Uniform temperature distribution and reduced convection losses: Top and bottom heating strategies for nanofluid and surface absorption-based solar thermal systems

Abstract

In direct absorption solar collectors (DASC), the suspension of nanoparticles in base fluid increases the solar absorptivity for volumetric absorption, which results in better efficiency when compared to transparent base fluids. However, a temperature gradient exists in the nanofluid volume while irradiated in the gravitational direction which can be diminished by changing the irradiation direction. The irradiation provided from opposite direction to gravity introduces natural convection in nanofluid and maintains a uniform temperature. The novelty of present experimental study is to compare and understand the effect of two directions of irradiation i.e., heating from the top (TH, in the gravitational direction) and heating from the bottom (BH, in the opposite direction of gravity) on both surface absorption-based (SAS) solar collector and volumetric absorption-based (VAS) solar collector. In VAS 6 different mass fractions of carbon-based nanofluids have been used. From the results, it was found that BH gives a higher and uniform temperature rise compared to TH for both the SAS and VAS. For VAS with TH, a maximum temperature rise of 8.51 °C is obtained at an optimum 5 mg/l mass fraction of carbon which is 1.46 times higher than the maximum temperature rise obtained for SAS with TH. For VAS with BH, a maximum temperature rise of 13.1 °C is obtained at an optimum 20 mg/l mass fraction of carbon which is 1.2 times higher than the maximum temperature rise obtained for SAS with BH. For BH a maximum value of Ra is obtained to be 1.12 × 10⁸. On comparing the temperature rise for BH and TH, the temperature rise for VAS with BH is 1.53 times than TH, whereas, for SAS, the BH gives a 1.88 times higher temperature rise than TH. The study concludes that buoyancy forces, Brownian motion, and thermophoresis play an important role in improving the thermal performance of VAS.