Thermal efficiency enhancement of direct absorption parabolic trough solar collector (DAPTSC) by using nanofluid and metal foam

In the present work, the effects of using nanofluid, metal foam and their combination on the thermal performance of direct absorption parabolic trough solar collector (DAPTSC) were examined experimentally. The use of these volumetric absorbers were introduced because of some of their characteristics such as superior optical properties and better flow mixing which can lead to a lower thermal losses. In order to compare the optical properties of nanofluids and metal foam, the spectrometric experiments were also performed. Obtained results confirm that both nanofluid and metal foam are good solar absorbers. Therefore, CuO/water nanofluids at different volume concentrations (0.01%, 0.05%, and 0.1%) were examined as volumetric solar absorbers. On the other hand, copper metal foam with 95% porosity and 10 PPI (pore per inch) pore density was used as volumetric absorber. Outcomes reveal that applying metal foam raises the friction factor drastically between 50 and 80 times of pure water without metal foam. Furthermore, the maximum temperature differences for metal foam, nanofluid and combination of them are 8.8 °C, 10.7 °C and 16.6 °C, respectively. Finally, in order to evaluate the heat transfer enhancement and pressure drop increment simultaneously, the thermal performance for different solar absorbers was assessed.