Due to global warming, the demand for energy for cooling buildings is dramatically increasing. Current mechanical refrigeration systems for cooling consume substantial amounts of electrical energy, leading to carbon emissions.

Approximately 12% of all electricity consumed worldwide is used for HVAC systems construction. Additionally, buildings are responsible for nearly 25% of CO2 emissions.

Therefore, the development of thin coatings that can be applied like paints, which reflect solar heat away from buildings and further cool them by emitting infrared radiation into space, can be a game-changer in the attempt to achieve net zero carbon emissions.

This is precisely why researchers are developing new technologies, such as special paints, as revealed in the review titled "Inventions, innovations, and new technologies: Paints and coatings for passive cooling," authored by Samantha Wijewardane from the University of South Florida College of Engineering.

Radiative cooling

Solar-reflective coatings, also known as "cool roofs," have been commercially available for several decades. However, their share is only 3% of the total paint and coating industry.

Recent developments in thin films exploit the infrared transparency window (the sky window) of the atmosphere, in the wavelength range of 8–13 μm, to directly transmit heat from an object through radiation to the cold outer space, which has a temperature of -270°C. This development of radiative cooling or sub-ambient radiative cooling is nearing commercialization.

Unlike most other solar energy-related technologies, thin films that can be applied as paints are practically a substitute for a product (paints and coatings) that has been used for generations. Therefore, awareness, knowledge, and perceptions are important when it comes to marketing and widespread use.

A key factor in paints is aesthetics, which should be considered as important as cost and functionality. Recent studies have shown the importance of aesthetic appearance, and in recent years, a number of researchers have cited it as important. Also, another non-technical factor is consumer awareness, which greatly affects widespread use. Below are some innovations in special paints that can be successfully used in facade technology.

Solar-reflective coatings

Solar-reflective coatings have been on the market for over four decades as roof coatings, and later, they have also attracted interest as wall coatings, to reflect direct and diffuse sunlight. These coatings are predominantly white, which includes glare effects that are harsh to the naked eye.

Also, due to weathering and dust, in a few months, these coatings become discolored, diminishing their effectiveness. To address the aesthetic issue, "cool-colored" coatings have been investigated and developed to meet the preferences of the construction market.

These coatings selectively absorb in the visible range of the solar spectrum, thus providing a certain color, and optimally reflect in the near-infrared spectral range, avoiding solar heating.

Conventionally, hollow spheres made of glass/silica/ceramics, which are transparent to the visible spectrum but efficiently reflect the infrared part of solar radiation, have been used with a color pigment in these coatings. However, these coatings absorb a large amount of the visible spectrum and, therefore, are not as effective as white coatings, but are more attractive than white paints.

Fluorescent pigments

One of the key innovations in solar-reflective coatings is the introduction of fluorescent pigment. Fluorescence, more precisely, the ability of certain compounds to absorb at a higher frequency and emit at a lower frequency, is well-known but has been recently introduced into cool-colored coatings as fluorescent pigments, to reduce absorption in the visible region.

One such type of pigment containing synthetic ruby crystals absorbs some of the energy from visible sunlight (400–700 nm) and spontaneously re-emits it in the near-infrared region (700–2500 nm). Since the emission is in the infrared region, it does not affect the color of the coating.

The most commercially available white paint, TiO2-acrylic paints, absorb UV light below the wavelength of 387 nm, due to the TiO2 band gap. As a result, the overall solar reflectance of these coatings is limited. To overcome this, a fluorescent pigment that absorbs below the wavelength of 400 nm and emits in the visible range is efficiently used.

Two-layer coating system

Apart from the introduction of fluorescent white pigment, another notable innovation in solar-reflective coatings is the development of a two-layer coating system where an upper layer absorbs suitable visible wavelengths to display specific colors, while the lower layer maximizes infrared reflection to reduce solar heating.

This two-layer system has worked better than single-layer colored paints with cooling effects. The reduced efficiency over a shorter time, compared to the lifespan of a paint layer due to environmental aging, mainly natural dirt and UV irradiation, is another critical factor affecting widespread use.

A superamphiphobic self-cleaning coating would be a final and preferred solution. However, none of the coatings are commercialized for larger applications, such as roof coatings. Therefore, there is room for innovation to solve the performance reduction due to environmental aging.

Such a recent strategy called anti-aging cooling paint has formed a hierarchical morphology, with the ability to self-clean. Inventors have demonstrated durability through accelerated weathering tests and under natural dirt and sun. They have also claimed that their product is scalable and efficient in real-world applications.

Radiative cooling coatings

Radiative cooling is a passive cooling technology that acts by reflecting solar light and emitting radiation into the sky window. As cooling demand generally occurs during the daytime in summer, daytime radiative cooling has recently attracted much scientific and technological interest as it can reduce the need for mechanical cooling.

One of the most common and effective methods of achieving spectral selectivity is by using a two-layer system, where a lower metal layer efficiently reflects solar radiation and an upper layer absorbs in the atmospheric window. A double-polymer-metal layer, a double-silica-metal layer, and a double silicon-metal nanocomposite layer are examples of such structures.

However, to deposit a uniform lower metal layer, deposition techniques that are not cost-effective for larger surface applications or are not industrially scalable have been used. Also, porous structures have been explored to achieve daytime cooling. In one study, a porous polymer similar

Cooling power

In most experiments, the cooling power achieved is about 50 W/m2. Ordinary white paint, TiO2 - acrylic paint, absorbs UV radiation, and the reflectance of acrylic resin in the near infrared of 0.7 to 2.5 μm is not perfect. Therefore, other commercially available pigments such as CaCO3, BaSO4 and SiO2 and other types of resins were also investigated.

However, these pigments have a lower reflection index in the solar frequency range and therefore the reflectance is not as high as TiO2. To achieve high reflectance, a higher concentration of pigments (CaCO3) up to 60% was successfully used in a recent trial. CaCO3 is a compatible and stable material used in paints as a spacer.

These pigments have been experimentally shown to have high solar reflectance, high normal emissivity in the sky window, and sub-ambient daytime radiative cooling in single-layer particle matrix paints. Also, it was shown that a higher concentration of CaCO3 reduces the volume fraction of the acrylic resin and therefore minimizes its absorption in the NIR band.

Additionally, a broad particle size distribution, instead of a single size, is used to efficiently scatter all wavelengths in the solar spectrum. The high emissivity of the sky window is achieved through the vibrational resonance peaks of the acrylic matrix.

The paint was made using a commercially compatible process. The 400 μm thickness makes the paint properties independent of the substrate and the paint can be applied using a conventional method.

Acrylic paints with nanoparticles

Also, another research found sub-ambient cooling performance with acrylic paints with BaSO4 nanoparticles. BaSO4 particles scatter solar radiation, as well as their phonon resonance at 9 𝜇m, provided a high sky window emissivity. Again, the higher concentration of BaSO4 was used to improve the scattering power and minimize the near-infrared absorption of the acrylic resin.

With an appropriate particle size and a wide particle size distribution, the BaSO4 nanoparticle film achieves an ultra-high solar reflectance of 97.6% and a high sky window emissivity of 0.96.

As an alternative to the acrylic resin material, to achieve high emissivity of the sky window, silicon particles can be used to improve the selective emissivity. By randomly distributing the silicon microspheres in the polymer matrix, the emissivity over the entire atmospheric window can be significantly enhanced due to the phonon-enhanced Frohlich resonances of the silicon microspheres.

In another notable attempt to achieve sub-ambient cooling, solar reflectance was achieved by a combination of commercially available pigments, such as hollow glass microspheres, barium sulfate nanoparticles, and calcium carbonate nanoparticles, in while silicon nanoparticles were used to achieve the sky window. spectral selectivity.

They achieved a solar reflectance of 0.986 and an emissivity of 0.954 in the atmospheric transparency window. In this study, an industrially scalable fabrication process was used, and the coatings were applied by spraying, rolling, or brushing methods, and a coating thickness of approximately 500 μm was achieved. The same research group developed a super-amphiphobic self-cleaning layer that could be effectively applied over a radiative cooling (RC) layer.

These self-cleaning water-alcohol superamphiphobic coatings have been introduced into RC coatings as a top layer. This transparent super-amphiphobic layer did not affect their initially high solar reflectance, but improved their emissivity values ​​in the atmospheric window due to the formation of silicon-oxygen bonds, which absorb infrared radiation.

Infrared frequency

Although a sky window emissivity coverage is widely believed to be better than broadband infrared emissivity (2.5 to 24 𝜇m), this is not always true in real environments. The sky temperature is always below the ambient temperature during summer and also the emissivity of the sky is not always unity.

Therefore, there is an advantage to using broadband infrared emission over sky window emission. This is practically proven recently by a modified photonic radiative cooler with identical solar reflectance and unity broadband emissivity above 4.5 𝜇m instead of just in the atmospheric transparency window.

Emission through the atmospheric window is very important in reducing global warming. Other researchers also found that the band emission wide infrared has been used effectively to improve daytime cooling. They used a common coating material, TiO2 rutile powder, to achieve solar reflectance and glass microspheres to achieve broadband IR emission.

To overcome UV absorption by TiO2 particles, they used fluorescent microparticles to absorb below 450 nm and emit in visible frequencies.

In another project using a completely different approach, a geopolymer coating was developed to achieve radiative cooling. The inorganic phosphoric acid (PGEO) geopolymer coating possesses a high average hemispheric infrared emissivity of 0.95 and reflects nearly 90% of solar radiation. The researchers attributed the higher spectral selectivity of the PGEO coating to its unique inorganic geopolymer network (-Si-O-Al-O-P-O-), which enabled multimode vibration.

They also claim that this suspension paint can be applied directly to various surfaces through scalable techniques such as spray coating and brushing.

Penalty in winter

The above two types of cold coverings, either reflective cooling or radiative cooling, can overcool buildings beyond the thermal comfort limit of the occupant, thus resulting in a heating penalty in winter, although the heating gain is much less than the corresponding cooling . energy savings in summer.

In recent years, thermochromic coatings have gained much attention due to their unique ability to modify optical properties.

These innovative coatings can adapt their color to the surrounding temperature, showing light colors in summer and dark colors in winter, which allows these coatings to reflect solar energy in summer and absorb solar energy in winter. Thermochromic pigments were developed by mixing different types of organic compounds and were incorporated into the common white layer.

The cost of these pigments, and the cost of thermochromic materials in general, are currently high and still may not be economical for incorporation into large-scale applications such as roofing paints.

Test standards

Experimental studies on radiative cooling have been conducted under different environmental conditions and methodologies, making it almost impossible to compare results presented as difference in temperature or cooling power.

The sub-ambient temperature difference is site and climate specific and the cooling power depends on the temperature of the emitting surface. Therefore, there is a need for a standard method of measuring the cooling potential performance that defines a standard thermal environment for the advancement of passive radiative cooling technology.

Currently, surface reflectance and emissivity values ​​are the only parameters that could be compared between published results, and therefore in this review we stick to those values ​​only. (Photo: Dreamstime)