Dynamic simulation of steam generation system in solar tower power plant

Concentrated solar power (CSP) plant with thermal energy storage can be operated as a peak load regulation plant. The steam generation system (SGS) is the central hub between the heat transfer fluid and the working fluid, of which the dynamic characteristics need to be further investigated. The SGS of Solar Two power tower plant was selected as the object. The mathematical model with lumped parameter method was developed and verified to analyze its dynamic characteristics. Five simulation tests were carried out under the disturbances that the solar tower power plant may encounter under various solar irradiations and output electrical loads. Both dynamic and static characteristics of SGS were analyzed with the response curves of the system state parameters. The dynamic response and time constants of the working fluids out of SGS was obtained when the step disturbances are imposed. It was indicated that the disturbances imposed to both working fluids lead to heat load reassignment to the preheater, evaporator and superheater. The proposed step-by-step disturbance method could reduce the fluid temperature and pressure fluctuations by 1.5 °C and 0.03 MPa, respectively. The results could be references for control strategies as well as the safe operation of and SGS.Peer reviewe

Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives

Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in concentrated solar power (CSP) plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-like storage concept, which aims at circumventing this issue. The proposed concept consists of a storage tank filled with a combination of solid material and encapsulated PCMs, forming a multi-layered packed bed, with molten salt as the heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until periodic state is achieved, i.e. when the same amount of energy is stored/released in consecutive charging/discharging cycles. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy and exergy stored/released, as well as the efficiencies of the storing process are compared for the different thermocline, single PCM, cascaded PCM and the proposed multi-layered solid-PCM (MLSPCM) configurations. The analysis shows that the multi-layered solid-PCM concept is a promising alternative for thermal storage in CSP plants.Peer ReviewedPostprint (author’s final draft

Thermo-mechanical parametric analysis of packed-bed thermocline energy storage tanks

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A packed-bed thermocline tank represents a proved cheaper thermal energy storage for concentrated solar power plants compared with the commonly-built two-tank system. However, its implementation has been stopped mainly due to the vessel’s thermal ratcheting concern, which would compromise its structural integrity. In order to have a better understanding of the commercial viability of thermocline approach, regarding energetic effectiveness and structural reliability, a new numerical simulation platform has been developed. The model dynamically solves and couples all the significant components of the subsystem, being able to evaluate its thermal and mechanical response over plant normal operation. The filler material is considered as a cohesionless bulk solid with thermal expansion. For the stresses on the tank wall the general thermoelastic theory is used. First, the numerical model is validated with the Solar One thermocline case, and then a parametric analysis is carried out by settling this storage technology in two real plants with a temperature rise of 100 °C and 275 °C. The numerical results show a better storage performance together with the lowest temperature difference, but both options achieve suitable structural factors of safety with a proper design.Peer ReviewedPostprint (author's final draft

Object-oriented modeling for the transient performance simulation of solar thermal power plants using parabolic trough collectors: a review and proposal of modeling approaches for thermal energy storage

La intención de este trabajo es extender las técnicas actuales de modelización del almacenamiento térmico activo directo y activo indirecto, con dos tanques y sales fundidas como medio de almacenamiento. Con el objetivo de conseguir aumentar el conocimiento sobre su comportamiento térmico y los aspectos operacionales, los modelos desarrollados deben permitir la evaluación del sistema de almacenamiento térmico en condiciones transitorias. Así, la parte principal de este trabajo (la Parte II) se centra en la modelización y evaluación del comportamiento de los intercambiadores de calor para la tecnología de almacenamiento térmico activo indirecto, que emplea sales fundidas (60% en peso de nitrato sódico, NaNO3, y 40% en peso de nitrato potásico, KNO3) como medio de almacenamiento y aceite térmico (una mezcla de difenilo, C12H10, y oxido de difenilo, C12H10O) como fluido caloportador. Asumiendo un diseño de intercambiador de calor del tipo carcasa y tubos, el comportamiento del proceso de intercambio de calor entre el medio de almacenamiento y el fluido caloportador se analiza en detalle, considerando condiciones de operación estacionarias y transitorias bajo cargas nominales y parciales. El modelo estacionario proporciona información útil sobre el coeficiente global de transmisión de calor y los rangos de variación de pérdidas de carga para dos configuraciones de intercambiadores de calor específicas. Se demuestra que la configuración de dos intercambiadores en paralelo supera a la configuración convencional de un único intercambiador en funcionamiento. Por otro lado, la evaluación del modelo transitorio suministra paráametros típicos del proceso como la ganancia, el tiempo muerto y la constante del tiempo para el modo de carga y descarga, en condiciones nominales y parciales. Además, se ha obtenido un modelo transitorio del tanque de almacenamiento a alta temperatura razonablemente simple, el cual es muy adecuado para simulaciones del comportamiento de centrales CSP en su conjunto. En el estudio se ha demostrado que las pérdidas térmicas por convección natural en la atmosfera de gas encima de la superficie libre de las sales fundidas se pueden omitir en el modelo, causando errores despreciables. También, se pueden asumir coeficientes de convección constantes entre la superficie de las paredes del tanque y las sales fundidas. Sin embargo, la transmisión de calor por radiación entre la superficie libre de las sales fundidas y las paredes interiores del tanque, que no están en contacto con las sales, deben de ser consideradas, dada su importante influencia en las pérdidas totales. Además, debido al modelado de la trasmisión de calor por las paredes del tanque en modo transitorio y al cálculo preciso de la temperatura de la superficie exterior, la influencia que las condiciones de contorno ambientales tienen sobre las pérdidas de calor, pueden ser caracterizadas de manera mucho más adecuada que mediante métodos cuasi-estacionarios, que solo tienen en cuenta la temperatura ambiente. Finalmente, la Parte III trata de la aplicación de los modelos desarrollados para los componentes del almacenamiento térmico, a un modelo exhaustivo y completo de una central de captadores cilindro-parabólicos a nivel global. De este modo se simula, no solo el comportamiento del sistema de almacenamiento térmico activo indirecto, sino también las respuestas de la central solar térmica al completo, debido a los cambios en las condiciones de contorno ambientales. Se observa que la inercia térmica del sistema de almacenamiento activo indirecto es muy considerable, influyendo de manera notable en los rápidos cambios de carga necesarios para capturar la mayor cantidad posible de la energía solar disponible, y para alimentar el bloque de potencia con una potencia térmica constante, independientemente de la actual radiación solar. Por último pero no menos importante, los modelos presentados han sido desarrollados de manera flexible, bien estructurada y con una programación orientada a objetos, particularmente dando importancia a una implementación independiente de la plataforma de simulación, hecho que ha sido llevado a cabo utilizando el lenguaje de modelación Modelica. Este es un lenguaje de modelizado de sistemas físicos multiobjetivo, que ha sido desarrollado en un esfuerzo internacional para unificar las técnicas de simulación ya existentes y para permitir el intercambio fácil de los modelos y librerías de modelos que se desarrollen. El concepto de Modelica se basa en modelos no causales que utilizan ecuaciones diferenciales ordinarias y algebraicas.This work's intention is to extend the current state-of-the-art regarding the modeling of the active direct and the active indirect two-tank moltensalt- based thermal energy storage (TES) concept. The aim is to widen the knowledge about their thermal behavior and operational aspects. In particular, the developed models shall enable the evaluation of the storage system's transient behavior. Thus, the main part of this work (Part II) focuses on the modeling and the performance evaluation of oil-to-molten salt heat exchangers for the active indirect thermal energy storage technology, applying molten salt (60%, by weight, sodium nitrate, NaNO3, and 40%, by weight, potassium nitrate, KNO3) as storage medium and thermal oil (a mixture of diphenyl, C12H10, and diphenyl oxide, C12H10O) as heat transfer fluid. Assuming a shell-andtube heat exchanger design, the performance of the heat exchange process between the storage medium and the heat transfer fluid is discussed in detail, considering steady-state as well as transient operating conditions under nominal as well as partial loads. On the one hand, the steady-state model gives useful information about overall heat transfer coefficient and pressure drop ranges for two specific heat exchanger setups. In particular, it is shown that two separate heat exchanger trains in parallel exceed the conventional single train setup in performance. On the other hand, the evaluation of the transient performance model yields typical process parameters as process gain, dead time and time constant for charging as well as for discharging mode at representative heat exchanger loads. In addition to this, a reasonable simple transient high-temperature storage tank model is derived, which is well suited for CSP performance simulations on system level due to reasonable model simplifications. For example, it is found in this work that the convective heat losses via the tank's gas atmosphere (usually nitrogen at ambient pressure) above the molten salt surface can be neglected by only introducing negligible calculation errors. Also, the convective heat transfer coefficients between the molten salt and the wetted parts of the tank's inner steel jacket may be set to constant values. However, the important radiative heat transfer between the surface of the molten salt and the non-wetted parts of the tank's inner steel jacket must be considered, which is done assuming an ideal cylindrical geometry. Furthermore, due to the transient modeling of the storage tank walls and a detailed estimation of the exterior surface temperature, the influence of altering environmental boundary conditions can be captured more accurately than by quasi-steadystate methods that only account for the current ambient air temperature. Finally, Part III treats the application of the developed TES model components in a comprehensive model of a parabolic trough collector plant on system level, showing not only the behavior of a typical active indirect twotank TES system under transient operating conditions, but also the responses of the entire solar thermal power plant to changing environmental boundary conditions. It is shown that the thermal inertia of the active indirect TES concept is considerable and forms a major obstacle for rapid load changes that are crucial for capturing as much solar energy as possible, and to supply the power block with constant thermal power, independently of the current solar irradiance. Last but not least, the presented models have been developed in a flexible, well-structured and object-oriented way, particularly giving importance to a simulation-platform-independent implementation, which has been accomplished applying Modelica, a multi-purpose physical system modeling language, developed in an international effort in order to unify already existing similar modeling approaches, and to enable developed models and model libraries to be easily exchanged. Modelica's concept is based on non-causal models featuring true ordinary differential and algebraic equations.Programa Oficial de Doctorado en Ingeniería y Arquitectura (RD 1393/2007)Ingeniaritzako eta Arkitekturako Doktoretza Programa Ofiziala (ED 1393/2007

Analytical approach to ground heat losses for high temperature thermal storage systems

A new approach to estimate the heat loss from thermal energy storage tank foundations is presented. Results are presented through analytical correlations based on numerical solutions for the steady-state heat conduction problem for thermal energy slab-on-grade tanks with uniform insulation. Model results were verified with other well-established benchmark problems with similar boundary conditions and validated with experimental data with excellent agreement. In addition to the TES foundation heat loss, new correlations for the maximum temperature and for the radial evolution of the temperature underneath the insulation layer are also provided, giving important information related to the tank foundation design. The correlated variables are of primordial importance in the tank foundation design because, due to the typical high operating storage temperatures, an inappropriate tank foundation insulation would lead not only to a not desired loss of energy but also to an inadmissible increase of the temperatures underneath the insulation layer, affecting the structural stability of the tank. The proposed correlations provide a quick method for the estimation of total tank foundation heat losses and soil maximum temperature reached underneath the insulation layer, saving time, and cost on the engineering tank foundation design process. Finally, a comprehensive parametric analysis of the variables of interest is made and a set of cases covering a wide range of tank sizes, insulation levels, depths to water table, and storage temperatures are solved

The Calcium-Looping (CaCO3/CaO) Process for Thermochemical Energy Storage in Concentrating Solar Power Plants

Articulo aceptado por la revista. * No publicado aún [28-06-2019]Energy storage based on thermochemical systems is gaining momentum as potential alternative to molten salts in Concentrating Solar Power (CSP) plants. This work is a detailed review about the promising integration of a CaCO3/CaO based system, the so-called Calcium-Looping (CaL) process, in CSP plants with tower technology. The CaL process relies on low cost, widely available and non-toxic natural materials (such as limestone or dolomite), which are necessary conditions for the commercial expansion of any energy storage technology at large scale. A comprehensive analysis of the advantages and challenges to be faced for the process to reach a commercial scale is carried out. The review includes a deep overview of reaction mechanisms and process integration schemes proposed in the recent literature. Enhancing the multicycle CaO conversion is a major challenge of the CaL process. Many lab-scale analyses carried out show that residual effective CaO conversion is highly dependent on the process conditions and CaO precursors used, reaching values as different as 0.07-0.82. The selection of the optimal operating conditions must be based on materials, process integration, technology and economics aspects. Global plant efficiencies over 45% (without considering solar-side losses) show the interest of the technology. Furthermore, the technological maturity and potential of the process is assessed. The direction towards which future works should be headed is discussed.Ministerio de Economia y Competitividad CTQ2014-52763-C2, CTQ2017- 83602-C2 (-1-R and -2-R)Unión Europea Horizon 2020 Grant agreement No 727348, project SOCRATCES

Dynamic Modeling of a Parabolic Trough Solar Power Plant

Models for dynamic simulation of a parabolic trough concentrating solar power plant were developed in Modelica for the simulation software Dymola. The parabolic trough power plant has a two-­‐tank indirect thermal storage with solar salt for the ability to dispatch electric power later in the evening and during the night when little or no solar irradiation is present. The complete system consists of models for incoming solar irradiation, a parabolic trough collector field, thermal storage and a simplified Rankine cycle. A parabolic trough power plant named Andasol located in Aldeire y La Calahorra, Spain, is chosen as a reference system when the complete system model is designed and built in Dymola. The system model is later validated against performance numbers from this reference system in order to make sure a correct implementation has been made. The collector and solar model have also been validated against different papers regarding solar collector performance and show good results. Finally the system in Dymola has been simulated during a day for different parts of the year with solar data from the same region as the Andasol power plant. The results from these simulations show similarities to how the Andasol power plant operates and how well the system performs under different solar irradiance conditions. By modeling the system dynamically the changes in insolation over time can be studied to gain information about how it affects the system model in detail. Plant startup time and how long the system can be running on the thermal storage is other parameters that could be studied with this dynamic model

Energy optimization of a concentrated solar power plant with thermal storage

One of the most relevant problems to solve at a planetary scale is the access to an affordable clean source of energy as CO2 equivalent emissions should be reduced significantly. Some authors aim for a zero emissions target for 2050. Renewable energies will play a leading role in this energy transition, and solar energy with storage is a promising technology exploring a renewable and worldwide available resource. Within the present thesis component development like a new thermal storage thermocline tank design or having latent heat storage capability are technological developments that have been pursued and analyzed on a system perspective basis, focusing on reducing the LCOE value of a commercial STE plant using TRNSYS software. Material research with molten salts mixtures and cement based materials has been performed at lab scale. A fully validation should occur through a 13 partners pan-European H2020 project called NEWSOL which has been developed supported on the laboratory data obtained. Moreover, incorporation of local available material, “modern slag” from an old mine of Alentejo region, was also studied. The material could be used as an aggregate incorporated into calcium aluminate cement (CAC) or as filler. This would help to solve a local environmental complex problem related to soil, air and water pollution due to heavy metals and mining activity in Mina de São Domingos, Southeast of Portugal. The integration of these results underlies a broad energy transition model, a proposal is presented in this thesis, with the aim to foster development towards a sustainable usage of resources and promote clean technologies especially in the energy sector. This model can be locally adapted depending on the pattern of existing industries. The goal is to achieve a smooth transition into a clean tech energy society in line with the target of achieving zero emissions for 2050; Optimização Energética de uma Central de Concentração Solar com Armazenamento de Energia Resumo: Um dos problemas mais relevantes a resolver a uma escala planetária é o acesso, com um custo moderado, a fontes limpas de energia considerando que as emissões equivalentes de CO2 derão ser reduzidas drasticamente. Alguns autores ambicionam mesmo um objetivo de zero emissões em 2050. As energias renováveis irão desempenhar um papel preponderante nesta transição energética, sendo que a energia solar com armazenamento é uma tecnologia promissora que aproveita um recurso renovável e disponível em boa parte do Planeta. Na presente tese foi realizado o desenvolvimento de componentes nomeadamente o design que um novo tanque do tipo termocline, ou de novos elementos recorrendo ao calor latente, desenvolvimentos tecnológicos que foram analizados de uma perspectiva de sistema, dando o enfoque na redução do custo nivelado da electricidade (LCOE) para uma planta Termosolar usando o software TRNSYS. Foi também realizada investigação em laboratório ao nível dos materiais com várias misturas de sais fundidos inclusivé em contacto directo com materiais de base cimenticia. Uma validação completa deverá ocorrer no projeto NEWSOL do programa H2020 que reúne um consórcio de 13 parceiros europeus e que foi preparado e submetido tendo por base os resultados laboratoriais obtidos. Adicionalmente, incorporação de material disponível (escória de minério) de uma mina abandonada da região do Alentejo foi outro dos aspectos estudados. Verificou-se que este material poderá ser utilizado como agregado num ligante do tipo cimento de aluminato de cálcio (CAC) ou como “filler”. Este re-aproveitamento resolveria um problema ambiental complexo derivado do elevado conteúdo de metais pesados resultantes da actividade de mineração e que actualamente provocam poluição do solo, água e ar na área da Mina de São Domingos, Sudeste de Portugal. Estes progressos deverão ser integrados num modelo de transição energética mais amplo. Na presente tese, uma proposta concreta é apresentada, com o objectivo de incentivar o desenvolvimento na direção de uma utilização sustentável dos recursos e a promoção de tecnologias limpas nomeadamente no sector da energia. Este modelo poderá ser adaptado localmente dependendo do padrão de indústrias existente. O objectivo é atingir uma transição suave para uma sociedade de energias limpas em linha com o objectivo de atingir zero emissões de CO2 equivalente em 2050

Solar thermal technology evaluation, fiscal year 1982. Volume 1: Executive summary

Three primary solar concepts the central receiver, parabolic dish, and parabolic trough are investigated. To a lesser extent, the hemispherical bowl and salt-gradient solar pond are also being studied. Each technology is described