Novel copper e Propylene glycol nanofluid as efficient thermic fluid for potential application in discharge cycle of thermal energy storage R. Yedhu Krishnan 1 , S. Manikandan 1 , K.S. Suganthi, V. Leela Vinodhan, K.S. Rajan * Centre for Nanotechnology & Advanced Biomaterials (CeNTAB), School of Chemical & Biotechnology, SASTRA University, Thanjavur 613401, India article info Article history: Received 24 December 2015 Received in revised form 31 March 2016 Accepted 9 April 2016 Keywords: Energy recovery Copper nanoparticles Nanofluid Thermal conductivity Discharge cycle Constant temperature abstract Nanofluids are promising heat transfer fluids for a wide range of energy management applications. Probe ultrasonication-mediated preparation of copper e propylene glycol nanofluid is accomplished through dispersion of biosurfactant-functionalized copper nanoparticles in propylene glycol. Copper e propylene glycol nanofluids are colloidally stable; retain their thermal conductivity enhancement despite repeated heatingecooling cycles and storage for more than 15 days. These nanofluids exhibit temperature- independent thermal conductivity enhancement, with about ~11% enhancement for 1 vol % nanofluid, attributable to Brownian motion and interfacial layering. The viscosity of nanofluids is lower than that of base fluid (propylene glycol) due to interactions between biosurfactant and propylene glycol. Our data clearly demonstrate that the use of 1 vol % copper e propylene glycol nanofluid as coolant can lead to 13.2% improvement in the rate of energy recovery from a constant-temperature hot bath when the heat transfer resistance in the hot fluid side is low. The results of the present study have implications for energy management in solar thermal systems. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction Heat transfer fluids with higher thermal conductivity, lower viscosity and higher specific heat are required to achieve enhanced heat removal rates from heat sources for energy recovery and sustainable performance of devices. The rate of heat removal from a constant temperature source, as in the case of condensation of vapors and freezing of liquids, depends on the thermophysical properties of the coolant, flow conditions and the geometry of heat exchanger. Latent heat storage using phase change materials is a well-established technique to store abundant solar energy during its availability in the day and utilize the same when the energy demand exceeds solar energy collection [1,2]. The recovery of latent heat stored in phase change materials is accomplished during discharge cycle through the use of a heat transfer fluid. Hence, the efficiency of discharge cycle can be improved through the use of heat transfer fluids with superior thermophysical properties. Nanofluids are two-phase colloids comprising a nanomaterial with particle size 100 nm dispersed in a liquid [3]. Nanofluids are well-known engineered heat transfer fluids [4e6] owing to their improved thermal conductivity. A considerable amount of work has been reported on noble metal-based nanofluids such as silver- based nanofluids [7e11], gold-based nanofluids [12e16] and tran- sition metal based nanofluids such as iron-based nanofluids [17,18], copper-based nanofluids [19e23] and nickel-based nanofluids [24,25]. The thermal conductivity of copper e ethylene glycol nanofluids has been reported [26,27] with varying degrees of thermal conductivity enhancements depending on the nano- particle size, method of nanoparticle preparation and strategy employed for nanofluid formulation. While 40% enhancement in thermal conductivity was reported at 0.3 vol % of nanoparticles for copper e ethylene glycol nanofluid prepared by one-step method [27], the nanofluid prepared by two-step method using PVP (pol- y(vinylpyrrolidone)) as surfactant followed by ultrasonication resulted in 50% thermal conductivity enhancement at 0.5 vol % [26]. The use of copper e ethylene glycol nanofluid in solar plate col- lector has resulted in improvement of solar energy collection effi- ciency with increasing nanoparticle concentration [28]. It is reasonably well-established that the nanofluids enhance heat transfer through improved thermal conductivity, particle migration and thermal dispersion effects. The use of nanofluids can lead to a reduction in coolant inventory or size of the heat exchanger due to higher heat transfer coefficients and heat transfer rates achievable. * Corresponding author. Tel.: þ91 9790377951; fax: þ91 4362 264120. E-mail address: ksrajan@chem.sastra.edu (K.S. Rajan). 1 Equal contribution. Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2016.04.047 0360-5442/© 2016 Elsevier Ltd. All rights reserved. Energy 107 (2016) 482e492