Properties of some salt hydrates for latent heat storage

For the utilization of low-grade heat the latent storage of thermal energy is of great advantage because the heat can be preserved at a constant temperature perfectly matched to the special purpose of application. Investigations on the heat capacities, enthalpies of fusion, densities, crystallization behaviour and other chemical and physical properties have shown that the following salt hydrates are especially suitable media for storing low-grade heat. The eutectic mixture of water and 3.92% by weight of sodium fluoride, melting point (MP) = - 3.5°C, is extremely convenient and cheap for refrigerating or other cooling purposes. Lithium chlorate trihydrate, LiClO₃. 3H₂O, MP = +8.1°C has an extremely high storage capacity and other advantageous properties as a storage medium in cooling systems, but a very high price will limit its application. Calcium chloride hexahydrate, CaCl₂. 6H₂O, MP = + 29.2°C, is a suitable and cheap storage medium for heating purposes. For the same application disodium hydrogen phosphate dodecahydrate, Na₂HPO₄. 12H₂O, MP = + 35.2°C, is even better because of the larger storage capacity per unit volume and other advantages which largely compensate the higher material cost. the unique properties of potassium fluoride tetrahydrate, KF. 4H₂O, MP = +18.5°C, make it especially suitable for storing low-grade heat. It can directly function as an energy sink and as an energy reservoir in heat collecting and consuming systems. Examples of the practical applicability for residential heating, temperature levelling and cooling are described.     

Mixtures of calcium chloride hexahydrate with some salt hydrates or anhydrous salts as latent heat storage materials

Mixtures of CaCl₂·6H₂O with some nitrate hydrates [Ca(NO₃)₂·4H₂O and Mg(NO₃)₂·6H₂O] or anhydrous nitrates NH₄NO₃ and KNO₃) were studied as latent heat storage materials for greenhouse purposes. The melting points and the heats of fusion of these mixtures showed no deterioration after thermal heat cycles (30°-10°C, 1000 times). Temperature data of solidification both for vertical and horizontal sample tube settings were given.     

A new system for heat storage utilizing salt hydrates

This is a first report on a new short term heat storage system which utilizes salt hydrates together with substances able to dehydrate them by heating below the transition temperature. The experimental results in progress show that the couple ammonium alum—ammonium nitrate, operating in the range of temperatures 27–62°C, allows storage capacities of the order of 50–60 kcal/kg, corresponding to a heat density per unity of volume of 2300–2500 kcal/m3°C. These figures are higher than those practically so far obtained with any other system. Tests are in progress to control the risk of degradation after many cycles.     

无机水合盐相变储热材料的过冷性研究

无机水合盐相变储能材料通常存在过冷现象,影响了该类材料所蓄热量的排放性能,如果过冷严重,储存的热量不能释放出来。就无机水合盐的过冷机理及非均匀形核的动力学机理进行了探讨,加入成核剂是降低过冷度的有效措施。     

Low temperature latent heat storage by reciprocal salt pairs

The double conversion of a reciprocal salt pair with favourable transition temperature and high enthalpy of transition may be suitable for low temperature latent heat storage.The behaviour during thermal cycling of two reciprocal salt pairs with transition temperatures of 45 and 65°C and very high enthalpies of transition has been studied. The heat storage capacities of these systems remain constant in some cases. However, a decrease of performance has been observed in most cases. Solutions to avoid this reduction are discussed.     

Low temperature latent heat thermal energy storage: Heat storage materials

Heat-of-fusion storage materials for low temperature latent heat storage in the temperature range 0–120°C are reviewed. Organic and inorganic heat storage materials classified as paraffins, fatty acids, inorganic salt hydrates and eutectic compounds are considered. The melting and freezing behaviour of the various substances is investigated using the techniques of Thermal Analysis and Differential Scanning Calorimetry. The importance of thermal cycling tests for establishing the long-term stability of the storage materials is discussed. Finally, some data pertaining to the corrosion compatibility of heat-of-fusion substances with conventional materials of construction is presented.     

Modeling of thermochemical energy storage by salt hydrates

We investigate the capability of salt hydrates to store thermochemical energy as they dissociate into anhydrous salts or lower hydrates and water vapor upon heating using the example of magnesium sulfate heptahydrate as a model salt. An anhydrous salt has a relatively higher energy content than its hydrated counterpart. It can be stably stored over long durations and transported at ambient temperatures. Thermal energy can be released by allowing water vapor to flow across the anhydrous salt, which transforms its chemically stored heat into a sensible form. This has potential as a technology for long-term thermal storage applications, e.g., for storing solar heat during summer months and releasing it in the winter to provide heat for buildings and infrastructure. Water desorption occurs from salt hydrates when they are heated to a critical temperature at which their dehydration is activated. While thermal diffusion governs heat transfer below this temperature, during the dehydration process it is influenced by the desorption kinetics. We model the overall thermochemical process using relations for the conservation of mass and energy, and a relation describing the desorption kinetics, employing a finite difference scheme to solve them. Different cases are considered, which provide guidance about process performance, such as the characteristics of optimally designed salts. The time required for hydrated salts to undergo complete desorption decreases nonlinearly with an increase in the input heat flux.     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

The capric and lauric acid mixture with chemical additives as latent heat storage materials for cooling application

The mixture of capric acid and lauric acid (C-L acid), with the respective mole composition of 65% and 35%, is a potential phase change material (PCM). Its melting point of 18.0°C, however, is considered high for cooling application of thermal energy storage. The thermophysical and heat transfer characteristics of the C-L acid with some organic additives are investigated. Compatibility of C-L acid combinations with additives in different proportions and their melting characteristics are analyzed using the differential scanning calorimeter (DSC). Among the chemical additives, methyl salicylate, eugenol, and cineole presented the relevant melting characteristics. The individual heat transfer behavior and thermal storage performance of 0.1 mole fraction of these additives in the C-L acid mixture are evaluated. The radial and axial temperature distribution during charging and discharging at different concentrations of selected PCM combinations are experimentally determined employing a vertical cylindrical shell and tube heat exchanger. The methyl salicylate in theC-L acid provided the most effective additive in the C-L acid. It demonstrated the least melting band width aimed at lowering the melting point of the C-L acid with the highest heat of fusion value with relatively comparable rate of heat transfer. Furthermore, the thermal performance based on the total amount of transferred energy and their rates, established the PCM’s latent heat storage capability.     

Stearic acid/polymethylmethacrylate composite as form-stable phase change materials for latent heat thermal energy storage

The aim of this research is to prepare of a novel form-stable composite phase change material (PCM) for the latent heat thermal energy storage (LHTES) in buildings, passive solar space heating or functional fluid by entrapping of SA into PMMA cell through ultraviolet curing dispersion polymerization. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. The results show that the form-stable microencapsulated PCM with core/shell structure was formed and the maximum encapsulated proportion of SA in the composite was 51.8 wt.% without melted PCM seepage from the composite. In the shape stabilized microencapsulated PCM, the polymer acts as supporting material to form the microcapsule cell preventing the leakage of PCM from the composite and the SA acts as a PCM encapsulated in the cell of PMMA resin. The oxygen atom of carbonyl group of skeleton is interacted with the hydrogen atom of hydroxyl group of SA. Thermal properties, thermal reliability and heat storage/release performance of the composite PCM were determined by differential scanning calorimetry (DSC), FT-IR and thermal cycling test analysis. The melting and freezing temperatures and the latent heats of the composite PCM were measured as 60.4 °C, 50.6 °C and 92.1 J/g, 95.9 J/g, respectively. The results of DSC, FT-IR and thermal cycling test are all show that the thermal reliability of the composite PCM has an imperceptible change. This conclusion indicates that the composite has a good thermal and chemical stability.     

Energy saving latent heat storage and environmental friendly humidity-controlled materials for indoor climate

This paper reviews the development and application of energy saving latent heat storage phase change materials (PCMs) and environmental friendly humidity-controlled materials for indoor thermal management and humidity control. Based on the studies reported in the literatures, we indicated that the super-efficient and innovative micro-encapsulated form-stable composite PCM and humidity-controlled materials with high moisture absorption and desorption capacity and intelligent self-humidity-control and related key techniques are worth to be expected.     

Durability of latent heat storage tube-sheets

Compatibility and durability of phase change materials (PCM) and packaging laminate films were tested in this study. The objective was to identify viable component materials for heat storage tube-sheets. The tube-sheets are the essential part of a patented latent heat storage device. The unique feature of the device is its offset arrangement of mass-producible units. The results of cycling tests on two PCMs and one laminate are presented in this paper. After more than 1000 accelerated cycles of charging and discharging tests, the tubes maintained operable stability and showed no sign of deterioration. The PCMs displayed the same temperature-time pattern during the entire cycling test. The packaging laminate was compatible with the PCM's and capable of maintaining satisfactory strength.     

Modeling and assessment of a thermochemical energy storage using salt hydrates

Several research studies have revealed the potential use of salt hydrates in thermal energy storage applications. These materials dissociate into anhydrous salts and release water vapor when subjected to heat source. The latter salt has the capability to store the energy that was supplied for dehydration upon heating. This thermal energy can be extracted by flowing cooler water or water vapor through the salt to obtain sensible heat that can be exploited for several applications, such as heating residential buildings during cold seasons. In this study, a numerical model that describes the overall thermochemical process of salt hydrates when being heated is developed. In comparison with previous published studies, the main contribution of the present work is to account for the impact of the temperature on the thermodynamic properties of the system. The obtained results agree well qualitatively and quantitatively with their experimental counterparts. A comparative study between three different salt hydrates, namely, the magnesium sulfate (MgSO₄ ? 7H₂O) , the cupric sulfate,(CuSO₄ ? 5H₂O) , and the gypsum (CaSO₄ ? 2H₂O) , is conducted in order to investigate their capabilities to efficiently store thermochemical energy. The present performance analysis aims at identifying the proper salt hydrates for the intended applications. It is shown that the cupric sulfate enables the best performance in terms of efficiency (defined as the ratio of stored energy over the supplied energy), and it requires the minimum heating time to initiate the chemical reaction. Copyright © 2017 John Wiley & Sons, Ltd.     

Corrosion of metals and salt hydrates used for thermochemical energy storage

Solar energy can be efficiently used if thermal energy storage systems are accordingly designed to match availability and demand. Thermal energy storage by thermochemical materials (TCM) is very attractive since these materials present a high storage density. Therefore, compact systems can be designed to provide both heating and cooling in dwellings. One of the main drawbacks of the TCM is corrosion with metals in contact. Hence, the objective of this study is to present the obtained results of an immersion corrosion test following ASTM G1 simulating an open TCM reactor, under humidity and temperature defined conditions. Four common metals: copper, aluminum, stainless steel 316, and carbon steel, and five TCM: CaCl₂, Na₂S, CaO, MgSO₄, and MgCl₂, were studied. Aluminum and copper show severe corrosion when combined with Na₂S, aluminum corrosion is more significant since the specimen was totally destroyed after 3 weeks. Stainless steel 316 is recommended to be used as a metal container material when storing all tested TCM.     

Comparison of heat storage systems employing sensible and latent heat

This study makes an evaluation of the performances of heat storage systems destined for power plants with a discontinuous power source such as nuclear fusion. Two classes of heat storage systems are investigated: heat storage systems based on first sensible heat storage and second latent heat storage. Both classes of heat storage systems are evaluated both from a thermodynamic point of view, inquiring whether the irreversibility of the system stays limited or not, and from an economic point of view, examining whether the system makes proper use of the heat storage capacity present. It is shown that an unambiguous conclusion that one is superior to the other is not possible and that the operating conditions and the configuration of the phase‐changing materials play an important role. Copyright © 1999 John Wiley & Sons, Ltd.     

The application of glauber salt in a new type of a latent heat storage unit

The application of Glauber salt as a latent heat storage material is a difficult technical problem owing to the separation of the coexisting phases (stratification) during melting. In order to avoid stratification and to improve the heat transfer in the course of the charge and discharge, a new storage type called ‘GLS’ was developed, in which the heat is transferred in a closed container to the latent heat storage material by means of the vapour of a heat transfer liquid. Surface-active materials which were added to the storage materials produce optimum conditions for the heat transfer and material transport during the heat transfer processes. Under the conditions of the GLS storage unit on application of Glauber salt, constant storable amounts of energy were found which are in good agreement with the theoretical values. The heat storage can be performed with high energy densities in narrow temperature differences and with high power of the transferred heat.     

Solar storage systems using salt hydrate latent heat and direct contact heat exchange—I preliminary design considerations

The operating characteristics of a salt hydrate latent heat storage system, using Glauber's salt and direct contact heat exchange through an immiscible heat transfer fluid, have been studied theoretically. Drop dynamics and heat transfer models from the literature were used to predict the system behaviour for a range of conditions involving heat transfer fluid inlet temperature and drop size, composition and crystallization temperature of the salt and vessel contact height. The results of these calculations are used to guide the specification of an approx. scale pilot heat storage unit which has been constructed and operated successfully.     

Thermoeconomics of seasonal latent heat storage system

A simple thermoeconomic analysis is performed for a seasonal latent heat storage system for heating a greenhouse. The system consists of three units that are a set of 18 packed‐bed solar air heaters, a latent heat storage tank with 6000 kg of technical grade paraffin wax as phase‐changing material, and a greenhouse of 180 m². The cost rate balance for the output of a unit is used to estimate the specific cost of exergy for a yearly operation. Based on the cost rate of exergy, fixed capital investment, operating cost, and economic data, approximate cash‐flow diagrams have been prepared. The systems feasibility depends on the cost rate of exergy, operating cost, internal interest rate, and rate of taxation strongly. A cash‐flow diagram based on exergy considerations may enhance the impact of thermoeconomic analysis in feasibility studies of thermal systems. Copyright © 2006 John Wiley & Sons, Ltd.     

The enhancement on heat capacity of nitrate salt heat storage materials by doping SiO2 nanoparticles

多元混合硝酸盐作为高温传热流体在聚光式太阳能热发电系统中具有良好的应用前景。本工作利用水溶液法,在具有较低熔点的NaNO₃-KNO₃-LiNO₃三元共混合硝酸盐中掺入SiO2纳米颗粒,有效地提高了混合硝酸盐储热材料的比热容,其中掺入较低质量分数（0.0625%）的纳米颗粒比掺入较高质量分数（1%、0.25%）的纳米颗粒能获得更高的比热容。利用聚乙烯吡咯烷酮（Polyvinylpyrrolidone,PVP）作为分散剂,增强了SiO2纳米颗粒在硝酸盐中的分散性,有效地缓解了制备过程中纳米流体中纳米颗粒的团聚。同时,本工作还研究了纳米颗粒的团聚对其增强比热容效果的影响,并通过使用分散剂,进一步提高了硝酸盐储热材料的比热容。最终结合上述关系,引入界面热阻和半固体层模型,讨论了掺入纳米颗粒增强硝酸盐储热材料比热容的机理。