Recent developments in phase change materials for energy storage applications: A review

A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed.

Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies

Latent heat energy storage system is one of the promising solutions for efficient way of storing excess thermal energy during low consumption periods. One of the challenges for latent heat storage systems is the proper selection of the phase change materials (PCMs) for the targeted applications. As compared to organic PCMs, inorganic PCMs have some drawbacks, such as corrosion potential and phase separation; however, there are available techniques to overcome or minimize these drawbacks. On the other hand, inorganic PCMs are found to have higher thermal conductivity and storage capacity over organic PCMs. As a result inorganic PCMs have a great potential in thermal energy storage field, especially in medium to high temperature applications where organic PCMs are not a viable option. In this study, a detailed review of research outcomes and recent technological advancements in the field of inorganic phase change materials is presented while focusing on providing solutions to the associated disadvantages of this class of PCMs. Long term stability, thermal cycling performance, and heat transfer enhancements are also discussed in the context of this review.

A review on current status and challenges of inorganic phase change materials for thermal energy storage systems

Phase change materials (PCMs) allow the storage of large amounts of latent heat during phase transition. They have the potential to both increase the efficiency of renewable energies such as solar power through storage of excess energy, which can be used at times of peak demand; and to reduce overall energy demand through passive thermal regulation. 198.3 million tons of oil equivalent were used in the EU in 2013 for heating. However, bulk PCMs are not suitable for use without prior encapsulation. Encapsulation in a shell material provides benefits such as protection of the PCM from the external environment and increased specific surface area to improve heat transfer. This review highlights techniques for the encapsulation of both organic and inorganic PCMs, paying particular attention to nanoencapsulation (capsules with sizes <1 μm). We also provide insight on future research, which should focus on (i) the development of multifunctional shell materials to improve lifespan and thermal properties and (ii) advanced mass manufacturing techniques for the economically viable production of PCM capsules, making it possible to utilize waste heat in intelligent passive thermal regulation systems, employing controlled, “on demand” energy release/uptake.

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Nanoencapsulation of phase change materials for advanced thermal energy storage systems

Phase change materials (PCM) are characterized for storing a large amount of thermal energy while changing from one phase to another (generally solid-liquid states) at a specific temperature and presenting a high specific heat of phase change process. PCMs can be classified as organic, inorganic and eutectic. Such materials present some limitations, including subcooling, phase segregation, flammability, low thermal conductivity and thermal instability, among others. In order to overcome these problems, encapsulation of PCMs is being successfully developed, providing decreased subcooling, large heat transfer area, and controlling the volume change of the storage materials when the phase transition occurs. A considerable amount of studies has been published in the field of encapsulation methods for organic PCMs. Nevertheless, the information available on inorganic PCMs is scattered. Furthermore, the influence of the encapsulation techniques on thermophysical properties of PCMs is not reported in these reviews most of the time. Hence, the aim of this review is to summarize the encapsulation and characterization techniques for inorganic PCMs and to provide the analysis about the influence of synthesis parameters on thermophysical properties of encapsulated PCMs. Two principal types of encapsulated inorganic PCMs were found: core-shell PCMs (core-shell EPCMs) and shape stabilized PCMs (SS-PCMs). Classification of encapsulation methods of core-shell EPCMs and SS-PCMs are reported in this work. Among all the microencapsulation methods, inverse Pickering emulsion, electroplating, solvent evaporation–precipitation method and mechanical packaging are the most common methods described in the literature for the production of core-shell EPCM. On the other hand, for SS-PCMs, mainly sol-gel process, infiltration and impregnation encapsulation methods were found. Scientific works report a reduction in the heat of phase change for core-shell EPCMs. This is mostly because of the low content of salt in the final material. Moreover, an improvement of thermal conductivity was procured for SS-PCMs. Finally, PCM percentage, particle size, stirring rate, type of crosslinking agent and solvent properties were established as principal factors influencing the final properties of the encapsulated materials. For the best of our knowledge, this is the first profound review of encapsulation techniques for inorganic PCM.

A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties

Encapsulation of copper-based phase change materials for high temperature thermal energy storage

Encapsulation of copper-based phase change materials for high temperature thermal energy storage / Sol. Energy Mater. Sol. Cells

Fe-shell/Cu-core encapsulated metallic phase change materials prepared by aerodynamic levitation method

The invention discloses a phase-change heat-storage energy-saving electric water heater and belongs to the technical field of household appliances. The water heater comprises a shell, an insulating layer, an inner tank, a magnesium rod, a sewage draining exit, a hot water outlet, a cold water inlet, a leakage protection switch, a temperature controller and an electric heating pipe, wherein the insulating layer between the shell and the inner tank is a composite insulating layer which consists of a polyurethane foam heat-preservation layer and a phase-change heat storage layer; the energy saving of a water storage electric water heater is realized by the double insulation of heat storage and heat insulation; the polyurethane foam heat-preservation layer is positioned between the shell and the phase-change heat storage layer which is tightly attached to the outer side of the inner tank; the composite insulating layer is 25 to 45mm thick; and the thickness ratio of the phase-change heat storage layer to the polyurethane foam heat-preservation layer is 0.4 to 0.8. The electric water heater has the advantages of capability of improving the energy saving level in a water storage and heat preservation process and reducing power consumption, high heat utilization ratio, safety and low cost, and is environmentally friendly and convenient for operation.

Phase-change heat-storage energy-saving electric water heater

To prepare the phase change materials for efficient low-temperature latent heat storage, microcapsules with commercial solid paraffin were synthesized by using in-situ polymerization process. Liquid paraffin was used first as core materials since it facilitates the direct observation of containment by the shell of melamine-formaldehyde resin (MF resin), and examines easily the optimal synthesizing condition. The effects of emulsion processes, pre-polymerization conditions, shell material content and reaction time on the quality of microcapsules were investigated. The experimental investigations show that the optimal process of synthesizing liquid paraffin microcapsules is adoptable to that of solid paraffin microcapsules. The as-prepared solid paraffin microcapsules show high quality. The microcapsules surface was smooth and dense, and be free of any adhesion. The measurements show that in the microcapsules solid paraffin was well encapsulated by MF resin. The microcapsule size was almost within the range of 6 similar to 33 mu m and most intense distribution at 23 mu m. DSC measurements gave two endothermic peaks with initial phase change temperature at 35 degrees C and 50 degrees C respectively, and the total enthalpy was above 134 J/g.

Zhe Xu

Papers

Encapsulation of copper-based phase change materials for high temperature thermal energy storage

Encapsulation of copper-based phase change materials for high temperature thermal energy storage / Sol. Energy Mater. Sol. Cells

Fe-shell/Cu-core encapsulated metallic phase change materials prepared by aerodynamic levitation method

Phase-change heat-storage energy-saving electric water heater

Synthesis and Properties of Microencapsulated Solid Paraffin Phase Change Materials