Lidia Roca

Robust Nonlinear Predictive Control Applied to a Solar Collector Field in a Solar Desalination Plant

This brief presents the application of a robust nonlinear predictive controller to the distributed collector field of a solar desalination plant. The main purpose of the controller is to manipulate the water flow rate to maintain the collector outlet-inlet temperature gradient constant in spite of disturbances. The controller uses a robust dead-time compensation structure and a nonlinear model predictive control to cope with time delay uncertainties and system nonlinearities, respectively. Simulation and real experimental results are shown to illustrate controller performance.

Modeling of a Solar Seawater Desalination Plant for Automatic Operation Purposes

Desalination plants play a fundamental role in the fight against fresh water shortage in places with seawater availability. On the other hand, optimization of industrial processes is an important goal in order to increase operation efficiency and to minimize costs. This paper deals with an experimental hybrid solar-gas desalination system designed, built, and operated at the Plataforma Solar de Almeria (Spain) under the framework of a European R&D project. This desalination system, based on a multieffect distillation process, is partially powered by a low temperature static solar collector field of compound parabolic concentrator type. With the aim of performing an optimal operation of the plant working with the solar resource, process dynamics have to be studied and modeled. This paper shows control-oriented low complexity models for the solar field and thermal storage subsystems as well as a model of the distillate production rate depending on process temperature. These models are validated with real data with the objective of being useful in the evaluation of optimal operating points, the design of operational procedures, and the establishment of adequate control references to maximize the use of the solar resource and to improve process efficiency.

Energy recovery using salinity differences in a multi-effect distillation system

AbstractThe use of Salinity Gradient methodologies to recover part of the osmotic energy in the brine of multi-effect distillation (MED) systems is explored here. Measurements from a membrane-based Pressure Retarded Osmosis laboratory system have been used to estimate the energy that would be recovered from this brine, when a source of low-salinity water is available locally (such as industrial or municipal wastewater). This methodology has been evaluated for a specific case study (72 m3/d solar/gas MED system) at different temperatures.

Practical MPC with Robust Dead-Time Compensation Applied to a Solar Desalination Plant

Abstract A practical model predictive control (MPC) for solar collector plants is proposed in this paper. By exploiting the non-linear structure of the solar collector model, the optimization problem is posed as a quadratic program similarly to a linear MPC. As applied in some recent publications, a robust dead-time compensation scheme is used to improve robustness. A different state-space interpretation of the robust compensation scheme is also presented. Simulation results including comparisons with other control schemes are shown in order to improve discussions.

Control strategies in a thermal oil – Molten salt heat exchanger

This paper presents a preliminary control scheme for a molten salt – thermal oil heat exchanger. This controller regulates the molten salt mass flow rate to reach and maintain the desired thermal oil temperature at the outlet of the heat exchanger. The controller architecture has been tested using an object-oriented heat exchanger model that has been validated with data from a molten salt testing facility located at CIEMAT-PSA. Different simulations are presented with three different goals: i) to analyze the controller response in the presence of disturbances, ii) to demonstrate the benefits of designing a setpoint generator and iii) to show the controller potential against electricity price variations.

Object-oriented modelling and simulation of ACUREX solar thermal power plant

Research and development of advanced control systems to optimize the overall performance of parabolic trough collector (PTC) solar power plants is a priority line of research at the CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas - Research Centre for Energy, Environment and Technology). These developments are underway at the CIEMATs Plataforma Solar de Almera (PSA), a highly specialized solar energy technology research centre. Some dynamic models for use in simulation and control of this type of solar power plant, developed in the PSA ACUREX facility, are presented in this article. The models developed are based on the ThermoFluid thermohydraulic modelling framework, in the Modelica modelling language. The ACUREX facility is presented with the main components modelled and their respective modelling assumptions. An operation strategy is presented and modelled based on the StateGraph Modelica library for discrete and reactive system modelling. A typical experiment is simulated with real experimental input data, a discrete operating strategy is designed and predicted model data are presented and discussed.

Solar membrane distillation: A control perspective

This paper presents a preliminary architecture for controlling solar membrane distillation facilities. Membrane distillation (MD) is a desalination technology under investigation, without industrial applications and with only few pilot systems. There is a general lack of information on how these systems perform during real solar-powered operation and how distillate quality deteriorates under intermittent conditions. One of the few systems that have been fully described in literature is the MD-solar pilot plant in Plataforma Solar de Almería (PSA), Spain. This paper presents a first step towards an optimal automatic operation of this system through the definition of operation modes and basic control loops. A discussion of control schemes to be implemented in the near future is also included in this work.

Filtered Smith Predictor with nonlinear model applied to a solar field

Model predictive control such as Generalized Predictive Control (GPC) combined with feedback linearization (GPC-FL) has shown to be a successful methodology to regulate the outlet temperature in a solar field. In addition, the benefits of input-output feedback linearization have been improved by a Dead Time Compensator (DTC) to avoid unstable or highly oscillatory responses caused by plant-model mismatch. In this paper, a Filtered Smith Predictor (FSP) controller is proposed using a nonlinear model (FSP-NLM) of the plant instead of a nominal linear one. Simulation studies compare DTC-GPC-FL with FSP-NLM and advantages of each one are explained.

Using a Nonlinear Model Predictive Control strategy for the efficient operation of a solar-powered membrane distillation system

This paper presents an optimal operation strategy in terms of thermal efficiency and distillate flux production for a Solar Membrane Distillation (SMD) system. Firstly, a study of the Membrane Distillation (MD) module is presented, revealing the optimal operation strategy. Secondly, a hierarchical control system with two layers is developed and tested. The upper layer consists on a Nonlinear Model Predictive Control (NMPC) scheme which allows us to obtain the maximum temperature at the inlet of the MD module, by optimizing the use of solar energy. The lower layer is composed by a direct control system that is in charge of reaching the setpoint calculated by the upper layer. Simulation results are shown in order to demonstrate the effectiveness of this control approach.

Control strategies applied in the HYSOL demonstrator: A simulation-based evaluation

HYSOL project was born under the idea of designing a new hybrid concentrating solar power plant concept fully based on renewable energies and able to provide stable power. Within this project, one of the main activities is the development of a demonstrator facility to evaluate experimentally the gas turbine heat recovery and its transference to a molten-salt storage system. This paper describes the main control loops proposed to maintain the facility at different operating points to study both nominal and off-design states.