Common fluids such as water, ethylene glycol, and heat transfer oil play an important role in many industrial processes such as power generation, heating or cooling processes, chemical processes, and microelectronics. However, these fluids have relatively low thermal conductivity and thus cannot reach high heat exchange rates in thermal engineering devices. A way to overcome this barrier is using ultra fine solid particles suspended in common fluids to improve their thermal conductivity. The suspension of nano-sized particles (1–100 nm) in a conventional base fluid is called a nanofluid. Choi first used the term ‘‘nanofluid’’ in 1995. Nanofluids, compared to suspensions with particles of millimeter-or-micrometer size, show better stability, rheological properties, and considerably higher thermal conductivities.

In recent years, many researchers have investigated the effects of nanofluids on the enhancement of heat transfer in thermal engineering devices, both experimentally and theoretically. Researchers have also applied a variety of preparation methods, characteristics, and different models used for the calculation of thermos physical properties of nanofluids (i.e., thermal conductivity, viscosity, density, specific heat capacity) [2–9]. Some investigators have also summarized the effects of nanofluids on flow and heat transfer in natural and forced convection in different systems [10–13]. The enhanced thermal behaviour of nanofluids could provide a basis for an enormous innovation for heat transfer intensification, which is of major importance to a number of industrial sectors including transportation, power generation, micro manufacturing, thermal therapy for cancer treatment, chemical and metallurgical sectors, as well as heating, cooling, ventilation and air-conditioning. Nanofluids are also important for the production of nanostructured materials for the engineering of complex fluids as well as for cleaning oil from surfaces due to their excellent wetting and spreading behavior (Ding et al. [14]). Another application of the nanofluid flow is in the delivery of nano-drug as suggested by Kleinstreuer et al. [15].

Collectors and solar water heaters - Solar collectors are particular kind of heat exchangers that transform solar radiation energy into internal energy of the transport medium. These devices absorb the incoming solar radiation, convert it into heat, and transfer the heat to a fluid (usually air, water, or oil) flowing through the collector. The energy collected is carried from the working fluid, either directly to the hot water or space conditioning equipment or to a thermal energy storage tank, from which it can be drawn for use at night or on cloudy days [26]. Solar water heaters are the most popular devices in the field of solar energy. As mentioned in the introduction, the nanofluid based solar collectors are investigated in two aspects. In the first, these devices are studied from the efficiency viewpoint, and in the second, from economic and environmental viewpoints. 

Nanofluids are advanced fluids containing nano-sized particles that have emerged during the last two decades. Nanofluids are used to improve system performance in many thermal engineering systems. This report presented a review of the applications of nanofluids in solar thermal engineering. The experimental and numerical studies for solar collectors showed that in some cases, the efficiency could increase remarkably by using nanofluids. Of course, it is found that using a nanofluid with higher volume fraction always is not the best option (Yousefi et al. [34]). Therefore, it is suggested that the nanofluids in different volume fractions should be tested to find the optimum volume fraction. It is also seen that the available theoretical works give different results on the effects of particle size on the efficiency of the collectors (see Refs. [27,28]). It is worth to carry out an experimental work on the effect of particle size on the collector efficiency. It is also concluded that some factors such as adding surfactant to nanofluid and a suitable selection of the pH of nanofluid are effective in the collector efficiency. From the economic and environmental point of view, the previous studies showed that using nanofluids in collectors leads to a reduction in CO2 emissions and annual electricity and fuel savings. Some other reported works of applications of nanofluids in solar cells, solar thermal energy storage, and solar stills are also reviewed. It is also stressed that for the numerical study of solar systems (for example cooling of solar cells), it is better to use the new thermophysical (temperature-dependent) models and two phase mixture models for the nanofluid to have a more exact prediction of the system performance. This review reveals that the application of nanofluids in solar energy is yet in its infancy. Therefore, some proposals are presented to develop the use of nanofluids in different solar systems such as solar ponds, solar thermoelectric cells, and so on. Finally, the most important challenges on the use of nanofluids in solar systems including high costs of production, instability and agglomeration problems, increased pumping power and erosion are mentioned. These challenges may be reduced with the development of nanotechnology in the future.