Experimental optimization of nanofluids based direct absorption solar collector by optical boundary conditions

Highlights: • The absorption performance of fluids could enhance ~50% with temperature increases. • The BI-DASC improves the collector efficiency from 24.9% to 43.6%. • The SI-DASC can save 40% of the time to reach steady-state. • Two kinds of modes have a uniform temperature field over 1.0 cm irradiation depth. Direct absorption solar collector (DASC) is regarded as one of the most promising next-generation solar energy collection technology. Most researches focus on the photothermal performance of working fluids. While the optical boundary condition, which is another important factor influencing the efficiency of DASC, receives little attention. In this paper, the ethylene glycol based TiN nanofluids are used as the research objective. The temperature-dependent optical properties of nanofluids are experimentally investigated in detail, and when the temperature increases from 0 °C to 60 °C, the optical absorption performance of fluids could enhance ~50%, which means that the heated fluids has stronger absorption capability. To improve the photothermal conversion efficiency of collector system, two types of irradiation directions have been studied for the collector, and different heat transfer mode of each type has been experimentally analyzed. The experimental results show that the added nanoparticles can significantly enhance the photothermal conversion efficiency of solar collectors. When the concentration of TiN is 0.003 wt.%, the photothermal conversion efficiency of bottom irradiation mode achieves ~45%, much higher than that of side irradiation. However, the side irradiation collector can save ~40% of time to reach steady-state compared with the bottom irradiation collector. Moreover, two kinds of collectors have a uniform temperature field (~10 °C difference between different depth) over 1.0 cm irradiation depth. Consequently, the prospects for possible applications of ethylene glycol based TiN nanofluids in high-efficiency DASC are presented.