Energetic and exergetic analysis of a heat pipe evacuated tube solar collector using MWCNT/water nanofluid

Increasing the energy demand and the limited availability of conventional energy sources are the main challenges to secure the human future. Solar energy is an attractive source of energy to ensure the sustainability of energy in this world for now and for the future. Evacuated tube solar collector is a solar thermal collector mostly used for domestic applications. The replacement of the working fluid with nanofluid can enhance the heat transfer even for low temperature applications. The effect of multi-walled carbon nanotubes and water nanofluid as working fluid in evacuated tube solar collector is used to experimentally investigate the energy and exergy efficiencies of the collector. Weight fractions of 0.005%, 0.01% and 0.05% were used in the investigation at flow rates from 1 to 3.5 L/min. The efficiency analysis of the collector was made as ASHRAE Standard 93-2010 stated. The results showed increase in energy and exergy efficiencies with the increase of volume flow rate and the concentration. The highest average energy and exergy efficiency achieved with 0.05 wt% MWCNT/water nanofluid were 55% and 10% respectively. The overall heat loss coefficient decreased with the increase of the concentration. It was found that 2 L/min were a critical flow rate and the behaviour of the system changed afterwards.