Utilizing Solar Vapour Energy by Use of Nanofluids in a Direct Absorption Solar Collector

Traditional solar thermal collectors and photovoltaic (PV) systems suffer from low energy conversion efficiencies. This is mainly due to high thermal losses from overheated absorber plates or solar cells. For steam generation purposes, certain nanofluids (NFs) have been proposed as an alternative due to their high photothermal response and possibility of direct light-to-steam conversion by volumetric absorption. High temperature steam in a direct absorption solar collector (DASC) can potentially be used for electricity generation, puri cation of drinking water and sterilization. However, efficient and cheap solutions are needed in order to compete against other energy production methods. Many researchers have focused on the absorption process within NFs, but few have investigated the boiling of NFs creating high temperature steam, especially the effect of varying the nanoparticle (NP) concentration. In this thesis, a low cost solution is proposed by use of carbon black (CB) and iron oxide (IO) NPs, both known to posses high thermal conductivities. Three volumetric absorbers with a variety of concentrations were experimentally investigated and compared with regard to: time to boiling, vapour temperature, evaporation ratio, absorption, and vapour generation efficiency. A surfactant, sodium dodecyl sulfate (SDS), was added to a range of CB NF samples in order to investigate how the dispersion stability effected the boiling. In addition, possible NP contamination of NF condensates were investigated by use of Raman spectroscopy and static light scattering (SLS). The findings from this study confi rm enhanced boiling of CB and IO NFs compared to water, and prove the importance of dispersion stability of NFs. A NP concentration of 2.00 wt.% resulted in the highest performance for all three NFs. The 2.00 wt.% IO NF achieved the highest steam temperature of 129°C and improved the vapour generation efficiency by 88.3 ± 0.08%. The other NFs also presented excellent performance. Nevertheless, the SLS analysis revealed NP contamination in all condensates of CB NF samples containing SDS, in addition to the NF condensates belonging to IO concentrations of ≽ 3.00 wt.%. This can be attributed to the wettability effects of both SDS and increased IO concentration. These findings can prevent the latter concentrations of NFs from being used in applications where clean water is important, or in systems where NP deposition needs to be avoided.Masteroppgave i energiENERGI39