separationHigh nucleation rateAdequate crystallization rate
In the light of the phase transition mode, PCMs can be classified into solid–solid, solid–liquid, solid–gas, and liquid–gas systems (Figure 2.2). Although solid–gas and liquid–gas PCMs possess a very high phase change latent heat, the large volume change during phase transition restricts their applications in TES systems.[7] Therefore, solid–solid and solid–liquid PCMs have received extensive attention in recent decades owing to their small volume change. Generally, solid–liquid PCMs exhibit a higher energy density than solid–solid PCMs. Solid–liquid PCMs can be broadly classified into organic PCMs and inorganic PCMs.[6] Compared with inorganic PCMs not discussed in this chapter, organic PCMs represented by low‐molecular paraffin wax (PW) or alkane and polyethylene glycol (PEG) have attracted ever‐growing interest owing to their high energy storage capacity, low or negligible supercooling, and desirable stability. However, the majorities of organic PCMs possess fatal disadvantages of poor formability and low thermal conductivity, which seriously limit their practical applications. More details are presented in Table 2.2. Although solid–solid PCMs possess excellent shape stability, they also exhibit inherently low thermal conductivity and limited energy conversion ability. Incorporating functional components (supporting or conductive materials) into these PCMs is regarded as a promising route to fabricate thermally conductive or leakage‐proof phase change composites for high‐efficiency TES systems.
Figure 2.2 Classification of PCMs.
Source: Based on [7, 12].
Table 2.2 Advantages and disadvantages of organic PCMs and inorganic PCMs.
| Solid–liquid PCMs | Advantages | Disadvantages |
|---|---|---|
| Inorganic PCMs | High energy storage densitiesHigh thermal conductivitiesLow costs | SupercoolingPhase separation |
| Organic PCMs | High energy storage densitiesWide range of phase change temperature for convenient useIsothermal characteristicsNo phase separationLow or negligible supercoolingGood compatibilityNontoxicity and noncorrosionDesirable thermal and chemical stability for long‐term useAbundant natural resources | Poor shape stability (leakage during phase transition)Low thermal conductivityWeak energy conversion ability |
Polymeric phase change composites, as TES units, can be roughly divided into two categories: phase change polymers presented in Figure 2.2 as working substance and common polymers as supporting component. In this chapter, we put our emphases on the innovations of shape‐stabilized and thermally conductive polymeric phase change composites, covering processing method, structural design, and materials selection. Also, energy conversion routes and potential applications associated with polymeric phase change composites are highlighted. Finally, a brief perspective on future opportunities and challenges for high‐performance and multifunctional polymeric phase change composites is proposed.
2.2 Shape‐stabilized Polymeric Phase Change Composites
Organic solid–liquid PCMs suffer from liquid phase leakage in the process of phase transition, resulting in decrease of energy storage density, pollution of related devices, and security risks. Shape‐stabilized PCMs are composed of working substance and supporting material, and the latter predominately includes polymers with good compatibility and thermal stability, nanomaterials with desirable structure, and porous scaffolds with excellent mechanical properties.[13] Micro/nanoencapsulated techniques, physical blending with supporting materials, incorporating porous scaffolds, and crosslinking‐solidity strategy have been adopted to improve the shape stability of PCMs, which are magnified in this chapter.
To the best of our knowledge, the methods used to evaluate the shape stability of PCMs have not been unified and standardized. Thermal mechanical analyzer (TMA) and dynamic rheometer are usually used to quantitatively measure the size change of materials to evaluate their shape stability. With the increase of temperature, the size or dimension of samples will automatically adjust when subjected to constant normal force, and thus the shape stability can be determined by the change degree of the size or dimension. Generally speaking, the smaller the change of the sample size, the better the shape‐stabilizing effect of the material.[14] In addition to monitoring change of sample size in rheological test, dynamic temperature scanning can be used to compare the difference of storage modulus between pure sample and composites, further to judge the shape‐stabilizing effect.[15] Moreover, facile and qualitative leakage experiments are conducted to intuitively reflect the shape‐stabilizing effect in most cases. The samples are directly placed on a hot stage or in an oven and then heated to different temperatures for varying durations, followed by tracking real‐time photography with a digital camera to record the change of shape and size and leakage status of samples. It is more reasonable to calculate the mass change of samples before and after leakage test.[16, 17] Additionally, Shi et al.[18] measured the dropping point to determine the shape‐stabilizing effect of samples.
2.2.1 Micro/Nanoencapsulated Method
Micro/nanoencapsulation is one of the common techniques to fabricate shape‐stabilized PCMs with core‐shell structure, in which the core and the shell are, respectively, the working medium performing energy storage and functional coating layer providing complete and stable structure. In general, PCMs with phase change temperature in the range of −10–80 °C can be encapsulated.[19] Micro/nanostructures of phase change capsules can be tuned by using different preparation methods and controlling synthesis conditions, mainly forming single core‐shell, multi‐shell and polynuclear structures (Figure 2.3).[20] Microencapsulated phase change products present different appearances, such as sphere, tubular, and other irregular shapes. The coating materials mainly include organic polymers, inorganics (e.g. silica, metal oxides, and hydroxides), and their hybrids.[21] The choice of coating material depends on the compatibility with the core materials and the synthesis methods. Usually, monomer polymerization (e.g. emulsion polymerization, interfacial polymerization, and in‐situ polymerization) and sol–gel method are adopted for preparing organic and inorganic shell layers, respectively.[21, 22]
Owing to their low cost, light weight, outstanding flexibility, good processability, and desirable compatibility, polymers, including polystyrene (PS), polymethylmethacrylate (PMMA), polyurethane (PU), urea–formaldehyde resin (UF), melamine–formaldehyde resin (MF), and many others, are widely used in the preparation of core‐shell phase change composites.[19, 23, 24] For example, a direct miniemulsion method has been adopted to prepare spherical poly(ethyl methacrylate) (PEMA)/n‐octadecane nanocapsules with the average size of 140 nm and PMMA/n‐octadecane