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 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 72 l/hr flow rate. the steady state performance tests were performed in different flow rates from 54 to 90 l/hr, based on the procedure of en 12975-2 standard. under similar operating conditions, a direct absorption solar collector using 100 ppm 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 on the results, the performance of a direct absorption solar collector using carbon nanotube nanofluids is better than a flat-plate solar collector.