Preparation and application of graphite nanofluid for greenhouse solar energy collection

Solar energy has is widely distributed, abundant, green, and pollution-free, leading to the best choice for the clean energy. Particularly, the greenhouse effect and energy crisis are ever ever-intensifying in the context of population growth and energy consumption. Among them, nanofluids exhibit the excellent photothermal conversion performance and thermal conductivity, indicating the promising potential in the field of solar heat collection. Therefore, the nanofluids can be expected to introduce be introduced as a collecting medium in the vacuum tube collectors. However, it is often required to improve the heat collection efficiency of the vacuum tube solar collectors. This study aims to prepare and screen the high-performance nanofluids with the high stability and the photothermal conversion properties. Application experiments were conducted to optimize the graphite nanofluids in the greenhouse solar collectors. Firstly, five heat transfer media were selected as the water, titanium dioxide, silicon dioxide, copper oxide, and graphene nanofluids. The photothermal conversion was ranked in the descending order of graphene nanofluid, copper oxide nanofluid, silicon dioxide nanofluid, titanium dioxide nanofluid, and water. Considering both cost and stability, the graphene nanofluid was selected, due to its strong photothermal conversion performance, excellent stability, and low cost. Secondly, a systematic investigation was made to explore the effects of dispersants, the order of material addition, ethylene glycol content, ultrasonic parameters, and dilution on the dispersion and stability of graphene nanofluids. An optimal preparation was developed for a stable ethylene glycol-water-based graphene nanofluid after dilution. Subsequently, the photothermal conversion experiments were performed on the ethylene glycol-water-based graphene nanofluid in both the direct-absorption and coated vacuum tubes. The results indicated that the instantaneous thermal efficiencies reached the maximum of 76.40% and 81.67%, respectively (compared with 41.70% for water) at the mass fractions of 0.0030% and 0.0048% in the direct-absorption vacuum tubes. The thermal efficiency of graphene nanofluid was consistently higher than that of water during heating. In the coated vacuum tubes, the graphene nanofluid served as the heat transfer medium, leading to a greater temperature increase, compared with water, although the difference in thermal efficiency was relatively small. Finally, the ethylene glycol-water-based graphite nanofluid was applied into greenhouse solar collectors. Its impact on the crop growth was verified after the test. The temperature of coated vacuum tubes was higher than that of the uncoated ones after heating of 500 L of collector fluid on a typical sunny day in winter. There was the a greater increase in temperature, when using graphite nanofluid as the medium, compared with the water. The "coating + 0.00 06% graphite" demonstrated the best heat collection among the configurations. The temperature of the "coating + 0.000 6% graphite" system increased by 0.48 ℃ after 4.5-hour hours of sunlight. The average substrate temperature increased by 1.53 ℃, and the average temperature was 5.01 ℃ higher than that of the unheated substrate. As such, the fruit harvest time was advanced, due to the plant height, stem thickness, fruit yield, dry weight, and fresh weight of the warmed tomato plants. In summary, the ethylene glycol-water-based graphite nanofluids also exhibited the excellent stability and photothermal conversion, effectively enhancing the heat collection efficiency of solar collectors. The mass production and application of graphite nanofluids can be achieved to prepare the concentrated and diluted solutions, thereby reducing manual labor. This finding can also provide some insights and valuable references to prepare the nanofluids in a greenhouse.