7.3. Inconsistent results among the researchers
Most of the studies indicated that the thermal performance of nanofluids operated solar collector increases with addition of nanoparticles. However, few studies have revealed that the thermal performance deteriorates once the certain particle loading is exceeded. For instance, Yousefi et al. [35] in their study implies that the efficiency of flat-plate solar collector for 0.2 wt% MWCNT nanofluid is lower than that for water as base fluid. However when added with 0.4 weight % of carbon nanotube, the solar collector improved significantly. The obtained results are based on the application of suspension without addition of surfactant.
7.4. Design of the solar collector
The design of the solar collector affects the particles sedimentation. This was studied by Colangelo et al. [37]. This study identified that by maintaining constant mass flow rate of the fluid, the particles׳ sedimentation can be addressed. Thus, modification of the solar collector design is required. Although these authors came out with a new design, more comprehensive study is still needed in this area.
7.5. Operation cost
In authors׳ opinion, the operation cost of nanofluids operated solar collector will definitely be higher than the same system operated with base fluid due to the increase in viscosity. This may not be the issue if the energy saving cost due to improved efficiency of solar collector is much higher than operation cost. Techno-economic analysis as well as the payback period analysis must be covered by the researchers in this field. To the author׳s best knowledge, there is no study related to this topic available in the open literatures.