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20 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 into three categories: physical, physico-chemical, and chemical methods. Li [119] studied the improvement in thermal conductivity of PCMs (paraffin) with nano-graphite. It was demonstrated that nano- graphite can be prepared easily from exfoliated graphite and expanded graphite. Moreover, the price of nano-graphite is low, while the improvement effect of nano-graphite on the thermal conduc- tivity is significant. Results showed that the thermal conductivity of the composite PCMs with 1% and 10% nano-graphite is 2.89 times and 7.41 times of that of paraffin, respectively. The phase change temperature of the nano-graphite/paraffin composite PCM was slightly less than that of pure paraffin. As the nano-graphite content increased, the thermal conductivity of the nano-graphite/paraffin composite increased while the latent heat gradually decreased. Parrado et al. [51] conducted a numerical study of a deform- able spherical copper shell containing solar salt at high temperatures. The thermo-mechanical analysis by Comsol Multiphysics was per- formed during charging and discharging, and showed that the volume expansion and pressure rise are below the acceptable me- chanical properties of copper and are not expected to crack the shell. 3.5.2.4. Macroencapsulation. A significant quantity of heat storage media is encapsulated, where the mass per unit may range from grams to kilograms [40]: by careful selection of the capsule geom- etry and the capsule material, the macroencapsulation can be used for a wide variety of energy storage needs. The shape of macrocapsules varies from rectangular panels to spheres or pouches without a defined shape. The macrocapsule shape selection will depend on the needs and design of each intended application. Ac- cording to Regin et al. [40] the most cost-effective containers are plastic bottles (high density and low density polyethylene bottles, polypropylene bottles), tin-plated metal cans and mild steel cans. In the DISTOR project using PCM and expanded graphite, a macroencapsulation of phase change materials was tested [104]. The extra volume required in this process, based on the thermal ex- pansion of PCM due to temperature changes, drastically reduced the volumetric storage capacity [101]. This design was compared with the graphite fin concept, mentioned above. Results showed that for the same amount of PCM, the same area and material for heat ex- change, heat transfer is more effective using fins. Cascades of macroencapsulated PCMs, as will be further de- tailed in Section 3.7, can store 50% more energy per unit volume than the conventional system using two tanks. 3.5.2.5. Metal matrix. A metal matrix, such as a porous matrix, has been used to enhance the heat transfer rate in TES. A porous matrix is characterized by two parameters, i.e. the porosity and the cell size which refer respectively to the percentage of volume that will be occupied by PCM and the size of the pore expressed in pores per inch (PPI). For the first parameter, Mesalhy et al. [120] indicated that the performance improvement depends on both the porosity of the matrix and its conductivity. Low values of porosity lead to a higher effective thermal conductivity, resulting in an increased perfor- mance enhancement. However, a decrease in the porosity has the opposite effect because the low porosity of the matrix hampers the movement of the liquid PCM and the natural convection. For the second parameter, Elgafy et al. [121] showed how the mean pore size of the additives could be of critical importance on the system performance. If it is too small, the PCM molecular motion will be hindered, thereby it will be very difficult to impregnate the porous media with the PCM, which will adversely affect the latent heat storage capacity. Metals with the highest thermal conductivities are silver, copper, gold and aluminium [122], which turns them into a desirable option for metal matrix production to latent heat thermal energy storage systems. Their characteristics are given in Table 13. Due to their lower Table 13 Metals with high thermal conductivities [42]. Metal Thermal Density Specific conductivity (kg/m3) heat (W/mK) (J/kg·K) Melting point (°C) 961.9 1084.6 °C 1064.4 660.2 °C 1450 °C prices in comparison with gold and silver, copper and aluminium are most commonly used. Also, nickel has been studied, as re- ported in Table 14. 3.5.2.6. Graphite composites. Another alternative to improve thermal energy storage materials is producing a new compound with the storage media. To improve the PCM thermal conductivity, the use of graphite to enhance the heat transfer rate has been investi- gated by numerous researchers. Mehling et al. [130] demonstrated that PCM/graphite compos- ites show a significant improvement in the heat transfer within latent heat storage materials. They studied two PCMs: the eutectic salt MgNO3/LiNO3·6H2O and paraffin RT50. In fact, the thermal conduc- tivity in the composite material is up to 100 times higher than in the pure PCM, leading to the phase front moving between 10 and 30 times faster. Py et al. [131] experimented a new supported PCM made of par- affin impregnated by capillary forces in compressed expanded natural graphite (CENG) matrix. High loads of paraffin were ob- tained: from 65% to 95% weight depending upon the bulk graphite matrix density. Composite PCM/CENG thermal conductivities were found to be equivalent to those of the sole graphite matrix from 4 to 70 W/mK instead of the 0.24 W/mK of the pure paraffin. Thermal power and capacity of the composite are theoretically compared to those of conventional systems in the case of two usual external ge- ometries: tubes and spherical hollow nodules. The compressed expanded natural graphite induced a decrease in overall solidifi- cation time and a stabilization of the thermal storage power. An optimization procedure of the composite composition was pro- posed according to the antagonistic behaviours of the thermal power and the thermal capacity in respect to the compressed expanded natural graphite content. Within the usual external heat transfer coefficient range, the estimated compressed expanded natural graph- ite matrix optimized densities fell within the practicable range. Cabeza et al. [132] investigated three heat enhancement methods to improve the low conductivity of the PCM water/ice. These were the addition of stainless steel pieces, copper pieces, and a graph- ite matrix impregnated with the PCM. The use of graphite composite allows an even larger increase in the heat transfer than with copper. The heat flux is about four times larger on heating and three times larger on cooling as compared to using pure ice. Lopez et al. [49] studied the behaviour of graphite with closed and open pores, made with salt as a PCM. A pore-thermo-elastic analysis was performed from which was concluded that varia- tions in expansion volumes of the salt due to fusion were limited by the matrix of graphite, which causes an increase of pressure in the pores. Higher pressure causes a progressive increase in the melting temperature of the salt and a gradual decrease of the latent heat. This implies that a significant part of the energy supplied to the material is used to heat it (sensible heat instead of latent heat). Zhao et al. [133] studied the addition of the highly thermal con- ductive ENG-TSA (expanded natural graphite treated with sulphuric acid) to solar salt (60% sodium nitrate and 40% potassium nitrate), in order to enhance the thermal conductivity of nitrates when used 429 10490 237 387.6 8978 381 401 19300 129 237 2800 910 Silver Copper Gold Aluminium Nickel 90.3 8910 440PDF Image | Thermal energy storage: Recent developments
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