818 research outputs found
Abstract State Machines 1988-1998: Commented ASM Bibliography
An annotated bibliography of papers which deal with or use Abstract State
Machines (ASMs), as of January 1998.Comment: Also maintained as a BibTeX file at http://www.eecs.umich.edu/gasm
Integration of CasADi and JModelica.org
This paper presents the integration of two open source
softwares: CasADi, which is a framework for efficient
evaluation of expressions and their derivatives, and the
Modelica-based platform JModelica.org. The integration of the tools is based on an XML format for exchange of DAE models. The JModelica.org platform supports export of model in this XML format, whereas CasADi supports import of models expressed in this format. Furthermore, we have carried out comparisons
with ACADO, which is a multiple shooting package for solving optimal control problems.
CasADi, in turn, has been interfaced with ACADO Toolkit, enabling users to define optimal control problems using Modelica and Optimica specifications, and use solve using direct multiple shooting.
In addition, a collocation algorithm targeted at solving large-
scale DAE constrained dynamic optimization problems has been implemented. This implementation explores CasADi’s Python and IPOPT interfaces, which offers a convenient, yet highly efficient environment for development of optimization algorithms. The algorithms are evaluated using industrially relevant benchmark problems
Verification of Timed Automata Using Rewrite Rules and Strategies
ELAN is a powerful language and environment for specifying and prototyping
deduction systems in a language based on rewrite rules controlled by
strategies. Timed automata is a class of continuous real-time models of
reactive systems for which efficient model-checking algorithms have been
devised. In this paper, we show that these algorithms can very easily be
prototyped in the ELAN system. This paper argues through this example that
rewriting based systems relying on rules and strategies are a good framework to
prototype, study and test rather efficiently symbolic model-checking
algorithms, i.e. algorithms which involve combination of graph exploration
rules, deduction rules, constraint solving techniques and decision procedures
Steady-state initialization of object-oriented thermo-fluid models by homotopy methods
The steady-state initialization of large object-oriented
thermo-hydraulic networks is a difficult problem, because of the sensitivity of the convergence to the initial guesses of the iteration variables. This paper proposes an approach to this problem based on homotopy transformation, detailing specific criteria for model simplifications in this physical domain. The approach is successfully demonstrated on large power plant test cases, having several hundreds of iteration variables
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
- …