Latent thermal energy storage for solar process heat applications at mediumhigh temperatures - A review

Abstract

Solar thermal energy has the potential to cover the heat demands of industrial processes. However, there may be a time mismatch between energy supplied by the solar field and the process demand. In this case, a thermal energy storage (TES) allows the use of heat at hours without solar irradiation available. Thermal energy storage for solar hot water or heating systems using low temperatures have been optimized since many decades and are in a mature stage. Developments at high temperatures (above 200°C) for CSP applications have also been deeply studied. However, until this present paper, limited attention has been paid to TES for solar thermal industrial applications at medium-high temperatures (120?400°C), where there is a potentially huge demand. When talking about TES several aspects have to be discussed: the heat demand that TES is going to be designed to supply, the material where the energy will be stored and the performance of the TES system which includes not only the material but also tanks, piping and connections. In this review, food, brewery and chemical industries were identified as the industries with higher potential in which TES andsolar energy could be integrated. Heat integration methodologies have been reviewed to optimize the use of the solar energy in the industrial processes. Regarding the material, latent heat storage or phase change materials (PCM) were selected for this study because they are a very promising type of storage to be integrated in thermal industrial processes, although the state of the art of latent heat thermal energy storage (LHTES) systems is still far from broad commercialization. Until now, no reviews of latent heat storage for industrial applications at medium-high temperatures (120?400°C) have been published. Therefore, literature related to PCM and latent heat storage (LHS) systems to be used in industrial thermal processes is here reviewed in order to have a general overview of the available technologies for their integration together with solar thermal energy in industrial processes at both experimental and numerical level. More than 100 potential PCMs for heat storage applications in the range of temperatures 120?400°C have been found. Inorganic eutectic compositions are the group with more potentially available PCM for these applications, with values of heat of fusion between 74 and 535kJ/kg. Finally, the works related to the performance of the system from the experimental and modelling point of view were presented. The review of experimental TES systems which include PCM in the studied range of temperatures 120?400°C showed that most of the experimental set-ups were developed for direct steam generation for CSP applications. Regarding numerical modelling, the type of configuration more simulated is the shell and tube configuration.