Tailoring Thermal Radiation from Near Field to Far Field

Citation

Kou, Junlong (2019) Tailoring Thermal Radiation from Near Field to Far Field. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/MCZC-7E85. https://resolver.caltech.edu/CaltechTHESIS:05302019-214849044

Abstract

Control of heat flow in both near and far field through thermal radiation is of fundamental interest for applications in thermal management and energy conversion.

One challenge is how we can realize high contrast control of heat flow with high temporal frequencies and without moving parts. We try to resolve this problem and propose two schemes in the near field: one based on electrical tuning of silicon and the other based on optical pumping of doped silicon slabs. Both methods rely on the change of free carriers, leading to tuning of the plasma frequency, resulting in modulation of near-field thermal radiation. Calculations based on fluctuational electrodynamics show that the electric method gives 10% tuning range. On the other hand, heat transfer coefficient between two silicon films can be tuned from near zero to 600 Wm^-2K^-1 with a gap distance of 100 nm at room temperature with the optical pumping method.

In the far field, we predict and demonstrate two spectrally selective absorbers based on semiconductors, by utilizing their band gap properties and dedicated photonic structure design. The germanium photonic crystals have around 95% absorption from 500 nm to 1000 µm and over 0.9 over the entire visible and near infrared spectrum. The effective absorptivity is as high as 0.91. The black silicon achieves 100% absorption for light with wavelength under 1 µm. The effective absorptivity is as high as 0.96. Field test shows that black silicon is able to maintain at 130 degrees Celcius under unconcentrated condition.

Another interesting topic is to achieve over 100 Wm^-2 electricity-free cooling power density with simple fabrication method by passive radiative cooling under direction sunlight. We theoretically predicted three schemes for achieving this goal and experimentally demonstrate that a polymer-coated fused silica mirror, as a near-ideal black-body in the mid-infrared and near-ideal reflector in the solar spectrum, achieves radiative cooling below ambient air temperature under direct sunlight (8.2 °C) and at night (8.4 °C). Its performance exceeds that of a multi-layer thin film stack fabricated using vacuum deposition methods by nearly 3 °C. Furthermore, we estimate the cooler has an average net cooling power of about 127 Wm^-2 during daytime at ambient temperature, more than twice that reported previously, even considering the significant influence of external conduction and convection. Our work demonstrates that abundant materials and straight-forward fabrication can be used to achieve daytime radiative cooling, advancing applications such as dry cooling of thermal power plants.

Thesis Files