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Today, one of the biggest challenges our society must face is the satisfactory supply, dispatchability and management of the energy. Thermal Energy Storage (TES) has been identified as a breakthrough concept in industrial heat recovery applications and development of renewable technologies such as concentrated solar power (CSP) plants or compressed air energy storage (CAES). A wide variety of potential heat storage materials has been identified depending on the implemented TES method: sensible, latent or thermochemical. Although no ideal storage material has been identified, several materials have shown a high potential depending on the mentioned considerations. Despite the amount of studied potential heat storage materials, the determination of new alternatives for next generation technologies is still open. One of the main drawbacks in the development of storage materials is their cost. In this regard, this paper presents the review of waste materials and by-products candidates which use contributes in lowering the total cost of the storage system and the valorization of waste industrial materials have strong environmental and societal benefits such as reducing the landfilled waste amounts, reducing the greenhouse emissions and others. This article reviews different industrial waste materials that have been considered as potential TES materials and have been characterized as such. Asbestos containing wastes, fly ashes, by-products from the salt industry and from the metal industry, wastes from recycling steel process and from copper refining process and dross from the aluminum industry, and municipal wastes (glass and nylon) have been considered. Themophysical properties, characterization and experiences using these candidates are discussed and compared. This review shows that the revalorization of wastes or by-products as TES materials is possible, and that more studies are needed to achieve industrial deployment of the idea.

a b s t r a c tLithium, mainly used in electrical energy storage, has also been studied in thermal energy storage. It is recognized as a "critical material" and is produced from minerals and from brines. Chile is one of the biggest producers, here from brine and with lower costs than in other countries. With sensible heat storage, in solar power plants lithium is seen as a way to improve the properties of molten salts used today. The low melting point in these ternary salts with lithium, represent a considerable reduction in the maintenance and operational costs associated with current solar technology, demonstrating that the fluids showed, are potential candidates for thermal energy storage (TES) in concentrated solar plants (CSP) plants. Many materials have been studied and proposed to be used as phase change materials (PCM). Between the multiple materials studied to be used in PCM, lithium materials and mixtures are listed as potential PCM for building applications and for high temperature applications. In thermochemical energy storage, lithium compounds have been used mainly in chemical heat pumps, following their use in absorption cooling.

Physical characterization and thermal properties of bischofite, a by-product from the nonmetallic industry, were determined and compared with those to MgCl 2 •6H 2 O with the idea of using it as phase change material in thermal energy storage applications. The melting point and heat of fusion were measured in the temperature range from 30°C to 150°C, where T fus and ΔH fus were 100°C and 115 kJ/kg for bischofite, and 114.5°C and 135 kJ/kg for MgCl 2 •6H 2 O. The solid heat capacity was determined from 25°C to 60°C, being 2.1 kJ/(kg•K) at 60°C for both samples. The measurements of the liquid heat capacity of bischofite were done from 105°C to 113°C and the Cp showed linear increase from 5.61 kJ/(kg•K) to 9.01 kJ/(kg•K). The thermal stability test (30 heating/cooling cycles) of bischofite and MgCl 2 •6H 2 O shows subcooling of about 37K and 29K, respectively. The solid and liquid densities were determined using the pycnometrically method; for bischofite, ρ solid decrease from 1686 (at 30°C) to 1513 kg/m 3 (at 50°C) and ρ liq was 1481 kg/m 3 (at 115°C). Based on the thermophysical properties evaluated, the energy storage density was evaluated for both materials, being 170 J/cm 3 for bischofite and 192 J/cm 3 for MgCl 2 •6H 2 O. This study established that bishofite is a promissory PCM with similar thermophysical characteristics to magnesium chloride hydrate, but with a lower cost. Nevertheless, further work is needed to overcome the two main problems found, subcooling and segregation of the material.