As renewable energy expands worldwide, one challenge remains stubbornly unresolved: how to store heat efficiently and sustainably when the sun is not shining or demand fluctuates. A new study shows that agricultural waste, specifically discarded neem seeds, can be transformed into a powerful and environmentally friendly thermal energy storage material.
Researchers have developed a biochar based phase change material that can capture, store, and release heat with high efficiency while also locking carbon away. The work demonstrates how the temperature used to produce biochar strongly controls its ability to store thermal energy, offering a new pathway for low cost and carbon negative energy storage technologies.
The team converted neem seed waste into biochar by heating it under low oxygen conditions at two different temperatures, 300 and 500 degrees Celsius. The resulting porous carbon material was then infused with lauric acid, a fatty acid commonly used in thermal energy storage. This combination creates a shape stabilized phase change material that can absorb heat as it melts and release heat as it solidifies, without leaking.
"Our goal was to turn an underused biomass waste into something that directly supports clean energy systems," said one of the corresponding authors. "By carefully tuning the biochar production temperature, we were able to dramatically improve how much heat the material can store and how stable it remains over time."
The difference between the two biochars was striking. Biochar produced at 500 degrees Celsius developed an exceptionally high internal surface area, more than 600 square meters per gram. This sponge like structure allowed much more lauric acid to be held securely inside the pores. As a result, the high temperature biochar composite stored nearly twice as much latent heat as the material made from lower temperature biochar.
Laboratory tests showed that the optimized composite could store almost 95 joules of heat per gram, while maintaining stable melting and solidification behavior over hundreds of heating and cooling cycles. Importantly, leakage tests confirmed that the phase change material remained locked inside the biochar matrix even when heated above its melting point.
"This kind of stability is essential for real world applications," the researchers noted. "Thermal energy storage materials must perform reliably for years without degrading or leaking, especially in buildings, solar energy systems, and industrial heat recovery."
Beyond performance, sustainability is a central advantage of this approach. Neem seeds are widely available agricultural residues in many tropical regions and are often discarded after oil extraction. Converting them into biochar not only adds value to waste biomass but also sequesters carbon that would otherwise return to the atmosphere.
Unlike conventional energy storage technologies that rely on mined materials or complex manufacturing, biochar based thermal storage can be produced at relatively low cost using locally available feedstocks. This makes it especially attractive for decentralized energy systems and regions seeking affordable clean energy solutions.
The researchers emphasize that their findings highlight the importance of controlling biochar production conditions to tailor materials for specific energy applications. With further development, biochar based phase change materials could play a key role in improving energy efficiency, reducing carbon emissions, and supporting the transition to a more sustainable energy future.
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Journal Reference: Mandal, S., Mendhe, A.C., Park, T. et al. Temperature-modulated surface features of neem seed biochar for sustainable thermal energy storage applications. Biochar 8, 9 (2026).
https://doi.org/10.1007/s42773-025-00510-x
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
