Performance characteristics of a residential-type direct absorption solar collector using MWCNT nanofluid

Abstract

In this study, the performance characteristics of a nanofluid-based direct absorption solar collector are investigated numerically and experimentally. The numerical model of nanofluid-based direct absorption solar collector was developed by solving radiative transfer equation combined with energy equation. The outlet temperature is analyzed by variation of internal emissivity of bottom wall, collector height, nanofluid volume fraction and flowrate. Then, a prototype of this new type of collector was built with applicability for domestic solar heating systems. As working fluid, different volume fractions of carboxyl functionalized multi wall carbon nanotubes in water and ethylene glycol mixture (70%:30% in volume) were prepared and their thermo-optical properties were presented. The procedure of EN 12975-2 standard was used for testing the thermal performance of the collector. The tests were performed in different flowrates from 54 to 90 l/h (0.015–0.025 kg/s) and two different internal surfaces (black and reflective) of bottom wall. By comparison of calculated and measured collector efficiencies for different volume fractions, it was shown that the numerical model was accurate within ±5% of the experimental results. The collector efficiency is increased by increasing nanofluid volume fraction and flowrates. The nanofluids improve the collector efficiency by 10–29% than the base fluid. The results of the study confirm that this new kind of collector can be best utilized in solar water heating applications.