Radiative properties of Al2O3 nanoparticles enhanced ionic liquids (NEILs) for direct absorption solar collectors

Nanoparticle Enhanced Ionic Liquids (NEILs) are the new heat transfer fluids for direct absorption solar collectors and are comprised of small vol/wt percentage of nanoparticles in base ionic liquids (ILs). In the present paper, NEILs are synthesized by dispersing Al2O3 nanoparticles in 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonylimide ([C(4)mim][NTf2]) ILs. The radiative properties of NEILs are investigated with different sizes and shapes (one rod-shaped type sized 10 x 100nm and four spherical sizes of 10 nm, 30 nm, 60 nm, and 90 nm) of Al2O3 nanoparticles. Five different concentrations (0.02%, 0.04%, 0.06%, 0.08%, and 0.1% wt) of NEILs were synthesized to investigate the concentration effect on radiative properties. The addition of Al2O3 nanoparticles greatly increased the light absorption capabilities of the ionic liquid [C(4)mim][NTf2], with at least a 26% decrease in transmittance observed with only 0.02% wt of nanoparticles. Differences in UV-vis absorption were more apparent in smaller concentrations; as much as 20% difference between nanoparticle types and larger concentrations varied in transmittance by as little as 5% between nanoparticle types. Nanofluids made of rod-shaped nanoparticles typically generated higher percentage transmittance values in 0.02%, 0.04%, and 0.06% wt of nanoparticles and were therefore less able to absorb light. Nanofluids made of 10 nm spherical nanoparticles yielded smaller percentage transmittance values at higher concentrations, as little as 2.7% transmission at 0.1% wt, and were therefore better able to absorb light. After completing the nanoparticle size and shape studies, three more NEILs were created from the ideal nanoparticle size and concentration in [C(2)mim][NTf2], [C(6)mim][NTf2], and [C(10)mim][NTf2] ILs. This investigation revealed an increase in percentage transmittance with increasing carbon chain length. [C(2)mim][NTf2] was determined to be the best-absorbing NEILs transmitting as little as 2.2% of light. Plausible reasons for the experimental results are discussed.