Revised receiver efficiency of molten-salt power towers

The demonstration power plant Solar Two was the pioneer design of a molten-salt power tower. In the report “Final Test and Evaluation Results from the Solar Two Project” (Pacheco, 2002, [15]) the efficiencies of the three main subsystems: heliostats, receiver and power block were measured or estimated. The efficiency of the global plant and the power block could be obtained with confidence, whereas the efficiencies of the heliostat field and the receiver could only be estimated because the solar flux reflected by the heliostats and intercepted by the receiver cannot be measured. The receiver efficiency was estimated using the Power-On Method. The authors themselves highlighted that this method contain an important assumption: the temperature distribution on the receiver surface is independent of the incident power level. This assumption is equivalent to have a Biot number much smaller than one for the solar receivers operation, fixed inlet and outlet salt temperature. For Solar Two reported data the Biot number is of order unity, and then the external tube temperature depends on the receiver load; and the thermal losses vary linearly with the incident solar flux rather than constant. Besides, our results show that receiver efficiency is around 76% for full load and 69% for half load instead of 87% and 80% reported when external tube temperature was assumed to be independent on the incident power.