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In this study, the performance of direct absorption solar collector is experimentally investigated using Fe3O4/Silica hybrid nanofluid based on deionized water. First, stability of prepared nanofluids is considered using spectral absorbency method. Then, spectrophotometry method is used for measuring optical properties of nanofluids. A prototype of this new type of collector was built with applicability for solar water heating systems. The procedure of EN 12975-2 standard was used for testing the thermal performance of the collector. Results show that collector efficiency is enhanced by nanofluid concentration, so that collector maximum efficiency is 73.9%, 79.8% and 83.7%using nanofluid with concentration of 500 ppm, 1000 ppm and 2000 ppm, respec/tively. This vaule is 63% using the base fluid as working fluid. Regarding very low volume fractions of nanofluids used in direct absorption solar collectors, the viscosity of the base fluid experience insignificant increase, therefore, pumping power will not increase significsantly. Such increase in efficiency show that direct absorption solar collector performance using hybrid nanofluid is much better than that of using the water at the same operating conditions. Application of stable hybrid nanofluid results in higher conversion efficiency of solar energy to useful energy.

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In this study, energy and exergy efficiency of residential-type direct absorption solar collector using PVP-coated silver nanofluid has been evaluated experimentally. First, stability and thermophysical and optical properties of nanofluid have been considered using the theoretical and experimental methods. Then, outdoor thermal performance of collector is investigated using the experimental setup based on EN12975-2. Results of energy analysis show that the collector efficiency is increased by increase of flowrate and concentration of nanofluid asymptotically. It is observed that exergy efficiency is firstly increased by nanofluid concentration and then, decreased after reaching the optimum value. The optimum concentration was 500 ppm for all flowrates. The variation of exergy efficiency by reduced temperature difference is similar to volume fraction. The optimum exergy efficiency is obtained at 0.01 m2K/W. The decrease of exergy efficiency by flowrate indicated that exergy losses due to pressure drop have the significant effect on the collector performance.