Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 150815

150815 - Thermal Radiation for Energy Applications: Passive Cooling and Nonreciprocal Radiation 

We will discuss recent progress of nonreciprocal control of thermal radiation, and simultaneous sub-ambient radiative cooling and photovoltaic power generation.

I will talk about recent progress in nonreciprocal thermal radiation, such as broadband nonreciprocal thermal radiation. We will show temporal coupled mode theory to elucidate the mechanism of nonreciprocal emission in the thin film emitter. We then introduce a general approach for achieving broadband nonreciprocal emission by using a gradient epsilon-near-zero magneto-optical metamaterial. We numerically demonstrate broadband nonreciprocal emission in gradient-doped semiconductor multilayer, as well as in a magnetic Weyl semimetal multilayer with gradient chemical potential. We will show our effort in experimental demonstration of nonreciprocal thermal emission.

Daytime radiative cooling can be achieved by reflecting most of the sunlight and emitting thermal radiation through the infrared atmospheric transparency window towards the cold universe. Existing approaches of passive daytime radiative cooling need to reflect most of the sunlight so as to achieve sub-ambient temperature, making them incompatible with solar energy harvesting. Hence, existing approaches cannot be utilized to achieve temperature below ambient and solar power generation. It will be desirable to achieve sub-ambient radiative cooling and photovoltaic energy generation simultaneously and from the same area. We develop a dual-energy harvesting device that achieves daytime sub-ambient radiative cooling and power generation simultaneously from the same projected area. The dual-energy system consists of a transparent radiative cooler at the top, separated from a solar cell at the bottom. A sealed air pocket is created around the transparent radiative cooler which minimizes the heat gain from the external solar heating of any surface that is adjacent to the cooler. To ensure the normal incidence of sunlight on both the radiative cooler and the solar cell, the whole setup has been tilted by 22.5˚ facing towards the south. Absorptivity measurement using ultraviolet-visible-near IR spectroscopy shows that transparent radiative cooler only absorbs ~0.9% of sunlight, and most of the sunlight passes to the solar cell. Emissivity measurement using Fourier-transform infrared spectroscopy shows that the cooler has high emissivity in the infrared. In outdoor experiments, the temperature of the transparent radiative cooler passively drops by more than 5 ˚C below the ambient air temperature under direct peak sunlight between 922 to 1090 W/m^2, and the solar power generation from the photovoltaic cell is more than 154 W/m^2 at the same time from the same projected area. The experiment is also performed to determine the radiative cooling power that could be extracted at ambient air temperature to be 61 W/m^2. Our demonstration of simultaneous sub-ambient radiative cooling and solar energy harvesting indicate untapped potential to utilize the two scalable renewable resources for harvesting renewable energy.

Presenting Author: Linxiao Zhu Penn State University

Presenting Author Biography: Dr. Linxiao Zhu is an assistant professor at the Penn State University. Dr. Zhu is interested in controlling heat and light for energy and information applications. Prior to that, Dr. Linxiao Zhu was a postdoctoral researcher in Mechanical Engineering in the University of Michigan, received his Ph.D. in Applied Physics from Stanford University, and a B.S. in Physics from the University Science and Technology of China. He has 35 peer-reviewed publications in journals on energy, nanotechnology, and photonics. He has received NSF CAREER Award, Kaufman New Investigator Award, and Penn State Engineering Alumni Society (PSEAS) Outstanding Research Award.

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