As can be seen from Fig. 7, before 180 min (12:00), the outlet temperature of nanofliuds is higher
than that of water. With the solar radiation decline (12:00e15:00),
the temperature difference of the outlet between nanofluids and
water is reduce gradually. In the end, the outlet temperature of
nanofliuds and water is almost equal. There is a possible explanation
to this phenomenon. During the high solar radiation, the heat
loss is very little compared with the solar thermal energy absorbed
by the fluids. The temperature of nanofluids enhanced faster than
that of water because of the superior heat transfer performance.
But in the process of solar radiation rapid decline, the heat loss can
not be ignored. The heat loss of nanofluids is more than that of
water. Hence, the outlet temperature of nanofluids is more and
more close to the outlet temperature of water.