Performance Investigation Of Water-ethylene Glycol Based Nanofluids For Direct Absorption Solar Collectors

Direct absorption solar collector(DASC)is a novel solar collector with less heat loss,and the development of high performance working fluid is the key to promote the development of this new collector.Nanofluids possess enhanced thermal conductivity and light absorption property,making them promising working fluids for DASCs.However,the common low-temperature nanofluids are thermodynamic instability systems,so they suffer from the deterioration in dispersion stability at elevated temperatures,and it becomes a bottleneck problem that restricts their use as working fluids.In order to expand the application temperature range and improve the performance of nanofluids used as working media for DASCs,water-ethylene glycol(EG)is selected as the basic fluid.At the same time,the dispersion stability of the nanofluids can be improved because hydroxyl group dissociates from ethylene glycol under heating process.Graphene oxide(GO)with good dispersion stability in water was applied as nano additive to prepare GO/water-EG nanofluids.First,a GO/water-EG nanofluid with the highest zeta potential was selected from a series of GO/water-EG ones at different loadings,followed by a UV irradiation for different durations to obtain a series of RGO/water-EG nanofluids.Second,the zeta potential and anti-freeze property of the obtained RGO/water-EG nanofluids were measured,together with investigating their optical absorption,thermophysical properties including thermal conductivity,specific heat and density,along with photothermal conversion performance.Finally,the RGO/water-EG nanofluid with the best photothermal conversion performance was used as working fluid,and the DASC based on this fluid was studied by numerical simulation.The results show that the zeta potential of water-EG based nanofluids increases with temperature,indicating their excellent dispersion stability at elevated temperatures.The RGO/water-EG nanofluid,obtained from the 0.06%GO/water-EG nanofluid,possess antifreeze properties and better light absorption ability than the previous reported RGO/water nanofluid.The RGO/water-EG nanofluids obtained with the irradiation time of 240 s manifest the best photothermal conversion performance,and the numerical simulation shows that DASC based on this nanofluid possess high receiver efficiency.It's found that graphene has better extinction properties than RGO,and water-glycol based nanofluids possess good dispersion stability at elevated temperatures,graphene/water-EG nanofluids were prepared and investigated.Firstly,the optical absorption property of carboxylated graphene(GCOOH)/water-EG nanofluids and hydroxylated graphene(GOH)/water-EG nanofluids was compared and the GOH with better light absorption ability was selected as nano additives.Then,the GOH/water-EG nanofluids with different mass fractions of GOH were prepared,and their optical absorption and thermophysical properties,photothermal conversion performance,and dispersion stability were systematically investigated.Finally,the dispersion stability of GOH/water-EG nanofluids with the best photothermal conversion performance was evaluated.It is shown that the transmittance of the GOH/EG nanofluid in visible band reduced to zero at a GOH loading of as low as 0.007 wt%,and its extinction coefficient is 100 times higher than that of the EG-water base fluid.Under the illumination of 1000W/m~2,the 0.007 wt%GOH/water-EG nanofluid can reach a high final temperature of55°C,which can meet the requirements of solar hot water systems.More significantly,the zeta potential of the 0.007 wt.%GOH/water-EG nanofluid increases significantly with temperature and reaches a maximum value of 47.63 mV at 60?.The excellent optical absorption property,the high photothermal conversion performance as well as the outstanding dispersion stability makes the GOH/EG-water nanofluid a very promising working fluid for a low temperature DASC.