In recent years, the energy problem has become increasingly serious due to the greenhouse effect caused by the depletion of fossil fuels and carbon dioxide emissions. Therefore, energy conservation and efficient use of energy are attracting attention. At the same time, the development of renewable energy as an alternative energy source has also become a research hotspot. Solar energy has the characteristics of abundant resources, low cost, clean and environmentally friendly, and is a promising renewable energy source. However, the intermittency and instability of solar energy still hinder the widespread use of solar energy.
In order to overcome the mismatch between solar supply and demand, energy storage technology that can alleviate the imbalance is a useful method. Energy storage methods include sensible heat energy storage, latent heat energy storage and chemical reaction energy storage. Latent heat energy storage has great advantages due to its high energy storage density and isothermal characteristics. Type C ordinary paraffin wax has large latent heat, good stability, no sub-coldness, and no toxicity, and is an excellent phase change material (PCM) for latent heat energy storage.
However, its widespread use is hampered by several bottlenecks. One of the main disadvantages is the low thermal conductivity, less than 0.4W/(mK), resulting in low heat storage/recovery and low utilization efficiency of stored energy. Therefore, many researchers are investigating ways to improve the thermal conductivity of PCM.
As a new material, metal foam has been widely studied and used because of its good mechanical and thermophysical properties. Its attractive advantages include: lower bulk density due to its high open porosity; It has high specific strength and specific stiffness; In particular, the continuous skeleton structure has high thermal conductivity.
Therefore, composite PCMs made of porous metal foam and pure PCM have been used in latent heat energy storage (LTES) systems, cooling water tanks, etc. The energy storage process of aluminum foam composite solar plate collector with pure paraffin wax as PCM was analyzed. The results show that the use of aluminum foam mixed with pure paraffin improves the heat transfer performance of the collector, and the temperature distribution in the collector is more uniform than that in the collector mixed with pure paraffin.
Through experiments and analysis, the mechanism of copper foam to improve the heat transfer performance of cylindrical solid-liquid LTES system was evaluated in detail. The time interval of the dotriane/copper foam system is significantly shortened during both the melting and freezing processes, and the natural convection during the melting process is also reduced.
From this, it can be concluded that the addition of metal foam to the PCM stock solution generates more heat. Provides an additional heat transfer path in pure PCM, enhancing heat conduction inside the PCM, ensuring that each part of the PCM is uniformly heated or cooled. However, it is clear that the above research is mainly focused on enhancing the heat transfer of PCM.
However, the morphological and thermal characteristics of PCM/metal foam composites are not well studied. In addition, the effective thermal conductivity predicted by theoretical analysis of PCM/metal foam composites has not been fully compared with the experimental results. The effect of porous metal foam on the PCM phase transition process is also unclear.
In this study, pure paraffin wax with a melting temperature of 60-62°C was used as PCM in order to use solar energy to store hot water. Paraffin/nickel foam and paraffin/copper foam composite PCM were prepared by vacuum impregnation method. The impregnation ratio was analyzed to reveal the interaction and compatibility between pure paraffin wax and metallic foam.
Firstly, the morphological characteristics of metal foams, such as surface porosity and volumetric porosity, were studied. Then, the effect of metal foam on the thermophysical properties of composite PCMs, such as thermal conductivity, phase transition temperature, and latent heat, was experimentally studied. Morphological and thermal characterization results are indispensable basic information that helps to reveal the mechanism of composite PCM and provide useful information for manufacturing composite PCM.
In addition, accurate data on the effective thermal conductivity and latent heat of composite PCMs play a key role in the design and modeling of LTES systems. The use of composite PCMs can greatly improve the output power and heat storage/recovery ratio of LTES systems, thereby improving the efficiency of LTES systems.
The figure above shows the equipment for preparing a composite PCM. Metal foam inevitably contains air due to its porous structure, and the air in the metal foam prevents PCM from penetrating into the porous structure, reducing the impregnation rate of PCM.
Therefore, it is necessary to evacuate the air during preparation. The vacuum pump (2XZ-4B) is provided by Shanghai Junlong Vacuum Equipment Co., Ltd. Vacuum pump (2XZ-4B) is used to evacuate air. The vacuum gauge is provided by Shanghai Nanxin Instrument Co., Ltd.
Nickel and copper foam are produced by Shanghai Zhongwei Innovative Materials Co., Ltd., and pure paraffin wax is produced by Sinopharm Chemical Reagent Co., Ltd. Pure paraffin wax is produced by Sinopharm Chemical Reagent Co., Ltd. Pure paraffin wax comes from Sinopharm Chemical Reagent Co., Ltd.
Tables 1 and 2 list the specifications of the materials used in this study, which are determined by measurements and data. The high porosity of metal foam is considered a compromise between balancing the heat storage density and heat transfer rate of LTES systems.
A vacuum-assisted impregnation processing process with or without vacuum is shown in the figure above. First, solid paraffin wax and a piece of metal foam cut into 100×100×10 mm in size are placed in a stainless steel container. As shown in fig. (1), a mesh is also placed in the container to support the specimen and facilitate the removal of the specimen. The specimen gauge valve vacuum pump is attached to the vessel to evacuate the air in the container, and the pressure is maintained below 50Pa by pumping air.
The container is then heated with a hot water tank maintained at 100 °C to melt the paraffin. When the metal foam and support net sink into the molten paraffin, the metal foam is immersed in the liquid paraffin during the sinking process. After the paraffin is completely immersed in the porous space of the metal foam, the heating process lasts about 60 minutes and the vacuum pump is turned off at the same time.
Subsequently, the stainless steel container is cooled with cold water at a constant temperature of 25°C for about 120 minutes. When the paraffin is completely solidified, the container walls are lightly heated for about 10 minutes to make them easy to separate. Finally, remove the porous metal foam impregnated with paraffin wax and carefully remove excess paraffin trapped on the surface of the composite. The impregnation process without vacuum assistance is similar to the process described above, the only difference being that there is no vacuum pump in the system.
The thermal conductivity of a paraffin/nickel foam composite PCM was measured at room temperature at 25°C using a hot plate thermal constant analyzer, based on the transient planar heat source method (TPS). Thermal plate thermal conductivity analyzers measure approximately ±3% accuracy and measure the thermal conductivity of materials in the range of 0.01 to 400 W/(mK) with highly sensitive sensors.
In this study, samples over 200 W/(mK) could not be measured due to the lack of probes. When measuring the thermal conductivity of paraffin/nickel foam composite PCM, two specimens of the same size (100×100×10 mm) are prepared with vacuum assistance and a probe with a radius of 6403 mm (Type 5501) inserted between the two specimens.
A specimen with a smooth surface is in uniform and tight contact with the probe to avoid heat leakage and reduce contact resistance between the two specimens. During the measurement process, the probe acts as both a temperature sensor and a heating source. Control parameters such as output power and time are 0.3W and 2.5-6s, respectively. A thermal conductivity model assuming the presence of a planar heat source in an infinite material can be used to evaluate thermal conductivity and thermal diffusivity in the same measurement.
Since the thermal conductivity of copper is significantly higher than that of pure paraffin, the anisotropic characteristics are more obvious in paraffin/copper foam composite PCM. In addition, current TPS devices cannot measure the thermal conductivity of copper because the thermal conductivity of copper exceeds 200W/(mK).
Therefore, the thermal conductivity of paraffin/copper foam composite PCM is measured by the steady-state method. In this study, a similar measurement system was constructed, first valuing stainless steel by measuring its thermal conductivity, and then using it to measure the thermal conductivity of paraffin/copper foam composite PCM.
The phase change temperature and latent heat are considered pure paraffin and paraffin/metal foam composite PCM. In this study, a DSC8000 analyzer was used to measure these two parameters. The temperature measurement accuracy is about ±0.01°C, and the calorimeter accuracy is ±0.03%. The sample volume analyzed is approximately 10-20 mg, and the sample is experimentally sealed in a standard aluminum DSC sample tray with a sample sealing assembly. Place an empty tray as a reference plate and a sample tray in two furnaces.
All vacuum-assisted impregnated specimens undergo a melt-freeze cycle under the same test conditions, heating and cooling at 5°C/min and a temperature range of 20-90°C. When performing DSC measurements at a nitrogen flow rate of 20.0 ml/min, the heat flow difference between the reference disk and the sample tray was recorded.
Five characteristic temperatures can be determined from the heat flow signal measured by the DSC, which are the start temperature, the end temperature, the extrapolated start and end temperature, and the peak temperature of the DSC curve. In general, extrapolated starting temperatures are used to characterize the starting point of the phase transition process. However, some researchers regard the peak temperature of the DSC curve as a characteristic temperature.
The figure above shows the morphology of nickel foam before preparation. AS CAN BE SEEN FROM THE FIGURE, THE APERTURES OF 5PPI (APERTURE PER INCH), 10PPI, AND 25PPI SPECIMENS ARE ABOUT 5 MM, 3 MM AND 1 MM, RESPECTIVELY.
In this study, three composite PCM specimens were prepared, and the impregnation ratio is shown above. As can be seen from the figure, the impregnation rate cannot reach 100%. This is because during the cooling process, the outer paraffin first solidifies and the inner paraffin then solidifies. Since pure paraffin wax has a higher solid density than liquid dense, it is inevitable that the inner paraffin will shrink slightly during cooling, forming small pieces of paraffin.
It can also be seen from the figure that the impregnation rate of copper foam composite PCM is slightly lower than that of nickel foam composite PCM, because copper foam has better heat transfer performance during the cooling process. In the manufacture of composite PCM, the bottom and sides of the container are in direct contact with the cooling water at 25 °C, so the paraffin around these two surfaces is cooled first.
However, the paraffin around the free top surface of the container gradually cools due to direct contact with air. Since the thermal conductivity of copper is greater than that of nickel, the copper foam skeleton cools faster, so the solidification process of liquid paraffin in the upper part of the container is accelerated. As a result, the liquid paraffin on the upper surface of the copper foam solidifies faster, and the solidified paraffin prevents more liquid paraffin from entering the inside of the copper foam. The liquid paraffin on the top surface of nickel foam solidifies slightly more slowly, allowing more liquid paraffin to enter the nickel foam.
25PPI specimens without vacuum assistance have less impregnation rates than 5PPI and 10PPI specimens. The smaller the pores, the more difficult it is for paraffin wax to impregnate into the pores. This phenomenon is not evident in the vacuum-assisted method. In addition, for nickel and copper composite PCMs, the impregnation rate with vacuum assistance is greater than the impregnation rate without vacuum assistance, regardless of the pore size. Therefore, the vacuum-assisted method is superior to the non-vacuum method in the preparation of paraffin/metal foam composite PCM.
The measurement was repeated six times to ensure accuracy and repeatability of the results. After the use of metal foam, the thermal conductivity of composite PCM is greatly improved.
As can be seen from the figure above, the thermal conductivity of paraffin/nickel foam composite PCM is close to 1.2W/(mK), which is about three times that of pure paraffin wax at 0.305W/(mK). The thermal conductivity of 5PPI paraffin/copper foam composite PCM is close to 4.9W/(mK), which is about 15 times that of pure paraffin. The high thermal conductivity of paraffin/copper foam composite PCM leads to its high melting rate in the carbonization process.
The melting curves of pure paraffin and paraffin/tar foam composite PCM are shown. The positive heat flow in the figure represents the heating process, while the negative heat flow in the figure represents the cooling process. The DSC curve of pure paraffin wax has two peaks: the low temperature peak corresponds to the solid-solid phase transition, and the high temperature peak corresponds to the solid-liquid phase transition with large latent heat
The detailed values of the phase temperature are shown in Tables 3 and 4. MELTING TEMPERATURES (INFERRED STARTING MELTING TEMPERATURE) AND PEAK MELTING TEMPERATURES WERE SLIGHTLY HIGHER AT 5PPI, 10PPI, AND 25PPI PARAFFIN/NICKEL FOAM COMPOSITE PCMs compared to pure paraffin. IN CONTRAST, THE FREEZING TEMPERATURE (INFERRED INITIAL FREEZING TEMPERATURE) AND PEAK FREEZING TEMPERATURE OF 5PPI, 10PPI, AND 25PPI PARAFFIN/NICKEL FOAM COMPOSITE PCM DECREASED SLIGHTLY COMPARED TO PURE PARAFFIN. The case of paraffin/copper foam composite PCM shows similar characteristics.
Since the metal skeleton cannot participate in the phase transition process, only paraffin impregnated in metal foam is analyzed. As shown in the figure above, the latent heat (LHPC) per unit mass of paraffin in the composite is equal to the mass fraction of LHC divided by pure paraffin.
The results showed that the LHPC of paraffin/metal foam composite PCM was reduced by about 2%-8%, for example, the LHPC of 25PPI paraffin/nickel foam composite PCM was 182.4kJ/kg, while the LHPC of pure paraffin wax was 189.4kJ/kg.
LHPC is derived from the theoretical mass fraction of pure paraffin in the composite PCM, which is calculated from the volume porosity. The slight decrease in LHPC values is due to a deviation between the theoretical mass fraction of pure paraffin wax and the actual mass fraction in the DSC measurement.
The reasons for the deviation are as follows. On the one hand, the mass of the sample per measurement is very small, only about 10-20 mg, so the mass fraction of pure paraffin wax in the sample may deviate from the theoretical value. On the other hand, small cavities may also reduce the actual mass fraction of pure paraffin wax in the composite PCM, resulting in a slight decrease in the LHPC of the composite PCM.
Paraffin/nickel foam and paraffin/copper foam composite PCMs were prepared with and without vacuum assistance, respectively. The morphological characteristics and thermophysical properties of composite PCM were studied and compared with pure paraffin.