Solar thermal energy is by far the largest exploitable resource. It
is a very convenient source of heating. It can be used free of charge,
and does not need to be transported [1e3]. The critical problem for
solar thermal utilization is how to improve the efficiency of the
solar collector. It can be performed with optimizing the structure of
collector or developing a new type of working medium. Currently,
water as a medium in solar thermal energy system is used widely.
But the thermal conductivity of water is not high.
With the development of nanotechnology, an innovative heat
transfer fluid arise. Nanofluids, a relatively newclass of fluids which
consist of a base fluid with nano-sized particles (1e100 nm) suspended
within them [4]. These particles are generally metals or
metal oxides. Nanofluids have been considered as a new-type heat
transfer fluid because of their enhanced thermal conductivities
[5e7]. Recently some researchers have put forward to use the
nanofluid as the working fluid for the solar collectors.