A study of how heat spreads through melting materials could change the way that we store energy. Ideally, a report in the Journal of Applied Physics finds, heat lines should spread through a material in a fractal manner, like branches on a tree.
Professor Adrian Bejan of Duke University has pioneered the study of branching systems over the last two decades, exploring the commonalities of river deltas, snowflake formation and lightning bolts. Now he has demonstrated that heat melts a solid object in a similar way.
Bejan has shown that when confronted with an initially cold material, heat spreads through it best when it can move in a branching pattern, with what he calls “fingers” of heat finding their way through paths of least resistance. When the fingers are allowed to evolve over time in number and branching angle they melt solid objects more quickly, and with less energy, than when constrained.
The finding has important implications for systems that rely on repeatedly melting objects before letting them cool. Solar thermal power plants store energy collected during the day by melting salts that release it as they are allowed to cool. Despite advances in battery technology, this is still often the cheapest way to store energy for easy release, but inefficiencies in the melting process are a significant drag on the commercial viability of clean power stored in this way.
According to Bejan, “The traditional architecture is to embed a heating and cooling coil in the phase change material, but our research shows that what happens naturally is also the best way to spread the heat into the volume: it is a dendritic structure, like a hand with many fingers.”
Bejan also found that more complex branching structures melt materials more quickly, but beyond a certain point it is not worth promoting further branching. “Imagine building a branching structure, with billions of tiny, tiny fingers touching at the tips,” Bejan says. “You'd say forget it!”