Thermal Properties of Alternative Phase Change Materials for Solar Thermal Applications

Thermal energy storage (TES) based on phase change materials (PCMs) is considered one of the most attractive techniques to store energy from renewable energy sources or waste heat from industrial applications [1]. PCMs are a broad group of materials able to store and retrieve a large amount of thermal energy in small temperature ranges since the stored energy corresponds to the latent heat involved in their phase transitions. Among the four phase transition processes that can occur in a PCM (i.e., solid-solid, solid-liquid, solid-gas, and liquid-gas), the materials comprising a transition between solid phases and from solid to liquid phase are the most suitable to be used in actual applications [1, 2].

In recent years, many solid-liquid PCMs and their eutectic mixtures have been broadly analyzed for various TES applications, such as solar energy, building sector, cold storage, and industrial sector [3-7]. The materials with solid-liquid phase transition properties suitable for the TES systems are classified into three main categories: organic, inorganic, and eutectic PCMs. However, each group has specific advantages and drawbacks that could hinder their use in different applications [1, 8]. Some of the main ones are high volume expansion, liquid leakage during multiple phase-change cycles, and phase segregation.

Consequently, PCMs with a solid-solid phase transition have been receiving increasing attention as promising alternatives since they have different advantages, such as negligible volume change during phase transition, no contamination, and long cyclic life [9, 10]. Another advantage of the solid-solid PCMs (S-S PCMs) is their possible direct use in TES systems without the need for encapsulation or heat exchangers [11].

The S-S PCMs can be divided into three main families: organic, inorganic, and hybrid materials. The organic S-S PCMs comprise polyalcohols, polymeric and organic-salt-based materials. This family's phase transition temperatures and enthalpies range between 25 to 185℃ and 15 and 270 kJ kg^-1, respectively [10]. The family of inorganic S-S PCMs is mainly composed of ceramic-based materials, having phase transition temperature and enthalpy ranges of 30-120℃ and 25-70 kJ kg^-1, respectively [10], and metallics and metal alloys, which are characterized by high phase transition temperatures (600-1000℃) [9]. Finally, hybrid S-S PCMs include perovskites and organometallic-based materials obtained by appropriately combining the first two families.

Among the organic S-S PCMs, polyalcohols are well-known and widely studied materials that seem suitable for low and medium-temperature TES systems, such as those used in solar thermal applications. Their solid phase transition temperatures are from 43 to 187℃, while their transition enthalpies are generally higher than other S-S PCMs [10]. Polyalcohols are also known as "plastic crystals" (PCs) since they undergo reversible solid phase transformations from low-temperature layered (or chained) structures (α-phase) to partially disordered ones, which form the plastic phase (γ’-phase) [10, 12]. Some of the most studied PCs are as follows [12]: pentaerythritol (PE), pentaglycerine (PG), neopentylglycol (NPG), 2-amino-2-methyl-1,3-propanediol (AMPL), and tris(hydroxyl methyl)aminomethane (TAM or TRIS). AMPL and TAM are amine derived from polyalcohols [12]. Moreover, the mixtures of PCs are considered promising S-S PCMs with phase transition properties adjusted for specific TES systems [12-14]. In particular, some of these mixtures are characterized by non-isothermal continuous phase transitions in temperature ranges specific for each composition. Examples are PE+PG+NPG ternary mixture and its binary subsystems that can be used in TES applications that do not exclusively need conventional isothermal phase transitions, such as different solar thermal systems [15].

Several studies regarding the phase transition properties and crystallographic properties of PCs and their mixtures have been carried out for many years [10, 16-20]. However, to the best of our knowledge, fundamental features to evaluate their actual use in TES applications (e.g., thermophysical properties, thermal stability, cycling stability, cost analysis) were not deeply analyzed. This could be due to the research on the use of solid-solid PCMs in actual TES applications, which still is at an early stage, and further studies are necessary [10]. In this regard, few works about the performance of PCs in TES systems for specific applications were found in the literature. Some of the analyzed applications are solar drying [21], solar cooking [22, 23], solar industrial process heat [24], and heat recovery [25, 26].

This study reviews the main characteristics of PE and PG and their mixture to evaluate if they can be suitable for various solar thermal applications at low-medium temperatures (80-150℃). In particular, the literature studies reporting the following characteristics of the mixture and their pure components are analyzed: phase transition properties, thermophysical properties, and thermal and cycling stability analyses. Measurements of the phase transition properties performed using differential scanning calorimetry (DSC) are also reported to confirm the accuracy of the literature data.

3.1 Phase transition properties

This section presents the S-S phase transition temperatures and enthalpies for PE and PG and their mixture collected from the literature. In particular, the experimental data provided by DSC in the heating and cooling scans were selected. Moreover, at this stage of this work, preliminary DSC measurements performed on the pure components are also reported and compared with the literature data.

Table 2 shows the phase transition temperatures in the heating (T_t,h) and cooling (T_t,c) scans for PE and PG reported in different literature studies, together with their enthalpies of S-S phase transitions (ΔH_t,h and ΔH_t,c). When available in the original works, both the onset and peak temperatures of the DSC peaks were selected and reported in the table.

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Figure 1. DSC thermograms of PE and PG

The measurements carried out in this work on pure PE and PG samples are shown in Figure 1 and the S-S transition properties are reported in Table 2. The results are comparable with those of literature. While the values of the phase transition temperatures are in good agreement with that of the literature, the measured enthalpies of S-S phase transitions are slightly lower than the literature data. These differences can be due to the different conditions and samples used in the measurements.

From Table 2, it can be observed that various measurements of T_t,h and ΔH_t,h for PE and PG were found. While the measured T_t,h of both PCs are generally concentrated in a small range, a higher discrepancy of the ΔH_t,h values is evident. Their differences can be related to the purity of the samples or the accuracy of the measurement setup and procedure. Only a limited number of works report also the values of T_t,c and ΔH_t,c. However, the measurements of T_t,c showed that PE and PG were affected by supercooling (i.e., the release of the enthalpy of phase transition during the cooling occurs at a temperature lower than the phase transition temperature during the heating). Although the values of T_t,c were not provided, the supercooling of the studied PCs was also evident in the measurements reported by Benson et al. [31]. They pointed out that it can be related to transformation kinetics and depends on the heating and cooling scan rates. The authors tested different nucleating agents that effectively decreased the degree of supercooling for the studied samples.

Currently, no well-established theory can explain the supercooling of the PCs, but it seems related to the high degree of directional disorder and mobility of the molecules in the plastic phase [12]. However, since their supercooling could hinder the use of PE and PG as PCMs, specific methods to reduce their supercooling degree should be applied.

Finally, it is worth pointing out that the two PCs have the following melting point temperatures [33]: 256-266℃ for PE and 195-203℃ for PG. However, they cannot be considered potential solid-liquid PCMs for high-temperature applications due to their low melting enthalpy (22-51 J g^-1 for PE and 38-45 J g^-1 for PG) [33].

Table 2. S-S phase transition properties of PE and PG

Notes: 1. Onset temperature. 2. Peak temperature.

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(a)

2b.png

(b)

Figure 2. Onset and final phase transition temperatures (a) and enthalpies of phase transition (b) of PE + PG mixtures measured using a DSC in the heating phase. (A) [17], (B) [18], (C) [15], (D) [12]

The literature phase transition properties of the PE+PG mixtures obtained by DSC analyses are shown in Figure 2 as a function of the PE composition. In detail, the phase transition temperatures measured in the heating phase (both onset and final) are shown in Figure 2a, while the enthalpies of phase transition for the heating phase are reported in Figure 2b.

It can be noted in Figure 2 that few studies presenting the phase transition properties of the PE+PG mixture were found in the literature. However, this figure also shows that the properties of the different mixture compositions generally vary within that of the pure components. While a slight discrepancy between the collected onset phase transition temperatures can be noted, the phase transition enthalpies generally agree. Only the enthalpy provided by Singh et al. [15] is lower than the values of the other studies. In addition, Figure 2a shows that the difference between the onset and final phase transition temperatures is also 50℃ for some compositions. As pointed out in various literature studies [12, 44], this behavior is ascribable to an isomorphous phase diagram of PE+PG mixture with complete solid miscibility for all compositions, as it can be deduced from Figure 2a. Consequently, the mixture is characterized by continuous S-S phase transitions, and its phase diagram shows temperature ranges (specific for each composition) where the low-temperature α-phase and the plastic γ’-phase coexist.

As the pure PE and PG, their mixtures also show the supercooling drawbacks [12, 18]. However, it was shown that the intermediate compositions had smaller supercooling degrees [18].

3.2 Other thermophysical properties

This section presents the density, specific heat, and thermal conductivity values for PE+PG mixture and its pure components collected from the literature.

Regarding the density, different values measured at room temperature were reported for the pure compounds. For PE, a density of 1.39 g cm^-3 was given in some works [12, 17], while Bushnell [22] provided a value of 1.107 g cm^-3. The following values were found for PG: 1.15 g cm^-3 [42] and 1.22 g cm^-3 [12, 17]. Instead, only two studies reporting the density of the PE+PG mixture were found in the literature. Benson et al. [17] showed that its density changes with the composition between the values of the pure components (1.39 g cm^-3 of PE and 1.22 g cm^-3 of PG). Serrano et al. [12] provided a density of 1.27 g cm^-3 for the 1:4 M ratio mixture of PE and PG, which is in good agreement with the data reported by Benson et al. [17].

Table 3. Specific heat values (c) for PE, PG, and their mixture available in the literature

Table 3 shows the specific heat values for the mixture and its pure components. Various sources reporting the data for the pure components, measured at different temperatures, were found. Although a certain discrepancy between the collected data is evident, the specific heat of the plastic γ’-phase is always higher than that of the α-phase. Instead, the specific heat data for the PE+PG mixture were found in only one study. It can be noted that the values of the different mixture compositions are quite close to that of the pure compounds.

Finally, the values of thermal conductivity for PE, PG and their mixture collected from the literature are reported in Table 4. One data source was found for PG and the mixture, while different values were collected for PE. It is worth noting that, for specific temperatures, the thermal conductivity values of PE reported in various studies vary in quite wide ranges. These differences could be due to the different samples and methods used in the measurements.

However, from Table 4, it can be noted that their thermal conductivity values are generally low. Consequently, this weakness was addressed in different studies that tried to enhance the thermal conductivity of the two PCs by combining them with materials with high thermal conductivity, such as graphite [42, 43], graphene [40], alumina [26, 38-40], titanium nitride [46], copper nanoparticles [41], indium [25]. The results of these studies showed that an increase of the thermal conductivity of the studied composite PCMs was generally obtained.

Table 4. Thermal conductivity values (λ) for PE, PG, and their mixture reported in the literature

3.3 Thermal, chemical, and cycling stability analyses

Regarding thermal stability, only studies reporting TGA measurements for PE were found [26, 34, 39, 46, 47]. In different studies of Venkitaraj and co-authors [26, 39, 47], it was shown that the analyzed samples had almost negligible weight loss up to 240℃, while the final degradation temperature was 350℃. The TG curves reported by Luo et al. [46] seem to agree with that of Venkitaraj and co-authors [26, 39, 47]. Instead, the TG behaviors presented by Sakamoto et al. [34] show that weight loss of PE began at lower temperatures than 240℃.

It is also worth pointing out that, since PCs have high vapor pressures, they could sublimate at high temperatures [34]. For this reason, the samples studied in different works [14, 17, 33, 34] were sealed in aluminum crucibles to prevent sublimation. However, it was shown that, at temperatures close to their S-S phase transition, the sublimation of PCs should occur only under vacuum conditions [12, 48]. However, additional investigations are necessary to understand if this phenomenon could happen in working conditions for TES applications at low-medium temperatures.

For cycling stability, different works studying PE samples were recently published in the literature [25, 26, 39, 46, 47], while the studies concerning PG and their mixture are limited [18, 43]. The results of these studies generally proved that the cycled samples had a limited reduction of the enthalpies of phase transitions and good thermal and chemical stability. For PE, Venkitaraj et al. [25] showed that its enthalpies of phase transitions decreased by about 10% after 500 repeated heating and cooling cycles. In contrast, the change of the phase transition temperatures was negligible. In the other works, the enthalpies of phase transitions of PE decreased by less than 10% since the samples were subjected to a limited number of thermal cycles (less than 500). Moreover, in some works [26, 38, 39, 47], it was demonstrated that the thermal conductivity and specific heat of PE only slightly changed due to thermal cycles. As concern PG, Zhang et al. [43] showed that its enthalpies of phase transitions decreased by about 1% after 100 cycles, the change of the phase transition temperatures was negligible.