Speaker
Description
Passive thermal control is essential for both terrestrial and space applications. In terrestrial applications, these coatings can reduce energy consumption for heating and cooling in buildings and automobiles. In space applications, they can drastically reduce the thermal load on the active cooling system. Thermal coatings with wavelength-dependent radiative characteristics, like reflectance and emittance, are desirable in both applications. In this study, we demonstrate the use of polymer nanofibers for passive temperature control for terrestrial and space applications. Specifically, we explore the use of polytetrafluoroethylene (PTFE) for terrestrial and space applications.
This study briefly describes the electrospinning fabrication process to create nanofibers and how specific process parameters can be varied to control the fiber geometry. To understand the role of material and fiber geometry, we measure the spectral reflectance, absorptance, and transmittance of various samples using spectrophotometers interfaced with integrating spheres. We also test if the samples show a change in radiative properties after exposure to ultraviolet light. We conducted calorimetric tests to determine the performance of various samples when exposed directly to sunlight.
We utilize multilayered structures with dynamically switchable optical properties for terrestrial applications to modulate their interaction with sunlight. Specifically, we test porous PTFE layers integrated with a spectrally selective (SS) absorber. This multilayered structure can provide tunable optical properties by wetting or dewetting the porous PTFE with a refractive index-matching liquid, allowing a highly reversible change in solar transmittance of 0.62. This variation allows the multilayered structure to switch between highly reflecting and absorbing states, which is tunable using different PTFE thicknesses. With this multilayered structure as the building exteriors, sunlight can be reflected or absorbed to reduce dependence on conventional heating and cooling systems driven by non-renewable energy sources. When exposed to 1 sun illumination under ideal conditions, this variation allows a 51℃ change in PTFE-SS steady temperatures. When applied to buildings as roofing materials, the PTFE-SS promises significant energy reduction with annual cooling and heating savings of around 77% and 27%, respectively.
For space applications, we combine polytetrafluoroethylene (PTFE) and polyethylene oxide (PEO) polymers to fabricate highly reflective coatings by electrospinning. By modifying the solution properties and coating thickness, we can control the solar reflectance of the coatings. Using different PTFE:PEO mixing ratios, nanofibers of different diameters, ranging from 1054±185 nm to 516±89 nm, were created on an aluminum substrate. As the coating thickness is increased from 0.3 to 1 mm, the average solar reflectance increases from 0.95 to 0.99. On the other hand, thermal emittance at room temperature was measured to be 0.7. Calorimetric testing of these samples was carried out under low vacuum conditions by exposing them to sunlight (1000 W/m2) inside a chamber maintained at 300 K. Relative to the incoming solar flux, only 1% of the energy was absorbed under these operating conditions. We anticipate the performance to be better in a cryogenic environment due to lower thermal radiation from the environment. With design optimization, further improvement or control over radiative properties is possible. Furthermore, these materials’ mechanical and radiative properties do not change significantly post-exposure to ultraviolet. Hence, these materials present a new paradigm for passive cooling in space and terrestrial applications.
Acknowledgments:
The authors acknowledge the support of the National Aeronautics and Space Administration under Grant No. 80NSSC21K0072 issued through the Space Technology Research Grants.
