Air Conditioning (AC) represents up to 70% of energy consumption in hot climatic zones [1]. In addition, in 2040 more than 65% of the global building stock will still exist [2]. It appears now urgent to deploy affordable and easy to implement solutions to enhance energy efficiency of buildings. Among existing solutions, cool roof coatings are a simple way to reduce energy consumption by limiting use of air conditioning thus improving thermal comfort of user while mitigating CO2 emissions.
The principle of radiative cooling
For many years, standard white paints have been used to cover buildings with the aim to reflect sunlight, especially in hot climatic zones. This phenomenon has been known for many years, as evidenced by the white facades of Mediterranean houses, for example. More recently, New York city has painted more than 9.2 million square feet of rooftop in white.
This cooling effect relies on a physical principle: radiative cooling. The use of radiative cooling technology is a great opportunity to reduce space cooling costs, thus alleviating the global warming. Unlike active cooling, which requires electricity to cool down a material, radiative cooling emits thermal radiation directly in the atmosphere without any electricity power demand [3]. Recently, researchers have been investigating several pathways to exploit radiative cooling. Among them, dual layers including a metal layer, polymers and silica nanocomposites showed good performances. Although efficient, these structures are either too complex, or requiring high thicknesses. These barriers prevent their application in multiple sectors. Today, there is a need for thin, easy to apply, low cost and scalable solution, that ultra-white paint seems to bring.
How does a reflective paint work?
Cool roof paints help reduce the heat up of buildings thanks to their solar reflecting properties. To understand how reflective paint works, one should first focus on the composition of sunlight: 6.6% of UV (Ultra Violet), 44.7% of visible and 48.7% of NIR (Near Infra-Red) radiations.
While visible light is the one helping us distinguish colors, NIR radiations are responsible for the transmission of heat, and UV rays have a mutagenic effect and are responsible of ageing effects.
Thus, it appears that a reflective paint formula should be designed to contain:
Thermo-reflecting components that will block NIR [4] in order to limit storage of heat,
Anti-UV materials to withstand ageing.
Standard white paints often contain TiO2 particles in an acrylic matrix. However, this type of composition fails to show relevant passive full daytime sub-ambient cooling for many reasons [3]:
TiO2 has a high absorption in UV band,
Acrylic has a high absorption in NIR band.
In 2021, researchers from Purdue University have recently shown that using BaSO4 (Barium Sulfate) as filler instead of TiO2 enhances the overall reflectance of the paint, due to its high electron band gap for low solar absorbance [3]. They also showed that using a broad particle size range further enhanced radiative cooling effect. This is likely to be due to air voids playing an insulator role. While standard paints reflect around 90% of solar rays, the developed material reached 98,1% [5]. The paint thus developed must still prove its resistance against ageing.
Commercial solutions already exist
Due to dramatic acceleration of global warming, the market of reflective paints is expected to experience a strong increase in the upcoming years. Several companies have already commercialized their products. Among them, the French company ENERCOOL, created in 2020, has launched its paint called Sunprotect, enabling to reduce inside temperature by 6°C and showing a great resistance against ageing [6]. The company Solar-Paint also commercializes a reflective paint, Solar-Coat, with similar properties.
Limitations and other cool roofing techniques
Painting a roof with reflective material can have drawbacks. In 2014, researchers showed that this solution should be adapted to climatic zones [7]. While cool roofs displayed high efficiency during summer, in winter this technology further cools environment, requiring additional heating, which cancels the savings made in summer. In parallel, they observed in some cases a reduction of rainfall, having negative consequences for water availability and ecosystems. Finally, if the reflective surface is close to another building, the reflected heat can also increase the temperature of the surroundings.
An alternative to this technology is the Green Roofing (covering roofs completely or partially with vegetation over a waterproofing membrane) [8]. Although more complex and costly to set up, Green Roofs bring additional advantages to capture CO2, thus improving air quality and drainage systems and supporting wildlife ecosystems.
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References
Georgescu, Matei; Morefield, Philip E.; Bierwagen, Britta G.; Weaver, Christopher P. (5 February 2014). "Urban adaptation can roll back warming of emerging megapolitan regions". Proceedings of the National Academy of Sciences. 111 (8): 2909–2914.