Phase Change Materials for Enhanced Thermal Energy Storage Applications

Thermal energy needs determined by the increase demand is a fundamental and strategic energy source and it is very important to recover and store its lost and surplus. In this frame thermal energy storage (TES) is a natural solution and it becomes the necessary bridge between the energy demand and availability mainly in renewable energy sources due to their intermittent nature, particularly solar energy, which is the most prospective energy source. Moreover, the use of TES determines the improvement of performance and reliability of the energy systems [1]. There are three types of thermal energy storage systems (TESS): chemical energy storage system (CESS), sensible heat thermal energy storage system (SHTESS) and latent heat thermal energy storage system (LHTESS). The first system stores the heat through reversible chemical reactions, so when it receives heat the behavior of the chemical process is endothermic and when it releases heat the chemical process is exothermic [2]. The second system is based on increasing the temperature of the material that the system is made. The third system uses the phase change materials (PCMs) to store thermal energy at quasi-constant temperature. During the phase change process, the heat is used to change phase and not to increase the temperature [3]. The LHTESS is better because it presents many advantages like the low range of the working temperature and high energy density. Three types of PCMs are employed: solid-solid, solid-liquid and liquid-gas. The solid-solid phase change process allows to store the heat during the crystallization and the solid material is subject to a change in its crystal lattice. The main characteristics are: a low variation of volume during the phase change process and a low latent heat value. During the solid-liquid phase change process the material melts storing the heat and whereas in the opposite process, liquid-solid, it solidifies releasing of the heat. In the solid-liquid process the latent heat in general is higher than in the solid-solid transformation but the variation of volume is higher. The liquid-gas phase change process has the highest value of latent heat far from the critical point and a corresponding very high difference between the saturated vapor and liquid specific volumes. The solid-liquid phase change process is more suitable to store thermal energy because it represents the best compromise among these phase transformations. There is not a unique classification of the solid-liquid PCMs, but generally the main subdivision is organic and inorganic materials, as hydrated salts [4]. The principal organic material is the paraffin, it is widely used in the literature because it is stable, not corrosive, nontoxic and the phase change process is reversible [5]. These characteristics are not sufficient to design a LHTESS because the worse disadvantage is the small value of the thermal conductivity, leading to a long time for melting respect to a working cycle and a huge temperature gradient in the system. [1] Cabeza, L. F., Advances in Thermal Energy Storage Systems: Methods and Applications, Woodhead Publishing, Cambridge UK, 2014. [2] Gil A, Medrano M, Martorell I, Lazaro A, Dolado P, Zalba B, Cabeza LF, State of the art on high temperature thermal energy storage for power generation. Part 1 – Concepts, materials and modellization. Renewable and Sustainable Energy Reviews, 14, pp. 31-55, 2010. [3] Zhou, D., Zhao, C.Y.,Tian, Y., Review on thermal energy storage with phase change materials (PCMs) in building applications, Applied Energy, 92, pp. 593-605, 2012. [4] Khudhair AM, Mohammed MF (2004) A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Convers Manag 45:263–275 [5] Abhat A (1983) Low temperature latent heat thermal energy storage: heat storage materials. Sol Energy 30:313–332