Study On Photothermal Conversion Characteristics Of MWCNT Nanofluid For Direct Absorption Solar Collectors

Solar energy,a renewable and sustainable energy source,is considered as the most promising substitution for fossil fuel.Solar thermal technology is a typical approach in practical exploitation of solar energy.However,the photothermal conversion efficiency of solar collector that is the most important component of solar thermal system is not high enough for practical applications.Direct absorption solar collector（DASC）serves as a promising technology to harvest solar energy,but the performance of DASC is largely affected by the working liquid.A few studies have shown that nanofluid applied to a direct absorption solar collector is one of the key to enhance the performance of direct absorption solar collectors.In this paper,the long-term stability,optical properties,thermal conductivity and kinetic viscosity of MWCNT-DW/EG nanofluid were measured and discussed,and the effects of mass fraction,p H,mass flow rate and work fluid height on the performance of nanofluid direct-absorption solar collector photothermal conversion were systematically and thoroughly investigated through the designed experimental test platform The potential of MWCNT nanofluid for direct absorption solar collectors was comprehensively evaluated.The results showed that the nanofluid can achieve long-term dispersion stability by choosing appropriate surface dispersant and ultrasonic vibration.As the absorbing medium,MWCNT nanofluids can significantly absorb the incident sunlight even at low mass fraction.As the heat transfer medium,the thermal conductivity of the nanofluids increased with increasing concentration and temperature but highly depended on temperature,and the effect of adding nanoparticles on the flow of nanofluids can be negligible.Moreover,the effect of mass fraction,p H,mass flow rate and liquid level height on MWCNT nanofluids photothermal conversion performance was analyzed,and found that the optimum mass fraction and p H value should be chosen to obtain higher temperature in DASCs.In this study,the optimum mass fraction and p H were 0.01wt% and 7 respectively.The photothermal conversion performance increased with the increase of mass flow rate and decreased with the increasing work fluid height.Due to more uniform internal temperature distribution and less heat loss,DASC had better photothermal conversion performance than that of evaluated tube solar collector under the same conditions.This study presented the great potential of MWCNT-DW/EG nanofluid for direct absorption solar thermal energy system.