Experimental optimization on the volume-filling ratio of a loop thermosyphon photovoltaic/thermal system

Loop thermosyphon (LT) is a two-phase closed device that provides an alternative solution to overcome the freezing problem associated with the traditional photovoltaic/thermal (PV/T) system. The overall performance of a LT-PV/T system is significantly affected by volume-filling ratio. To reveal this effect, a prototype of the LT-PV/T system was designed and constructed in this paper; and long-term outdoor tests under 24%, 32%, 40% and 48% volume-filling ratios were carried out. The results show that the daily average photothermal performance of the LT-PV/T system presented a trend of increased first and then decreased with the volume-filling ratio increasing on the selected sample days. The highest performance was generated under the 32% filling condition. The solar evaporator had an even temperature distribution when the volume-filling ratio was no less than 32%; the temperature difference of the solar evaporator decreased with the volume-filling ratio increased, the values were 1.5 °C, 1.2 °C and 0.6 °C, respectively. Performances of the collector and system were linear fitted based on the outdoor tests, the fitting results showed that both of them shared the same variation trend, the optimum volume-filling ratio lied in between 32% and 40% of the whole system volume, but nearer to 32%.