On the design of solar external receivers

Mención Internacional en el título de doctorSolar external receivers with molten salt as heat transfer fluid are the most critical subsystem of a Solar Power Tower (SPT). Receiver tubes work under extreme conditions due to the high incident solar flux and the potentially corrosive environments. These demanding conditions of operation usually produce the failure of the receiver by stress corrosion cracking. The unsteady solar flux and the large size of the heliostat field and the receiver make very complicated accurate measurement of the spatial heat flux on the receiver tubes. Hence, modelling accurately the solar flux onto the receiver and the heat transfer in the tubes is required. This PhD thesis consists in the development and validation of several thermal models of external receivers to improve the estimation of the temperature distribution on the receiver tubes and the thermal efficiency. The application of the models has enabled to establish the guidelines for the accurate and safety design of the external receivers. In this thesis there are presented two simplified and two-dimensional models. The first model assumes homogeneous heat flux in the tubes, while the other assumes homogeneous temperature. The main characteristic of the models is that they consider circumferential and axial distribution of the temperature in the receiver tubes. In addition, they take into account the main heat exchange mechanisms, as well as the temperature dependence of the thermo-mechanical properties of tube materials and heat transfer fluid. Firstly, the SPT operation modes and weakness were analysed. Subsequently, the viability of installing a system to reduce the parasitic energy consumption of the SPT was studied. This system, named Potential Energy Recovery System (PERS), recovers the potential energy from the downcomer of the receiver. The PERS was included in the models of two different actual SPT resulting in important energy savings in both plants. The simplified models were validated with CFD simulations, other simplified models, and experimental data. Regarding the CFD, the accuracy of the results is similar, but the simplified models proposed here have a significant lower computational cost, which is a notable advantage for the pre-design of the receiver where many geometrical parameters must be analysed. Regarding experimental data, given the inlet temperature of the heat transfer fluid, the direct normal irradiance, and an approximation of the aiming strategy of the heliostat field, the results obtained for the outlet temperature of the salt and the mass flow rate in the receiver are very close. Comparing with previous simplified models the thermal efficiency obtained is around 10% lower than in previous studies. The key of this difference is the thermal resistance for the heat transfer process related to the fluid and the tube material. It was also seen that the Biot number is large, and therefore the circumferential temperature must be taken into account for proper receiver efficiency estimation. In addition, different receiver geometries were analysed to find the optimum receiver design. It was determined that the most restrictive variables are the mechanical stresses and the film temperature. Regarding the receiver flow path, the best option is to implement two symmetrical paths that in the north hemisphere go from north - to - south of the receiver assuring the peak flux far from southern panels. Finally, the feasibility of employing SPT that uses supercritical or ultrasupercritical power blocks was analysed using the developed thermal models. However, the increase of the power block efficiency implies higher heat losses in the receiver. Therefore, the new generation of SPT will be only advisable when the cost of materials and systems decrease considerably.Los receptores solares de sales fundidas son el subsistema más crítico de las centrales termosolares tipo torre. Estos receptores están sometidos a unas condiciones de trabajo extremas, destacando la gran concentración de flujo solar incidente y un ambiente de trabajo potencialmente corrosivo. Estas condiciones tan exigentes suelen producir roturas en el receptor por corrosión bajo tensión. Además debido a la gran inestabilidad del flujo solar y a las grandes dimensiones tanto del receptor como de los heliostatos es muy complicado determinar de forma precisa la distribución espacial del flujo de calor sobre los tubos del receptor, resultando imprescindible el modelado del flujo de calor sobre el receptor y la transferencia de calor en sus tubos. Esta tesis doctoral se basa en el desarrollo y validación de varios modelos térmicos de receptores centrales que intentan mejorar la estimación de la distribución de temperatura en los tubos del receptor y su eficiencia térmica global. Mediante la aplicación de estos modelos se han establecido las pautas para el diseño de receptores que aseguran un funcionamiento fiable del mismo. Los modelos térmicos desarrollados son simplificados y bidimensionales, uno de ellos asume flujo de calor constante en los tubos y el otro temperatura constante. La característica principal de estos modelos bidimensionales es que tienen en cuenta las variaciones circunferenciales y axiales de temperatura en los tubos del receptor. Además, estos modelos aunque sencillos y rápidos tienen en cuenta los principales mecanismos de intercambio de calor, y que las propiedades termomecánicas de materiales y del fluido caloportador dependen de la temperatura. En primer lugar se estudió el funcionamiento de las centrales solares de torre, analizando sus fortalezas y debilidades. Surge así la idea de evaluar la viabilidad de implantar un sistema que reduzca su auto-consumo energético. Este sistema, denominado PERS, consiste en recuperar la energía potencial del fluido caliente que baja del receptor a los tanques de almacenamiento. El PERS se ha incluido en el modelo de dos centrales solares de torre diferentes, y en ambos casos se han encontrado unos importantes ahorros energéticos. Los modelos simplificados desarrollados han sido validados con CFD, otros modelos simplificados y datos experimentales. Con respecto a las simulaciones CFD los resultados obtenidos son del mismo orden pero con una notable reducción del coste computacional, lo que significa una ventaja notable para el pre-diseño de los receptores centrales, donde son analizados numerosos parámetros geométricos. Comparando con los escasos datos experimentales publicados, conocidas la temperatura de entrada del fluido de trabajo, la irradiación solar directa y una aproximación de la estrategia de apuntamiento del campo de heliostatos, se han obtenido unos flujos másicos y unas temperaturas de salida del fluido muy similares a los experimentales. Para completar el estudio, se han comparado nuestros modelos con otros modelos simplificados de la bibliografía. En este caso la eficiencia térmica del receptor obtenida es alrededor de un 10% menor a los obtenidos previamente. La clave de esta diferencia es la resistencia térmica en el proceso de transferencia de calor, relacionada tanto con el fluido como con el material de los tubos. Además se ha visto que el número de Biot es elevado, y por lo tanto las variaciones circunferenciales de temperatura deben tenerse en cuenta para estimar la eficiencia térmica del receptor correctamente. En esta tesis se han analizado diferentes geometrías del receptor bajo diversos modos de funcionamiento en orden de encontrar un diseño óptimo. Se ha determinado que las variables más restrictivas para el diseño del receptor son el estrés mecánico y la temperatura de película. En cuanto a los canales de flujo, en el hemisferio norte la mejor opción es implementar dos canales simétricos que circulen de norte a sur, asegurando que el pico de densidad solar se encuentre lejos de la zona de salida del receptor, lado sur. Finalmente, la posibilidad de utilizar una nueva generación de centrales solares tipo torre que emplee bloques de potencia supercríticos y ultra-supercríticos ha sido analizada con el empleo de los modelos simplificados previamente desarrollados. Sin embargo, el aumento de eficiencia en el bloque de potencia implica mayores temperaturas y pérdidas de calor en el receptor. Por lo tanto, esta nueva generación de centrales de torre sólo será recomendada cuando los precios de los materiales y de los sistemas supercríticos desciendan considerablemente.Programa Oficial de Doctorado en Ingeniería Mecánica y de Organización IndustrialPresidente: José María Martínez-Val Peñalosa.- Secretario: Eduardo Zarza Moya.- Vocal: Manuel J. Blanco Murie

Aiming strategy model based on allowable flux densities for molten salt central receivers

This study presents an aiming model to properly point heliostats at cylindrical molten salt receivers in Solar Power Tower. By means of two iterative algorithms (search and fit), the proposed strategy attempts to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints are translated into allowable flux densities (AFD) that are handled by the model. The computer code accommodates the flux images produced by each heliostat in a field to accurately fit the AFD limit. In this paper, a Gemasolar-like field-receiver system serves to illustrate the aiming model. Compared to the equatorial aiming, receiver interception is slightly lower using the proposed strategy, but the receiver integrity is ensured; peak flux is significantly reduced up to 23%. It has been found that a favorable flux density profile generally has its peak displaced to the salt entrance at each receiver panel. Since external cylindrical receivers consist of a combination of up-flow and down-flow panels, the optimal flux profile is challenging for contiguous panels with contrary demands. In spite of that, remarkable matching is achieved by the fit algorithm. Because of its fast computation and automatic operation, the resulting tool can be applied to real-time control of existing heliostat fields and the integrated design of the coupled systems field and receiver

Field-receiver model validation against Solar Two tests

Validation is a crucial aspect in the reliability assessment of models. Validation accuracy is measured with respect to experimental data. In this regard, there is not any experimental technique with sufficient spatial resolution capable of measuring the incident solar flux onto the receiver of solar power tower plants to model validation. Therefore, the individual optical efficiency of the field and the thermal performance of the receiver cannot be accurately obtained experimentally. To calculate these efficiencies, the development of numerical models is mandatory. Although, numerous receiver models can be found in the literature, the accuracy of most of them is not checked because of the scarcity of experimental data to compare with. In this study, the simulations of a model that includes the heliostat field and an external tubular receiver, taking into account all the receiver tubes, have been compared with available experimental data from Solar Two plant, in order to check its accuracy. It was obtained that the model overestimates 1.42% the total mass flow rate and 0.73% the global efficiency of Solar Two, which is almost negligible. Besides, the field-receiver efficiency can be calculated with confidence using Solar Two experimental data, and then it can be taken into account in the validation of the model. The model error, with respect to the experimental data are of 1.1% at full load and 1.4% at 50% partial load

Feasibility study of a new concept of solar external receiver: Variable velocity receiver

The deployment of new solar power tower plants mainly depends on becoming cost-competitive with traditional forms of electricity generation. The solar field represents around 40% of the solar power tower investment cost, thus the cost reduction of such subsystems is mandatory to achieve that goal. This reduction could be done by increasing the solar flux intercepted by the receiver, which would increase the peak flux. Therefore, new concepts of solar receivers are required to accommodate such high peak flux. The proposed receiver, which withstands high peak flux, consists on a Traditional External Tubular Receiver (TETR) equipped with valves that allow the division of each panel of the receiver in two independent panels, increasing the velocity of the heat transfer fluid in specific zones of the receiver. This receiver configuration, named Variable Velocity Receiver (VVR), avoids tube overheating. Moreover, this novel receiver allows more concentrated aiming strategies, which increases the optical efficiency of the solar field and permits to reduce the number of heliostats in the field. Given a specific generation capacity, the size of the solar field required by a VVR is 12.5% smaller in comparison to a TETR. Such efficiency improvement has a negligible effect in tube mechanical stresses; even though pressure drop and parasitic consumption of the power plant increase. This new receiver configuration also gains hours of operation, even in winter: in hours with low solar irradiance all the panels can be split in two, increasing the number of passes and the velocity of the heat transfer fluid and accomplishing the transition from laminar to turbulent regime. Therefore, this receiver is able to reduce the levelized cost of energy

Aiming factor to flatten the flux distribution on cylindrical receivers

High incident flux gradients and hot spots lead to extreme thermal stresses that may damage and reduce the lifetime of central receivers. An aiming strategy based on a single parameter, k, named aiming factor, is developed to generate symmetric flux maps about the receiver equator. By means of this k factor, ranging between 3 (generally equivalent to equatorial aiming) and 0 (alternatively aiming to top and bottom borders), the solar flux incident on the receiver and the spillage losses can be controlled. For each sector in a heliostat field, the aiming factor values causing the flattest symmetric flux maps, kflat, are deterministically found with a sweep and mesh shifting procedure. Results for Dunhuang solar power tower plant show that kflat is fairly constant throughout the year, except near sunrise and sunset in east and west sectors, respectively./This work has been supported by the Spanish Ministry of Economy and Competitiveness under the project ENE2015-69486-R (MINECO/FEDER, UE)

Allowable solar flux densities for molten-salt receivers: Input to the aiming strategy

Solar Power Tower technology requires accurate models and tools to assist in design and operation stages. The heliostat field aiming strategy seeks the maximization of the thermal output from the receiver, while preventing its permanent damage because of thermal stress and corrosion in molten salt receivers. These two limitations are translated into Allowable Flux Densities (AFD), which can be handled by the aiming strategy. This paper explains the methodology to determine AFDs, and analyzes the influence of tube geometry and material. AFD by corrosion is slightly lower in Haynes 230 than Inconel 625 and austenitic alloys. On the contrary, HA230 has better performance than In625 under thermal stress. Increment of tube wall thickness diminishes the AFD: slightly by corrosion, but significantly by thermal stress. The generated AFD databases feed the aiming model, herein applied to Gemasolar case study. In the cylindrical receiver, first northern panels are limited by thermal stress, while the last ones by corrosion. Under optimized aiming, HA230 receiver tubes produce equivalent thermal output than In625.This study has been supported and financed by Iberdrola España Foundation under the program Ayudas a la investigación en Energía y Medio Ambiente 2018, project: “Diseño y evaluación de un Nuevo receptor solar exterior de tubos ovalados”. The authors acknowledge to 11CNIT (XI National and II International Engineering Thermodynamics Congress) committee for selecting the previous conference contribution for further dissemination through this special issue

Revised receiver efficiency of molten-salt power towers

The demonstration power plant Solar Two was the pioneer design of a molten-salt power tower. In the report "Final Test and Evaluation Results from the Solar Two Project" (Pacheco, 2002, [151) the efficiencies of the three main subsystems: heliostats, receiver and power block were measured or estimated. The efficiency of the global plant and the power block could be obtained with confidence, whereas the efficiencies of the heliostat field and the receiver could only be estimated because the solar flux reflected by the heliostats and intercepted by the receiver cannot be measured. The receiver efficiency was estimated using the Power-On Method. The authors themselves highlighted that this method contain an important assumption: the temperature distribution on the receiver surface is independent of the incident power level. This assumption is equivalent to have a Blot number much smaller than one for the solar receivers operation, fixed inlet and outlet salt temperature. For Solar Two reported data the hot number is of order unity and then the external tube temperature depends on the receiver load; and the thermal losses vary linearly with the incident solar flux rather than constant. Besides, our results show that receiver efficiency is around 76% for full load and 69% for half load instead of 87% and 80% reported when external tube temperature was assumed to be independent on the incident power

Experimental study of honeycomb SiCSi under highly concentrated solar flux: Evolution of its thermo-radiative properties

The material that is used in solar receivers is subjected to intense cyclic thermal stresses and extreme temperatures, which are directly dependent on the intermittence of the solar resource. These factors accelerate the ageing mechanisms and reduce the durability of the receivers because of a reduction of their thermal performance. This study presents guidelines to study the thermo-radiative properties of an absorber material that is subjected to a highly concentrated solar flux. The material was a square honeycomb SiCSi structure that is typically used in volumetric air receivers. Accelerated ageing tests were performed by means of crashing thermal treatments, in which the modulus and period of the incident flux and the boundary conditions of the material were varied. The reflectivity and absorptivity of the material were experimentally characterized before and after the thermal treatments. The measurements were performed using two different reflectometers, one monochromatic and one in the solar band; the latter can measure at ambient temperature or high temperature that is representative of the operational conditions (400-700 degrees C). However, only the solar band reflectometer working at high temperature was able to detect the evolution of the thermo-radiative properties of the material, which highlights the important role of the temperature and the wavelength. Furthermore, the thermal treatments in which the samples were water-cooled and in which the solar flux was medial more quickly accelerated the ageing mechanism of the material and reduced its absorptivity.The authors acknowledge the financial support of the research programme of the Universidad Carlos III de Madrid, 2015, which made this study possible through a mobility grant. Moreover, this work was supported by the French "Investments for the future" programme, which is managed by the National Agency for Research under contract ANR-10-LABX-22-01 by Labex SOLSTICE and by the Spanish government under the project ENE2012-34255

Saving assessment using the PERS in solar power towers

The improvement of the solar power tower using solar salt is one of the main goals of researchers. Any method or invention to improve the efficiency of this technology contributes to promote the renewable energies. The use of a Potential Energy Recovery System (PERS) in two different solar power tower plants of 20 and 100 MW has been analysed. The PERS is formed, at least, by one turbine, located at the hot salt pipe coming from the receiver. The turbine is engaged to the shaft of the feed pump, which raises the heat transfer fluid from the cold tank to the receiver. It reduces the parasitic power consumption of the plant, and increases its global efficiency. Different PERS configurations have been modelled. Based on an energetic and economic analysis, the optimal configuration is a geometrical similar turbine of three times the volume flow rate of one feed pump. The PERS has been proven to be a cost reductive and clean tool. For a 100 MW power plant of 30-year lifetime the investment cost is 1.26 M$and the annual cash flow is 0.89 M$, while for a plant of 20 MW these values are 0.26 M$and 0.19 M$, respectively.The financial support from CDTI and S2m Solutions for the MOSARELA Project (Molten salt receiver lab), whose reference is IDI-20120128. Finally, the financial support of the ENE2012-34255 Project is also acknowledged

Development of a new method to estimate the incident solar flux on central receivers from deteriorated heliostats

This work proposes a new empirical direct methodology to estimate both the solar flux distribution and intensity on the surface of central receivers. In solar power tower plants with deteriorated heliostats, the numerical simulations to estimate the incident solar flux are not precise. Hence the thermal behaviour of the receivers cannot be determined. In those cases, direct measurement or semi-empirical methodologies are required to characterize the radiant power on the receiver. The new methodology proposed, named "Superposition method", consists in the hourly characterization of the reflected solar beam of each individual heliostat by means of a pyrheliometer, a passive screen, a flux sensor, a camera and digital image analysis. According to the aiming strategy used during receiver operation, each individual solar flux distribution and intensity can be gathered to obtain the total incident radiant power on the solar receiver. This non-real-time method has the advantage of reproducing any solar flux distribution on the receiver at present and past time