Nanofluids for Energy Applications
Nanofluids are gaining attention in the exchange, conversion, and storage of energy, owing to their enhanced thermophysical characteristics. Since the thermal conductivity of metals are far higher than that of fluids such as water and oil, adding metal nanoparticles into such fluids can result in a new medium with improved heat transfer properties. Apart from pure metals, nanoparticles made of metal-oxides, silicon carbide, composites, bio-based materials, etc. are also used for this purpose. Nanofluids are produced by engineered-dispersion of nanoparticles in a base fluid, which significantly enhances the thermal conductivity of conventional heat transfer fluids (Ganji et al. 2018; Sheikholeslami 2019). However, the thermophysical characteristics of nanofluids depend on the size, shape, and volumetric fraction of the dispersed nanoparticles. An interesting breakthrough in nanofluids is the advent of hybrid nanofluids wherein two or more nanoparticles are immersed in the base fluid, which has shown further enhancement of the thermal and rheological properties (Kumar and Arasu 2018).
Nanofluids play important role in energy exchange and conversion applications (Jama et al. 2016;Nagarajan et al. 2014; Said et al. 2014; Reddy et al. 2017; Hussain et al. 2019; Muñoz-Sánchez et al. 2018; Bhalla and Tyagi 2017; Islam et al. 2015). Holkar et al. (2018) listed different nanofluids employed for energy harvesting and their advantages. Apart from these, cupric oxide nanofluid was proposed to improve the heat transfer performance of asphalt solar collectors that harvest solar energy from roads (Hashim 2014). The efficiency of direct absorption solar collectors could be improved by the use of nanofluids as the absorption medium (Nagarajan et al. 2014;Otanicar et al. 2010).
A major challenge in the use of nanofluids for energy applications is the contradictory behavior of thermal conductivity and specific heat. While the enhancement of thermal conductivity of base-fluid by the dispersion of nanoparticles is well established, there are no consistent findings on the enhancement of specific heat capacity (SHC) (Lu and Huang 2013), which is a desirable property for energy storage. Several studies reported decline of SHC of aqueous nanofluids, whereas non-aqueous nanofluids showed improvement of SHC (Shin and Banerjee 2010). This issue has been tackled in many studies, and it remains an ongoing challenge. Adding nanoparticles to molten salts that are used as heat transfer fluids in concentrated solar power (CSP) system could enhance the thermophysical properties including SHC and enable the CSP plants operate at higher temperatures (Shin and Banerjee 2010; Lu and Huang 2013). Muñoz-Sánchez et al. (2018) provided a comprehensive review on the use of molten salt-based nanofluids for energy storage and transfer at higher temperatures.
Owing to the growing environmental concerns on the conventional synthesis techniques of nanoparticles, the emerging trend is to adopt green synthesis approaches (Genuino et al. 2013; Ghulam 2016). The nanoparticles produced by green synthesis are termed as green nanoparticles, and the nanofluids prepared from them are known as green nanofluids (Narayanan and Rakesh 2019). Several researchers have reported development and characterization of green and eco-friendly nanofluids such as silver and gold nanofluids (Mollick et al. 2014)(John et al. 2015), grapheme-based nanofluids (Mehrali et al. 2016), and coconut fiber-based nanofluids (Adewumi et al. 2018). These green nanofluids would be promising alternatives for their conventional counterparts for various energy conversion applications.