High-Efficiency Thermal Fluids for Solar Systems: Silica-Encapsulated Paraffin with Nano-Graphite for Enhanced Heat Storage and Conductivity

As the push for clean, sustainable energy ramps up, solar power continues to stand out as one of the best options we have—largely because it’s both abundant and endlessly renewable. However, making the most of solar energy isn’t as straightforward as it sounds, particularly when it comes to storing and transferring that energy as heat. In solar thermal collectors, for instance, capturing sunlight and converting it into usable heat relies heavily on the working fluids inside the system. These fluids are responsible for storing the heat and moving it where it needs to go, but commonly used ones like water, ethylene glycol, and silicone oil can only do so much. They have relatively low thermal storage capacities, and they aren’t the best at transferring heat, which ultimately puts a cap on how efficient solar thermal systems can be. One interesting approach researchers have been exploring to work around these limitations is the use of phase-change materials, or PCMs, as part of these working fluids. PCMs are pretty unique—they absorb and release large amounts of heat by changing their phase, like going from solid to liquid. This allows them to store much more heat than typical fluids. But as promising as they sound, PCMs also have their own challenges. For example, organic PCMs like paraffin are fantastic for storing heat, but they tend to leak when they go through phase changes, which makes them less stable and more challenging to use in practical settings. They also tend to have low thermal conductivity, so they don’t transfer heat as efficiently, which can be an issue for applications that need quick thermal responses. To tackle these problems, researchers have been looking at ways to encapsulate PCMs within stable shell materials. Encapsulating a PCM can prevent leaks, make the material more stable, and, with the right additives, improve its ability to transfer heat. Silica is a popular choice for encapsulating PCMs because it’s thermally stable and compatible with various additives. However, on its own, silica doesn’t really address the conductivity issue.

Recent research paper published in Journal of Materials Chemistry A and led by Associate Professor Kunjie Yuan, Qiuyang Chen, Aijia Zhang and Nan Xiao  from the University of Science and Technology Beijing together with Zhiqun Lin from the National University of Singapore and Dr. Xuelin Zou from Dongguan University of Technology, came up with a promising solution. In their study, published in the Journal of Materials Chemistry A, they developed a novel encapsulated PCM composite that combines the benefits of multiple materials. They created microcapsules with a core of paraffin—an excellent heat-storing PCM—encased in a silica shell, with nano-graphite particles added to boost thermal conductivity and light absorption. Their goal was to create a material that not only stores a lot of heat but also transfers it effectively, all while maintaining stability and preventing leakage. By addressing each of these specific challenges, the team hoped to produce a highly efficient thermal fluid that could work well in direct absorption solar collectors (DASCs), systems that convert sunlight directly into thermal energy. This approach, if successful, could have a real impact on solar thermal technology and how effectively we harness and use solar power.

The researchers carried out an in-depth series of experiments, not only to create their innovative silica-encapsulated paraffin microcapsules but also to thoroughly test how well they would perform. These microcapsules, enhanced with nano-graphite particles, were designed to tackle the common challenges faced in heat transfer and energy storage. The synthesis process was crafted with precision—aimed at making sure the paraffin phase-change material (PCM) would stay secure within a silica shell, so it wouldn’t leak during its repeated shifts between phases. The nano-graphite addition wasn’t arbitrary either; it was key to solving the low thermal conductivity that typically limits the effectiveness of PCMs. By employing a polycondensation technique with tetraethoxysilane, they created stable, durable microcapsules. Their examination under scanning electron microscopy confirmed that each microcapsule had a strong core-shell structure, with nano-graphite particles evenly distributed within the paraffin core—a necessary foundation for consistent, smooth heat transfer. To understand how well these microcapsules could handle thermal demands, the team observed their behavior in various heat conditions. They found that the paraffin melted at around 49.4 °C, while the microcapsules’ latent heat capacity hit 82.5 J/g—an impressive figure that means they could store and release significant amounts of heat. These results not only met but often exceeded the performance of current PCM materials, making a clear case for their effectiveness. A standout finding was the 500% boost in thermal conductivity thanks to the embedded nano-graphite. This improvement was a game-changer: it showed that the composite could efficiently overcome the heat transfer challenges that typically plague traditional PCMs, positioning it as a highly responsive material ideal for applications requiring rapid heat flow.

This study’s impact really comes from its fresh approach to creating a heat transfer fluid that’s both efficient in storing heat and effective in moving it around—something that has been missing in solar energy systems. Professor Kunjie Yuan  and colleagues managed to combine silica-encapsulated paraffin with nano-graphite, resulting in a composite material that holds up under the demanding conditions of DASCs. Essentially, this innovation means solar systems can now run more smoothly, using a fluid that captures, stores, and transfers heat better than what’s currently available. For renewable energy setups that rely on efficient heat management, this improvement could be a real boost to reliability and overall performance. We believe the benefits here go beyond just technical improvements. The durable silica shell, combined with nano-graphite, creates a structure that resists leaks and maintains its properties over time, potentially reducing the need for frequent maintenance or replacement parts in solar systems. This resilience also makes it suitable for other applications, like industrial heating or waste heat recovery, where efficient heat transfer can lead to lower energy costs and less environmental impact. The composite’s strong light absorption capacity adds another layer of efficiency, making it ideal for systems that need to capture and convert as much solar energy as possible. This study doesn’t just add a new tool to the field—it opens up exciting possibilities for future work in PCMs. Successfully combining thermal conductivity with durability here shows how conductive additives can be used in PCMs without sacrificing stability. The potential applications could go far beyond solar energy, offering improvements in areas like building insulation and thermal storage in electronics. By addressing issues that have held traditional PCMs back, this study takes an important step towards the kind of sustainable, efficient materials that can support a transition to greener technology on a much broader scale.