Water or mixture of water and ethylene glycol are heat transfer
fluids generally used in flat solar thermal collectors. Their thermal
conductivity is a limiting factor, because it is lower than thermal
conductivity of heat exchange solid surface. Innovative heat
transfer fluids have been proposed to enhance efficiency of energy
systems, based on mixing of solid nanoparticles (o100 nm) of
metal or metal oxide with traditional heat transfer fluids as water,
ethylene glycol, oil. These suspensions are called nanofluids, word
used by Choi for the first time [50].