Experimental study on solar thermal conversion based on supercritical natural convection

In this paper, experimental investigation into the basic characteristics of solar thermal conversion using supercritical CO2 natural convection are presented. Natural circulation of supercritical fluids can be easily induced and even a small change in temperature can result in large change in density close to the critical point. The supercritical experimental system carefully designed and operated in this study. It is found that an obvious and continuous long-time drop of solar radiation would not affect the CO2 flow rate, temperature and pressure very much, if the solar radiation is in a relatively high-value level. This continuous drop can induce obvious drops in the CO2 flow rate, temperature and pressure only when the solar radiation is in a low-value level. Furthermore, it is observed that a long-time drop and low-value in the solar radiation may make the flow rate temporarily become zero, which should be paid more attention in future system design and operation. The collecting efficiency increases with the comprehensive coefficient and this pattern is contrary to that of water based system. In addition, it is found that there exist optimal flow rate and CO2 charge amount for system overall performance. This kind of solar thermal conversion has a higher collecting efficiency in spring and winter than summer and autumn; a better performance in cold and low-radiation region than hot and high-radiation region.