Experiments on Natural Convective Solar Thermal Achieved by Supercritical CO₂/Dimethyl Ether Mixture Fluid

The current study proposed an experimental investigation into the basic characteristics of solar thermal conversion using supercritical CO₂–dimethyl ether (DME) natural convection. The main goals are to reduce the operation pressure while maintaining relative high solar thermal conversion efficiency. Experimental systems were established and tested in Shaoxing area (around N 30.0 deg, E 120.6 deg) of Zhejiang Province, China. Due to the preferable properties of supercritical fluids, very high Reynolds number natural convective flow can be achieved. Typical summer day results are presented and analyzed into detail in this paper. It is found that the introduction of DME has successfully reduced the operation pressure and the increase in DME fraction leads to further reduction. Different from pure supercritical CO₂ systems, the collector pressure follows the trend of solar radiation with its peak value at noon, instead of continuously increasing mode. The mass flow rate and temperature are typically more stable and also more sensitive than pure supercritical CO₂ tests due to the moderation of supercritical fluid properties when DME is introduced. At the same time, the averaged collector efficiency is less affected by the DME mass addition. It is also found that there possibly exist some optimal of DME mass fraction when both the system suitability and stable natural circulation can be achieved.