24th ABCM International Congress of Mechanical Engineering December 3-8, 2017, Curitiba, PR, Brazil COBEM-2017-1426 PHOTOTHERMAL CONVERSION USING GRAPHENE/WATER NANOFLUIDS Carolina Lau Lins Beicker Universidade Federal de Uberlândia. Av. João Naves de Ávila, 2121, Santa Mônica. Uberlândia-MG, Brasil carolina.beicker@ufu.br Enio Pedone Bandarra Filho Universidade Federal de Uberlândia. Av. João Naves de Ávila, 2121, Santa Mônica. Uberlândia-MG, Brasil bandarra@ufu.br Abstract. The dispersion of particles in the nanometer range - 1 to 100 nm - in a liquid medium, known as nanofluids, has been shown by several authors as an optimization alternative for conventional solar absorption systems and heat transfer systems due to its unique properties. The present study experimentally investigated the behavior of Graphene/Water nanofluids under realistic conditions of photothermal conversion using direct absorption solar collector (DASC). Nanofluids at low volumetric concentrations – 1 to 300ppm - were obtained by dilution and sonication process. An apparatus was built for the simultaneous exposure of different concentrations of nanofluids and the base fluid to solar radiation without any solar concentrating device. The results showed that nanoparticles have excellent photo-thermal conversion capacity, even at low concentrations, however, the Specific Absorption Rate (SAR) decreased with nanoparticle’s addition to the base fluid. After ten hours of testing, the samples showed agglomeration and precipitation problems, a challenge for future applications. Keywords: Nanofluids, Graphene, DASC, Photothermal Conversion. 1. INTRODUCTION Usual fluids used in thermal systems, like water and ethylene-glycol have been very useful in energy generation. However, these fluids have limitations due to its relative low thermal conductivity. In this way, recently were developed techniques to produce a new class of fluids, called nanofluids. Nanofluids are colloidal dispersions were the solid particles are sized between and 100 nm (Choi 1995). The main advantage of nanofluids is the enhanced thermal conductivity compared to base fluid. Thus, the principal studies involving nanofluids are related to the synthesis and characterization of thermal and transport properties, like thermal conductivity, viscosity and density (Li et al., 2009; Lee et al., 2010; Ghadimi et al., 2011; Ramesh e Prabu, 2011; Khanafer et al., 2011; Fan e Wang, 2011; Vajjha et al., 2012; Trisaksri et al., 2007). Another characteristic of nanoparticle dispersions is the improvement of fluid’s optical properties, like higher radiation absorption capacity and better energy storage ability. These characteristics has motivated the search for applications of nanofluids as solar radiation absorbers, such as its use as working fluid in direct absorption solar collectors (DASC’s). DASC’s, known for having, instead of selective surfaces, transparent coating and a fluid responsible for the photothermal conversion, became promising substitutes to the traditional solar collector design, which has efficiency limitations due to heat loss in the absorbing surface with temperature rise. This substitution would lead to the reduction of the heat losses and consequently increasing device efficiency. Natarajan et al. (2009) investigated the possibility of increasing efficiency of conventional solar water heaters using nanofluids as a heat transfer medium and concluded that nanofluids are more effective than conventional fluids. Otanicar et al. (2010) investigated experimentally the performance of solar thermal collectors using nanofluids as the absorption method, and concluded that it is possible an enhancement up to 5% in the collector efficiency. The use of carbon-based nanoparticles has also been reported by many authors as great candidates for use in solar collectors due to their high spectral absorptivity over the entire solar range, good photothermal conversion efficiency even at low concentrations and the low cost compared with noble nanoparticle materials (He et al., 2011; Taylor et al., 2011a; Taylor et al. 2011b; Ladjevardi et al., 2013; Hordy et al., 2014; Sabiha et al., 2015; Delfani et al., 2016; Vakili et al., 2016).