Publication: Heliostat field aiming strategies for solar central receivers
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2016-12
Defense date
2016-12-16
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Abstract
La presente tesis está dedicada al desarrollo de modelos ópticos aplicados a la tecnología de central solar
tipo torre. Más concretamente, este trabajo se centra en el modelado de mapas de flujo y estrategias de
apuntamiento para sistemas de receptor central. Los códigos resultantes son de utilidad en las fases de
diseño y operación de centrales solares de torre. Esta memoria fundamentalmente presenta cuatro modelos
computacionales.
El primer modelo, sobre el cual se construyen el resto de modelos, calcula la distribución de densidad
de flujo incidente en cualquier tipo de receptor central y que es causada por un único heliostato. El
procedimiento se basa en la proyección oblicua de la malla de cálculo desde el receptor hasta el plano imagen,
en donde se evalúa una función analítica de precisión conocida, e.g. UNIZAR. La proyección oblicua
se obtiene mediante un adecuado cambio de sistemas de coordenadas. El método de proyección reproduce
notablemente la distorsión presente en la mancha de luz concentrada cuando el ángulo de incidencia con el
receptor es elevado. Este modelo básico ha sido validado con medidas de distribución de flujo en un receptor
plano y con simulaciones de Monte Carlo de trazado de rayos para un receptor cilíndrico. En comparación
con SolTrace, el modelo propuesto requiere un tiempo de computación 50 veces inferior y con un nivel de
resolución aún mayor.
El segundo modelo determina los errores de canteo en las facetas de heliostatos reales. En base a un
algoritmo de optimización determinista, se ha establecido un procedimiento que ajusta los mapas de flujo
simulados con las imágenes tomadas en un blanco lambertiano. Se han empleado imágenes experimentales
tomadas en la planta THEMIS para encontrar los errores de reglaje en tres heliostatos CETHEL seleccionados.
A partir de los resultados del modelo, uno de los heliostatos ha sido satisfactoriamente reajustado,
mejorando de forma significativa su calidad óptica y validando la metodología propuesta.
El tercer modelo es una ampliación del primero de ellos para superponer los mapas de flujo producidos por
cada uno de los heliostatos en un campo completo. Las pérdidas ópticas por sombras y bloqueos se calculan
mediante proyección paralela de los heliostatos vecinos. Se ha desarrollado una estrategia de apuntamiento
que da lugar a mapas de flujo simétricos respecto de la línea media ecuatorial del receptor y que depende
de un solo parámetro: k, factor de apuntamiento. Con k = 3 se obtienen mapas de flujo similares a los
de apuntamiento simple al ecuador, mientras que con k = 0 los heliostatos apuntan a los bordes inferior y
superior del receptor. Para el caso de estudio basado en Gemasolar, un factor de apuntamiento igual a 2
da lugar a las distribuciones de flujo más uniformes, i.e. perfil plano en la región central, sin menoscabo del
factor de intercepción en comparación con el apuntamiento ecuatorial simple.
En el cuarto de los modelos se ha implementado una estrategia de apuntamiento óptima para receptores
centrales de sales fundidas. Se ha desarrollado un algoritmo que maximiza la potencia térmica instantánea
del receptor, al mismo tiempo que se cumplen sus límites de operación. Los límites de corrosión y estrés
térmico se traducen en flujos máximos admisibles; AFD, por sus siglas en inglés. En comparación con el
apuntamiento simple, habitualmente inviable, la estrategia de apuntamiento optimizado asegura la integridad
del receptor, a la vez que las pérdidas por desbordamiento sólo se incrementan en 4 puntos porcentuales.
Se ha comprobado que la posición óptima de los apuntamientos en cada panel se encuentra, en promedio,
ligeramente desplazada hacia el lado de entrada de las sales. A pesar de los requisitos contradictorios entre paneles adyacentes de receptores multi-panel con flujo de sales en serpentín, el algoritmo consigue un buen
ajuste al perfil AFD instantáneo. El código resultante requiere alrededor de 2 minutos de cálculo en una
computadora estándar para determinar los apuntamientos óptimos en un campo de 2650 heliostatos.
This thesis deals with the development of optical models for solar power tower technology. Specifically, this work is focused on modeling flux mapping and aiming strategies for central receiver systems (CRS). The resulting codes are applicable to CRS design and operation. This dissertation essentially presents four computational models. The first model, on which the rest of the models are built up, computes the flux density distribution incident on any kind of central receiver which is caused by a single heliostat. The procedure relies on the oblique projection of the receiver mesh onto the image plane, where an accurate analytic function, e.g. UNIZAR, is evaluated. Oblique projection is accomplished by transformation of coordinate systems. The 4-step projection method remarkably reproduces the distorted spot found for large incidence angles on the heliostat and the receiver. This basic model was validated against flux measurements on a flat receiver and Monte Carlo Ray Tracing simulations on a cylindrical receiver. Compared to SolTrace, the model takes 50 times less computation time and higher level of resolution. The second model was developed to determine canting errors in the facets of real heliostats. Based on a deterministic optimization algorithm, a procedure was set up to minimize the difference between computed flux maps and captured images on a lambertian target. Experimental images from THEMIS plant were employed to find out canting errors in selected CETHEL heliostats. From results of the model, one of the heliostats was successfully readjusted, significantly improving its optical quality, and validating the proposed methodology. The third model extends the basic model to superpose single heliostat flux maps in a whole field of heliostats. Shading and blocking losses are computed by parallel projection of neighbor heliostats. An aiming strategy, symmetric about the receiver equator, was developed on the basis of only one parameter: k, aiming factor. Nearly single equatorial aiming is achieved with k = 3, while k = 0 results in pointing to either upper or lower receiver edges. For the Gemasolar case study, an aiming factor equal to 2 yielded the most uniform flux maps, i.e. flat profile in the central region, and negligible increase in spillage losses compared to equatorial aiming. An optimal aiming strategy for molten salt receivers was implemented in the fourth model. An algorithm was developed to maximize receiver thermal output, while meeting at the same time corrosion and thermal stress limits; which were translated into allowable flux densities, AFD. Compared to unreliable single aiming, the optimized aiming strategy ensures receiver integrity and spillage losses only increase up to 4 percentage points. It was found that optimal aim points are, on average, slightly shifted towards the panel entrance. Despite the conflicting demand between adjacent panels in multi-panel receivers with serpentine flow pattern, the fit algorithm performs noticeable matching to the AFD profile. The resulting code takes around 2 minutes in a standard PC to compute the optimal aim points for a field made up of 2650 heliostats.
This thesis deals with the development of optical models for solar power tower technology. Specifically, this work is focused on modeling flux mapping and aiming strategies for central receiver systems (CRS). The resulting codes are applicable to CRS design and operation. This dissertation essentially presents four computational models. The first model, on which the rest of the models are built up, computes the flux density distribution incident on any kind of central receiver which is caused by a single heliostat. The procedure relies on the oblique projection of the receiver mesh onto the image plane, where an accurate analytic function, e.g. UNIZAR, is evaluated. Oblique projection is accomplished by transformation of coordinate systems. The 4-step projection method remarkably reproduces the distorted spot found for large incidence angles on the heliostat and the receiver. This basic model was validated against flux measurements on a flat receiver and Monte Carlo Ray Tracing simulations on a cylindrical receiver. Compared to SolTrace, the model takes 50 times less computation time and higher level of resolution. The second model was developed to determine canting errors in the facets of real heliostats. Based on a deterministic optimization algorithm, a procedure was set up to minimize the difference between computed flux maps and captured images on a lambertian target. Experimental images from THEMIS plant were employed to find out canting errors in selected CETHEL heliostats. From results of the model, one of the heliostats was successfully readjusted, significantly improving its optical quality, and validating the proposed methodology. The third model extends the basic model to superpose single heliostat flux maps in a whole field of heliostats. Shading and blocking losses are computed by parallel projection of neighbor heliostats. An aiming strategy, symmetric about the receiver equator, was developed on the basis of only one parameter: k, aiming factor. Nearly single equatorial aiming is achieved with k = 3, while k = 0 results in pointing to either upper or lower receiver edges. For the Gemasolar case study, an aiming factor equal to 2 yielded the most uniform flux maps, i.e. flat profile in the central region, and negligible increase in spillage losses compared to equatorial aiming. An optimal aiming strategy for molten salt receivers was implemented in the fourth model. An algorithm was developed to maximize receiver thermal output, while meeting at the same time corrosion and thermal stress limits; which were translated into allowable flux densities, AFD. Compared to unreliable single aiming, the optimized aiming strategy ensures receiver integrity and spillage losses only increase up to 4 percentage points. It was found that optimal aim points are, on average, slightly shifted towards the panel entrance. Despite the conflicting demand between adjacent panels in multi-panel receivers with serpentine flow pattern, the fit algorithm performs noticeable matching to the AFD profile. The resulting code takes around 2 minutes in a standard PC to compute the optimal aim points for a field made up of 2650 heliostats.
Description
Mención Internacional en el título de doctor
Keywords
Concentrating solar power tower, Central receiver system, Flux density distribution, Multi-aiming strategy, Heliostat optical quality, Torre solar de concentración, Sistema de receptor central, Distribución de flujo, Estrategia de apuntamiento, Calidad óptica de heliostatos