(PDA),[121] Fe3O4@graphene,[122] and MXene[70] have been employed as photothermal absorbers for PCMs to realize the efficient conversion and storage of solar energy. The solar‐to‐heat conversion and storage efficiency (ηs) can be calculated from the ratio of the stored heat with respect to the input solar energy according to the light‐thermal calculation Eq. (2.3).
Figure 2.7 Energy conversion routes associated with polymeric phase change composites: (a) electro‐to‐heat. (b) photo‐to‐heat.
Source: Wei et al. [95]. Reproduced with permission from Elsevier Ltd.
(c) magnetism‐to‐heat.
Source: Based on Wang et al. [107].
(d) heat‐to‐electricity.
Source: Jiang et al. [108]. Reproduced with permission from the Royal Society of Chemistry
and (e) photo‐to‐heat‐to‐electricity.
Source: Yang et al. [105]. Reproduced with permission from the Royal Society of Chemistry.
where ρ and S are the light irradiation intensity and the surface area, respectively.
Note that carbon materials are mainly incorporated in the above‐mentioned electric‐driven phase change composites, and thus most of them can achieve electro‐to‐heat and light‐to‐heat conversions simultaneously. Considering that PEG with abundant oxygen‐containing functional groups, dyes, GO, PDA, and MXene serving as photon harvesters and molecular heaters have been proved to be good candidates for the fabrication of photodriven polymeric phase change composites. The working mechanism and experimental setup of solar‐to‐heat conversion associated with polymeric phase change composites are shown in Figure 2.7b. During the solar radiation from simulated light source, phase change composites can absorb thermal energy in the form of sensible heat, leading to a rapid increase in temperature. When the temperature reaches the phase change temperature, an inflection point emerges in the temperature evolution curves, indicating that phase transition and TES in the form of latent heat take place with small temperature fluctuation. After the phase transformation, the temperature rises again, and the converted heat energy is expressed as sensible heat in this phase. Upon removing the incident light, the temperature drops sharply and another phase change platform emerges later, demonstrating that the stored heat energy is released to maintain the temperature during the solidification. After that the temperature of the system slowly decreases to room temperature.
Aside from photodriven polymeric solid–liquid phase change composites, Zhou et al.[75] developed a photothermal energy conversion system by introducing graphene into crosslinked PU solid–solid PCMs. When the content of graphene in the phase change composites increased from 0.5 to 9.1 wt%, the photothermal energy conversion efficiency of the system increased from 42.9 to 78.7% accordingly. Furthermore, multifunctional PU‐based phase change composites with zinc oxide (ZnO) and graphene aerogel were fabricated.[116] Owing to the enhanced photoabsorption from ZnO and graphene aerogel and electrical conductivity from graphene aerogel, the photo‐/electro‐thermal energy conversion efficiencies of the system containing the resultant polymeric solid–solid phase change composites were up to 80.1 and 84.4%, respectively.
2.4.3 Magnetism‐to‐Heat Conversion
Tang’s group has proposed a magnetism‐to‐heat conversion system through incorporating magnetic particles into phase change composites for the first time.[107] Magnetic Fe3O4 was deposited on the surface of graphene and then introduced into PEG matrix to yield phase change composites. The obtained nanocomposites can not only absorb solar energy to perform light‐to‐heat conversion but also realize magnetism‐to‐heat conversion under an alternating magnetic field. The energy conversion process and experimental apparatus are illustrated in Figure 2.7c. Together with Fe3O4/graphene hybrids as nanoheaters, alternating magnetic field induced by an alternating current generator can trigger magnetism‐to‐heat conversion of phase change composites. The magnetic thermal conversion efficiency (ηm) can be defined by the ratio of stored heat energy to energy exerted by magnetic field on the basis of the magnetism‐thermal calculation Eq. (2.4), which increases with the increase of content of functional materials in the composites, up to 41.7%. Moreover, the results and conclusions have been confirmed in another magnetic thermal energy conversion system containing a novel PEG/SiO2/Fe3O4 ternary phase change composite.[123]
where P is alternating magnetic field power.
2.4.4 Heat‐to‐Electricity Conversion
The above three systems are capable of converting other forms of energy into heat energy for storage. How to use the thermal energy collected and stored by PCMs? In 2015, Jiang et al.[108] creatively proposed a proof‐of‐concept thermoelectric conversion by combining phase change energy storage materials with n/p‐type semiconductors, achieving the conversion of heat into applicable electricity. As shown in Figure 2.7d, the thermal energy from heat source can be collected and stored by PCMs, and then the collected heat is transferred to a commercial thermoelectric device, forming a temperature difference at its two ends and generating electricity via Seebeck effect. Similarly, Yu et al.[124] designed a thermoelectric energy conversion system based on graphene aerogel embedded shape‐stabilized PCMs. Two PCMs (PEG and 1‐tetradecanol) with different phase transition temperatures are, respectively, used as heat and cold sources at the ends of thermoelectric devices to tune the temperature difference. Based on the experimental data and numerical predictions, the heating and cooling energy conversion efficiencies are 52.81 and 30.29%, respectively. Furthermore, one‐step photo‐to‐electricity conversion systems (Figure 2.7e)[66, 104, 105] integrating light‐to‐heat with heat‐to‐electricity have been developed. To achieve light‐to‐heat‐to‐electricity energy conversion, a prerequisite is that phase change composites ought to possess excellent photoabsorption.
Compared with electro/photo‐to‐heat energy conversion efficiency, magnetism‐to‐heat and heat‐to‐electricity conversion efficiencies are relatively low and need to be further improved. As the most of the above‐mentioned energy conversion systems are exposed to the surrounding environment, a portion of the converted heat energy is more or less dissipated, affecting the energy conversion efficiency. One solution is to carry out these energy conversion operations in a vacuum (thermal insulation) environment. Additionally, there is a lack of evaluation index for light‐to‐electricity energy conversion. For the case of photodriven phase change composites, polymer‐based photothermal absorbers such as artificial melanin, polyaniline (PANI), and polypyrrole (PPy) are competitive candidates. In heat/solar‐to‐electricity route, a commercial thermoelectric module is used directly, and thus an integrated device consisting of PCMs‐based component and thermoelectric