where m is the mass, 𝛥H is the phase change enthalpy determined by differential scanning calorimeter (DSC), U and I, respectively, represent the applied voltage and current, and t is phase change time.
The majorities of the systems used to conduct electro‐to‐heat conversion are organic non‐polymeric solid–liquid PCMs. Leakage‐proof phase change composites composed of microcrystalline cellulose/GNPs aerogel and PEG have been reported. The conductive composites have the ability of electrical energy transition and release (Figure 2.7a).[95] When an electrical field is applied, electrical energy can be inverted into thermal energy by generating Joule heat. Once the accumulated heat reaches the phase transition temperature of working substance, phase change and heat storage behaviors occur. Likewise, shape‐stabilized PW‐based composites containing commercial melamine foam incorporated by GO and GNPs exhibited high electrical conductivity (2.787 S cm−1) at a filler loading of 4.89 wt% and efficient electro‐to‐heat conversion capacity with an efficiency of 62.5%.[113] In addition, Chen et al.[114] employed solid–solid PCMs to realize electro‐to‐thermal energy conversion. PEG was introduced into graphite foam and then in‐situ polymerized, giving rise to the formation of PU‐based solid–solid phase change composites. When a relatively low voltage of 1.2 or 1.4 V is applied, the phase change composites can complete the electro‐to‐heat conversion, and the estimated conversion efficiency is above 80%. Also, an efficient electro‐to‐heat conversion for PU‐based solid–solid phase change composites has been achieved after introducing electrically conductive graphene aerogel.[115, 116]
2.4.2 Light‐to‐Heat Conversion
Not only can electricity be converted into thermal energy, but also solar energy can be transformed into heat and stored in PCMs. Solar energy, a renewable and clean energy source, is considered to be one of the most effective methods to solve the energy shortage issue. However, there are still technical challenges in the effective utilization of solar energy owing to the intermittence and discontinuity of solar radiation in time and space, which can be exactly solved with the assistance of PCMs storing and releasing heat during the phase transition process. The weak photoabsorption capacity of PCMs makes them unable to convert solar energy into heat directly and effectively. In recent years, dyes,[117] carbon materials (biomass carbon,[118] CNT,[88] EG,[119] graphene,[94] and GO[120]), polydopamine (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
