Watching how coconut oil behaves gives us a clear picture of what a phase change material (PCM) is. Without being touched, moved, or supplied with any extra energy, the oil signals the arrival and end of summer with clockwork precision: it solidifies when temperatures drop and turns into a clear liquid when it warms up. How can this marvel of physics be harnessed for building climate control?

The idea has been tested for decades in laboratories around the world in search of a more efficient way to regulate temperature with less reliance on electricity, gas and, ultimately, fossil fuels. These materials release heat as they solidify and absorb it when they melt.
Several studies have shown that “integrating phase change materials into bricks reduces indoor temperature fluctuations as well as the demand for heating and air conditioning, thereby improving the thermal comfort and energy efficiency of buildings,” notes Nazareth Ruíz Marín, PhD in Nanoscience and Materials Technology, in a paper published in Energy and Buildings in June.

To put it in perspective, in periods of high temperatures, reductions of up to 6°C have been recorded in homes built using this method, with energy savings of between 20% and 60% for cooling. With the associated lower environmental impact, this can translate into a reduction of up to 700 kilograms of CO₂ emissions per household per year.

Another point in their favour is their longevity. “These materials perform well over time. They last more than 50 years, which is about the same period after which a traditional brick structure is typically updated,” says Nazareth Ruíz, a researcher in the Department of Technical Machines and Engines at the University of Cádiz.

The concept involves incorporating this type of phase change material into buildings, whether in bricks, paints, panels or the concrete itself. “The key to their effective application in construction is that the maximum and minimum daily temperatures must differ by at least 10 degrees Celsius – as they do in many parts of Spain,” says Ruíz.

This way, the phase change material cools down and solidifies at night, and during the day, when the sun is at its strongest, it absorbs heat as it melts. This process is known as thermal inertia, or “the capacity of a material to store thermal energy,” explains José Antonio Tenorio, coordinator of the Construction Quality Unit at the Eduardo Torroja Institute of Construction Sciences (CSIC).

In his words, they are “very interesting materials,” although they do not always entail zero energy consumption – even if they consume significantly less than conventional air conditioning systems. When environmental conditions are less than ideal, “they can be thermally activated – heated or cooled – which in our experiment we did using water and air,” he says, referring to the INPHASE project, a collaboration between the Spanish Cement Institute, the University of Lleida and the Eduardo Torroja Institute of Construction Sciences (CSIC).

So far, the most widely used PCMs in experiments have been paraffin-based – and therefore petroleum-derived – making them more polluting. However, that is set to change.

Ruíz’s team is developing a new line of research using organic and biological phase change materials such as coconut or palm oil, “which are affordable for laboratories and more sustainable than petroleum derivatives.” Their plan is to apply them as thin layers on transparent, flexible panels. These oil-filled panels could be installed either inside or outside a building’s walls, depending on the climate: “In hot climates, the phase change material performs its temperature-regulating role more effectively on the exterior wall, and in cold climates, the opposite is true,” she explains.

“Using more sustainable materials is the future. The biggest challenge is ensuring that every component used in the construction process is biodegradable and non-polluting,” she stresses.

The main challenges? According to Ruíz, “the biggest issue is that when these compounds melt and become liquid, leaks can occur. One way to solve this is to inject them into cylindrical nanocapsules made from polymer materials, which are then integrated into the cement, bricks or paints.”

But this is not the only difficulty. The INPHASE project sought to create concrete panels containing 20% paraffin to cover building façades. The experiment was halted when researchers discovered that the material was highly flammable. “We could not move on to production. During fire tests, the panels failed to meet façade fire-safety regulations,” she explains. “The solution could be to encapsulate these flammable materials in metal casings rather than polyethylene capsules,” she suggests.

It is a “perfectly feasible” technology. PCMs have already been successfully tested in hot regions such as Mexico, Algeria and Morocco. What is needed now for wider adoption, Ruíz emphasises, is investment from industry.

“The next step would be to make them economically competitive, or so effective at maintaining comfortable temperatures that they become cost-effective. In other words, if the phase change material saves me money on heating or air conditioning, it will still be worth it, even if it is more expensive at the construction stage,” she concludes.

Sources:

• Hamidi, Y., & col. (2021). Integrating PCM into hollow brick walls: Toward energy savings in Mediterranean buildings. Energy and Buildings.  ScienceDirect
• Ait Laasri, I., Es-sakali, N., Outzourhit, A., & Mghazli, M. O. (2023). Evaluation of phase change materials for a light-weight building in Morocco: Effect of building’s volume, window orientation & infiltration. Energy and Built Environment. (Artículo aceptado / en prensa) ResearchGate
• García-Pérez, D., & col. (2023). Annual evaluation of a modified wall with PCM to reduce thermal loads. Energy and Buildings. ScienceDirect
• Mourid, A., & col. (2018). Experimental investigation on thermal behavior and PCM application in residential rooms. Energy Procedia / Energy and Buildings / Journal correspondiente. ScienceDirect
• Ruiz-Marín, N. (2025). Selection and integration strategies of PCMs in traditional bricks for thermal comfort and energy efficiency: A comprehensive review. Energy & Buildings. ScienceDirect
• García-Pérez, D., & col. (2023). Annual evaluation of a modified wall with PCM to reduce thermal loads. Energy and Buildings. ScienceDirect