Electronic International Standard Serial Number (EISSN)
The receiver tubes of Solar Power Tower plants are typically attached through clips and sliding rods to the reradiating wall of the receiver in order to reduce the thermal deflection of the tubes. However, as a side effect, these mechanical attachments increase heat dissipation from the tube to the exterior and can reduce the local temperature of the tube during the receiver start-up, an operation performed every morning or after a severe drop of solar irradiation due to passing clouds. In the start-up operation, the empty tubes of molten salt receivers must be preheated with a careful aiming sequence of the heliostat field until their temperature is high enough to avoid the freezing of the incoming flow of the molten salt. This work numerically addresses for the first time the influence of the mechanical attachment on the preheating temperature of a receiver tube. To achieve this goal, a set of three-dimensional thermal fluid simulations of the temperature distribution in the tube and the mechanical attachment, described with different levels of detail, has been performed for the receiver of the Gemasolar Solar Power Tower plant. The results of this work show that the mechanical attachment locally affects the tube temperature, leading to a temperature overestimation above 25.1% when the tube is modeled without a mechanical attachment. In addition, the temperature measured separated from the attachment at the rear face of the tube to control the preheating sequence may not be very representative of the minimum tube temperature, which occurs in the internal face of tube just beneath the mechanical attachment. The results indicate that accumulation of heat not only by the clip but also by the guide and the rod of the mechanical attachment plays a significant role in the dissipation of heat from the tube to the attachment. This reveals the need of considering all the elements of the mechanical attachment if an accurate modeling of the preheat temperatures of the tube is needed. Besides, the use of a simplification in the modeling of the air and radiative heat transfer outside the tube reduces by a factor of 4.9 the computational cost involved in a full simulation of these transfer processes, with a temperature discrepancy between models below 14.3%. Finally, the effect of the size of the attachment clip on the minimum tube temperature is investigated, revealing that the minimum tube temperature of the preheated tube can decrease below the freezing temperature of the molten salt for clip heights larger than 170 mm.
solar tower plant; central tubular receiver; non-uniform transient heating; receiver preheat; mechanical attachment; numerical modeling