**1. Introduction**

Energy is an indispensable component of human survival and social development. The excessive exploitation and abuse of fossil energy such as coal, oil and natural gas has caused the current global energy crisis and environmental pollution [1, 2]. Therefore, the development of renewable energy (solar energy) and energy conservation are considered the main directions of future energy development. "Green energy" is limited by its constraints showing intermittency and instability [3–5]. Solving the time-space mismatch between the supply and demand of thermal energy is one of the most critical obstacles, which makes thermal energy storage (TES) technology become an important support to promote the energy revolution. Thermal storage materials are a central part of TES [6]. Real-time thermal energy distribution with thermal storage material as the medium for the demand of the energy-using

side can significantly improve the energy efficiency, stability, and economic benefits of TES [7, 8].

Solar photo-thermal and industrial waste heat are the main targets in the strategy of practical thermal energy storage conversion and their temperature ranges are shown in **Table 1** [11]. Medium-high temperature latent heat TES technology (>120°C) to store excess thermal energy is the most ideal choice for packed bed latent heat TES technology (PLTES). The high operating temperature range makes the available phase change materials (PCMs) mainly divided into inorganic salts and alloys [12]. For medium-high temperature PCMs, inorganic salts are the most widely accepted objects, including chlorinated salts, nitrates, carbonates, and fluorinated salts, referenced as shown in **Table 2** [14–16]. Molten salts are bound to undergo a solid-liquid phase transition during charging/discharging cycling in practical


#### **Table 1.**

*Temperature range of practical thermal energy storage-conversion.*


#### **Table 2.**

*Performance parameters of common molten salts.*

### *Medium-High Temperature Composite Phase Change Materials Based on Porous Ceramics DOI: http://dx.doi.org/10.5772/intechopen.114185*

applications, which makes the structural requirements of PLTES particularly high and difficult to massively promote. Therefore, there is a need to prepare inorganic salt-based PCMs into shape-stable composite phase change materials (CPCMs) with high energy density, wide operating temperature range, stable physical/chemical properties, and low prices through rational encapsulation methods.

It should be noted that the inorganic salt-based CPCMs have the dilemma of low thermal conductivity and severe corrosion [17]. Mating inorganic salts with porous skeletons to obtain inorganic salt-based CPCMs can effectively solve these problems. Inorganic salt provides sufficient heat capacity, and the porous skeleton acts as a stable carrier and reduces overcooling, so CPCMs have excellent heat storage and release effects [18]. A skeleton matrix generally requires high porosity and a large surface area to provide more PCM loading volume. Ceramic is known to be one of the best choices for porous skeletons with high-temperature corrosion resistance, and high thermal conductivity. Preparation of CPCMs from porous ceramics and inorganic salts can enhance heat transfer and support-encapsulation, with higher corrosion resistance and economic benefits compared to alloy carriers. However, there are fewer comprehensive reviews focusing on medium-high temperature ceramic-based CPCMs, with some content bias: the preparation process is not specific; the materials selection is not comprehensive, and system-level studies and applications are not mentioned.

Our group has been focusing on the study of medium-high temperature CPCMs based porous ceramic (C-CPMs), and thus have a comprehensive understanding of that field. This chapter summarizes the recent contents of C-CPCMs in detail. Firstly, the preparation process and the influence of thermal properties of medium-high temperature CPCMs are described in detail, followed by a summary of the material types and selection principles of porous ceramic skeletons; finally, the CPCMs are described for system-level research and application.
