Properties of solar energy absorption and photothermal conversion at medium temperature based on magneticnanofluids

Abstract

As a kind of clean and renewable energy, solar energy has been widely used in photovoltaic power generation, photothermal utilization, photocatalysis and other fields. According to its working temperature range, photothermal utilization can be divided into three areas: Low-temperature heat utilization ( medium-temperature heat utilization (80−250°C) and high-temperature heat utilization (>250°C). At present, the technologies of low-temperature heat utilization and high-temperature heat utilization have been relatively mature, while the development of medium-temperature heat utilization technology is relatively slow. Furthermore, the medium-temperature heat utilization is one of the effective technologies to solve the current energy shortage, environmental pollution and other problems. The key problem in solar energy heat utilization is how to improve the efficiency of solar collector. Nanofluids-based direct absorption of solar radiation energy can effectively improve the collection efficiency of solar collectors. However, nanofluids are easy to settle in the long-term cold-hot alternating process, which will deposit on the pipe wall of heat exchanger and even cause blockage, resulting in the increase of thermal resistance and serious heat loss in the process of heat exchange. In this paper, a way of magnetic nanofluid-based direct absorption system for medium-temperature heat utilization is proposed. Firstly, a heat-conducting oil-based nanofluid with magnetic Co@NC is prepared by “two-step” method, and high temperature fluids are obtained under high-power simulated solar irradiation by its excellent optical absorption performance. Secondly, based on magnetic separation technology, Co@NC nanoparticles are separated from the heat conducting oil matrix to obtain pure base liquids with high temperature. Under five solar radiations, the highest temperature of the nanofluids could reach to 120.37°C. Moreover, various liquids with higher temperature can be obtained by the process of heat exchange. Finally, the effects of illumination on the photothermal conversion properties of nanofluids are studied, and the mechanism is explained. This method achieves high photothermal conversion efficiency, and can effectively solve the problem of sedimentation caused by the instability of nanofluids. In addition, it provides a scheme for promoting new development of nanofluids in the use of solar energy at medium temperature. The results show that the photothermal conversion efficiency of magnetic nanofluids is significantly higher than that of base fluids. When the illumination intensity is 5000 W/m2 and the concentration is 100 ppm, the highest temperature can reach to 120.37°C, which is higher than that of pure thermal conductive oil (90.7°C). In addition, the highest photothermal conversion efficiency is 65.23% when the illumination intensity is 1000 W/m2 and the concentration is 100 ppm. With the increase of illumination intensity at the same concentration, the final temperature of the nanofluids increases, but the photothermal conversion efficiency decreases significantly. When the illumination intensity is 5000 W/m2 and the concentration is 100 ppm, the highest photothermal conversion efficiency is only 33.84%. The main reason for this phenomenon is that part of the sunlight can’t be absorbed. With the increase of light intensity, the absorptivity of nanoparticles in nanofluids quickly reaches saturation. Moreover, the stronger the illumination is, the stronger the penetration ability is. The colorimetric dish with a thickness of 1 cm is used, and the maximum concentration of nanofluids is 100 ppm , which is relatively low. Some light passes through the whole nanofluidic layer and can’t be fully absorbed. Therefore, the results show that the photothermal conversion efficiency decreases with the increase of light intensity.