experimental investigation on performance comparison of nanofluid-based direct absorption and flat plate solar collectors

in the present work, a prototype of a new type of solar collectors, which called direct absorption solar collector, was built and its thermal performance is experimentally compared with conventional flat plate solar collector under transient and steady state conditions. different volume fractions of multi wall carbon nanotubes in water and ethylene glycol mixture (70%:30% in volume) were used as working fluid of direct absorption solar collector.the transient comparison show that the efficiency of the direct absorption solar collector becomes about 7% (in average) more than that of flat plate solar collector at 72l/hr flowrate. thesteady state performance tests were performed in different flowrates from 54to 90l/hr,based on the procedure of en 12975-2 standard.under similar operating conditions, adirect absorption solar collector using 100ppm carbon nanotube nanofluid has the zero-loss efficiency of 23% higher than that of a flat plate collector;whereas, the zero-loss efficiency of a direct absorption solar collector using the base fluid is 4.4% lower than that of a flat plate collector. based onthe results, the performance of a direct absorption solar collector using carbon nanotube nanofluidsis better than a flat-plate solar collector.