Hiroo YUGAMI and Keiko NISHI, Toshimitsu ITOKO and Yoshiaki KANAMORI

Department of Mechanical Systems and Design Engineering,
Graduate School of Engineering, Tohoku University
Aoba01, Aramaki, Aoba-ku, Sendai 980-8579, JAPAN
e-mail:h_yugami@energy. mech. tohoku. ac. jp

The spectrally selective optical properties of wavelength selective radiation based on periodically microstructured metal surface were investigated. The surface grating optics fabricated on several metallic substrates is applied to make high performance sly radiation cooling systems. The characteristic features of this technique are the flexibility of optical property control of substrates and the zenithal angular dependence of optical refractivity. The optimum surface structures are determined by numerical calculations by RCWA. The cooling power is estimated by a simple atmospheric model using the simulated emission properties.

1. Introduction

Growing energy demands in the household sector is one of serious problem to the reduction of the carbon dioxide emission in Japan. Especially, the peak power in the summertime is caused by electric power demand for the cooling. The sky radiators have been considered as a useful passive cooling system for household, and have been investigated by many researchers [1-3].

The wavelength region between 8-13pm, which is called the atmospheric window, shows high transmittance in thermal emission spectrum of the atmosphere as shown in Fig.1. In other word, the thermal radiation from ground in this wavelength region can reach to outer space more efficiently than that in other wavelength regions. This atmospheric window plays an important role to the sky radiation cooling. Using this atmospheric window, if the more waste heats can be emitted to the space, the warming phenomenon will be defused.

Some drawbacks of conventional sky radiators are low efficiency in summer season with high humidity, heavy weight and large body. These drawbacks prevent spreading of the practical use of sky radiators. To improve these problems, a sky radiator with surface grating structures is proposed in this study.

The spectrally selective optical properties of wavelength selective radiation based on periodically microstructured metal surface were investigated [4-7]. The surface grating optics fabricated on several metallic substrates is applied to make high performance sly radiation cooling systems. The characteristic features of this technique are the flexibility of optical property control of substrates and the zenithal angular dependence of optical refractivity. These are quite advantage features for the sky radiation cooling application.

In this study, we have developed the programming code based on RCWA (Rigorous coupled-wave analysis) to find the optimum structure for the sky radiator. The cooling power of the designed sky radiator is estimated by the simulated emission spectra. A microstructured sky radiator surface is fabricated by the Si micromachining technique, and the optical properties are measured on these samples.