Thermal properties on nanofluids based on a high temperature-heat transfer fluid and metallic nanoparticles for concentrating solar energy

In this work, nanofluids were prepared using commercial Cu nanoparticles and a commercial Heat Transfer Fluid (eutectic mixture of diphenyl oxide and biphenyl) as the base fluid. This fluid is used in Concentrating Solar Power plants. Different properties such as density, viscosity, heat capacity, and thermal conductivity were characterized. Nanofluids showed enhanced heat transfer efficiency. In detail, the incorporation of Cu nanoparticles led to an increase of the heat capacity up to 14%. Also, thermal conductivity was increased up to 13%. Finally, the performance of the nanofluids prepared increased up to 11% according to the Dittus-Boelter correlation. INTRODUCTION Concentrating Solar Power is one of the most interesting alternative to conventional energy sources. But actually, it is necessary to improve the global efficiency of these plants in order to decrease the costs of the energy production. Thus, there are many lines of research to improve the efficiency of this technology. In plants based on parabolic mirrors, a thermal fluid flowing through a tube covered with a coating capable of absorbing radiation is used. The absorbed radiation heats the thermal fluid. One option to improve the efficiency of these plants is to improve properties of the Heat Transfer Fluid (HTF) usually used. It is well-known that the suspension of solids in liquids can improve some thermal properties of the liquid [1-3]. Specifically, colloidal suspension of some kind of nanostructures, such as nanoparticles, nanotubes, nanofibers, nanorod, in a base fluid is known as nanofluids [4]. An increase of the thermal conductivity of nanofluids based on metallic nanoparticles have been reported [1,2,5-7]. Also, for applications at lower temperature, nanofluids based on water and ethylene glycol as base fluid, and metallic nanoparticles have been studied [2,8]. In turn, the use of metal oxide nanoparticles has been reported too [2,9]. But, there are many studies related with lower temperature applications using water and ethylene glycol, but analysis of the nanofluids based on the HTF used in Concentrating Solar Power plants is clearly needed. So, in this work, we report the preparation of nanofluids based on Cu nanoparticles and a HTF used typically in CSP plants as the base fluid. The nanofluids were prepared using the two-step method, and several properties were characterized such as density, viscosity, heat capacity, and thermal conductivity. The analysis of the heat transfer for the nanofluids prepared in relation to the base fluid was develop. We observed that the Cu nanofluids improved the thermal properties and the heat transfer coefficient compared with the HTF. EXPERIMENTAL In this work, nanofluids based on Cu nanoparticles and a commercial Heat Transfer Fluid were prepared using a two steps method [10]. This method consists in the use of the previously synthesized nanoparticles and a base fluid used as a HTF. The base fluid used was the eutectic mixture of diphenyl oxide (73.5%) and biphenyl (26.5%). This was supplied by The Dow Chemical Company©, model Dowtherm A. Figure 1 shows the main properties of this fluid, that is the evolution of vapour pressure, density, viscosity, heat capacity and thermal conductivity with respect of the temperature. This fluid is used in some CSP plants in the southern of Spain. The properties shown in Figure 1 give us information of the reason for its use in this kind of plants. For example, the low vapour pressure or the high heat capacity are interesting properties for this application. On the other hand, the Cu nanoparticle (purity 99.5%, density 8940 kg/m at room temperature) used were supplied by SigmaAldrich©, and they show a particle size of 40-60 nm. Nanofluids were prepared according to the next procedure. A nanofluid with a Cu concentration of 0.01wt. % was prepared. From this, nanofluids with a concentration of 0.5·10, 1.0·10 and 5.0·10 wt. % were prepared. The nanofluids were prepared under sonication for 3 h using a Sonics Vibra Cell VCX 750 sonicator. On the other hand, several properties of the nanofluid prepared such as density, viscosity, heat capacity and thermal conductivity were characterized in order to evaluate the heat transfer performance. Density was determined using a pycnometer thermostatized using a thermal bath supplied by 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics