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2 Polymer Composites for Thermal Energy Storage
Jie Yang, Chang‐Ping Feng, Lu Bai, Rui‐Ying Bao, Ming‐Bo Yang, and Wei Yang
College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, Sichuan, P. R. China
Jie Yang and Chang‐Ping Feng contribute equally.
2.1 Introduction
It is predicted that there will be a one‐third increase in energy demand by 2035 from 2011.[1] Although fossil fuels, as limited and nonrenewable resources, can provide vast quantities of energy, the irreversible burning releases large amounts of pollutants, resulting in the deterioration of environment and global warming. The focus of the energy battlefield is also gradually shifting to green and renewable energy. However, the utilization efficiency of renewable energy sources, including solar energy, wind energy, tidal energy, geothermal energy, etc., only accounts for approximately 20% of today’s energy consumption owing to the limitations of modern technologies and geographical environments.[2] The discontinuity and regionalism seriously affect their utilization efficiency. More importantly, during the process of using various energy resources, a large amount of energy is used in the form of thermal energy or converted from thermal energy for reuse. Therefore, the development of thermal energy storage (TES) technologies and systems through sensible heat, latent heat, and thermochemical reaction is of great significance to solve the issue of spatiotemporal mismatch between energy supply and demand, improve energy utilization efficiency, alleviate energy crisis, and manage environmental pollution.[3–5] TES not only serves as an integrated buffer in energy conversion systems but also can be incorporated into thermal management technologies for electronics, human body, and buildings.
Phase change materials (PCMs), as an advanced energy storage technique, can absorb or release a lot of energy in the form of latent heat during phase transition for efficient storage and utilization of energy, while maintaining constant temperature (Figure 2.1).[6–8] The selection criteria and characteristics of ideal PCMs feasible for TES are listed in Table 2.1.[7–11] However, it is troublesome for the majorities of PCMs to fully satisfy these criteria. Recent advancements in the innovations of nanomaterials and nanotechnologies for energy‐related applications provide possibilities for PCMs with enhanced comprehensive performance.
Figure 2.1 Working principle of PCMs.
Table 2.1 The features of ideal PCMs feasible for TES.
| Aspects | Details |
|---|---|
| Thermal properties | Appropriate phase transition temperature to satisfy practical applicationsLarge phase change enthalpy to provide high latent heat storage capacityHigh thermal conductivity to achieve fast charging or discharging rateExcellent thermal stability to prolong service life |
| Kinetic properties | No/negligible supercoolingNo phase separationHigh nucleation rateAdequate crystallization rate |
| Physical properties | Subtle volume variation during phase transformationLow vapor pressure at the operating temperature |
| Chemical properties | Long‐term chemical stabilityNoncorrosive, nontoxic, nonpolluting, nonflammable, and nonexplosive to ensure safety and harmless to the surroundings |
| Economics | AbundanceEasy availabilityLow costGood recyclability |
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
|
