How phase-changing materials can build green structures

The statement ‘70 per cent of the buildings India will have by 2030 are yet to be built’ has been repeated so often that it has become a cliché. Still, it underscores the importance of ‘greening’ the construction industry, which has a large carbon footprint. The International Energy Agency has determined that carbon dioxide emissions from existing and new buildings may grow from 194 million tonnes in 2020 to 245 million tonnes by 2040.

Can buildings double as energy storage devices? Yes — with the help of materials that change phases while absorbing or releasing energy. Water becoming vapour or ice is a good example of such ‘phase-changing materials’, or PCMs.

The idea of incorporating PCMs into construction material has been around for some years, driven by climate action imperatives.

PCMs can be embedded into building materials either through macro encapsulation (hollows in slabs, walls or bricks filled with PCMs) or micro encapsulation (PCMs powdered and mixed with construction materials). A recent study by A Aridi of the University of Angers, France, and A Yehya of Harvard University notes that PCMs can save five to 14 times more energy in one unit volume than conventional sensible storage materials (water, masonry, or rock). PCMs can store a considerable amount of thermal energy in a building during off-peak load periods to balance the on-peak demand situation, it says. “Furthermore, latent heat devices are better than sensible because they can store a large amount of heat with only a small to no temperature difference,” the study notes. (Latent heat is the heat energy required for a phase change without changing the temperature.)

Using PCMs in buildings reduces energy consumption and, consequently, carbon dioxide emissions. However, the trade-off is between the cost of the PCM and the emissions produced during its manufacture. The study has delved into the relative merits of different types of PCMs in buildings — and has determined that coconut oil is the best.

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The cost factor

PCMs can come from various sources — crude oil (paraffin wax), chemicals (salt hydrates) or plants (palm, soya or coconut oil). The study assessed their relative merits on parameters such as technical appropriateness, economic viability, environmental impact, and social fairness. It looked at each material’s ‘environment cost indicator’ or ECI, and social aspects such as hazards in the mining of metals and minerals.

Four types of PCMs were studied: magnesium nitrate hexahydrate as salt hydrates, octadecane as paraffins, coconut oil, and coconut oil produced with biofertilisers.

In terms of effectiveness, all the candidates showed promise. However, when it came to cost and environmental impact, they varied widely. Octadecane was found costliest, at around $8 a kg, followed by coconut oil ($2 per kg), magnesium nitrate hexahydrate ($0.3 per kg). Octadecane, derived from crude oil, has the highest environmental impact among the four PCMs, but stores and releases the highest amount of energy because of its relatively high latent heat. “This makes it desirable for small spaces,” the study’s authors say.

“The coconut oil PCM using biofertilisers is ecofriendly, non-toxic, transparent, and has excellent chemical and thermo-physical properties for TES [thermal energy storage]. This makes it suitable for various applications. Besides, coconut oil is relatively cheap and is obtainable, renewable, and biodegradable, unlike paraffin, which requires decades to be fully decomposed. Its production positively affects the economy of the agricultural communities that produce it, despite some issues related to cheap and child labour. Consequently, relying on bio-based/plant-based materials is recommended for improving the sustainability of PCM production, keeping in mind the need to enhance the socio-economic conditions of the labour,” says the study.

Waste to PCM

The study notes that there are other bio-based PCMs, too, which may be used in combo, such as beef tallow combined with coconut oil, rapeseed oil, palm kernel oil, palm oil, and soyabean oil, among others. There are PCM materials sourced from waste or by-products such as animal fats, fish wastes, pork lard, beef tallow, chicken fat, plastics and carbon PCM (C-PCM). “New findings and research conducted on these waste products can pave the way for the creation of resilient and inexpensive PCM alternatives in the near future,” the study says.

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