Experimental investigation of the specific heat of a nitrate-alumina nanofluid for solar thermal energy storage systems

We measured the change in specific heat of nitrate salt-alumina nanoparticle nanofluids at low nanoparticle concentration (less than 2% by mass) to understand how adding small amounts of nanoparticles affected this property. Alumina nanoparticles were dispersed in a eutectic of sodium nitrate and potassium nitrate (60:40 mole fraction) to create nanofluids using a two-step method. Neutron activation analysis was used to measure the actual mass fraction of the alumina nanoparticles in the nanofluids. The nominal mass fraction was always larger than the actual mass fraction, with differences up to 41%. The specific heat was measured using a modulated differential scanning calorimeter (MDSC). The results showed that there exists a parabolic relation between specific heat and mass fraction of alumina nanoparticles (maximum 30.6% enhancement at 0.78% actual mass fraction of alumina nanoparticles). The measurement uncertainty for the specific heat values was less than 4%. The stability of the specific heat values of the nanofluids was also examined; we found the nanoparticle concentration with the highest specific heat value shifted from 0.78% to 0.3% when the same samples were tested after one and two months.