Seaweed-based material stores heat for reuse

Maria Burke

A material derived from seaweed could be used to store heat for re-use, UK researchers report.

Thermochemical heat storage uses a reversible endothermic reaction to store heat energy. It is one of the most attractive technologies to store heat because of its high energy density and long-term storage capability.

In this study, a team from the University of Swansea combined small beads of sodium alginate derived from seaweed with expanded graphite to create a highly porous structure that can hold thermochemically active salts (hydrated calcium chloride).

Through a reversible hydration and dehydration reaction, the incorporated CaCl₂ salt can store and release heat (Journal of Materials Science, 2023, 58, 5610). They report that thermal energy from solar thermal generators or low-grade waste heat sources (< 200°C) was enough to trigger dehydration of CaCl₂.

The researchers believe this is the first time that alginate and graphite have been successfully combined and used in this way. The process starts with dissolving sodium alginate in water, then adding expanded graphite. The team tested two methods of making their porous structure (gelation). One involves transferring the solution into a mould for freezing where it is stored at -20°C for over two hours. The beads are formed and transferred to a saturated calcium chloride solution. The other method involves dropping the mixture into calcium salt, causing gelation on contact.

Once sufficient salt diffusion has occurred, the synthesised beads are filtered and dried at 120°C.

Seaweed, micro- and macroalgae are increasingly being used across industries to provide sustainable alternative materials.

The team compared the new alginate materials with a more conventional composite made from the naturally occurring mineral vermiculite and CaCl₂. With both gelation methods, they found the spherical beads of alginate boosted salt capacity, achieving up to four times greater energy density than the vermiculite carrier. In other words, the new material achieved the same heat energy storage capacity in a quarter of the volume.

‘The ability to recover and store otherwise wasted heat from various sources presents an exciting opportunity in the quest for sustainable and affordable energy resources,’ says Jack Reynolds, who led the research as part of his doctorate. ‘Our new heat storage material marks a significant step forward in realising this potential.’

The team is preparing for a trial at Tata Steel UK’s Trostre steelworks to investigate ways of capturing waste heat from industrial processes.

‘Alginate can be used to absorb moisture and hence store energy, similar to zeolite,’ says Mohammed Farid of The University of Auckland, New Zealand. ‘However, based on the work we have done, I do not believe there is a potential of commercialisation since the salt hydrates have higher energy density.’