Development, Characterization, and Latent Heat Thermal Energy

Article pubs.acs.org/IECR

Development, Characterization, and Latent Heat Thermal Energy Storage Properties of Neopentyl Glycol-Fatty Acid Esters as New Solid−Liquid PCMs Ahmet Sarı,* Cemil Alkan, and Alper Bicer Department of Chemistry, Gaziosmanpaşa University, Tokat, Tokat 60240, Turkey ABSTRACT: Neopentyl glycol (NPG) is known as a solid−solid phase change material (PCM) and shows a solid−solid phase transition at 41.9 °C. Although it has a good latent heat capacity of 141.91 J/g, the low degradation temperature interval (80−150 °C) is close to its solid−liquid phase change temperature (129.46 °C), which lessens its latent heat thermal energy storage (LHTES) potential. The present study is aimed to eradicate these disadvantages by synthesizing its ester compounds with three kinds of fatty acid chlorides. The developed NPG-DL, NPG-DM, and NPG-DP esters were characterized using FT-IR and 1H NMR spectroscopy techniques. The DSC results showed that the esters had a melting point and latent heat value in the range of about 11−35 °C and 98−124 J/g. The synthesized PCMs had high decomposition temperature ranges, noncorrosivity, and good odor properties. The PCMs also exhibit good thermal reliability and chemical stability besides their increased thermal conductivity.

1. INTRODUCTION The use of fossil energy sources is known as one of the main reasons associated with emissions of the harmful gas emission, climate change, and environmental pollution. The dependency on fossil fuels can be reduced with the utility of thermal energy storage systems. Thermal energy can be stored and released in sensible, latent, and chemical forms.1 The latent heat storage by using phase change material (PCM) is the most attractive one for thermal energy storage and release at high amount and narrow temperature range.2 Therefore, during the last thirty years, the PCMs have been used for various latent heat thermal energy storage (LHTES) fields such as waste heat recovery,3 air conditioning of buildings4 and greenhouses,5 smart textiles,6,7 cooling of electronics,8 and thermal preservation of foods.9 The PCMs have thermal energy storage and discharge abilities in latent heat form by means of solid−liquid, liquid− gas, solid−gas, and solid−solid phase changes. Compared with the other types, solid−liquid PCMs are more preferred in LHTES systems because they provide adequate thermophysical, kinetic, and chemical properties as well as economic value for thermal applications.10−12 Therefore, selection of a suitable solid−liquid for LHTES systems is mainly based on several criteria that include phase change temperature, phase change enthalpy, other variations in its LHTES properties after extended thermal cycles, melting-solidifying behaviors, thermal durability, chemical stability, thermal conductivity, volume change, odor, and cost.13−15 In recent years, most of the studies about the PCMs used in LHTES applications have been directed to the development of new kinds of solid−liquid PCMs that have good characteristics mentioned above. In this regard, some ester compounds of the fatty acids with butanediol,16 some diols,17 ethylene glycol,18 hexamethylene daimine,19 ethylene diamine,20 mono-, di-, and polyalcohols,21−26 hexadecanol,27 myristyl alcohol (1-tetradecanol),28−30 and octadecanol31 were synthesized and characterized as novel solid−liquid PCMs. © 2013 American Chemical Society

Neopentyl glycol (NPG; IUPAC name: 2,2-dimethyl-1,3propanediol) is polyhydric alcohol and heterogeneous at low temperature, but it has homogeneous face-centered cubic crystals at its solid−solid phase change temperatures.32,33 NPG is known as a solid−solid PCM and shows phase transition at 41.9 °C. Although it has a good latent heat value of 141.9 J/g, the low decomposition temperature interval (80−150 °C) is close to its melting temperature and thus restricted considerably its usage feasibility in LHTES applications. In this regard, in order to eradicate these drawbacks of the NPG, this study is focused on the development of NPG esters as new solid−liquid PCMs by means of its esterification with three kinds of fatty acid chlorides. The synthesized NPG-dilaurate (NPG-DL), NPG-dimyristate (NPGDM), and NPG-palmitate (NPG-DP) esters were characterized chemically by using FT-IR and 1H NMR spectroscopy methods. The phase change enthalpies, phase change temperatures, and thermal reliability properties of the NPG-DL, NPG-DM, and NPG-DP esters were measured by the DSC technique. The chemical stability of the PCMs under extended thermal cycling was evaluated by examining their FT-IR spectral results. Thermal degradation temperatures of the new PCMs were determined by using the thermogravimetry (TG) analysis method. The thermal conductivities of the PCMs were also improved appreciably by graphite addition.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Developed PCMs. NPG and the fatty acids (lauric acid, myristic acid, and palmitic acid) used in the obtaining of fatty acid chlorides (FACs) were supplied from Sigma-Aldrich company. The other chemicals used in the experiments were purchased from Sigma Company. The same Received: Revised: Accepted: Published: 18269

September 13, 2013 November 29, 2013 December 5, 2013 December 5, 2013 dx.doi.org/10.1021/ie403039n | Ind. Eng. Chem. Res. 2013, 52, 18269−18275

Industrial & Engineering Chemistry Research

Article

Figure 1. The reaction mechanisms used in the synthesis of the developed new PCMs.

Figure 3. 1H NMR spectra of the developed new PCMs. Figure 2. 1H NMR spectra of NPG and fatty acid chlorides.

2.2. Characterization of the Developed PCMs. The chemical structures of the synthesized ester compounds were verified by using FT-IR (JASCO 430 model) and 1H NMR spectroscopy (AVANCE III 400 MHz BRUKER model) techniques. The spectroscopic measurements were carried out by using the same procedure reported in our previous studies.23−25 2.3. Determination of LHTES Properties of the Developed PCMs. The phase change (melting and freezing) temperatures and latent heats of PCMs were measured by using a DSC instrument (PerkinElmer Jade DSC) at a heating rate of 2 °C/min. The evaluation procedures of the DSC results were

procedure reported in our early studies23−25 was used in the synthesis of the FACs. Figure 1 show the reaction mechanism used in the synthesis of NPG-DL, NPG-DM, and NPG-DP esters. The synthesis reaction was started by taking 2.5 mol of FAC and 1 mol of NPG solved in toluene and then maintained at 85 °C for 5 h. After the evaporation process of the solvent at the end of the reaction, the obtained product was washed several times by deionized water and then dried at room temperature. 18270

dx.doi.org/10.1021/ie403039n | Ind. Eng. Chem. Res. 2013, 52, 18269−18275


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Figure 6. DSC thermograms of the developed new PCMs at temperature interval of (−20)−(160 °C).

Figure 4. (a) FT-IR spectra of fatty acid chlorides and NPG (b) FT-IR spectra of the developed new PCMs.

esters. The a, b, c, and d protons due to the protons of FACs also shifted to 0.9, 2.3, 1.8, and 1.6 ppm in the spectrum of all the ester compounds, respectively. Moreover, the protons of CH2 and CH3 groups associated with the NPG was observed at 3.9 and 1.0 ppm for all the esters. These results showed that the NPG-DL, NPG-DM, and NPG-DP esters were successfully synthesized as new PCMs. On the other hand, the FT-IR spectra of FACs and NPG are presented in Figure 4(a). As can be seen from these spectra, the O−H vibration bands of NPG were observed in the range of 3309−3476 cm−1, while any peak regarding this group was not seen in the spectra of the synthesized esters. Moreover, as can be seen from the FT-IR spectra of the esters in Figure 4(b), the stretching bands of CO groups of FACs and the C−O group of NPG shifted from 1733 cm−1 to 1797 cm−1 and from 1101 cm−1 to 1058 cm−1, respectively. All of these findings confirmed that all of the OH groups of NPG were successfully transformed to ester groups. 3.2. Thermal Energy Storage Properties of the Developed PCMs. DSC thermograms of NPG at temperature intervals of 0−60 °C and (−20)−(180) °C are presented in Figure 5. As clearly seen from these heating and cooling curves, the NPG shows a solid−solid phase transition at 41.92 ± 0.14 °C and solid−liquid phase change at 129.46 °C. However, it could not show any peak during cooling period because it volatilized above 150 °C. On the other hand, as clearly observed in the DSC curves in Figure 6 and the thermal data in Table 1, the NPG-DL, NPG-DM, and NPG-DP esters maintained their chemical structures and exhibited both melting peaks at 10.74 ± 0.16 °C, 20.93 ± 0.11 °C, and 34.95 ± 0.10 °C,

the same as in our previous studies.23−25 The new PCMs were also tested in terms of thermal reliability by using a thermal cycler (BIOER TC-25/H model). The sealed samples were introduced into the chamber of the thermal cycler. They were heated up to their melting temperatures and then cooled up to their freezing temperatures. A thermal cycle consists of a heating (melting) and a cooling (freezing) process. The cycling test was applied consecutively until the numbers of thermal cycle would be 1000. After the thermal cycling test, the DSC and FT-IR analyses were repeated to observe whether the thermal reliability and chemical stability of the PCMs is changed. TG analysis of the PCMs (Perkin-Elmer TGA7 model) was conducted at a heating rate of 10 °C min−1 in a static air atmosphere. In addition, the thermal conductivity of the PCMs was increased by using graphite (10 wt %), and the measurements were made at 20 °C by using a KD2 thermal property analyzer.

3. RESULTS AND DISCUSSION 3.1. Chemical Characterization of the Developed PCMs. The chemical structures of the synthesized PCMs were confirmed by 1H NMR and FT-IR spectroscopic methods. The 1H NMR spectra of NPG, FACs, NPG-DL, NPG-DM, and NPG-DP esters are shown in Figure 2 and Figure 3. As seen from Figure 2, the protons (a, b, and c, respectively) associated with OH, CH2, and CH3 groups of NPG were observed at 3.2, 3.4, and 0.9 ppm, respectively, while the a, b, c, and d protons of FACs were recorded at 0.7, 2.7, 1.7, and 1.2 ppm, respectively. As also seen from Figure 3, none of the OH protons of NPG are observed in the spectra of the synthesized

Figure 5. DSC thermograms of NPG at temperature intervals of 0−60 °C and (−20)−(180) °C. 18271



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Table 1. LHTES Properties of the Developed New PCMs PCM

phase change temperature for melting (°C)

latent heat of melting (J/g)

phase change temperature for freezing (°C)

latent heat of freezing (J/g)

NPG NPG-DL NPG-DM NPG-DP

129.46 ± 0.14 10.74 ± 0.16 20.93 ± 0.11 34.95 ± 0.10

141.91 ± 1.24 97.62 ± 1.86 106.44 ± 1.37 123.71 ± 1.82

not measured −10.22 ± 0.13 1.03 ± 0.10 17.42 ± 0.12

−127.42 ± 1.14 −94.30 ± 1.44 −99.32 ± 1.56 −110.93 ± 1.72

Figure 7. DSC thermograms of the developed new PCMs after thermal cycling.

Table 2. LHTES Properties of the Developed New PCMs Measured after Thermal Cycling PCM





NPG-DL NPG-DM NPG-DP

10.42 ± 0.13 21.26 ± 0.14 34.71 ± 0.11

93.70 ± 1.24 103.52 ± 1.86 118.24 ± 1.23

−10.71 ± 0.12 0.40 ± 0.11 17.02 ± 0.15

−92.47 ± 1.64 −98.74 ± 1.43 −96.93 ± 1.75

respectively, and freezing peaks at −10.22 ± 0.13 °C, 1.03 ± 0.10 °C, and 17.42 ± 0.12 °C, respectively, even when they were heated above 150 °C. Compared with NPG, these properties make the developed new esters more promising PCMs for low temperature-LHTES applications. Moreover, the developed new PCMs have considerably high latent heat of melting (97.62 ± 1.86, 106.44 ± 1.37, and 123.71 ± 1.82 J/g, respectively) and latent heat of freezing (−94.30 ± 1.44, −99.32 ± 1.56, and −110.93 ± 1.72 J/g, respectively). 3.3. Thermal Reliability and Chemical Stability Properties of the Developed PCMs. Thermal stability of a PCM can be determined with the comparison of its LHTES properties obtained before and after thermal cycles. This property is needed to ensure the long-term life and economic feasibility of PCM in LHTES systems. After repeated 1000 melting/freezing cycling, the DSC thermograms and the thermal data obtained for the developed new PCMs were given in Figure 7 and Table 2. When comparing them with that given in Table 1, it can be concluded that the melting temperatures of NPG-DL, NPG-DM, and NPG-DP were changed by −0.32 °C, 0.33 °C, and −0.24 °C, respectively. Moreover, the decrease in the latent heats of melting of the PCMs before and after thermal cycling were determined as about 4.5, 2.8, and −4.4%, respectively. Based on these results, it can be remarkably noted that the developed new PCMs have good thermal reliability in terms of their long-term LHTES performance. On the other hand, a PCM should be stable chemically despite it being subjected to a large number of thermal cycles. The FT-IR analysis was repeated after the thermal cycle test to decide whether the developed new PCMs have chemical

Figure 8. FT-IR spectra of the developed new PCMs after 1000 thermal cycling.

stability. It can be seen from Figure 8, there was not any new peak on the spectrum as well as the shape and wavenumber 18272



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Figure 9. TG curves of NPG and the developed new PCMs.

values of the specific bands remaining unchanged after thermal cycling. These results mean that the developed new PCMs had good chemical stability even when they were exposed to 1000 melting/freezing cycles. 3.4. Thermal Durability of the Developed PCMs. Thermal decomposition temperature is one of the key parameters for a PCM in terms of knowing its resistance against the chemical degradation. So, the initial and ending temperatures of the degradation process should be taken into account when choosing the PCM for LHTES systems. For this purpose, the thermal durability temperatures of the developed new PCMs were determined by using the TG analysis method. As seen from Figure 9, the degradation of the NPG begins at 80 °C and ends at 150 °C that is in agreement with its DSC data. The low initial decomposition temperature is an important disadvantage in the use of NPG as PCM. On the other hand, the developed new PCMs demonstrate two decomposition steps, the first one of which corresponds to 180−320 °C for NPG-DL, 180−330 °C for NPG-DM, and 180−310 °C for NPG-DP as the second one falls into 320−370 °C for NPG-DL, 330−365 °C for NPG-DM, and 310−350 °C for NPG-DP. Moreover, the first stages of the TG curves represent the weight loss steps (in the range of about 83−88% for three PCMs) associated with the fatty acid groups of the PCMs as the second stages are regarded with the NPG group. These results mean that the developed new PCMs have much higher decomposition temperatures than NPG and fatty acids34 and thus good thermal durability for LHTES applications. 3.5. Thermal Conductivity of the Developed PCMs. Thermal conductivity of a PCM is a significant parameter that needs to be taken into consideration because of its effects on the energy charging and discharging times in LHTES systems.35

Figure 10. DSC thermograms of the developed new PCMs with graphite (10 wt %) additive.

In this study, the expanded graphite was preferred as thermal conductivity promoter due to its favorable properties such as light-weightiness, good thermal conductivity, low-density, binder free passing capacity, and chemical inertness, resistance of oxidation, corrosion and radiation, and good physical compatibility with PCMs.36,37 The preliminary test indicated that the optimum weight ratio of graphite was 10% to achieve a reasonable increase in thermal conductivity values of the PCMs without a significant reduction in latent heat capacity of them. The thermal conductivity values (at 20 °C) of NPG-DL, NPG-DM, and NPG-DP were measured to be 0.15, 0.15, and 0.13 Wm−1 K−1 and 0.21, 0.20, and 0.17 Wm−1 K−1, respectively, after graphite addition. These results revealed that the thermal conductivity properties of NPG-DL, NPG-DM, and NPG-DP were enhanced by 40%, 33%, and 31%, respectively. On the other hand, Figure 10 shows the DSC thermograms of the developed new PCMs with graphite (10 wt %) additive, and Table 3 summarizes the thermal 18273



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Table 3. LHTES Properties of the Developed New PCMs after EG (10 wt %) Addition PCM





NPG-DL NPG-DM NPG-DP

9.74 ± 0.16 21.05 ± 0.11 33.77 ± 0.10

92.62 ± 1.86 98.74 ± 1.37 112.42 ± 1.82

−11.22 ± 0.13 10.03 ± 0.10 23.56 ± 0.12

−83.45 ± 1.44 −92.62 ± 1.56 −102.12 ± 1.72

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data obtained from these curves. As can be clearly seen from these results, the dropping effect of graphite additive (10 wt %) on the latent heat capacity of the PCMs was not as high as the theoretical expectation. Moreover, after graphite addition, the extent of supercooling of the PCMs was reduced as about 7−9 °C. Based on these results, it can be also concluded that especially NPG-DM and NPG-DP esters with graphite additive have a better potential for LHTES applications compared to the PCMs without graphite additive.

4. CONCLUSIONS In this study, three new solid−solid PCMs that have more advantageous LHTES properties compared to the NPG and fatty acids were successfully synthesized. The chemical structures of the developed PCMs (NPG-DL, NPG-DM, and NPG-DP) were corrected by the NMR and FT-IR spectroscopic results. The DSC results showed that the developed new PCMs melt in the temperature range of 10.74−34.95 °C and freeze in the range of −10.22−17.42 °C, while they have latent heat of melting between 97.62 J/g and 123.71 J/g and latent heat of freezing between 94.30 J/g and 110.93 J/g. These properties make them more functional solid−liquid PCMs for low-temperature LHTES applications compared to NPG. In addition, other improved properties of these PCMs are to be having high decomposition temperatures in comparison to the NPG, noncorrosivity, and good odor in relation to the fatty acids. Besides, they exhibited good thermal and chemical stability performance under extended thermal cycling as well as they have enhanced thermal conductivity after the graphite addition in mass fraction of 10%. After graphite (10 wt %) addition, the decrease in latent heat capacity of the PCMs was below the theoretical expectation. Moreover, by addition of graphite, the extent of supercooling of the PCMs was reduced significantly. Based on these results, it can be also concluded that especially NPG-DM and NPG-DP esters with graphite additive have better potential for LHTES applications compared to the PCMs without graphite additive.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +903562521616. Fax: +903562521285. E-mail: ahmet. [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Ayşegül Nazlı Ö zcan for her help during some parts of the experiments.

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REFERENCES

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