*6.1. Parabolic Trough Solar Collectors*

A PTSC is part of the existing energy solar systems that utilizes solar radiation (usually emitted from the sun) to generate thermal energy with high efficiency [294]. This happens when reflecting concentrated incident sunlight from its reflector surface, which consists of a parabolically curved mirror polished metal, to a focal line where the receiver or absorber tube containing the working fluid is located. The lower temperature heat transfer fluid, which is usually water or oil, then absorbs the solar heat flux from the attached inner tube surface, and thus causes its temperature to raise. Figure 19 shows an example of a real life PTSC system and its working mechanism in a schematic diagram. Based on the system configuration and the application used, the working fluid temperature in a PTSC can exceed 500 ◦C at concentrated solar power plants (CSPP), for steam power cycles; or can be lower than 100 ◦C, for industrial process heat (IPH) applications, such as domestic and industrial water heating [295]. Examples of low temperature requirements (i.e., temperature starting from ≤100 ◦C) for different industrial processes are shown in Table 6 [296]. Most of the modern designs of PTSC contain a sunlight tracking system that helps improve the efficiency of these systems [297].

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 39 of 79

**Figure 19.** Example of a parabolic trough solar collector system, where (**a**) shows the physical device, (**b**) illustrates its schematic diagram, and (**c**) demonstrates the reflection mechanism of solar radia-**Figure 19.** Example of a parabolic trough solar collector system, where (**a**) shows the physical device, (**b**) illustrates its schematic diagram, and (**c**) demonstrates the reflection mechanism of solar radiation on the absorber tube [298].

**Table 6.** Examples of different industrial processes that utilize parabolic trough solar collector systems and their temperature requirements. **Table 6.** Examples of different industrial processes that utilize parabolic trough solar collector systems and their temperature requirements.

tion on the absorber tube [298].


Desalinization Heat transfer fluid 100–250

Since the primary goals in industrial applications are to reduce the processing time, increase the lifetime of the equipment, and decrease the amount of energy consumption, using PTSC systems, these goals can be fulfilled through improving the rate of heat transfer between the absorber tube and the working fluid. One way of achieving this is by utilizing nanofluids as the heat transfer fluid in the PTSC system [299,300]. This is because, as mentioned earlier, nanofluids have higher thermal conductivity than any known conventional heat transfer fluid, which makes them potential candidates for the future of such heat transfer applications. When using carbon-based particles (e.g., MWCNTs, graphene, or NDs), the effective thermal conductivity significantly increases along with the rate of thermal diffusion and effective viscosity of the suspension. Subsequently, this causes the fluid heat capacity, Reynold's number (), and Prandtl number () to decrease. In the case of turbulent flow, the Nusselt number () depends on both and . Thus, a decrease in the two aforementioned parameters would result in fewer or smaller eddy formations within the fluid, and hence the level of turbulently in the flow would reduce. Furthermore, since the effective viscosity of a nanofluid is higher than its base fluid, the pressure drop that will be experienced from using such category of fluids in a PTSC system would be higher than that of the conventional base fluids. To overcome this issue, the Since the primary goals in industrial applications are to reduce the processing time, increase the lifetime of the equipment, and decrease the amount of energy consumption, using PTSC systems, these goals can be fulfilled through improving the rate of heat transfer between the absorber tube and the working fluid. One way of achieving this is by utilizing nanofluids as the heat transfer fluid in the PTSC system [299,300]. This is because, as mentioned earlier, nanofluids have higher thermal conductivity than any known conventional heat transfer fluid, which makes them potential candidates for the future of such heat transfer applications. When using carbon-based particles (e.g., MWCNTs, graphene, or NDs), the effective thermal conductivity significantly increases along with the rate of thermal diffusion and effective viscosity of the suspension. Subsequently, this causes the fluid heat capacity, Reynold's number (*Re*), and Prandtl number (*Pr*) to decrease. In the case of turbulent flow, the Nusselt number (*Nu*) depends on both *Re* and *Pr*. Thus, a decrease in the two aforementioned parameters would result in fewer or smaller eddy formations within the fluid, and hence the level of turbulently in the flow would reduce. Furthermore, since the effective viscosity of a nanofluid is higher than its base fluid, the pressure drop that will be experienced from using such category of fluids in a PTSC system would be higher than that of the conventional base fluids. To overcome this issue, the PTSC system should take into account the thermophysical properties of the suspension

PTSC system should take into account the thermophysical properties of the suspension

used at its design phase. One important thing to consider is that when using nanofluids, as the PTSC working fluid, the absorber tube needs to be transparent so that the dispersed particles can directly absorb the sunlight throughout their cycle [301].

Although the previous facts showed how promising nanofluids could be when used in PTSC systems, the scientific field is still scarce with the amount of published works that investigate carbon-based nanofluids in such system. Most of the work covered on nanofluids were those involving nanoparticles of Al2O3, CuO, TiO2, Fe2O3, SiO2, Cu, SiC, Fe3O4, and limited other literature were found for CNTs, MWCNTs, and SWCNTs [298]. For instance, Kasaeian et al. [302] explored the overall efficiency enhancement of a pilot PTSC system using MWCNTs–mineral oil suspensions of 0.1 wt % and 0.3 wt %. The researchers found that the 0.1 wt % and 0.3 wt % dispersions had improved the system efficiency by 4–5% and 5–7%, respectively, compared to conventional base fluid (i.e., mineral oil). Furthermore, Kasaeian et al. [303] studied the effect of 0.1, 0.2, and 0.3 vol. % of MWCNTs dispersed in EG, as the working fluid, for a direct absorber solar collector attached to a parabolic trough. They found that the optical efficiency reached up to 71.4%, due to the 0.3 vol. % of MWCNTs particles employed in their heat transfer fluid. In addition, the thermal efficiency of their system was found to be 17% higher, when using the 0.3 vol. % nanofluid, than that obtained from pure EG. Moreover, Mwesigye et al. [304] coupled a Monte Carlo ray tracing (MCRT) optical model along with a computational fluid dynamics (CFD) finite volume method (FVM)-based model to analyze a PTSC, hosting a SWCNTs– Therminon VP-1 suspension, thermal performance. The authors found that raising the particles concentration from 0 to 2.5 vol. % caused the entropy generation to reduce by 70%, with the heat transfer rate to increase by 234%, and the thermal efficiency of the system to improve by 4.4%. In addition, Dugaria et al. [305] designed and modeled the optical efficiency of a direct absorber solar collector (DASC) that is connected to a parabolic trough system. In their experiment, they used 0.006, 0.01, 0.02, and 0.05 g/L of SWCNTs to fabricate their aqueous nanofluids. Their results showed that increasing the nanoparticles concentration to more than 0.05 g/L would cause the thermal efficiency to reduce due to the thermal radiation being mostly contained in the surrounding area between the absorber tube inner surface and the nanofluid. In addition, using nanofluids made of 0.05 g/L SWCNTs caused the thermal efficiency of the system, including the optical losses of the concentrating trough, to reach 90.6% at a reduced temperature range (*T* ∗ *<sup>m</sup>*) = 0 K·m2/W and 77.2% at *T* ∗ *<sup>m</sup>* = 0.128 K·m2/W. It is important to note that the thermal efficiency of solar collectors is usually shown in a graph as a function of *T* ∗ *<sup>m</sup>*, which is defined for the case of nanofluids as:

$$T\_m^\* = \frac{\left(T\_{m\_{nf}} - T\_{amb}\right)}{DNI} \tag{12}$$

where *Tmn f* , *Tamb*, and *DNI* are the mean temperature of the nanofluid, ambient air temperature, and direct normal irradiance, respectively. One of the main aspects for the enhancement in the thermal performance of the two aforementioned published works [304,305] was due to the fact that CNTs, along with other carbon-based materials, possessed extremely high solar absorption characteristics (i.e., more than 90%) [306]. Despite the research investigations that were covered in this section on carbon-based nanofluids usage in PTSC's, there are only a few other alternatives [298]. To the best of the authors of this article knowledge, there is still a lack of exploration on utilizing ND's and graphene nanofluids for PTSC's. This shows that further investigation is required from the researchers working in the solar energy field; especially since, for example, nanofluids of ND base showed to contain remarkable optical and thermal properties when studied in other similar applications [307].
