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Because of the harsh environmental impacts of fossil fuels, price fluctuation, and resource limitation, renewable energy resources (RERs) are considered the ultimate solution to overcome these challenges. RERs are also abundant and environmentally friendly. It has been reported that the growth rates in renewable energy installations have increased by 8–9% annually since 2010, reaching a cumulative capacity of 2.9 TW by 2020 and exceeding the growth rate for fossil resources. In this work, the different RERs have been critically discussed in terms of their operation principles, advantages, disadvantages, and associated environmental impacts, focusing on wind and solar energies as the most deployed RERs. Barriers to the full deployment of RERs to replace fossil resources are discussed. Based on the discussion presented on RERs, the contribution of deploying RERs towards the achievement of the United Nations (UN) Sustainable Development Goals (SDGs) is also critically discussed. A total of 80 indicators have been identified for RERs' contribution toward the achievement of SDGs.

Heat transfer operations are very common in the process industry to transfer a huge amount of thermal energy, i.e., heat, from one fluid to another for different purposes. Many fluids are used as heat transfer fluid (HTF), in which water is the most common HTF due to its high specific heat, availability, and affordability. However, conventional HTFs, including water, have a lower thermal conductivity, which is the most critical thermophysical property, hence decreased heat transfer efficiency. The addition of solid particles of highly thermally conductive material, specifically at nano-size, i.e., nanoparticles NPs, result in nanofluid NF, which has evolved over the last two decades as efficient HTF and have been investigated in a wide range of applications. Among NPs, graphene (Gr) based materials have shown very high potential as NF due to the very high thermal conductivity up to 5,000 W/m.K, hence higher thermal conductivity NF. This work aims to thoroughly discuss the thermophysical properties of Gr-based NFs, including thermal conductivity, heat capacity, density, and viscosity. The discussion focus on the thermophysical properties as it is the ultimate determinator of the heat transfer characteristics of the HTF, such as the convective and the overall heat transfer coefficient as well as the heat transfer capacity of the NF. The discussion expands to the relative enhancement in such thermophysical properties reaching up to a 40% increase in thermal conductivity, as the most critical thermophysical property. The discussion shows that Gr-based NF has a much higher thermal conductivity relative to widely studied metal oxide NF and at much lower content, and lower density and viscosity increase, which is critical for determining the pumping power requirements. Critical challenges facing the application of Gr-based NFs such as cost, stability, increased density and viscosity, and environmental impacts are thoroughly discussed with mitigation recommendations given.

This investigation summarized the application of nanofluids (NFs) in heat exchangers (HExs) with different geometries. The quest for heat devices with quick response for the industrial sector is still a major challenge that has been an active research direction over the years. Addressing this issue is likely to increase the capacity of several industries. There is a direct relationship between expanding the heat capacity and the pressure drop. The common approach in increasing the rate of heat transfer often leads to an increment in pressure drop. This study reviews and summarizes the investigations on various geometrical effects inside the channel combined with NF in HExs. This review explored the potential of NFs as possible heat transfer fluid in HExs. From a detailed literature review compiled and evaluated, it has been deduced that NFs application significantly improves the thermal efficiency of HExs. The investigation further evaluated plate, helical, as well as shell and tube HExs. The review explored NFs application in HExs and how they can significantly improve the HExs' thermal characteristics. It was deduced that the use of NFs improved the heat transfer both experimentally and numerically . This equally has a direct relation to energy savings as well as industrial waste heat.

Heat exchangers are widely utilized in different thermal systems for diverse industrial aspects. The selection of HEx depends on the thermal efficiency, operating load, size, flexibility in operation, compatibility with working fluids, better temperature and flow controls, and comparatively low capital and maintenance costs. Heat transfer intensification of heat exchangers can be fulfilled using passive, active, or combined approaches. Utilizing nanofluids as working fluids for heat exchangers have evolved recently. The performance of heat exchangers employed different nanofluids depends mainly on the characteristics and improvement of thermophysical properties. Regarding the unique behavior of different nanofluids, researchers have attended noteworthy progress. The current study reviews and summarizes the recent implementations carried out on utilizing nanofluids in different types of heat exchangers, including plate heat exchangers, double-pipe heat exchangers, shell and tube heat exchangers, and cross-flow heat exchangers. The results showed that nanofluids with enhanced thermal conductivity, although accompanied by a considerable decrease in the heat capacity and raising viscosity, has resulted in performance enhancement of different heat exchangers types. So, the performance evaluation criterion that combines the thermal enhancement and increases the pumping power for any type of heat exchangers is requisite to evaluate the overall performance properly. The challenges and opportunities for future work of heat transfer and fluid flow for different types of heat exchangers utilizing nanofluids are discussed and presented.