He et al. [8] studied the light-heat conversion characteristics of
two nanofluids, water–TiO2 and water–carbon nanotube (CNT), in
a vacuum tube solar collector under sunny and cloudy weather
conditions. They showed that the CNT–H2O nanofluid is more suitable
than the TiO2–H2O nanofluid to be utilized in a vacuum tube
solar collector.
Khullar et al. [9] investigated theoretically a nanofluid-based
concentrating parabolic solar collector (NCPSC) and compared
the obtained results with experimental results of conventional
concentrating parabolic solar collectors operating under similar
conditions. They used Aluminum nanoparticles with 0.05 vol.%
suspended in Therminol VP-1 as the base fluid, and found that
the thermal efficiency of NCPSC compared to a conventional parabolic
solar collector is about 5–10% higher under the same weather
conditions.
Tyagi et al. [10] studied theoretically the effects of different
parameters on the efficiency of a low-temperature nanofluid-based
direct absorption solar collector (DAC) with a mixture of water and
aluminum nanoparticles as the working fluid. They showed that by
adding nanoparticles to the working fluid, the efficiency increases
remarkably for low values of volume fraction of nanoparticles.
Moreover, Mahian et al. [11] have reviewed and discussed utilizing
nanofluids in solar energy devices.
To obtain stable nanofluids, some physical or chemical methods
are recommended. pH control of nanofluid is an approved technique
for dispersing the aggregated nanoparticles in liquid and
finally preparing of a stable nanofluid. During the last decade, some
studies showed that the variation of pH in nanofluid is an important
parameter for enhancing of the stability and thermal conductivity
of nanofluids.
Lee et al. [12] showed that as the pH of the water based CuO
nanofluids varied far from the isoelectric point of particles, thecolloidal particles become more stable and finally alter the thermal
conductivity of the fluid.
Wamkam et al. [13] have studied pH influences of zeta potential
and stability on heat transfer nanofluids, and showed that thermal
conductivity enhanced significantly in 3 wt% ZrO2 and TiO2 near
the isoelectric point which indicated that the stability of nanofluids
are influenced by pH values.