For passive building air conditioning: phase change materials
Phase change materials (MCPs) have the ability to store heat before releasing it. In times of overheating, the MCP melts and the heat is stored. When the building cools, the MCP solidifies and the stored energy is returned. This new material therefore gives the possibility of increasing thermal inertia and reducing the need for air conditioning. An ecological and economical solution.
Vincent Pessey d'Alcimed, a consulting and decision support company applied to life sciences and chemistry, explains that “PCMs (Phase Change Materials), placed in partitions, melt and absorb thermal energy excess when the outside temperature exceeds their melting temperature (during the day) and solidify by restoring the energy accumulated when the temperature goes down (at night) ”. Energy is therefore stored as latent heat. MCP can be used in passive air conditioning or even in off-season heating.
The objectives
Due to global warming, the energy consumption linked to air conditioning is only increasing, strongly impacting the environment. Limiting the need for air conditioning has become a real challenge. Feedback shows that temperature peaks in a room equipped with PCM can be reduced by 3 to 4 ° C and the electricity consumption linked to air conditioning can drop by 30%. Being able to store energy also avoids having to use thick layers of heavy materials such as concrete and creating high inertia which can lead to overheating during heavy internal loads (computers in offices).
Daniel Quénard, of CSTB, underlines the interest of the PCM in the renovation of tertiary buildings with light structure, in order to reinforce their thermal inertia and improve their summer comfort. If PCMs are ecological, they are also economic: a study carried out in 2007 by the Lyon Thermal Center assesses a return on investment at 8 years, which is interesting in the current context of energy prices and scarcity fossil fuels, especially since the PCMs have a very long lifespan, identical to that of the building.
Integration with building materials
The phase change takes place depending on the materials between 19 and 27 ° C, temperatures corresponding to the limit values respectively fixed for winter and summer comfort. MCPs (paraffin, polymer, fatty acid, etc.) can be incorporated into construction materials (plaster, concrete, certain plastics) and even into a graphite matrix, which has the advantage of having excellent thermal conductivity. Paraffins, chemically stable, are the most common substances.
They can be packaged in microscopic plastic beads (microencapsulation) mixed with plaster or concrete, or else incorporated into the pores of a carrier material (impregnation), which allows them to be added during a renovation. 3 cm of plaster containing 30% of MCP is equivalent to 18 cm of concrete or 23 cm of brick in terms of thermal inertia.
MCPs are indestructible, inert and non-toxic; they do not require maintenance. In addition to the constituent walls of buildings, MCPs can also be integrated into false ceilings. CSTB specifies that a night ventilation system optimizes the release of calories, which makes it possible to regenerate MCP more efficiently.
Research and first experiences
Products are starting to be marketed (around € 50 / m2), such as Micronal® (paraffin in polymer microcapsules) from BASF or Energain® (paraffin / polyethylene composite) from Dupont de Nemours: references exist abroad and in situ experimental monitoring takes place in France. Many other types of materials and forms are also being studied. Another type of MCP with a solid / solid transition is being researched, because moving from one solid phase to another allows more heat to be stored than the solid / liquid phases; the fact that they are permanently solid is interesting for construction, because they are more easily packaged and usable as they are.
CSTB considers that specific points still need to be improved and specific research: better definition of the melting / solidification range, fire behavior, characterization of any secondary products emitted (potential harmfulness), conditioning and durability to cycles merger / solidification. Research is also underway on the coupling of phase change and super-insulating material (VIP) to produce lightweight envelopes with both good insulation and significant thermal inertia. MCP has no doubt a promising future to reduce air conditioning consumption while preserving a good level of comfort for buildings with low inertia.
By Pascale Maes, Journalist
Source: http://www.techniques-ingenieur.fr/actu ... cle_73628/
