Optimización termodinámica de plantas termosolares híbridas de ciclo Brayton

[ES] La actual intensificación antropogénica del cambio climático junto con el agotamiento de los combustibles fósiles han impuesto un nuevo paradigma energético en el que destaca la necesidad de generar más potencia eléctrica, pero a partir de fuentes de energía más limpias y que reduzcan las emisiones contaminantes asociadas. La energía solar de concentración (Concentrated Solar Power, CSP), que emplea la radiación solar como principal fuente de energía, es una de las opciones más interesantes entre las diferentes energías renovables. En los sistemas que emplean esta tecnología, se concentra la radiación solar normal para obtener energía térmica a altas temperaturas y, a continuación, esta energía se transforma en energía eléctrica mediante un ciclo termodinámico y un generador eléctrico. De este modo, la energía termosolar de concentración permite producir energía de forma fiable, estable, segura, eficiente y limpia puesto que reduce, o incluso elimina por completo, las emisiones contaminantes de efecto invernadero asociadas con los combustibles convencionales y los problemas derivados de ellas. Una de las principales ventajas de los sistemas de energía termosolar de concentración radica en su potencial para ser hibridados con otras fuentes de energía y para almacenar energía solar en forma de calor, de modo que se pueda producir energía eléctrica cuando se desee y que se complete y rectifique el aporte de calor solar, que es intrínsecamente variable. Esta tesis doctoral está dedicada a estudiar una planta de concentración termosolar (CSP) desde el punto de vista termodinámico; en concreto, una planta solar de torre central (Solar Power Tower, SPT) acoplada a un ciclo Brayton híbrido. La planta en estudio está formada por un campo de heliostatos que apuntan hacia un receptor solar, donde se absorbe la radiación solar. A continuación, se intercambia este calor solar concentrado con un fluido de trabajo que lo absorbe y desarrolla un ciclo Brayton. El objetivo de la planta es funcionar como planta de generación de carga base (baseload), es decir, producir y entregar a la red eléctrica una potencia neta constante e independiente de la radiación solar. Para ello, se hibrida la turbina de gas en serie con una cámara de combustión, lo cual asegura una temperatura de entrada a la turbina constante y, como consecuencia, una potencia de salida constante. Si el aporte de calor solar no es suficiente para alcanzar la temperatura de entrada a la turbina impuesta, entonces la cámara de combustión quema gas natural completando y rectificando así la entrada de calor solar. Respecto al estado de la cuestión, existe una escasez significativa de estudios que se centren en integrar todos los subsistemas y en analizar sus inter-relaciones y cómo afectan estas a la planta global. Por consiguiente, los objetivos de la tesis comprenden el desarrollo de un modelo teórico y su implementación en un código propio para realizar simulaciones, tanto en el punto de diseño como dinámicas, que puedan ofrecer información valiosa sobre las pérdidas de energía y sobre las configuraciones que traen consigo mejores registros de salida. En esta tesis doctoral se analizan dos tipos de sistemas diferentes teniendo en cuenta el tamaño de la planta y la simetría del campo de heliostatos. Primero se evalúa una planta similar a SOLUGAS con una potencia de alredor de 5 MW y un campo polar. En segundo lugar, se examina una planta más grande, de aproximadamente 20 MW, y con un campo circundante. En este caso, para el dimensionamiento de los parámetros se emplea la planta GEMASOLAR, aunque se simula un ciclo Brayton en vez del ciclo Rankine propio de GEMA- SOLAR. Asimismo, otro objetivo de este estudio es la comparación de dos unidades de potencia diferentes (turbina de gas y turbina de vapor), pero con potencias similares, y con dimensiones del subsistema solar también similares. Por otro lado, se validan las predicciones del modelo mediante varios paquetes de software comerciales y utilizando datos de la literatura. Además de esta validación, también se realizan una comparación y una simple contextualización de las variables de salida de los modelos de los diferentes subsistemas. En el código se implementan datos meteorológicos reales de la localización específica, tales como irradiancia solar directa normal (Direct Normal Irradiance, DNI) y temperatura ambiente. Asimismo, en el subsistema solar se tienen en cuenta otros parámetros de entrada como la altura de la torre, el tamaño del receptor, la reflectividad de los heliostatos o el área de los espejos. Los principales parámetros de la máquina térmica incluidos para la modelización de la planta son la relación de presiones, la temperatura de entrada a la turbina, el flujo de masa del fluido de trabajo, las eficiencias de la turbina y el compresor y las caídas de presión asociadas con la cesión y absorción de calor. De entre los parámetros de salida analizados, varios están relacionados con diferentes eficiencias: eficiencia térmica global, eficiencia óptica del campo de heliostatos, eficiencia del subsistema solar y eficiencia de la máquina térmica. Igualmente, se calculan todas las tempera- turas y todos los flujos de calor del ciclo. Al mismo tiempo, se estudian otras variables como la fracción solar o solar share, la potencia de salida, el consumo específico de combustible y las emisiones de efecto invernadero correspondientes. Desde la perspectiva termo-económica, se evalúan la energía neta, el Coste Normalizado de la Electricidad (Levelised Cost of Electricity, LCoE) y sus componentes. Con el objetivo de analizar el comportamiento de diferentes fluidos de trabajo, se simula un ciclo cerrado mediante intercambiadores de calor. Así, se estudian aire seco, nitrógeno, dióxido de carbono y helio. También se examina la influencia del número de etapas de compresión y expansión. Por otro lado, se lleva a cabo un proceso de pre-optimización buscando configuraciones óptimas para la relación de presiones que impliquen mejores valores de las variables de salida. Las simulaciones diarias confirman que se ha cumplido el objetivo de generar una potencia de salida estable. Por otro lado, el comportamiento estacional se traslada directamente a la anchura y a la altura de las curvas de evolución diaria de variables de salida tales como eficiencias y temperaturas. Una conclusión clave de las simulaciones anuales fuera de diseño es que, entre todos los subsistemas, la máquina térmica se asocia potencialmente con la mejora más significativa de los registros de salida analizados. Asimismo, se investiga la influencia del recuperador en el esquema de la planta y se ha demostrado que su presencia es positiva tanto desde el punto de vista termodinámico como termo-económico. Igualmente, también se varía la localización de la planta para evaluar su efecto en las variables del modelo. Finalmente, los análisis de sensibilidad llevados a cabo permiten demostrar que, respecto de la relación de presiones, el Coste Normalizado de la Electricidad presenta todavía potencial para su reducción. Aparte de estos resultados concretos, la tesis doctoral revela la importancia de diseñar como un todo los sistemas solares de torre central acoplados a una turbina de gas híbrida, teniendo en cuenta la interacción entre los diferentes subsistemas. Por tanto, esta tesis doctoral puede ser útil en una etapa inicial de diseño de futuros sistemas solares de concentración de torre central que realicen ciclos Brayton híbridos

Seasonal thermodynamic prediction of the performance of a hybrid solar gas-turbine power plant

[EN] An entirely thermodynamic model is developed for predicting the performance records of a solar hybrid gas turbine power plant with variable irradiance and ambient temperature conditions. The model considers a serial solar hybridization in those periods when solar irradiance is high enough. A combustion chamber allows to maintain an approximately constant inlet temperature in the turbine ensuring a stable power output. The overall plant thermal efficiency is written as a combination of the thermal efficiencies of the involved subsystems and the required heat exchangers. Numerical values of model input parameters are taken from a central tower installation recently developed near Seville, Spain. Real data for irradiance and external temperature are taken in hourly terms. The curves of several variables are obtained for representative days of all seasons: overall plant efficiency, solar subsystem efficiency, solar share, fuel conversion rate, and power output. The fuel consumption assuming natural gas fueling is calculated and the reduction in greenhouse emissions is discussed. The model can be applied to predict the daily and seasonal evolution of the performance of real installations in terms of a reduced set of parameters.MINECO of Spai

Thermodynamic model of a hybrid Brayton thermosolar plant

[EN]We present a thermodynamic model for the prediction of the performance records of a solar hybrid gas turbine power plant. Variable irradiance and ambient temperature conditions are considered. A serial hybridization is modeled with the aim to get an approximately constant turbine inlet temperature, and thus to deliver to the grid a stable power output. The overall thermal efficiency depends on the efficiencies of the involved subsystems and the required heat exchangers in a straightforward analytical way. Numerical values for input parameters are taken from a central tower heliostat field recently developed near Seville, Spain. Real data for irradiance and external temperature are taken in hourly terms. Curves for the evolution of plant efficiencies (solar, gas turbine, fuel conversion efficiency, overall efficiency, etc.) and solar share are presented for representative days of each season. The cases of non-recuperative and recuperative plant configurations are shown. Estimations of the hourly evolution of fuel consumption are simulated as well as savings between the hybrid solar operation model and the pure combustion mode. During summer, fuel saving can reach about 11.5% for a recuperative plant layout. In addition, plant emissions for several configurations are presented

On- and off-design thermodynamic analysis of a hybrid polar solar thermal tower power plant

[EN]Concentrated solar power (CSP) is one challenging renewable technology for the production of electricity. Within this concept central receiver solar plants combined with gas turbines are being investigated because of their promising efficiencies and reduced water consumption. Hybrid plants incorporate a combustion chamber in such a way that in periods of low solar irradiance power output can be kept approximately constant and so, electricity production is predictable. An integrated, non-complex solar thermodynamic model of a hybrid gas turbine solar plant is developed employing a reduced number of parameters with a clear physical meaning. The solar subsystem is modeled in detail, taking into account the main heliostats field losses factors as cosine effect, blocking and shadowing, or attenuation. An heliostat field with polar symmetry together with a cavity receiver are considered. The model is implemented in our own software, developed in Mathematica language, considering as reference SOLUGAS solar field (Seville, Spain). Heliostats field configuration is determined for the design point and its associated efficiency is computed. First, an on-design analysis is performed for two different working fluids (dry air and carbon dioxide), for recuperative and non-recuperative modes. A pre-optimization process is carried out regarding the pressure ratio of the gas turbine for different configurations. Some significant efficiency and power rises can be obtained when pressure ratio is adapted for each specific configuration and working fluid. Maximum achievable plant overall efficiency is 0.302 for both fluids in the recuperative mode, taking a pressure ratio of 7 for dry air and 16 for carbon dioxide. In non-recuperative configurations maximum overall efficiency is obtained for dry air, about 0.246. Moreover, a dynamic study is performed for four representative days of each season. Then, efficiencies and solar share are plotted against time. In addition, fuel consumption and greenhouse emissions are computed for all seasons

Thermo-economic and sensitivity analysis of a central tower hybrid Brayton solar power plant

[EN]A hybrid central tower thermo-solar plant working with a gas turbine is simulated by means of an in-house developed model and software. The model considers the integration of all plant subsystems. The calculation of the heliostat solar field efficiency includes the main losses factors as blocking, shadowing, attenuation, interception, and cosine effect. The simulation considers a Brayton cycle for the power unit with irreversibilities in the compressor and turbine, and pressure drops in the heat absorption and extraction processes. A combustion chamber burning natural gas ensures an approximately constant power output. The model is flexible and precise. At the same time it is fast enough to perform sensitivity studies on the efficiency of any subsystem and the overall plant. Thus, it allows for performing a thermo-economic analysis of the plant checking the influence of the main plant design parameters. The focal objective is to analyze the importance on the levelized cost of electricity (LCoE) of the key plant design parameters. The direct influence of parameters from the heliostat field and receiver (as tower height, distance to the first row of heliostats, heliostats size, receiver size and heat losses, etc.) on final LCoE is surveyed. Similarly, parameters from the turbine as pressure ratio, turbine inlet temperature, influence of recuperation and others, are also analyzed. The dimensions of the plant are taken from SOLUGAS prototype near Seville, Spain, although another location with quite different solar conditions in Spain is also considered. LCoE values predicted are about 158 USD/MWh. The analysis concludes that among several parameters surveyed, two of them are key in LCoE predicted values: turbine inlet temperature and solar receiver aperture size.Junta de Castilla y León of Spain (project SA017P17

On-design and off-design thermodynamic analysis of a hybrid multi-stage solar thermal tower power plant

[ENConcentrated solar power (CSP) is one challenging renewable technology for the future production of electricity. Within this concept central receiver solar plants combined with gas turbines are being investigated because of their promising efficiencies and reduced water consumption. Hybrid plants incorporate a combustion chamber in such a way that in periods of low solar irradiance power output can be kept approximately constant and so, electricity production is predictable. An integrated, non-complex solar thermodynamic model of a hybrid multi-stage gas turbine solar plant is developed employing a reduced number of parameters with a clear physical meaning. The solar subsystem is modelled in detail, taking into account the main heliostats field losses factors as cosine effect, blocking, or attenuation. The model is implemented in our own software, developed in Mathematica® language, considering as reference Gemasolar solar field (Seville, Spain). First, an on-design analysis is performed for four different working fluids (dry air, nitrogen, carbon dioxide, and helium), for different number of expansion and compression stages, and for recuperative and non-recuperative modes. Moreover, heliostats field configuration is determined for the design point and its associated efficiency is computed. A pre-optimization process is carried out regarding the pressure ratio of the gas turbine for different configurations. Some significant efficiency and power rises can be obtained when pressure ratio is adapted for each specific configuration and working fluid. Three particular plant configurations are chosen for the off-design analysis due to their interesting behaviours. For these configurations, a dynamic study is performed for four representative of each season. Then, efficiencies and solar share are plotted against time. In addition, fuel consumption and greenhouse emissions are computed for all seasons. Heliostats efficiency varying with the season and the solar time is also forecasted. Keywords: Dynamic analysis, On-design pre-optimization, Multi-stage gasJunta de Castilla y León of Spain (project SA017P17

Towards a Sustainable Future through Renewable Energies at Secondary School: An Educational Proposal

[EN]A compilation of innovative educational activities to work on concepts related to the production of electrical energy is presented. To approach the real-life secondary education curriculum, they are grouped to be performed during a week denominated Renewable Energy Week: an educational proposal aimed to promote the respect for the environment through the insight on Sustainable Development Goals (SDG) and renewable energy sources. The students would build and perform low-cost experiments so as to deeply understand the essence of energetic transformations, as well as electricity generation. Learning by discovery, collaborative learning and experimentation, are the methodological pillars that characterize Renewable Energy Week, since they have been proven to be efficient methodologies to promote students’ learning. Innovative techniques for pupils evaluation are employed, including a rubric, Socrative application and a set of sheets regarding experiments. Through this educational proposal, the students are expected to achieve a deep understanding of some key concepts related to electricity and awaken their interest in scientific subjects, making them conscious of the transition to sustainable development that our planet urgently requires. At the same time, this project offers to teachers a series of experiments and innovative activities to work on the SDG in Physics, Chemistry and Technology subjects.University of Salamanca through Innovation and teaching improvement project ID 2019/16

Thermo-economic study of hybrid parabolic dish solar power plants in different regions of Spain

[EN]Small-scale hybrid parabolic dish Concentrated Solar Power (CSP) systems coupled to a micro-gas turbine are a promising option to obtain electrical energy in a distributed manner. During the day, solar energy is used to produce electricity and the absence of sunlight can be overcome with the combustion of a fossil or renewable fuel. This study presents the technical feasibility and thermo-economic model of a hybridized power plant in different regions of Spain, considering the local climatic conditions. The implemented model aims to provide a realistic view of the behaviour of the system, using a reduced number of selected parameters with a clear physical meaning. The irreversibilities taking place in all subsystems (solar part, combustion chamber, micro-gas turbine, and the corresponding heat exchangers) have been considered in the model, developed in Mathematica® language. The model considers the instant solar irradiance and ambient temperature dynamically, providing an estimation of the power output, the associated fuel consumption, and the most relevant pollutant emissions (CO2, CH4 and NO2) linked to combustion, for hybrid and combustion only operating modes at selected geographical locations in Spain. The considered power output ranges between 7 to 30 kWe which is achieved by varying the design specifications. The levelized cost of electricity (LCoE) indicator is estimated as a function of investment, interest rate, maintenance and fuel consumption actual costs in Spain. The electricity costs from hybrid parabolic dish are between 22% and 27% lower compared to pure combustion power plant, while specific fuel consumption and therefore CO2 emissions can be reduced up to 33%. This model shows the potential of hybrid solar dishes to become cost-competitive against non-renewable ones from the point of view of electricity costs and significant reduction in gas emission levels in regions with high solar radiation and low water resources.Junta de Castilla y Leó

On-design pre-optimization and off-design analysis of hybrid Brayton thermosolar tower power plants for different fluids and plant configurations

[EN]A working fluid performs a Brayton cycle that is fed by a heat input from a solar power tower and from a combustion chamber, which burns natural gas. This hybrid system is described by a complete model that includes all the main losses and irreversibility sources (optical and thermodynamic). Numerical implementation and validation is performed based on a Spanish commercial plant. On-design computations are carried out varying the pressure ratio for four working fluids (dry air, nitrogen, carbon dioxide, and helium), for different number of stages and for recuperative and non-recuperative configurations. When adjusting the pressure ratio, an improvement of about 7% in overall thermal efficiency is predicted for a dry air single-stage recuperative configuration with respect to a standard commercial gas turbine. A study about the main energy losses in each plant subsystem for some particular plant layouts is accomplished. A two-compression and expansion stages recuperative Brayton cycle working with air is expected to give overall thermal efficiencies about 0.29 at design conditions, which is about a 47% increase with respect to the simplest single-stage configuration. It is stressing that fuel consumption from the reheaters maybe higher than that of the main combustion chamber for multi-stage layouts. Off-design hourly curves of output records for the four seasons throughout a day are analyzed. Greenhouse emissions are also analyzed. Specific carbon dioxide emissions are smaller for helium than for dry air, when they both work in a single-stage non-recuperative configurationJunta de Castilla y León SA017P1

Micro Gas Turbine and Solar Parabolic Dish for distributed generation

[EN]A thermodynamic model for a Brayton-like microturbine in combination with a solar parabolic dish is analyzed in order to evaluate its efficiency under any ambient condition. The thermodynamic cycle is a recuperative Brayton cycle with internal irreversibilities in the recuperator, compressor and turbine and external losses associated to the heat transfers in the solar receiver, the combustion chamber, and the environment. All the irreversibilities have been taken into account in the model with home-software elaborated using Mathematicaâ. The model validation is done by comparison with results provided by Semprini et al. [1]. An analysis of hybrid and sunless performance is carried out for four different microturbine power outlets (30, 23, 15 and 7 kWe) and for four days of the year (corresponding to each season). The greenhouse emissions are also calculated for both off-design performance and for the four power output levels