Experimental investigation of the thermal performance of a horizontal two-phase loop thermosiphon suitable for solar parabolic trough receivers operating at 200–400 °C

A horizontal two-phase loop thermosiphon (HLTS) has been developed as a potential receiver for parabolic trough collectors (PTCs). The design consists of an evaporator (which is horizontally arranged), a condenser, a riser, and a downcomer with a U-turn. This HLTS was designed to push to higher temperatures than previous HLTS studies (200–400 °C) by using Dowtherm A as the working fluid. An indoor experimental prototype was built to investigate its heat transfer performance. Three regimes: start-up, transition and steady operation were analyzed. A unique feature of this design, the U-turn compensation tube, was shown be helpful during the transition and steady operation regimes since it forms a liquid seal to avoid bidirectional flow in the loop. However, solidification of the working fluid in the U-turn section was found to adversely impact the start-up regime in the case of cold (e.g. frozen) initial conditions. The system was tested up to a heat flux value 11.22 kW/m². The thermal resistance and the two-phase heat transfer coefficient were demonstrated to be considerably better than prior literature. Moreover, the present HLTS was shown to be theoretically limited to 85.6 kW/m², thus demonstrating that this type of system can meet the needs of intermediate temperature PTC receivers.