Solar Energy 227 (2021) 365–425 0038-092X/© 2021 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. Progress, challenges and future prospects of plasmonic nanofuid based direct absorption solar collectors – A state-of-the-art review Sanjay Kumar a, * , Nikhil Chander b , Varun Kumar Gupta a , Rajeev Kukreja a a Department of Mechanical Engineering, Dr B R Ambedkar National Institute of Technology, Jalandhar 144011, Punjab, India b Department of Electrical Engineering and Computer Sciences, Indian Institute of Technology Bhilai, GEC Campus, Sejbahar, Raipur 492015, Chhattisgarh, India A R T I C L E INFO Keywords: Solar energy Direct absorption Plasmonic nanofuids Thermal effciency Beam splitting ABSTRACT The use of nanofuids improves the overall performance of solar thermal collectors and has been studied explicitly in the last decade. More recently, to overcome the limitations of surface based solar collectors, novel nanofuid seeded direct absorption solar collectors (DASCs) have been proposed for effective solar energy con- version. Plasmonic nanofuids, the colloids of plasmonic nanoparticles such as Au, Ag, Cu, Al etc. in base fuids, have emerged as promising thermal media for novel DASCs. Due to the inherent localized surface plasmon resonance effect (LSPR), these novel media show high thermal gain and photo thermal conversion potentials compared to other types of nanofuids. Therefore, this review focuses on recent progress and challenges in the use of plasmonic nanofuids in DASC-based solar thermal applications. The state-of-the-art includes reporting the recent experimental and numerical results in low, medium and high-temperature DASCs as well as hybrid photovoltaic/thermal (PV/T) technology, which utilises nanofuids as beam splitter. We have also tried to provide a review of optical characteristics of plasmonic nanoparticles along with the latest developments in the synthesis of complex nanoparticle morphologies and blends for broadband absorption of solar spectrum. Finally, authors have tried to highlight the challenges, grey areas and possible future research directions for the potential applications of plasmonic nanoparticles in futuristic solar harvesting applications. 1. Introduction In the last two decades, unprecedented growth in developed as well as in developing countries has increased the living standards of people and hence the energy consumption has also increased signifcantly (Ahmed and Azam, 2016; Esso and Keho, 2016). It is expected that by 2040, energy demand can rise by more than one third compared to the present value (IEA, 2017). To meet this escalated energy demand, renewable energy technologies are gaining much attention from research community, given the growing awareness about climate change and rapid depletion of primary energy resources (Kalogirou, 2004; Ohler and Fetters, 2014; Tugcu et al., 2012; Alper and Oguz, 2016). The other reasons include limited availability of fossil fuels and their adverse environmental impacts such as carbon dioxide emissions leading to global warming effect (Esso and Keho, 2016; Muhammad, 2019). Solar energy is a relatively clean, free, inexhaustible and non- localized energy source with negligible environmental impacts (Kabir et al., 2018). Currently, solar energy is harnessed in different forms–(i) Photovoltaic, (ii) Photo-catalytic, and (iii) photo thermal conversion systems (Mallah et al., 2019). Solar photovoltaic (SPV) systems convert intercepted solar energy directly into electricity. Whereas, photo ther- mal conversion systems convert sunlight into heat energy which can be further used to heat water, space heating, and cooking or used to generate steam for electricity production. Solar PV technologies have shown advantage of direct electricity generation compared to solar thermal technologies at a much lower levelized cost of electricity (Goel et al., 2020; Joshi and Dhoble, 2018). 1.1. Overview and concept of direct absorption solar collector (DASC) Solar thermal systems are designed for optimum solar absorption, and to obtain maximum thermal performance with minimum heat losses through the collector (Kalogirou, 2004; Goel et al., 2020; Bhalla and Tyagi, 2018). In surface-based absorption systems (SASC), also referred to as conventional solar thermal collectors, the incident solar irradiation is frst absorbed by absorber material (tubes) and the heat is transferred to the working fuid, generally water or ethylene glycol. Considering the low cost, simple design and high durability, SASCs are widely utilized in low temperature solar thermal applications like water or air heating * Corresponding author. E-mail address: sanjay@nitj.ac.in (S. Kumar). Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener https://doi.org/10.1016/j.solener.2021.09.008 Received 26 October 2020; Received in revised form 1 September 2021; Accepted 3 September 2021