Experimental study of transparent oscillating heat pipes filled with solar absorptive nanofluids

Nanoparticle-based volumetric solar absorption has been shown to be a potential technique to realize efficient solar harvesting. However, most of studied systems are stationary and cannot achieve long distance solar energy transport, which limits their applications. A novel idea of using directive absorptive nanofluids in oscillating heat pipes (OHP) is investigated in this work, which would achieve efficient solar energy capture and transportation simultaneously without the use of any additional pumping power. The influence of a variety of parameters such as nanoparticle type, nanoparticle concentration, nanofluids filling ratio and solar radiation intensity on the performance of OHPs are investigated. It is found that there exists an optimal filling ratio of the nanofluid for the OHP (i.e., 83%), under which single direction circulation of the working fluid is achieved and the thermal resistance of the OHP reaches the minimum. An extremely high thermal conductivity, i.e., 6000 W/(m·K), is obtained when the OHP is filled with 3.0 wt% MWCNT nanofluid. The maximum energy conversion efficiency reaches to 92% under the current experimental settings. Strong absorption of solar energy, efficient vapor generation inside the OHP and proper configuration of the OHP should be responsible for the efficient operation of such a system.