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A metal foam/paraffin composite modular thermal storage unit has been developed for efficient energy storage

Phase-change materials (PCMs) have been utilized in various applications such as thermal energy storage, electronics cooling, and data storage due to their remarkable latent heat storage capacity. However, the practical implementation of PCMs is hindered by two primary concerns: the low intrinsic thermal conductivities that result in a slow thermal storage/retrieval process, and the leakage problem caused by volume expansion during the phase change process. To address the first concern, researchers have attempted to enhance the thermal conductivity of PCMs by incorporating high thermal conductivity particles or porous matrixes, such as metal powder, expanded graphite, carbon nanotube, graphene, and metal foam. The effectiveness of the PCM’s thermal conductivity enhancement depends on the corresponding filling material and percentage. Regarding the second concern, PCMs are typically encapsulated with organic polymers to overcome the leakage problem due to their high ductility and compatibility with PCMs. Huang et al. utilized PET plastic pipes to encapsulate paraffin and float stones to increase thermal conductivity, resulting in a reduction of nearly one-third of the melting time. Alam et al. employed polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) as shell materials to encapsulate PCM, resulting in significantly shorter melting/solidification times without degradation in thermophysical properties on cycling. Additionally, many researchers have utilized epoxy resin as an encapsulation material due to its high tensile strength, good adhesive properties, low cost, and high compatibility with PCMs.

Copper Foam 1 (6)

However, a significant drawback shared by all of these approaches is that once all of the phase change materials (PCMs) have completely undergone phase change, their thermal storage capacity will approach its maximum and heat will no longer be stored. Furthermore, the relatively low intrinsic thermal conductivity of PCMs compared to metal or semiconductor substrates makes them the key bottleneck for heat dissipation. At this point, electronic devices must stop working and wait for the slow re-solidification process of the PCMs before restarting. This drawback significantly restricts the implementation of PCMs for thermal storage applications.

Inspired by the online-discharging/offline-charging working characteristics of driving batteries and the modular thermal energy storage concept, we propose a similar modularized thermal storage unit (MTSU) to overcome this drawback and realize online thermal charging and offline thermal discharging working characteristics. Figure 1 illustrates the working principle of the MTSU. During the operating time of electronic devices, the MTSU absorbs heat from the electronics and maintains the designed constant temperature before completely undergoing phase change. Once the MTSU reaches its thermal storage limitation, it can be replaced with a fresh one directly, allowing the electronics to continue working without waiting for the long re-solidification process. Therefore, the MTSU can overcome the aforementioned drawback and enable the continuous working of electronic devices.

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In this paper, we fabricate the MTSU using epoxy resin encapsulated paraffin to validate the online thermal charging and offline thermal discharging working characteristics. We adopt vacuum impregnation and casting molding to prepare the phase-change core and encapsulate shell, respectively. Paraffin is chosen due to its advantages of high latent heat (approximately 250 J/g), appropriate melting point (30-70°C), high stability, low negative environmental impact, and low cost. Copper and nickel metal foams with different porosities are employed as thermal enhancers due to their high thermal conductivity and good mechanical strength of the skeleton. We develop a series-parallel model to predict the effective thermal conductivity (ETC) of the MTSU samples, and the accuracy is verified experimentally. We also assess the thermal storage performance of the MTSU and verify and discuss the advantages of the proposed MTSU by comparison with conventional thermal storage units.

Preparation of MTSUThe present study investigates the fabrication processes of paraffin/metal foam core through vacuum impregnation. Initially, solid paraffin was placed in a beaker and melted in a water bath at a temperature of 80°C. Subsequently, a metal foam measuring 26 mm×26 mm×10 mm was immersed in the liquid paraffin. The beaker was then placed in a vacuum oven at 80°C, and the pressure inside the oven was maintained below 100 Pa for a duration of 2 h using a vacuum pump to achieve the desired impregnation effect. Following this, the beaker was cooled to ambient temperature until complete solidification. The beaker was then reheated in the water bath, and upon wetting of the inner surfaces of the beaker by the liquid paraffin, the paraffin/metal foam composite was extracted. Finally, any excess paraffin outside the metal foam was removed. It is observed that both the paraffin and epoxy resin exhibit low thermal conductivity, highlighting the necessity of thermal enhancement through the use of metal foam.

The thermally enhanced MTSU was produced using the casting molding method. The preparation process involved several steps. Firstly, a vacuum chamber was used to mix epoxy resin, consisting of resin and hardener, in a mass ratio of 5:1 to eliminate air bubbles. Secondly, Mold 1 was utilized to mold resin gaskets with a thickness of 2 mm. Thirdly, Mold 2 was employed to house a paraffin/metal foam core, with the mold-releasing agent, polyvinyl alcohol (PVA), uniformly coated on the surfaces of the molds. The gaskets were used to maintain a fixed distance between the core and mold surfaces. Subsequently, liquid epoxy resin was injected into Mold 2 to fill the reserved space around the paraffin core. Mold 2 was then placed in a vacuum chamber to eliminate air bubbles inside. Finally, the specimen was subjected to curing treatment at 50°C for 6 hours in the vacuum chamber.

In the course of the preparation procedure, copper and nickel foams of varying porosities were utilized in the production of the thermally enhanced MTSU. The foam’s internal structure is reticulated and interconnected in a layered fashion, thereby creating pathways of high thermal conductivity within the metal foam.

In this study, we propose a thermally enhanced Multi-Tube Storage Unit (MTSU) to achieve online charging and offline discharging working characteristics. The MTSU samples were fabricated using the vacuum impregnation and casting molding method. A series-parallel model was developed to predict the Effective Thermal Conductivity (ETC) of the MTSU samples. An experimental setup based on the steady-state method was established to verify the accuracy of the prediction results. The findings indicate that the predictions align well with experimental results, with deviations within 9.7%.

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Based on the model, we observed that the ETC of the MTSU samples increased significantly with the addition of metal foams. Specifically, the ETC of copper foam-enhanced MTSU increased by 376% when the porosity was 95.52%, and the enhancing effect increased with decreasing porosity. The repeated heat storage/release cycles demonstrated that the proposed MTSU possesses good thermal stability.

Furthermore, we evaluated the heat storage performance of the MTSU experimentally and compared it with that of the conventional Tube Storage Unit (TSU). The MTSU avoids the slow re-solidification process and enables a continuous thermal storage process for electronics, maintaining the electronics temperature consistently below the design temperature. Therefore, the MTSU exhibits potential for continuous heat storage over long periods, making it suitable for fields with continuous heat storage demands, such as driving batteries and solar-thermal conversion systems.

Solid-liquid phase change materials (PCMs) are promising for thermal energy storage and electronics cooling. However, when PCMs reach their thermal storage limit, they become a bottleneck for heat dissipation, causing electronics to stop working. To address this issue, a modularized thermal storage unit (MTSU) was proposed. The MTSU can be replaced once the PCMs reach their limit, enabling continuous electronics operation. The MTSU is made by encapsulating paraffin with epoxy resin and thermally enhancing it with copper or nickel foams. The proposed MTSU increases effective thermal conductivity by up to 376% and exhibits good thermal stability. It has potential for continuous thermal storage over long periods and can be applied in driving batteries and solar-thermal conversion systems.

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Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

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WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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