Adoption of nanofluids in low-enthalpy parabolic trough solar collectors: Numerical simulation of the yearly yield

Energy demand in the world is continuously increasing and fossil fuels resources must be replaced by renewable resources with lower environmental risk factors, in particular CO2 emissions. Concentrating solar collectors appear to be very promising for that purpose. Thus, this work presents a numerical analysis for the evaluation of the yearly yield of a low-enthalpy parabolic trough solar collector (PTC). To increase the thermal efficiency of such systems, six water-based nanofluids at different weight concentrations are investigated: Fe2O3 (5, 10, 20 wt%), SiO2 (1, 5, 25 wt%), TiO2 (1, 10, 20, 35 wt%), ZnO (1, 5, 10 wt%), Al2O3 (0.1, 1, 2 wt%), and Au (0.01 wt%). The simulation environment was validated by experimental tests using water as heat transfer fluid, in two prototypes of PTC located in the city of Ancona (central Italy), while the convective heat transfer coefficient of nanofluids was measured through a dedicated apparatus. A typical meteorological year was built to perform the simulation, which presents a time-resolution of one hour. A specific arrangement for the PTC was defined, while different inlet fluid temperatures were considered at a mass flow rate of 0.50 kg/s: 40, 50, 60, 70, and 80 °C. For this last temperature, the variation in flow rate was also studied (at 1 kg/s and 1.5 kg/s). Results show that only Au, TiO2, ZnO, and Al2O3 nanofluids at the lower concentrations, present very small improvements compared to the use of water, while increasing the concentration of nanoparticles no advantage with respect to water appears.