Innovative strategy of passive sub-ambient radiative cooler through incorporation of a thermal rectifier to double-layer nanoparticle-based coating

Abstract

Although various types of radiative cooling have been investigated, providing more cooling efficiency is always a challenge. One of the most critical challenges of radiative coolers is the ability to cool during the daytime against direct sunlight. In this work, the efficiency of the radiative cooler is increased by proposing an innovative design consisting of a thermal rectifier, an emitter with high emissivity in the “sky window,” and a solar reflector. Radiative and optical properties of selective surfaces were used to achieve thermal rectification. The spectroscopy (IR-Vis) tests were used to measure the optical-radiative properties of the present design structure, as well as their performance in the simulation software, was analyzed. An apparatus with the ability to measure the temperature at five simultaneous points and a digital data logger were designed and built to compare the cooling power of different samples experimentally. First, an experiment was performed to measure the temperature of different samples in winter (February) and summer (September) on the roof of a building in the southwest sky of Tehran (Iran). Site measurements showed that the temperature of the rectifier-cooler was at least 4.6 °C lower than the sample without the rectifier during the day. Also, the temperature difference between the no-coated and black samples reaches up to 7 and 17°C, respectively. Moreover, the resistance of the samples to the thermal shock was evaluated and characterized. Results show more thermal shock resistance in the rectifier-cooler, which reduces the body's stress and thus decreases its fatigue and failures. Further studies showed this structure has a reflectance of about 90% in the solar range and an emittance of about 0.7 in the sky transparency (8–13 μm spectrum). This new method has the potential to apply in a confined or extensive area, which provides a practical step towards achieving an industrial approach for radiation cooling technology, including building air conditioning, automotive industries, space industries, etc.