Daytime radiative cooling technology implementation for residential buildings

Daytime overheating of building roofs is a vital issue in the Indian summer. As a solution, we have developed the daytime passive (no need of electricity) radiative cool coating technology, which has been field-tested for building roof applications. In this article, Dr Jahar Sarkar from the Indian Institute of Technology at Banaras Hindu University (BHU) shares his findings about tested performance and related issues in this article.

Cooling is the main requirement for a wide range of building applications. Conventional cooling – mainly compressor-based air-conditioning systems- evaporative cooling and vapour-absorption cooling systems considered in many cases – is used in buildings, which involves the utilisation of energy, water or refrigerant, which has many negative impacts. Moreover, the conventional cooling system rejects heat to the ambient river or ocean, which leads to local or global warming (local warming or heat island effect created by the summer air-conditioner is an issue for particularly dense populated cities). Hence, we need passive cooling devices with no such local warming issue. In this direction, eco-friendliness and the absence of energy or water make passive radiative cooling the ultimate solution.

Overview

Ancient applications of radiative cooling include natural ice production and cooling buildings through radiative heat rejection. Daytime radiative cooling, proposed in 1967, involves emitting radiation to space through the atmospheric window (8-13 µm) while reflecting solar and atmospheric radiation. This technology gained traction after a successful 2014 demonstration in the USA. Effective cooling depends on minimal solar absorption and atmospheric radiation, with maximum sky emission and minimal non-radiative transfer. Factors like temperature, humidity, and wind impact performance. Daytime radiative cooling is ideal for reducing excessive roof heat in summer, offering passive cooling below ambient temperatures without water loss. We developed and tested radiative cooling coatings for building roofs, with field results discussed here.

Daytime radiative coating development

A three-layered polymer-particle composite-based coating structure has been developed for daytime radiative cooling. The initial selection of materials is based on the refractive index mismatch between polymer and particles in the solar spectrum for lossless materials. Polymer PMMA is lossless in the solar spectrum with a refractive index 1.45. Also, the particle BaSO4 is lossless in the solar spectrum with a refractive index of 1.65. We have purchased polymer from OttoKemi and particle from Sigma Aldrich. We first dissolved PMMA in acetone (solvent) and mixed BaSO4 powder. After that, we sonicated for half an hour and then stirred in a magnetic stirrer for half an hour to prepare a precursor solution for the coating. The polymer to BaSO4 powder ratio is optimised and taken as 70:30 percent by volume.

For coating preparation, the thin adhesive polymer (primer) layers are coated first on the substrate. The precursor solution is then spray-coated on the thin adhesive layer. The coatings can be dried at room temperature with acetone as a solvent. The direct spray coating is fast and yields a smooth surface of the radiative cooler without any cracks. Hence, it is a better option for radiative coating. The coating thickness has been optimised as 0.4-0.5 mm. The coating structure and sample are shown in Figure 1. The emissivity characteristics of the developed radiative coating have been done using an FTIR spectrometer with an integrating sphere for the infrared range and a UV-Vis spectrometer with an integrating sphere for the UV and Visible spectrum. The spectral characterisation of the developed radiative coating is depicted in Figure 2. An average reflectivity of 96 percent in the solar spectrum and emissivity of 95 percent in the atmospheric window has been found.

Figure 1. Radiative coating structure and developed sample for testing

Figure 2. Reflectivity (Emissivity=1-Reflectivity) spectrum of developed coating

The degradation test against environmental hazards has also been performed to check the long-term sustainability of the developed radiative coating. After two months of exposure to the sun, rain, and dust, a minor change in the emissivity profile in the solar spectrum and the infrared spectrum was observed. As a change of emissivity in the solar spectrum is undesirable, proper maintenance is recommended.

Field testing for building roof coaling

Furthermore, the developed radiative coating has been applied in daytime building roof cooling. We have selected two identical rooms in the local area of Varanasi. We have coated the roof of one room and kept another room uncoated to check the effectiveness of our developed radiative coating, shown in Figure 3. We have measured the daily hourly variation of bulk temperatures of both rooms and outside ambient temperature on different days in June 2023. The variations of indoor and outdoor temperature on 24.06.2023 are depicted in Figure 4. The figure clearly shows that the bulk temperature of the room can be reduced by 2-3oC by using a developed radiative coating on the roof. The bulk temperature of the coated room was observed 5 to 10oC lower than the ambient temperature. The present field test reveals the developed radiative cool coating is very effective in the Varanasi climate. It has significant energy-saving potential for an air-conditioning room. After the long-term use of the radiative coating on the roof, the bacterial effect has been found on the surface in case of rainfall. Mixing of some anti-bacterial agents is recommended to prevent this.

It is observed that the passive radiative coating can be used for building in different ways: (i) it can be directly coated on a sky-facing building envelope, (ii) it can be used as an air cooler, and the cooled air can be directly used for indoor cooling, (iii) it can be integrated with compressor-based air-conditioning (condenser cooling or subcooling purpose) and (iv) evaporative cooling (pre-cooling) system. It will help to cool the building by rejecting heat through radiation and absorbing minimum heat from solar radiation. Hence, it acts as an insulator that will ultimately reduce the load on a conventional air-conditioner. So, it has a significant energy-saving potential for air-conditioning buildings. The energy-saving potential may be achieved in the 35-45 percent range by using a passive radiative cooler envelope in buildings.

Conclusion

The developed daytime radiative cooling coating has been field-tested for residential roof cooling in the present study. The bulk temperature of the room can be reduced by 2-3oC by using a developed radiative coating on the roof and 5-10oC lower than the ambient temperature. Hence, its energy-saving potential for building and mixing anti-bacterial agents is recommended. Many barriers to market adoption and commercialisation may exist related to technical issues, economic issues, retrofit issues, familiarity, customer acceptance levels, and climate constraints. However, these can be overcome with the proper selection of radiative cooling material and design, better compatibility with the building structure, and proper coordination with the building designer.