To address the growing problem of pollution and global warming, it is necessary to steer the development of innovative technologies towards systems with minimal carbon dioxide production. Thermal storage plays a crucial role in solar systems as it bridges the gap between resource availability and energy demand, thereby enhancing the economic viability of the system and ensuring energy continuity during periods of usage. Thermal energy storage methods consist of sensible heat storage, which involves storing energy using temperature differences; latent heat storage, which utilizes the latent heat of phase change materials; and thermochemical heat storage, which utilizes reversible chemical reactions through thermochemical materials. The objective of this review paper is to explore significant research contributions that focus on practical applications and scientific aspects of thermal energy storage materials and procedures. For each type of storage, different materials have been examined, taking into consideration the most recent studies, both for medium and long-term storage and, when possible, comparing methodologies for the same purpose. It has been observed that TCHS systems have the potential to reduce the volume of chemical storage tanks by 34 times using chemical reactions. Among the SHS materials, water, molten salts, and graphite exhibit the highest energy density, with graphite also possessing remarkable thermal conductivity. Nanoparticles can enhance the thermophysical properties of TES materials by increasing their thermal conductivity and wettability and improving intermolecular characteristics. The use of biobased PCMs for applications that do not require very high temperatures allows for maximizing the efficiency of such storage systems.

The Mild Combustion is a relatively novel combustion technique characicrized by both an elevated temperature of reactants and an adiabatic flame temperature not higher than 1600K. These features are the results of several technological demands coming form different application fields. The main advantages derived by Mild Combustion concern both the combustion process itself and its applications. In the former case the pollution reduction, the increase of efficiency process and fuel flexibility has to be considered whereas the latter one is principally related to heat treatment process and wall confinement. In this paper, characteristics of Mild Combustion arc discussed with particular regards towards the aspects of plant design. A general classification of the processes, where initial and adiabatic temperature values are different from characteristic ones of classical combustion system, is given. This classification leads to a definition of Mild Combustion process as a direct consequence of technological evolution and practical constrains, especially related to environmental problems. In this sense, the high temperature of reactants and the low adiabatic temperature, principally obtained using exhausts for both heat recovery and dilution, can not be considered independently. Application of Mild Combustion in practical systems infers the use of particular solutions because of high temperature of reactants involved. In the final part of the paper, specific solutions utilized in a laboratory mild combustor are presented.