Infrared-Transparent Visible-Opaque Fabrics for Wearable Personal Thermal Management (1507.04269v1)

Abstract: Personal cooling technologies locally control the temperature of an individual rather than a large space, thus providing personal thermal comfort while supplementing cooling loads in thermally regulated environments. This can lead to significant energy and cost savings. In this study, a new approach to personal cooling was developed using an infrared-transparent visible-opaque fabric (ITVOF), which provides passive cooling via the transmission of thermal radiation emitted by the human body directly to the environment. Here, we present a conceptual framework to thermally and optically design an ITVOF. Using a heat transfer model, the fabric was found to require a minimum infrared (IR) transmittance of 0.644 and a maximum IR reflectance of 0.2 to ensure thermal comfort at ambient temperatures as high as 26.1oC (79oF). To meet these requirements, an ITVOF design was developed using synthetic polymer fibers with an intrinsically low IR absorptance. These fibers were then structured to minimize IR reflection via weak Rayleigh scattering while maintaining visible opaqueness via strong Mie scattering. For a fabric composed of parallel-aligned polyethylene fibers, numerical finite element simulations predict 1 {\mu}m diameter fibers bundled into 30 {\mu}m yarns can achieve a total hemispherical IR transmittance of 0.972, which is nearly perfectly transparent to mid- and far-IR radiation. The visible wavelength properties of the ITVOF are comparable to conventional textiles ensuring opaqueness to the human eye. By providing personal cooling in a form amenable to everyday use, ITVOF-based clothing offers a simple, low-cost solution to reduce energy consumption in HVAC systems.

• The paper introduces ITVOFs that enable personal cooling by achieving a minimum infrared transmittance of 0.644 for thermal comfort.
• It employs a rigorous heat transfer model and photonic design using polyethylene fibers to optimize IR transparency while maintaining visible opaqueness.
• Experimental comparisons indicate these fabrics can reduce HVAC loads by enhancing indoor energy efficiency through passive cooling.

Analysis of Infrared-Transparent Visible-Opaque Fabrics for Personal Thermal Management

The paper "Infrared-Transparent Visible-Opaque Fabrics for Wearable Personal Thermal Management" provides an in-depth exploration of an innovative strategy to augment personal thermal regulation. The primary focus is on the development of fabrics that effectively balance infrared transparency with visible opaqueness, thereby facilitating personal cooling through radiation without compromising privacy or aesthetic quality.

Key Contributions

The researchers have innovated the concept of infrared-transparent visible-opaque fabrics (ITVOFs) to leverage passive cooling mechanisms. The study employs a rigorous heat transfer model to derive specific infrared optical properties necessary for such fabrics to deliver thermal comfort. The researchers ascertain that a minimum infrared transmittance of 0.644 and a maximum reflectance of 0.2 are requisite for comfort at elevated ambient temperatures around 26.1°C (79°F).

Material and Structural Considerations

To achieve the desired infrared optical properties, the study introduces a design using synthetic polymer fibers, namely polyethylene, known for low intrinsic infrared absorptance due to its simple molecular structure which supports fewer vibrational modes. Polyethylene fibers, arranged into a fabric structure, display optimal interaction with light. The fabric design manipulates fiber dimensions to strategically employ weak Rayleigh scattering for infrared wavelengths and strong Mie scattering for visible wavelengths. Specifically, the simulations predict that fibers with a diameter of 1 μm bundled into 30 μm diameter yarns can achieve a hemispherical infrared transmittance of 0.972, presenting near-perfect transparency in the mid- and far-infrared spectra.

Experimental Methodology

The research integrates sophisticated modeling with empirical characterization to benchmark current fabric materials. Utilizing both UV/visible and FTIR spectrometry, the optical properties of common textiles like cotton and polyester are analyzed, indicating their inherent opacity in the infrared spectrum despite high visible light transmittance. This opposes the requirements established for ITVOF, reinforcing the necessity for alternative polymer usage and structural photonic enhancements as proposed by the authors.

Practical and Theoretical Implications

The engineering of ITVOF has substantial implications for both energy efficiency and thermal comfort in controlled environments. The potential applications extend to indoor settings where energy savings could be realized by reducing HVAC loads without sacrificing user comfort. The research highlights a practical, straightforward solution for improving energy efficiency in buildings, particularly during warmer seasons, by elevating allowable indoor temperature thresholds without loss of comfort.

Future Directions

The paper invites future research to rigorously evaluate real-world performance of ITVOF designs under various environmental conditions. The challenge remains to develop a scalable manufacturing process for producing these fabrics at reasonable costs, maintaining durability and mechanical comfort. Further experimental verification using human trials and thermal manikins can provide insight into the efficacy of these fabrics for widespread adoption. Additionally, exploring mixed-material fiber compositions may afford enhancements in comfort and breathability, crucial factors for daily wearability.

Conclusion

This research establishes a solid groundwork for utilizing structural photonic techniques to crafting fabrics that harmonize thermal management with practical and aesthetic functionalities. By effectively engaging with both the scientific principles and engineering practices, the paper contributes a novel perspective on personal cooling technologies, advocating for their integration in everyday clothing to potentially transform the way energy efficiency and personal comfort are approached.