Photo-thermal Conversion Performance Of Carbon Nanotubes Enhanced By Light Guide Materials

How to effectively use solar energy has always been the top priority of new energy development.In recent years,a new type of direct absorption solar collector（DASC）has entered the field of research of researchers.DASC not only has low manufacturing cost,but also has relatively low heat loss,so it has a good development prospect.At the heart of DASC is a fluid that absorbs solar energy and transfers heat.It requires excellent light absorption,thermal conductivity,long-term stability,and photothermal conversion.In this paper,a multi-walled carbon nanotube-water（MWCNT-H₂O）nanofluid with long-term dispersion stability was prepared by using a two-step method and used as a collector.It is committed to pass through a cylindrical DASC with a certain longitudinal depth.The introduction of the light guiding material improves the uniformity of the longitudinal temperature distribution and avoids the excessive surface temperature,so that the solar energy is more effectively utilized and the heat collecting efficiency of the DASC is improved.At the same time,phase change microcapsules with phase change energy storage performance were introduced into DASC,and a mixed suspension of phase change microcapsules and nanofluids was prepared.Combining the advantages of both,the energy storage function of phase change microcapsules was applied.Solar collectors.The main findings show that:（1）In the cylindrical DASC,the light guide plate is used as the light guide material,the simulated sunlight is introduced into the collector working medium,and the MWCNT-H₂O nanofluid is used as the heat collecting working medium to explore the impact of longitudinal temperature distribution and the photothermal conversion performance of the working medium in the DASC.The experimental results show that the MWCNT concentration in the MWCNT-H₂O nanofluid is not as large as possible,and it has the best photo-thermal conversion effect at a concentration of 0.01 wt%.The uniformity of the longitudinal temperature distribution of the working fluid in the cylindrical DASC follows the light guide plate.The insertion depth increases and gradually becomes better,indicating that the introduction of the light guide plate can effectively improve the temperature distribution and improve the heat collection efficiency;the collector efficiency of the DASC decreases with the increase of the solar radiation intensity,but under the large solar radiation intensity higher heat collection efficiency is still guaranteed.（2）In the cylindrical DASC,the optical fiber is used as the light guide material,the simulated sunlight is introduced into the collector working medium,and the MWCNT-H₂O nanofluid is used as the heat collecting working medium to explore the impact of longitudinal temperature distribution and the photothermal conversion performance of the working medium in the DASC.The experimental results show that the collector efficiency of DASC with fiber insertion decreases with the increase of solar radiation intensity,but it can maintain high collector efficiency.The concentration of MWCNT in the working fluid is too high,but it has an adverse effect.The optimal concentration is about 0.01 wt%;the effect of fiber on the uniformity of temperature in the cylindrical DASC is better than that of the light guide plate,which can better improve the solar energy utilization rate.（3）The MWCNT-ethanol water nanofluid containing phase change microcapsules was prepared by adding phase change microcapsules and MWCNT with the appropriate ratio of ethanol and deionized water to investigate the photothermal conversion and energy storage performance of the working fluid in the cylindrical DASC with optical fiber insertion.The experimental results show that although the solar radiation intensity increases gradually,although the heat can be stored through the microcapsule phase change,the heat collection efficiency decreases.In the experimental research range,the DASC heat collection and energy storage performance increase with the concentration of MWCNT.The enhancement is also enhanced with the increase of the concentration of the phase change microcapsules;when the insertion position of the optical fiber in the cylindrical DASC is different,it will have a great influence on the photo-thermal conversion and energy storage effect of different height working fluids,by adjusting the insertion of the optical fiber.The position can be temperature controlled at different depths.