Mechanical and thermal properties of cement composite graphite for solar thermal storage materials

Mechanical and thermal properties of cement composite graphite for solar thermal storage materials

Available online at www.sciencedirect.com Solar Energy 86 (2012) 3227–3233 www.elsevier.com/locate/solener Mechanical and thermal properties of ceme...

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Available online at www.sciencedirect.com

Solar Energy 86 (2012) 3227–3233 www.elsevier.com/locate/solener

Mechanical and thermal properties of cement composite graphite for solar thermal storage materials Hui-Wen Yuan, Chun-Hua Lu ⇑, Zhong-Zi Xu ⇑, Ya-Ru Ni, Xiang-Hui Lan State key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China Received 8 November 2011; received in revised form 26 July 2012; accepted 15 August 2012 Available online 26 September 2012 Communicated by: Associate Editor D. Laing

Abstract The comprehensive survey on an attractive thermal storage material consisted of aluminate cement and graphite is obtained in this paper. The effect of different water/cement (w/c) ratio and graphite content on compressive strength and thermal properties including thermal conductivity, volume heat capacity and thermal expansion coefficient of hardened aluminate cement pastes were investigated to pursue the optimum material design for solar parabolic trough power plant. It is observed that thermal conductivity and volume heat capacity were improved with the decrease of w/c and the increase of graphite content. The results show that w/c is a key factor affecting thermal properties of pastes and graphite even has some influence on the hydration process. After heat treatment at 350 °C for 6 h, compressive strength and thermal properties descended in a certain extent. XRD and FTIR were used to characterize the evolution of hydration products together. Furthermore, the properties obtained from the paper will lay the foundation for thermal storage materials of solar thermal power plants in the future. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Thermal energy storage; Cement; Graphite; Solid sensible heat

1. Introduction Nowadays, solar thermal power is an attractive way to produce electricity hardly with any polluting or emissions of carbon dioxide, which uses solar radiation as energy input. Four main sections are required in the solar thermal power plant: concentrator, receiver, transport/storage media system, and power conversion device (Laing et al., 2010). The obtaining of solar energy is strongly affected by climate, so thermal energy storage (TES) balancing energy supply and demand in cloudy day and night is a necessary component in four sections. TES systems can ⇑ Corresponding authors. Tel.: +86 25 83587252; fax: +86 25 83587220 (C.-H. Lu), tel./fax: +86 25 83172128 (Z.-Z. Xu). E-mail addresses: [email protected] (C.-H. Lu), [email protected] (Z.-Z. Xu).

0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.08.011

facilitate the integration of solar thermal power plants into the electrical grids by smoothing out fluctuations, thus avoiding instability problems and increasing electricity production (Tamme et al., 2004). Therefore, long service life and low cost of efficient storage materials are quite necessary to be investigated in the TES systems. There are several kinds of thermal energy storage materials which have been considered for storage systems. Liquid materials such as synthetic oil are unreasonable to be used as a large volume storage material due to high vapor pressure at high temperature. However, synthetic oil can be used for solar thermal power plants as the heat transfer medium owing to its good heat transfer ability. Nitrate molten salt is the most common material employed in the TES systems, nonetheless, there still exist corrosive and expensive investment problems. Concrete storage is regenerative storage concept with cyclic charging and

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Table 1 Thermal properties of high temperature concrete on solar storage. High temperature concrete (heating temperature)

Volume heat capacity (kJ m3 K1)

Thermal conductivity (W m1 K1)

Coefficient thermal expansion (106 K1)

200 °C (Laing et al., 2009) 350 °C (Laing et al., 2006) 370 °C (Laing et al., 2008)

2457

1

9.3

2519

1

9.3

2475

1.3

11.6

Table 2 Chemical compositions (by mass) of aluminate cement (%). Materials

CaO

SiO2

Al2O3

Fe2O3

R2O

LOI

Aluminate cement

38.79

7.17

51.68

2.07

0.29

0.30

discharging systems, meanwhile, it has the advantage of low cost, easy processing, and fine mechanical properties (Fernandez et al., 2010). A first concrete storage unit was tested successfully by the German Aerospace Center in Spain within a project funded by the German government (Laing et al., 2006). Then one of the largest German building company, Ed. Zu¨blin AG, joined the follow-up project which concentrated on cost reduction (Laing et al., 2008). The part thermal properties of storage materials used in the project are listed in Table 1. There have been a few attempts to further research of the storage materials and storage design in order to optimize storage capacity and keep TES continuous. For instance, a start-up process for improving stability was added to measure the vapor pressure which may cause serious damage if it exceeded a critical value (Laing et al., 2009). In the aspect of thermal conductivity, graphite seems to be a good candidate as high thermal conductivity material that has been attempted to add in the storage materials. So then a thermal storage concrete material consisted of aluminate cement, basalt, bauxite, graphite and other materials was produced. Even though thermal conductivity of the material was up to 2.43 W m1 K1, the material properties were obtained only at room temperature (Guo et al., 2010). The results could be convinced if changes under high temperature were elucidated. Material parameters and storage performance have been validated between 200 °C and 400 °C and more than 370 thermal cycles (Laing et al., 2011). In this paper we have mainly studied thermal properties and compressive strength of the aluminate cement pastes before and after heat treatment at 350 °C for thermal energy storage materials in storage system of parabolic trough power plants. John et al. (2011) have present compressive strength evaluations of seven concrete mixtures of various material components in excess of 500 °C and found

that the ones with calcium aluminate cement reveal better residual strength. And they have also found that the heating rate shows no significant impact on residual compressive strength (John et al., 2010). Cement pastes are important constituent parts of concrete. It should be noted that during the heat-up process of concrete thermal storage materials, the hydration products of cement acting as cementing agent exhibit more great influence on volume stability than other materials such as aggregate and rock, which is a potential danger in future applications. To date, few records have been provided on comprehensive study of mechanical and thermal properties of cement paste under major operating temperature of TES system. So in this paper we aim to provide the mechanical and thermal properties of hydration products along with variation of w/c and graphite content. In addition to those properties, Scanning Electron Microscope (SEM), X-ray Powder Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) were obtained to characterize the fracture section, the hydration phases and the bonds, respectively. The optimum properties of pure paste and graphite composite paste will express better performance for further preparation of high temperature concrete thermal storage materials and lay theory foundation for actual project of thermal energy storage in solar thermal power plants. 2. Experimental 2.1. Materials Aluminate cement performing better corrosion resistance than traditional silicate cement was used as cementing agent. Ground Expanded Graphite (G) with high thermal conductivity of 129 W m1 K1 and volume heat capacity of 2122 kJ m3 K1 was used to improve thermal properties of composite materials. Chemical compositions of aluminate cement are given in Table 2 (R2O represents equivalent oxidation sodium content and LOI means loss on ignition.).

Fig. 1. XRD pattern of aluminate cement.

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The main mineral phase is CaOAl2O3(CA), then the minor phase is CaO2Al2O3(CA2), 2CaOAl2O3SiO2(C2AS) and 12CaO7Al2O3(C12A7), which are shown in Fig. 1. The max size of graphite that we used is 30 lm, and the color is gray black with metallic luster. 2.2. Preparation and characterization of specimens

Fig. 2. TG–DTA curves of aluminate hardened cement paste.

Fig. 3. Effect of different w/c on the compressive strength of pure paste.

Aluminate pure cement pastes were cast with water to cement ratio (w/c) of 0.30, 0.33, 0.36 and 0.40. And the fraction of graphite to aluminate cement is 1%, 5%, 10% and 15% in mass, and the pastes were denoted as 1%G, 5%G, 10%G and 15%G, respectively. The composite pastes mixed by aluminate cement and graphite were molded with w/c of 0.3 at room temperature. The pure and composite pastes cast for compressive strength, thermal conductivity and thermal expansion coefficient had molds of 2 cm  2 cm  2 cm, 4.8 cm  2 cm  8 cm, 0.5 cm  0.5 cm  4 cm, respectively. After being molded, the specimens were cured under water condition at room temperature for 7 days and then to be ready to test. Compressive strength was obtained by automatic pressure test machine (HualongWHY-200, Hualong Ltd., China) at a rate of 500 N s1. In order to eliminate the influence of free water on pastes, the specimens were dried at 105 °C for 24 h in the oven before the specimens were tested. Thermal conductivity and volume heat capacity were measured by thermal conductivity constant tester (TPS2500, Hot Disk Ltd., Sweden) with Probe 5465 at 25 °C, and thermal expansion coefficient was measured by thermal expansion coefficient apparatus (PCY-II, Xiangtan Xiangyu Instrument Ltd., China) at a heating rate of 5 °C min1. It can be seen that an endothermic peak at 260 °C appeared and the endothermic reaction ended approximately at the temperature of 350 °C with certain quality loss according to the TG–DTA (Diamond Perkin–Elmer Ltd., America) curves at a heating rate of 10 °C min1 ranging from room temperature to 1000 °C in an air atmosphere of aluminate hardened cement paste (Fig. 2). Consequently, the specimens were heat-treated in muffle furnace at 350 °C for 6 h. The temperature is the same as the operating temperature of solar parabolic trough power plants. After that mechanical properties and thermal properties were tested again. The morphologies of fracture section of composite paste were observed on a SEM (JSM-5900, JEOL, Tokyo, Japan). XRD (Rigaku D/Max-2500, Rigaku Ltd., Japan) and FTIR Spectroscopy (Nexus670, Nicolet Ltd., America) were used to characterize the evolution of phases and the bonds, respectively. 3. Results and discussion 3.1. Mechanical properties

Fig. 4. Effect of graphite and heat treatment on compressive strength of composite pastes.

Compressive strength of pure pastes with w/c of 0.30, 0.33, 0.36 and 0.40 curing for 7 days before and after heat-

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Table 3 Thermal conductivity of paste with different w/c curing for 7 days before and after heating.

Table 5 Volume heat capacity of paste with different w/c curing for 7 days before and after heating.

w/c

Before heating (W m1V K1)

After heating (W m1 K1)

w/c

Before heating (kJ m3 K1)

After heating (kJ m3 K1)

0.30 0.33 0.36 0.40

0.741 0.650 0.562 0.483

0.405 0.392 0.349 0.338

0.30 0.33 0.36 0.40

1785 1730 1572 1439

1384 1296 1195 1043

Table 4 Thermal conductivity of composite paste before and after heating. Content (%)

Before heating (W m

0 1 5 10 15

0.741 0.830 1.083 1.967 2.197

1

1

K )

After heating (W m

1

1

K )

0.405 0.501 0.587 0.746 0.767

ing is shown in Fig. 3. It is observed that compressive strength is decreasing with the increase of w/c, which is the same as traditional cement paste. Meanwhile, after heating at 350 °C for 6 h all of the pastes reveal the decrease of compressive strength to a great extent. Fig. 4 shows the effect of graphite content and heat treatment on the compressive strength. The results indicate that the variation trend of compressive strength before heating is the same as the one after heating. Obviously it can be seen that the compressive strength descended quite a bit after heat treatment. There was a turning point in each curve. When graphite content was added to 1%, the compressive strength showed improvement due to the physical filling of the graphite powder in a small amount. With the addition of graphite content up to 5%, the compressive strength of the specimens of both curves decreased to 36 MPa and 20 MPa, respectively, which was slightly higher than the non-graphite specimens. Moreover, when the content of graphite replacing aluminate cement was to 10%, the compressive strength continued to decrease even lower than the non-graphite specimens, demonstrating the excess graphite powder was not good for the compressive strength because of its soft characteristic. Furthermore, when the addition of graphite content was up to 15%, the compressive strength decreased rapidly because the graphite powder could not take much more pressure than the hydration products of aluminate paste, and the change of the hydration products caused by the graphite.

Table 6 Volume heat capacity of composite paste before and after heating. Content (%)

Before heating (kJ m3 K1)

After heating (kJ m3 K1)

0 1 5 10

1785 1704 2598 4801

1284 1611 1848 2585

sensible storage material, compaction paste will present better thermal conductivity. Thermal conductivity data of graphite composite pastes before and after heat treatment is listed in Table 4. It is notable that the thermal conductivity of graphite composite paste is increasing with the increasing of graphite content. When the graphite content is 1%, the thermal conductivity is slightly improved compared to the pure paste. Afterwards when the graphite content is up to 10%, the thermal conductivity is 2.65 times than the pure paste. Unfortunately, heat treatment at 350 °C for 6 h on pastes significantly reduces thermal conductivity, which results from the change of the hydration products. Although graphite remains stable at 350 °C, the change of entire structure due to the transformation of hydration phase would affect thermal conductivity of graphite. Even so, the thermal conductivity of 10% graphite composite paste after heating still can obtain 0.746 W m1 K1 which is higher than that of pure paste.

3.2. Thermal properties 3.2.1. Thermal conductivity Thermal conductivities of pure pastes with different w/c curing for 7 days before and after heating are shown in Table 3. It indicates that thermal conductivity is decreasing with the increase of w/c. That is probably a consequence of the rise of porosity with the increasing w/c. As a solid

Fig. 5. Thermal expansion coefficient curves of pure pastes with different w/c before and after heating.

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Fig. 6. Thermal expansion coefficient curves of composite pastes before and after heating.

Fig. 7. SEM micrographs of pure graphite.

3.2.2. Volume heat capacity Volume heat capacity of paste with different w/c curing for 7 days before and after heating is shown in Table 5. It indicates that volume heat capacity is decreasing with the increase of w/c just the same as thermal conductivity.

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Volume heat capacity data of pure paste and graphite composite pastes before and after heating is listed in Table 6. The volume heat capacity of graphite is approximately 1600 kJ m3 K1 reported generally. Results indicate that there was no improvement of volume heat capacity with 1% graphite. However, as the graphite content is increasing, the composite pastes begin to reveal higher volume heat capacity. Before heat treatment volume heat capacities of 5% and 10% graphite composite pastes are 1.4 and 2.7 times than that of pure paste, respectively. It demonstrates that the mixture of graphite not only elevates the thermal conductivity, but also affects the hydration process greatly. Similarly, volume heat capacity descends a lot as well as thermal conductivity after heat treatment. Moreover, when the graphite content is 10%, after heat treatment volume heat capacity of 2585 kJ m3 K1 of composite paste exhibits much higher than that of pure paste, which will be useful in the further optimization of storage materials. 3.2.3. Thermal expansion coefficient Thermal expansion coefficient curves of pure pastes with different w/c before and after heating are recorded in Fig. 5. The symbol of 0.3-H in figure means paste casted at 0.3 of w/c and heated at 350 °C for 6 h. The results indicate that the pastes without pre-heating present unstable with the rise of temperature, while the ones suffered heat treatment can keep stable. Fig. 6 shows thermal expansion coefficient curves of composite paste before and after heating. The graphite content is 0–10%. It can be observed that the curves of pastes without heat treatment present large fluctuations than that of non-heating pastes. Moreover thermal expansion coefficient curves of pastes before heating decreased with the rise of temperature ranging from 100 °C to 400 °C. And the negative value means a shrinking of the pastes. During the heating-up, the free water and a certain amount of chemically bonded water evaporated from the specimens, which resulted in the instability of the integral structure of the pastes. When the temperature was up to 300 °C, the thermal expansion coefficient curves of nonheating specimens began to keep flat. That was probably because the chemically bonded water would not evaporate any more. The comparison between composite pastes and

Fig. 8. SEM micrographs of the fracture sections of 10%G composite paste.

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can be observed that the layered graphite is embedded in the paste (see Fig. 8a). One magnification part of the graphite is shown in Fig. 8b. 3.3.2. XRD XRD patterns of pure and graphite doped hardened cement pastes before and after heating are shown in Figs. 9 and 10, respectively. The characteristic peaks of phases are marked by different symbols through the comparison with PDF cards. It exhibits (see Fig. 9) that CA, the main phase of aluminate cement, hydrated to 3CaOAl2O36H2O (C3AH6) and Al(OH)3, whose reaction hydration equation is present as follows: 3CaO  Al2 O3 þ 12H2 O ! 3CaO  Al2 O3  6H2 O Fig. 9. XRD patterns of pure paste before and after heating.

pure paste demonstrated that adding graphite would affect the thermal stability of the pastes. After heat treatment of the specimens, it appeared that the thermal expansion coefficient curves almost kept a line. The mean thermal expansion coefficients of 0%G, 1%G, 5%G, 10%G and 15%G are 7.5  106/°C, 8.9  106/°C, 3.7  106/°C, 6.9  106/ °C and 6.3  106/°C, respectively. The phenomenon further showed that thermal stability would be better after a part of water removing from the paste, which means that there should be a preheating process before the applications of the cementitious energy storage materials.

3.3. Characterization and analysis 3.3.1. SEM The SEM figure of graphite raw material is shown in Fig. 7. It shows that the graphite seems layered-like. And Fig. 8 illustrates SEM micrographs of the fracture sections of 10%G composite paste. Compared Fig. 7 with Fig. 8, it

þ 4AlðOHÞ3

ð1Þ

After heating at 350 °C for 6 h, the characteristic peaks of C3AH6 at 20.04°, 26.49°, 28.45°, 31.91°, 39.29° and 44.36° (2h) disappeared as a result of the decomposition of C3AH6 (see Fig. 9). Meanwhile, the characteristic peaks of Al(OH)3 at around 18.33°, 20.90°, 36.84°, 37.39° and 44.36° (2h) also disappeared, which demonstrated that Al(OH)3 also could not keep stable at 350 °C. Those two reaction equations are shown as follows: 3CaO  Al2 O3  6H2 O ! 3CaO  Al2 O3 þ 6H2 O

ð2Þ

2AlðOHÞ3 ! Al2 O3 þ 3H2 O

ð3Þ

It can be seen from Fig. 10a that with the increase of graphite content, the characteristic peak of graphite at 26.49° was enhanced while the other phases were almost not transformed, which demonstrated that graphite did not participate in the hydration directly. However, adding graphite indeed influenced the mechanical and thermal properties. After heating at 350 °C for 6 h the graphite phase still can exist in the paste, while C3AH6 and Al(OH)3

Fig. 10. XRD patterns of composite paste: (a) before heating (b) after heating.

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of w/c, meanwhile, thermal conductivity and volume heat capacity increased with the addition of graphite and the decrease of w/c. The volume heat capacity and thermal expansion coefficient properties of our composite cement pastes with the adjustment of w/c and graphite content will be promising matrix materials. Certainly, more work about improving thermal conductivity and volume heat capacity of heat treated pastes still need to be carried out. In a word, our composite paste as a substrate material can combine with other good thermal properties materials in future, which will be a significant candidate for thermal storage materials in the applications of solar thermal parabolic trough power plant. Acknowledgements Fig. 11. FTIR spectra of pure paste and 10%G composite paste before and after heating.

begin to decompose. Consequently, the main reason of the decease of mechanical and thermal properties after heating is the dehydration of the hydration products. 3.3.3. FTIR FTIR spectra (Fig. 11) for pure paste and 10%G composite paste before and after heating are discussed in this section. For pure paste, the intensity bands at 525.7 cm1, 807.9 cm1 and 3662.3 cm1 are associated with the characteristic peaks of C3AH6, and the other intensity bands at 3455.5 cm1 and 1019.9 cm1 are associated with the characteristic peaks of Al(OH)3. After heat treatment, the characteristic peaks of C3AH6 at 525.7 cm1, 807.9 cm1 and 3662.3 cm1 are blue-shifted to 527.5 cm1, 811.7 cm1, 3667.4 cm1, respectively, declaring expelling of the chemical bonded water of C3AH6. Meanwhile, the O–H–O stretching vibration and the –OH bending vibration of Al(OH)3 at 3455.5 cm1 and 1019.9 cm1 are red-shifted to 3444.2 cm1 and 1016.4 cm1, respectively. The change of FTIR spectra for 10%G composite paste before and after heating is just the same as that of pure paste. The characteristic peaks of C3AH6 at 528.4 cm1, 807.5 cm1 and 3649.1 cm1 are blue-shifted to 529.1 cm1, 809.6 cm1 and 3651.1 cm1, respectively. Then the characteristic peaks of Al(OH)3 at 3461.5 cm1 and 1020.0 cm1 are red-shifted to 3446.9 cm1 and 1017.0 cm1, respectively. Obviously, FTIR spectra corroborate the XRD patterns in the last section. All of the results confirm that dehydration of C3AH6 and Al(OH)3 is the main reason for the decline of properties. 4. Conclusions Mechanical and thermal properties of pure and graphite doped hardened aluminate cement pastes were investigated in the paper. It indicates that compressive strength decreased with the addition of graphite and the increase

The authors would like to express sincere thanks to Jiangsu innovation scholars climbing project (SBK200910148), Graduate Innovation Foundation of Jiangsu Province (CXLX11_0347) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for Financial Support. References Laing, D., Steinmann, W.D., Viebahn, P., Grater, F., Bahl, C., 2010. Economic analysis and life cycle assessment of concrete thermal energy storage for parabolic trough power plants. ASME Journal of Solar Energy Engineering 132, 041013. Tamme, R., Laing, D., Steinmann, W.D., 2004. Advanced thermal energy storage technology for parabolic trough. ASME Journal of Solar Energy Engineering 126, 794–800. Fernandez, A.I., Martı´Nez, M., Segarra, M., Martorell, I., Cabeza, L.F., 2010. Selection of materials with potential in sensible thermal energy storage. Solar Energy Materials and Solar Cells 94, 1723–1729. Laing, D., Steinmann, W.D., Tamme, R., Richter, C., 2006. Solid media thermal storage for parabolic trough power plants. Solar Energy 80, 1283–1289. Laing, D., Steinmann, W.D., Fib, M., Tamme, R., 2008. Solid media thermal storage development and analysis of modular storage operation concepts for parabolic trough power plants. ASME Journal of Solar Energy Engineering 130, 011006. Laing, D., Lehmann, D., Fiss, M., Bahl, C., 2009. Test results of concrete thermal energy storage for parabolic trough power plants. ASME Journal of Solar Energy Engineering 131, 041007. Guo, C.Z., Zhu, J.Q., Zhou, W.B., Chen, W., 2010. Fabrication and thermal properties of a new heat storage concrete material. Journal of Wuhan University Technology–Material Science Edition 25, 628–630. Laing, Doerte, Bahl, Carsten, Bauer, Thomas, Fiss, Michael, Breidenbach, Nils, Hempe, Matthias, 2011. High-temperature solid-media thermal energy storage for solar thermal power plants. Proceedings of the IEEE 100, 516–524. John E.E., Hale W.M., Selvam R.P., 2011. Development of a highperformance concrete to store thermal energy for concentrating solar power plants. In: Proceedings of ASME 2011 5th International Conference on Energy Sustainability, ESFuelCell 2011-54177, August 7–10, Washington, DC. John E.E., Hale W.M., Selvam R.P., 2010. Effects of high temperatures and heating rate on high strength concrete for use as thermal energy storage, Paper no: ES2010-90096. In: Proceedings of ASME 2010, 4th International Conference on Energy Sustainability ES2010, May 17– 22, Phoenix, AZ.