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• A methodology to take into account tube elbows in stress and deflection is presented. • The proposed methodology is faster and gives similar results to finite element models. • It is feasible to assume straight tubes and plane strain when clips distance is small. • Stress distribution along tubes varies according the clips distance. • Deflection is higher for flat aiming strategies. Mechanical boundary conditions in tubular receivers of solar power tower plants have a main role in the thermal stress distribution and tube deflection. Longitudinal supports, particularly, has an strong influence on stress and displacements, since they prevent the tube bending. In this work, the influence of longitudinal supports, on tube deflection and stress has been studied in external-cylindrical receivers, using an analytical methodology, which it is able to take into account the tube geometry in the deflection calculation. Therefore, real tube geometry with elbows can be considered. Results for two aiming strategies, one equatorial and another that flattens the heat flux, have been compared for different clips distances, from 1 to 9 meters. The analytical methodology developed in Matlab provides lower computational cost than the numerical model developed in Abaqus. Results show that clip distribution has a significant impact on thermal stress. For clips distance of 2 meters or lower, the generalised plane strain solution provides the stress distribution along the tube accurately, with a tube deflection lower than 1 millimetrer. When clips distance increases, the longitudinal stress distribution differs from the plane strain case, and the deflection increases to non-desirable values. Deflection is greater at tube ends, and aiming strategies that flatten the heat flux increases the displacement in that regions. [ABSTRACT FROM AUTHOR]

Highlights • Mechanical constraints from longitudinal supports have to be taken into account to obtain total stresses. • Longitudinal supports prevent the tube deflection, increasing thermal stress. • Circumferential temperature variation has a leading role in the tube stress. • Axial temperature variation is needed for locating the maximum stress point. Abstract Failure in solar external receivers is mainly originated from the thermal stress, caused by the high non-uniform transient solar flux. The heat-up and cooldown of tube receivers in daily cycles produce low-cycle fatigue that limits the lifetime of tubes. The corrosion of tube materials produced by incompatibility between the decomposed heat transfer fluid and tube material may increase this issue. The temperature spatial distribution in these tubes has strong variations in radial, circumferential, and axial directions. The stress field, produced by the temperature gradients, has been commonly analyzed using bidimensional models in isolated tube cross sections, without taking into account the axial temperature variation, the mechanical boundary conditions, and the temperature-dependent thermomechanical properties. In this work, a three-dimensional finite element model has been developed in order to calculate the stress field distribution, without performing any geometrical simplification. In addition, appropriate mechanical boundary conditions have been imposed in order to adequately simulate the tube behavior. Besides, radial, circumferential and axial temperature variations have been studied separately to analyze how each of them influences the maximum stress distribution. This 3D model has been compared with analytical solutions for the two-dimensional thermal stress problem in circular hollow cylinders. The results show that the boundary conditions have a significant effect on the tube stresses, increasing the axial stress component and therefore the equivalent stress. The analysis of each of the temperature variations showed that the circumferential variation temperature is the one that produces most of the stress, since it tries to strongly bend the tube, which is impeded by the boundary conditions. The results also present that 2D models are not capable of obtaining the correct stress distribution along the tube, since they are not taking into account the longitudinal supports. By contrast, the maximum stress can be obtained with confidence using the analytical stress solution of the angular and radial temperature variation around a hollow circular cylinder. [ABSTRACT FROM AUTHOR]

The receiver is a key component of concentrated solar power plants, whose operation can be interrupted by its early failure. In this research, a novel approach to the enhancement of solar receiver lifetime is presented, studying the use of linear actuators in the longitudinal supports of the tubes to induce forced displacements, reducing thermal stresses. Existing analytical models have been improved and joined together, in order to be implemented in a genetic algorithm to optimize the solar receiver lifetime. In a first approach, the maximum equivalent elastic stress, produced by the thermal gradients over the receiver, has been minimized. This approach reduces the maximum thermoelastic stress up to a 9.5%, and increases the lifetime of the receiver to an 8% when a forced displacement of 100 mm (1% of the tube length) is considered. When the rupture time due to creep is optimized, with the same forced displacement of 100 mm, the tube lifetime is enhanced up to a 25%. Small forced displacements (0.1% of the tube length) may lead to a lifetime improvement up to 6%. The small amount of force necessary to carry out the forced displacement of the supports makes feasible the implementation of linear actuators in future solar power plants. [Display omitted] • A novel method to reduce stress in solar receivers via linear actuators is presented. • A Genetic algorithm is used to enhance the receiver lifetime via forced displacements. • Equivalent elastic stress and creep rupture time has been optimized. • The receiver lifetime is improved a 25% with forced displacements of 1% of its length. • Reaction forces are small enough to make the use of linear actuators feasible. [ABSTRACT FROM AUTHOR]