Full Daytime Sub-ambient Radiative Cooling with High Figure of Merit in Commercial-like Paints (2008.03372v1)

Abstract: Radiative cooling is a passive cooling technology by reflecting sunlight and emitting radiation in the atmospheric sky window. Although highly desired, full daytime sub-ambient radiative cooling in commercial-like single-layer particle-matrix paints is yet to be achieved. In this work, we have demonstrated full daytime sub-ambient radiative cooling in CaCO3-acrylic paint by adopting large bandgap fillers, a high particle concentration and a broad size distribution. Our paint shows the highest solar reflectance of 95.5% among paints and a high sky-window emissivity of 0.94. Field tests show cooling power exceeding 37 W/m2 and lower surface temperature more than 1.7C below ambient at noon. A figure of merit RC is proposed to compare the cooling performance under different weather conditions. The RC of our cooling paint is 0.62, among the best radiative cooling performance while offering unprecedented benefits of the convenient paint form, low cost, and the compatibility with commercial paint fabrication process.

• The paper demonstrates that a CaCO₃-acrylic paint can achieve sub-ambient cooling with surfaces reaching 1.7°C below ambient and an RC figure of merit of 0.62.
• It employs innovative material selection with high particle concentration and varied particle sizes to achieve 95.5% solar reflectance and 0.94 sky-window emissivity.
• The study highlights a cost-effective passive cooling solution with significant energy-saving potential for urban and commercial applications.

Full Daytime Sub-ambient Radiative Cooling with High Figure of Merit in Commercial-like Paints

The paper described in the paper investigates the potential of a novel passive radiative cooling technology using a CaCO$_3$-acrylic paint that achieves sub-ambient temperature during full daylight. The main goal is the realization of an efficient, practical, and commercially viable solution for passive cooling applications, which has significant implications for reducing energy consumption in various buildings.

Core Findings

Key results demonstrate that the CaCO$_3$-acrylic paint formulated in this research exhibits a remarkable solar reflectance of 95.5% and a substantial sky-window emissivity of 0.94. When tested in field conditions, the paint performed impressively, achieving a cooling power of over 37 W/m$^2$ and maintaining a surface temperature that is 1.7˚C below ambient at midday. Notably, the proposed figure of merit, denoted as RC, reached 0.62, a benchmark that situates this paint amongst the top-performing radiative cooling materials explored in contemporary studies. Importantly, the paper advances the body of knowledge by showing that single-layer particle-matrix paints, often considered cost-prohibitive or inefficient, can indeed deliver sub-ambient cooling.

Methodological Innovation

The paint's effectiveness is attributed to a few central innovations:

1. Material Selection: The use of CaCO$_3$ with its larger bandgap (> 5 eV) reduces UV absorption better than traditional TiO$_2$ and other materials.
2. Particle Concentration: A high particle volume concentration of 60% is employed to leverage the crowding effect beyond the critical particle volume, enhancing solar reflection while maintaining the structural integrity of the film.
3. Particle Size Distribution: A broad distribution of particle sizes optimizes the scattering across the solar spectrum, thus increasing the overall solar reflectance efficacy.

Practical and Theoretical Implications

Practically, the implementation of this paint can lead to meaningful energy savings by reducing reliance on active cooling systems such as air conditioning, which represents a significant energy consumer in infrastructure worldwide. This is vital in the context of urban heat islands and combating global warming.

Theoretically, the work opens new avenues for optimizing particle-matrix composites in thermal applications by providing evidence for the advantages of high-concentration and size-distributed particle systems. This could inspire further research into the fine-tuning of filler material properties to maximize performance.

Future Directions

Though the incorporation of high-bandgap dielectric particles has proven beneficial, future research directions could include exploring other composite materials and fabrication techniques to further lower film thickness while maintaining or enhancing cooling efficiency. Additionally, understanding the long-term durability and weathering effects on these paints will be crucial for large-scale adoption.

The work sets a precedent for evaluating passive cooling systems not just based on empirical tests but also on methodologies like the RC figure of merit that allow for standardized performance assessments irrespective of geographic and climatic variances.

Conclusion

This research significantly advances the field of radiative cooling technologies, showing promise for integration into existing commercial paint infrastructures while retaining cost-effectiveness and high performance. The implications both in terms of energy savings and environmental impact are considerable, characterizing the CaCO$_3$-acrylic paint as a noteworthy development in energy-efficient materials technology. Future explorations should focus on refining this approach, probing various other nanoparticle and matrix combinations, and further scaling the applications to enhance the energy-saving potential globally.