Experimental thermal evaluation of a novel solar collector using magnetic nano-particles

A novel solar thermal collector using magnetic nano-particles to create a special array structure to capture solar light to enhance the thermal efficiency is put forward. The Gradient-Index Optics Theory is used to explain the physics of the proposed system. A set of experimental facility is set up to test the thermal collecting efficiency and a comparison between the coating vacuum tube and the conventional vacuum tube is conducted. 180 nm sphere iron nanoparticles were used under the stagnation experiment with water as the working medium. Results show that the magnetic tube performs as well as the coating tubes in lower temperatures and better than ordinary tubes all the time. Heat loss analysis shows the magnetic array structure has a larger ability to capture solar light while a lower ability to prevent heat loss due to the low emissivity layer in the coating. The normalized temperature difference instantaneous efficiency analysis shows that the magnetic tube has a higher top instantaneous efficiency as well as a higher heat loss coefficient, thus resulting a lower thermal efficiency as time passes and temperature rises. The temperature when the efficiency for the coating tube equals to that for the magnetic tube is about 53.8 °C and the temperature when the coating tube and the magnetic tube reach the same is about 73 °C. A comparison between the experimental results and what was available in literatures on the application of nanofluids to solar energy and a similar performance was observed.