Experimental comparison of the photothermal conversion performance of coal and plant soot nanofluids for direct absorption solar collectors

Direct absorption solar collectors (DASCs) based on nanofluids improve the thermal utilization efficiency of solar energy. In this study, the waste soot materials obtained from coal and plant biomass combustion were recycled. Deionized water was used as the basic fluid. A two-stage method was used to prepare coal soot (CS) and plant soot (PS) nanofluids. Detailed investigations and comparisons of the thermal conductivity and light-absorbing characteristics of the two fluids were performed. The results suggested that the introduction of soot nanoparticles improved the heat transfer and light-absorbing capacities of the base fluid and both thermal conductivity and optical absorption of soot nanofluids were positively correlated with the soot nanoparticle loading. Subsequently, photothermal conversion experiments were conducted internally and externally. The experimental results indicated that the PS nanofluids could absorb incident light radiation more efficiently and converted it into thermal energy. Specifically, the overall photothermal conversion efficiency of the PS nanofluid at a maximum mass fraction of 0.02% in the solar simulator was 55.7%, which was approximately 9.8% higher than that of the CS nanofluid. In addition, 42.3 degrees C and 44 degrees C were the maximum temperature increases under natural sunlight for the CS nanofluid and PS nanofluid, respectively. Notably, the waste soot materials are low cost and easily available. Therefore, this work demonstrates that recycled soot nanofluids, especially PS nanofluids, are promising candidates for application in DASCs systems.