Phase change energy storage or phase change temperature control is realized by utilizing the characteristics of PCM to store and release latent heat during phase change. However, the thermal conductivity of most PCMs (such as paraffin and fatty acid PCMs) is relatively poor, which restricts the practical application of PCMs in the fields of phase change energy storage and phase change temperature control. After strengthening the heat transfer of the phase change material, the heat conduction rate of the phase change material can be improved, the storage and release of energy can be accelerated, and the utilization efficiency of energy can be improved. Therefore, further strengthening the heat transfer of PCM is the key technology to promote the application of phase change materials. When selecting a high thermal conductivity material as an additive to enhance heat transfer of a phase change material, the following points should be met: the thermal conductivity is relatively high; the density of the material should not be too high; additives should be compatible with phase change materials; have certain corrosion resistance, non-toxic, etc.
At present, many studies enhance heat transfer by adding high thermal conductivity materials into PCM. Commonly used high thermal conductivity materials include carbon nanotubes, metals and their oxides, metal fin structures, high thermal conductivity porous media, and carbon fibers.
1. Enhanced heat transfer of metal materials to phase change materials
Generally, the thermal conductivity of most metals is relatively high. Adding metal structures or metal particles to PCM can enhance heat transfer to PCM. Commonly used metal additive particles include copper, aluminum, iron and metal oxides. Eastman et al. studied the thermal conductivity of nanofluids composed of copper oxide particles and ethylene glycol vinyl. When the volume fraction of copper oxide reached 0.3%, the thermal conductivity increased by 40%. Long Jianyou et al. added TiO2 particles to the eutectic salt BaCl2 aqueous solution. When the added TiO2 volume fraction was 1.13%, the thermal conductivity of the composite material increased by 16.74%, thereby improving the thermal conductivity of the material. Zhang Yanping added copper powder and aluminum powder with a mass ratio of 5% to 20% into the PCM. The results show that the thermal conductivity of PCM can be increased by 10%-26% after adding copper powder, and the thermal conductivity of PCM can be increased by 20%-56% after adding aluminum powder. Hu Waping added nano-zinc oxide and nano-alumina to butane erythritol. When the mass fractions of zinc oxide and aluminum oxide are both 12%, the thermal conductivity of butane erythritol increased from 0.268W·m-1·K-1 to 0.611W·m-1·K-1 and 0.658W·m-1·K-1, respectively, increased by 126% and 145%, indicating that the addition of nano-zinc oxide and nano-alumina significantly improved the thermal conductivity of butane erythritol.
The addition of metal fins to phase change materials to improve the heat transfer performance of their energy storage systems is also a commonly used method. In order to achieve the best enhanced heat transfer effect in application, fins with appropriate shape, size, arrangement and metal material should be selected according to the actual situation of the energy storage system.
Ismail et al. added fins in PCM to enhance heat transfer, and compared the results with the experimental results by establishing a model, and analyzed the effects of the number, length, thickness of fins and the size of the annular space on the heat transfer efficiency. Liu et al. designed a thermal energy storage unit with new fins using stearic acid as the energy storage material, and analyzed the effects of fin size and fin pitch on thermal conductivity. The research results show that the increase of the fin structure increases the thermal conductivity by 67%, effectively improving the heat transfer efficiency, and reducing the width and fin pitch can improve the thermal conductivity. Velraj et al. studied the fin structure in the cylindrical tube, and established a two-dimensional model to simulate the heat transfer process of the fin structure with different tube diameters and fin thicknesses, and it is believed that a V-shaped fin structure can maximize the heat transfer capacity.
Although metal has high thermal conductivity, adding metal material to phase change material can improve the thermal conductivity of phase change material, but for metal additives in PCM, the compatibility of metal and PCM should be considered. Some metals have good compatibility with PCM, such as aluminum and paraffin, but some metals are not compatible with PCM, such as copper and nickel, which are incompatible with paraffin. In addition, the density of metal is larger than that of PCM, and the addition of metal will lead to an increase in the mass of the energy storage system. When the PCM is melted, the metal ions tend to settle under the action of gravity, which greatly limits the application of metal materials in the enhanced heat transfer of PCM.
2. Enhanced heat transfer of porous media to PCM
The skeleton structure of porous media is mainly composed of solids, and due to the separation of the skeleton structure, a large number of dense pores are formed, and these tiny pores may be interconnected, partially connected or partially disconnected. Due to the high porosity of porous media, a large amount of PCM can be stored, and the interconnected skeleton structure can effectively improve the heat transfer efficiency of PCM. The pores of porous media are generally relatively small, and the confinement under the capillary effect of the pores can make the PCM maintain the solid state macroscopically after the phase transition.
There are many types of porous media. At present, the porous media commonly used as the carrier matrix of PCM include expanded graphite, graphite foam, ceramics, expanded perlite, gypsum, bentonite, porous concrete, metal foam, etc. for thermal management of PCM.
Expanded graphite is also known as vermicular graphite, and its structure is porous worm-like. Expanded graphite is prepared from natural graphite after a series of oxidation intercalation reactions and then expanded at high temperature. Since a large number of pore structures are generated during the expansion process, it has strong adsorption performance. Zhang Guoqing et al. used expanded graphite as a carrier matrix to adsorb paraffin to prepare an expanded graphite/paraffin composite material with a paraffin content of 80%, and its thermal conductivity was greatly increased to 12.346W·m-1·K-1, which was more than 50 times that of pure paraffin. Zhang Zhengguo et al. prepared four kinds of expanded graphite/paraffin composites with different mass fractions, and their heat storage and heat release efficiencies were greatly improved compared with pure paraffin. Xavier Py et al. studied the thermal conductivity of expanded graphite/paraffin composites with different paraffin mass fractions. The results show that the thermal conductivity increases from 0.24W·m-1·K-1 of single paraffin to 4~70W·m-1·K-1 when the mass fraction of paraffin is 65%~95%. Sari et al. analyzed the relationship between thermal conductivity and melting time, storage capacity and phase transition temperature, and studied the thermal conductivity of expanded graphite with different mass fractions added to graphite, and considered that the mass fraction of expanded graphite was the best at 10%, and the thermal conductivity at this time increased to 272.2%.
Graphite foam is a porous carbon material with high thermal conductivity and certain adsorption properties. Therefore, many researchers use this characteristic of graphite foam to enhance heat transfer of phase change materials. Zhong Yajuan et al. used graphite foam as the enhanced heat transfer carrier of the phase change material, prepared a graphite foam/paraffin phase change energy storage material, and used a laser thermal conductivity meter to test the thermal conductivity of the composite material and pure paraffin. The results show that the thermal conductivity of graphite foam/paraffin wax composite is 437 times higher than that of pure paraffin wax. Zhong et al. prepared paraffin/graphite foam composites using graphite foams with different pore sizes, and tested and analyzed the latent heat of phase transition and thermal diffusivity. The results show that the thermal diffusivity and heat storage capacity of the composites are affected by the ligament thickness and pore size of the graphite foam.
Metal foam has the advantages of small specific gravity, large specific surface area, high porosity and high thermal conductivity, so many researchers use metal foam for enhanced heat transfer of phase change materials. The commonly used metal foams are copper foam, aluminum foam and nickel foam. Zhang Jiangyun used foamed copper as the carrier matrix and paraffin as the phase change material, and prepared four kinds of foamed copper/paraffin composite phase change materials with different mass ratios by vacuum infusion method. After the paraffin and foamed copper are compounded, the thermal conductivity of the composite material has been greatly improved. The mass fraction of the foamed copper in the four composite materials is 36.1%, 32.5%, 32.3%, and 36.8%, respectively. Compared with pure paraffin, the thermal conductivity is increased by 15.2 times, 14.44 times, 14.07 times and 15.93 times, respectively. Zhao Mingwei et al. used several aluminum foams with different porosity to prepare foamed aluminum/paraffin composite phase change materials by infiltration method, and studied their heat storage and release properties. The results show that the composite phase change material shortens the time of paraffin solid-liquid phase transition, and the heat storage and release efficiencies increase with the decrease of the porosity of the foamed aluminum framework. When the porosity of the foamed aluminum framework is between 54.81% and 69.74% of the foamed aluminum/paraffin composite phase change material, its equivalent thermal conductivity is 91.40~61.16W·m-1·K-1, and the thermal conductivity is obviously improved compared with pure paraffin. In order to improve the cavity distribution and heat transfer performance of the solid-liquid phase change energy storage device, Xu Weiqiang et al. filled the solid-liquid phase change heat storage container with nickel foam, and compared it with the heat storage container without foam brocade. The research results show that the filled nickel foam can significantly disperse the cavity distribution in the solid-liquid phase transition process, and can significantly improve the thermal conductivity of the phase change material, improve the heat transfer efficiency of the heat storage container, and improve the temperature uniformity in the container. Karaipekli et al. composited the blend of tetradecanoic acid and hexanoic acid with expanded perlite, and mixed a certain amount of expanded graphite to prepare a composite phase change material. The research shows that in the case of no leakage of the phase change material in the composite material, the mass fraction of the highest adsorption capacity of the expanded perlite to the phase change material is 55%, and when the mass fraction of expanded graphite is 10%, the thermal conductivity is increased by 58%.
3. Enhanced heat transfer of PCM by other materials
The effect of enhancing heat transfer in PCM by adding other high thermal conductivity materials (such as carbon fiber, carbon nanotubes, and graphene, etc.). Carbon fiber is a new type of fiber material with high strength and high modulus fiber with a carbon content of more than 95%. It is a new type of material gradually developed in the early 1960s, and are widely used in construction, transportation and aerospace and other fields. Carbon fiber not only has the advantages of high strength, low density, high specific performance, good electrical conductivity, corrosion resistance, etc., but its thermal conductivity is also between non-metal and metal, so it is gradually used to improve the thermal conductivity of PCM. Karaipekli et al. prepared four ratios of stearic acid/carbon fiber composites and conducted thermal conductivity analysis. The results show that when the mass fraction of carbon fiber is below 10%, the thermal conductivity increases with the increase of the mass fraction of carbon fiber. Fukai et al. added carbon fibers to phase change materials using two different distribution methods. The first method is to randomly distribute carbon fibers in the phase change material, the second method is to use carbon fiber brushes, and the direction distribution of carbon fiber filaments is consistent with the direction of heat flow, and then the thermal diffusivity is calculated by a one-dimensional thermal conductivity model, and compared the enhanced heat transfer effect of the two cases. The research shows that after adding carbon fiber, the thermal conductivity of the phase change material is greatly improved, and the heat transfer enhancement effect of the second method is more obvious. When the first method is used to place carbon fibers with a volume fraction of 3%, the thermal conductivity of the phase change material is increased by a factor of 10, while the second method can achieve the same enhanced heat transfer effect by placing a carbon fiber brush with a volume fraction of 1%.
Carbon fiber has good compatibility with most PCMs, and has strong corrosion resistance, and the diameter of the fiber is relatively small, which is conducive to the uniform arrangement in the PCM. Based on its excellent physical and chemical properties, carbon fiber has a larger application space as one of the reinforced heat transfer materials.
The diameter of carbon nanotubes is generally between several nanometers and tens of nanometers, and can be divided into single-walled carbon nanotubes and multi-walled carbon nanotubes according to the number of layers of graphene sheets. Carbon nanotubes have excellent thermal properties and are ideal additives for enhancing heat transfer. Wang et al. prepared a hexadecanoic acid/carbon nanotube composite phase change material by incorporating carbon nanotubes into hexadecanoic acid. The thermal conductivity was significantly improved after hexadecanoic acid was incorporated into carbon nanotubes. The thermal conductivity of the composites increases with the increase in the mass fraction of carbon nanotubes, both in the solid state and in the liquid state. The thermal conductivity of pure hexadecanoic acid is 0.22W·m-1·K-1 in solid state and 0.16W·m-1·K-1 in liquid state. When the mass fraction of carbon nanotubes in the composite material is 1%, the thermal conductivity of the composite material in the solid state is 0.33W·m-1·K-1, and the thermal conductivity is increased by 30%.
Graphene has very high thermal conductivity and is a good heat transfer material. Hu Waping uniformly blends graphene and polyethylene glycol to increase the thermal conductivity of polyethylene glycol. The test results show that due to the high thermal conductivity of graphene, when the mass fraction of graphene added is 1%, 2% and 4%, respectively, the thermal conductivity of polyethylene glycol increased from 0.263W·m-1·K-1 to 0.613%, 0.785% and 1.042% by 133%, 171% and 296%, respectively, and the latent heat of polyethylene glycol after graphene filling is still as high as 170.29 J·g-1, and the crystal structure and chemical structure are basically not affected.