Ultra-stable carbon quantum dot nanofluids for direct absorption solar collectors

Nanofluids show great interest in the direct absorption solar collector (DASC) due to their excellent optical, thermal, and flow properties. However, the stability of nanofluid in the terms of the long period and the working temperature has limited its commercial application in DASCs. In this work, ultra-stable carbon quantum dot (CQD) nanofluids were prepared by microwave heating polyethylene glycol 200 (PEG-200), which shows excellent stability over a long period (such as 30 days) and repeatable performance at the working temperature. The solar thermal conversion efficiency eta increases with extending the microwave duration t, and eta can reach 81% at 1 sun and t = 24 min, which is triple the base fluid PEG-200 (eta = 27%). In addition, the solar intensity and working temperature have little effect on eta due to the ultra-stability of CQD nanofluids. Finally, eta can be further enhanced by changing the base fluid or mixing with the plasmonic nanofluids. These results show that the CQD nanofluids can be a potential working fluid used in solar thermal applications.