Experimental and numerical study of the heat transfer process during the startup of molten salt tower receivers

During the startup operation of molten salt solar tower plants, the tubes of the receiver are subjected to a high nonhomogeneous heat flux along with a low heat transfer coefficient to the air inside the tube, which results in a highly uneven angular temperature distribution. This uneven temperature distribution causes thermal stresses and tube deflection in the receiver. The most important constraint for the design and operation of central receivers is to keep the intercepting solar flux within the tube mechanical safety limits. In this work, an experimental facility consisting of a molten salt loop that simulates a solar tower receiver tube is used to measure the outer surface temperature and the deflection of the tube under the startup operating conditions of a solar tower power plant. An inverse heat transfer problem is applied to obtain the heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient three-dimensional numerical model of an empty circular pipe subjected to a nonhomogeneous heat flux is developed. A good agreement between the experimental and calculated tube temperatures and deflection is observed, with differences of 7% and 10%, respectively. Moreover, the thermal stresses are calculated. It has been found that higher thermal stresses are obtained when the tube is preheated compared to the stress when the molten salt is flowing under similar heat flux conditions.