Publication: Análisis de las tensiones térmicas en tubos bayoneta del receptor de una central termosolar de torre central
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2019-07
Defense date
2019-07-01
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Abstract
La energía termosolar de concentración es un tipo de energía renovable que utiliza el calor solar para producir electricidad. Dentro de este tipo de tecnología se encuentran las centrales termosolares de torre central, en las que un sistema de espejos refleja los rayos solares en un receptor ubicado en la zona más alta de una torre, donde se calienta un fluido caloportador que es utilizado posteriormente para generar vapor a alta temperatura, el cual se usa para accionar una turbina y producir así energía eléctrica.
Las centrales termosolares de torre central están experimentando un gran crecimiento en los últimos años, no sólo expandiéndose, sino también optimizando sus características y producción energética. El elemento más crítico de estas centrales es el receptor solar, ya que está sometido a altas temperaturas que provocan elevadas tensiones térmicas en los tubos del receptor, ocasionando elevados costes de mantenimiento. Una de las soluciones planteadas recientemente para mejorar las condiciones del receptor y disminuir los efectos de las elevadas temperaturas es utilizar tubos bayoneta en lugar de los tubos simples tradicionales.
En este trabajo se ha realizado un diseño de tubos bayoneta para un receptor de central termosolar de torre central que ha sido simulado, analizando posteriormente las tensiones térmicas que se producen en dichos tubos. Para este diseño se han simulado diferentes casos con distintas separaciones entre tubos y distinta separación de éstos con la pared de alta reflectividad que se ubica tras ellos, eligiendo el caso óptimo en el que se producen menores tensiones térmicas. Dicho caso óptimo de separación en el que se producen menos tensiones ha resultado ser, para separación lateral entre tubos, el más pequeño de los analizados, con una separación lateral de un 1% del diámetro exterior del tubo, mientras que la separación trasera ha resultado no representar apenas variación de tensiones entre unos casos y otros.
Además, ha sido estudiado el flujo incidente de radiación mínimo para el que comienza a producirse daño en los tubos, y también la longitud crítica de grieta, a partir de la cual las grietas de la superficie pasan de experimentar un crecimiento estable a una propagación descontrolada hasta la fractura.
Concentrating Solar Power uses solar heat to generate high temperature steam, which moves a turbine that produces electricity. Within this technology, in the Solar Power Tower plants a system of mirrors reflects the solar rays on a receiver located in the highest area of a tower, where a heat transfer fluid is heated and used to generate steam. Solar Power Tower plants are growing rapidly in recent years, not only expanding, but also optimizing their characteristics and energy production. The most critical element of these power plants is the solar receiver, since it is exposed to high temperatures that cause high thermal stresses in the receiver tubes, causing high maintenance costs. One of the solutions recently proposed to improve receiver conditions and reduce the effects of high temperatures is to use bayonet tubes instead of traditional simple tubes. In this project, a bayonet tube design for a Solar Power Tower receiver has been developed and simulated in order to analyse the thermal stresses produced in these tubes. For this design, different cases have been simulated with different separations between tubes and different separation between the tubes and the wall of high reflectivity that is located behind them, choosing the optimal case in which lower thermal stresses are produced. The best case of separation between tubes in which less stress is produced is the smallest of the cases analyzed, with a lateral separation of 1% of the outer diameter of the tube. On the other hand, the rear separation hardly represents variation of stresses between some cases and others. In addition, it has been studied the minimum heat flux when damage appears on the receiver tube, as well as the critical crack length, which is the crack length where the crack starts unstable propagation to fracture.
Concentrating Solar Power uses solar heat to generate high temperature steam, which moves a turbine that produces electricity. Within this technology, in the Solar Power Tower plants a system of mirrors reflects the solar rays on a receiver located in the highest area of a tower, where a heat transfer fluid is heated and used to generate steam. Solar Power Tower plants are growing rapidly in recent years, not only expanding, but also optimizing their characteristics and energy production. The most critical element of these power plants is the solar receiver, since it is exposed to high temperatures that cause high thermal stresses in the receiver tubes, causing high maintenance costs. One of the solutions recently proposed to improve receiver conditions and reduce the effects of high temperatures is to use bayonet tubes instead of traditional simple tubes. In this project, a bayonet tube design for a Solar Power Tower receiver has been developed and simulated in order to analyse the thermal stresses produced in these tubes. For this design, different cases have been simulated with different separations between tubes and different separation between the tubes and the wall of high reflectivity that is located behind them, choosing the optimal case in which lower thermal stresses are produced. The best case of separation between tubes in which less stress is produced is the smallest of the cases analyzed, with a lateral separation of 1% of the outer diameter of the tube. On the other hand, the rear separation hardly represents variation of stresses between some cases and others. In addition, it has been studied the minimum heat flux when damage appears on the receiver tube, as well as the critical crack length, which is the crack length where the crack starts unstable propagation to fracture.
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Keywords
Energías renovables, Energía termosolar, Centrales solares tipo torre, Receptor solar, Tubo bayoneta, Tensión térmica