"PCM" radiant surfaces (Phase Change Materials)

They are based on the use of so-called "PCM" materials: Phase Change Materials. These are particular materials, such as paraffin (main component in the production of candles), or hydrated salts or propylene glycol (contained in the "tablets" sold in pharmacies for the treatment of localized trauma, replacing the traditional ice pack or hot water bag), all materials that have a great thermal capacity.

PCM materials have the characteristic of preserving the solid state as long as they remain at room temperature. But exceeding a certain limit value, which generally corresponds to a temperature of 25°C, these materials pass into the liquid state storing a large amount of heat. On the contrary, when the temperature drops below 25°C, they solidify releasing the previously accumulated thermal energy. Their peculiarity is precisely this phase transition (solid - liquid - solid) with an associated "sponge effect", which is an accumulation of heat at higher temperatures and a release of heat at lower temperatures. 

The use of PCM radiant surfaces inside buildings allows to obtain, as the main result of their sponge effect, a significant attenuation and a phase shift of the thermal load peaks: the excess heat, which develops inside the rooms during the hottest hours of the day, it is stored by these surfaces to be released at night, when the temperature drops and when it is possible to open the windows to keep the air temperature at a comfort value.

In this way it is possible to reduce the daily fluctuations of the ambient temperature and consequently reduce the requirements for air conditioning, especially in cooling. Analyzing the diagram above, comparing the red curve (daily fluctuation of the ambient temperature in the absence of PCM surfaces) and the green curve (fluctuation in the presence of PCM surfaces), it is clear that the use of PCM radiant surfaces allows to attenuate the variations of temperature and undersize the current air conditioning system.

In construction, PCM materials and surfaces already find various applications:

1) MICROSPHERES

The containment solutions most used for phase change substances are micro and macrosphere (2-50 μm: a micrometer corresponds to the thousandth part of a millimeter). The uses of these spheres are varied, including adding to the cement mixture for the creation of innovative plasters. Another solution is to aggregate the PCM capsules to the construction mix of the plasterboard or wood fiber panel for wall and ceiling applications.

This same technique can be used in the production of prefabricated modules for the construction of raised floors (to be applied, for example, in the portions of the floor adjacent to large glass surfaces, in order to absorb a greater amount of energy due to solar radiation).

2) TABLETS

Another containment system is that of the "tablets": sealed bags of reduced thickness generally filled with propylene glycol. These tablets can be used as a surface coating not visible on the panels to create a false ceiling or false wall.

An exemplary application is that adopted in a building on the university campus of Seattle (USA), where it was possible to demonstrate that the effect of the thermal accumulation of the structures made in this way is absolutely surprising: 1,25 cm of material thickness PCM is equivalent to the thermal mass of 25 cm of concrete!

It is quite evident that PCM radiant surfaces represent an optimal solution in the design of passive buildings or light buildings (wooden constructions), where it is still desired to guarantee a phase shift of the heat wave. Although having put in place structures with low mass, such as a wooden structure roof (problem of overheating of the attic), it is possible to associate to the normal thermal insulation layer an additional layer of PCM material, of reduced thickness, capable of "simulate”a large thermal mass to guarantee a correct phase shift of the thermal peaks.

3) CLOSED LOOP RINGS

The worst defect of PCM materials is extreme flammability (paraffin in particular) and therefore special attention must be paid in the fire prevention project (for example by reducing the quantities used). From this point of view, the safest application consists in the creation of closed loop rings (possibly made of metal), filled with PCM substances and subsequently plastered or incorporated into the vertical and horizontal structures.

4) INTEGRATION INTO TRANSPARENT SURFACES

A) Liquid state (high temperatures)

B) Intermediate phase change state

C) Solid state (low temperatures)

A last very interesting application regards the integration of phase change materials in transparent surfaces: both glazed ones (exploiting the interspace between the various layers) and plexiglass ones (aggregating the microspheres with polycarbonate). The result is to obtain dynamic behavior windows: transparent in the hottest hours (liquid PCM), with the advantage of absorbing most of the energy that tries to pass through them and opaque in the colder hours (solidification of the PCM), with the advantage of filtering light according to external climatic conditions.

Advantages of PCM radiant surfaces

Whatever the type of application, PCM radiant surfaces are certainly destined for greater success. Faced with the general interest in energy saving and the race to reduce polluting emissions into the atmosphere, their use in buildings represents a more than winning solution thanks to the multiple advantages:

• Attenuation of temperature fluctuations in the rooms.
• Thermal wave phase shift.
• Obtaining a large thermal mass with reduced thickness.
• Downsizing of the air conditioning system (PCM systems do not replace but integrate the actual air conditioning system!).
• Consequent energy saving and reduction of polluting emissions into the atmosphere.

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