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Abstract Thermal energy storage systems provide a means to store energy for use in heating and cooling applications at a later time. The storage of thermal energy allows renewable sources of energy to be stored if the time of demand does not coincide with the time of production. It also enables access to off-peak electricity tariffs offered during times of low electricity demand. Storage systems can be charged during the low-cost tariff period and provide heating or cooling later when required. This benefits consumers with lower electricity costs and power generators with demand levelling. Thermal energy storage systems predominantly store heat as sensible heat in a substance. However, during a phase change heat energy can be stored as latent heat. Phase change material (PCM) thermal storage systems can store a greater amount of thermal energy per unit volume than sensible heat storage systems. Historically a drawback of using PCMs as a storage medium has been the low rates of heat transfer. Heat transfer enhancement techniques studied have included the use of additional metallic material and increasing heat transfer surface area such as fins to improve heat transfer rates of the PCM. Although these techniques are effective, they add significant cost and reduce the compactness factor of the thermal energy storage system. Recent research has been conducted on heat transfer enhancement that makes use of moving or transporting the PCM. This method is not only effective for increasing the heat transfer; it is less expensive and maintains a high compactness factor for the thermal energy storage system. This review paper presents the different heat transfer enhancement techniques reported in the literature. It also summarises the research conducted on phase change storage systems where the PCM is moved in the storage system.

Considerable effort has been devoted to the characterization of thermal properties of the different types of materials that can be used as thermal energy storage (TES) media, but scarce literature exists concerning the materials to manufacture the tanks that can be used to contain these storage media. One of the main concerns when selecting the most suitable material for these tanks is its resistance to corrosion due to molten salts that constitute the TES system. Dynamic gravimetric analysis is a newly proposed method for the study of corrosion on metals, which optimizes the standard procedure described by ASTM G1-03. The new technique avoids the direct handling of samples, so more accurate values can be obtained. In this work, the resistance to corrosion of AISI 316 stainless steel samples in contact with commercial grade molten salts of the Li2CO3-Na2CO3-K2CO3 system, at 600 °C for different exposure times, has been determined by using this new methodology. The results show that the initial corrosion rate is lower at higher amounts of lithium carbonate present in the molten salts mixture.

In the present paper, an experimental study is carried out to evaluate the effect of the dynamic melting concept in a cylindrical shell-and-tube heat exchanger using water as the phase change material (PCM) and a potassium formate/water solution as the heat transfer fluid (HTF). The dynamic melting concept is a new heat transfer enhancement technique which consists of recirculating the liquid PCM during the melting process with a pump and thus increasing the overall heat transfer coefficient as a result of the dominance of the forced convection. The HTF flow rate was kept constant at 1 l/min and four different PCM flow rates of 0, 0.5, 1 and 2 l/min were tested. Results from the experimental analysis showed enhancements up to 65.3% on the melting period, up to 56.4% on the effectiveness, and 66% on the heat transfer rates when the PCM flow rate was twice the HTF flow rate. From these experiments, it can be concluded that dynamic melting is an effective technique for enhancing heat transfer during melting of PCM. The authors acknowledge the South Australian Department of State Development who have funded this research through the Premier’s Research Industry Fund - International Research Grant Program (IRGP 33). This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement N° PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Jaume Gasia would like to thank the Departament d’Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2016FI_B 00047). The work is partially funded by the Spanish Government (ENE2015-64117-C5-1-R). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123).

Abstract A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses.