a power output of 283–675 kW/m3. Na2S and MgCl2 on the other hand are chemically not robust in heat storage application, although having a higher energy density, output power and temperature in one cycle.

For use as a heat storage material, CaCl2 is often impregnated into porous materials. This is done to stabilize the salt against conglomeration and its dissolution due to the low deliquescence relative humidity. However, CaCl2 has overlapping temperature and water vapor pressure conditions for its trito- and monohydrate, which are kinetically hindered against each other creating path-dependent (de-)hydration steps. These pathways may change under the influence of confinement. These changes can influence the temperature output for heat batteries using CaCl2 composites and could make the taken pathways for hydration and dehydration either more complex or simpler than the pure salt. So, in this research, the hydration and dehydration steps of CaCl2 inside different clays (Vermiculite, Halloysite, and Sepiolite) and silica gels were investigated with respect to their transformations compared to the bulk salt. Therefore, the kinetic phase transition onsets were determined with isobaric TGA measurements together with PXRD in situ experiments to confirm or identify the crystalline phases. This showed that inside pores, CaCl2 forms the monohydrate rather than the tritohydrate. The decrease of pore diameter leads to easier formation of monohydrate over tritohydrate. This trend can be explained by the crystal structures of the hydrates and their unit cell volumes considering that larger crystals are difficult to form in the limited space inside the pore systems. This change in phase transition steps influences the transition temperatures, which affects its application for heat storage.

Efficient and cheap storage of energy from renewable resources presents a key technology to facilitate the ongoing energy transition. Storing heat in thermochemical materials (TCMs), such as salt hydrates, provides a promising concept to meet this demand. TCMs can capture heat reversibly and loss-free by relying on equilibrium hydration reactions of the salts. Persistent bottlenecks in the full-scale application of this technology are the low mechanical resilience of salt grains and their tendency to coagulate or dissolve when in contact with water vapor. To overcome this, the salt grains can be encapsulated by a stabilizing polymer coating. Ideal coatings combine high water vapor permeability with reversible deformability to minimize the resistance for water transport and to accommodate the volumetric changes of the TCM during repetitive (de)hydration, respectively. Here, a systematic study into the applicability of commercially available polymers as coating materials is presented. Mechanical analysis and wet-cup experiments on freestanding polymer films revealed that cellulose-based coatings successfully combine permeability and ductility and meet the engineering demands for domestic TCM-based heat storage applications. The validity of using freestanding films as model system was confirmed by encapsulating granular TCMs in ethyl and hydroxyl propyl cellulose using fluidized bed coating. The permeability was retained and an enhanced structural integrity of the TCM grains during (de)hydration cycles was observed.

Potassium carbonate (K2CO3) is a promising thermochemical heat storage material (TCM). However, it suffers from hysteresis between (de)hydration temperatures and poor reaction kinetics close to equilibrium conditions. Both aspects are caused by a nucleation barrier and low ionic mobility close to equilibrium. This study investigates the impact of caesium fluoride (CsF) incorporated through recrystallisation on the phase transitions. The composition studies show that K2CO3and CsF react during synthesis, forming KF, which points to the formation of Cs2CO3. The secondary phases are not incorporated into the crystal structure but reside between the main phase's grain cracks due to capillary forces. Because the secondary phases are highly hygroscopic, they promote surface mobility by forming a liquid-like layer even at low water vapour pressures. As the effect of their presence, hydration kinetics are enhanced significantly in all investigated conditions, with the most pronounced impact when hydration of K2CO3is inherently inhibited. The benefits manifest themselves through a faster reaction rate and shorter induction period. The dehydration is enhanced by the presence of the additive mainly far away from equilibrium conditions. Close to the equilibrium, the dehydration of the composite proceeds in an unusual 2-step manner, where the second step is much slower than the dehydration of pure K2CO3. The enhancement of dehydration kinetics is ascribed to the formation of defects during recrystallisation. The lowering of dehydration rates close to equilibrium is attributed to diffusion issues due to excess of a deliquescent phase present in the system.